Seed Priming with Phytohormones

*Musa Saheed Ibrahim, Nathan Moses and Beckley Ikhajiagbe*

### **Abstract**

Improving growth and yield properties of plats has been the major aim of most researchers in plat science field. Several strategies have been suggested in order to sustainably improve crop yield. Among these strategies is biopriming, has gained the highest attention being the most effective strategy. Biopriming is a technique involving pre-soaking of plant seed into a solution in order for the metabolic processes to be enhanced before to germination, thereby improving the percentage and rate of germination and increase seedling growth and crop yield under normal and different environmental stresses. The most important aspects of phytohormones is that they are very essential in the regulation of plant development and growth and also functions as an essential chemical messengers, allowing plants to thrive even during exposure to various stresses. Priming plant seeds with phytohormones has led to improved growth and yield of plants in developing countries. Furthermore, it has emerged as an important tool for mitigating the effects of environmental stress. However, this innovation has received less attention from local farmers and merger work has been reported. Therefore, this review discusses the mechanism and potential role of priming with phytohormones to enhance crop productivity and improve plant tolerance to biotic and abiotic stressors.

**Keywords:** biopriming, phytohormones, plant growth regulators, biotic stress, abiotic stress

### **1. Introduction**

Plant hormones since their discovery in seventeenth century have been used extensively in crop production. Advent of technology allowed the researchers to study more about different plant hormones and their endogenous and exogenous uses [1]. Plant hormones are a group of naturally occurring organic substances that are produced by plants and has effects on plant physiological processes when released at low concentrations. Plant hormones also referred to as phytohormones have the ability to influence growth, differentiation, development, and stomatal movement [2]. These hormones are well known for their important roles in plant physiology such as regulation of plant growth and development and important chemical messenger [3]. The first plant hormone that was identified was auxin. This plant hormone has a wide range effect on plant growth and development. Recently, it has been clearly shown that plant hormones are not exactly like the mammalian hormones. Even though the synthesis of plant hormone may be localized as in the case of animal hormones, but plant hormones may be transported to a long distance where it is needed most [4]. Auxins (IAAs), cytokinins (CKs), gibberellins (GAs),

abscisic acid (ABA), salicylic acid (SA), and ethylene (ET) are of the most essential phytohormones that are important for plant growth and development [5, 6]. These plant hormones have series of ways by which they improve growth and development of plants.

Strategies and mechanisms of growth promotion by phytohormones have been assessed, and several hormones have been categorized in different classes [7]. Major functions of these phytohormones are cell enlargement such as in auxin [6]. Cell division such as auxin, which stimulates the division of cells in the cambium and sometimes together with cytokinin in tissue culture media [3]. Vascular tissue differentiation such as in indole acetic acid which stimulates fast differentiation of phloem and xylem, initiation of plant root, and also the development of secondary roots under normal growth media and tissue culture media [6]. In most cases for plants, synthesis of hormones and plant response determines plant health and may in turn improve soil fertility [8]. Plant hormones have been very helpful in sustainable agriculture by stimulate fast growth and development in plants, which can be achieved through processes such as by spraying on the leaf and other plant tissues [7]. It is now important to consider introducing the endogenous and exogenous plant hormones into the growing seeds to improve stimulating rates. This process is called seed biopriming.

Seed biopriming is a unique innovation where seeds are treated with biological substances (such as bacteria, fungi, and hormones) that assimilates plant morphology and physiological facets [9]. This process has also been used to fortify plant against diseases [10]. Hormonal priming is a situation where plant seed are prime with phytohormones. This process has been documented to influence seed metabolism and germination rate. This technique has now been adopted in developing countries to enhance seed germination, growth of seedlings, and crop yield in environmentally disturbed arid lands [11, 12]. Ensuring better germination, improved plant growth and seedling vigor through seed biopriming would result in healthy and productive plants. There are several approaches to seed priming, however, all followed similar mechanisms and are used in improving plant growth properties [8]. Therefore, the purpose of this review is to summarize the effectiveness of seed priming using phytohormones in enhancing crop productivity along with future prospects of this innovative technology. In order to achieve this, this review discusses mechanisms involved in hormonal seed priming, hormone specific in seed priming, biopriming and crop productivity, role of hormonal priming in plant stress mitigation, economics of hormonal priming and future prospects of hormonal priming.

### **2. Mechanisms involved in seed biopriming**

There are several approaches used in biopriming such as hydro priming which involve soaking seeds in distilled water and oven drying at low temperature before sowing. This process does not involve the use synthetic chemicals which makes it faster, cheaper, and eco-friendlier [13]. Osmo-priming has also been considered by local farmers in Brazil where seeds are soaked in salt-containing solution. This process allows slow imbibition of water into the seed and that initiate energy activation [14]. Hormopriming which involves the soaking of seeds in naturally occurring plant growth regulators. This process has direct effect on processes of seed metabolism. Usually, scientist considered abscisic acid, auxins, gibberellins, kinetin, ethylene, polyamines, and salicylic acid as the hormones widely used in priming [15]. For example, Galhaut et al. [16] confirmed the effectiveness of gibberellic acid in improving photosynthetic properties, antioxidant system, seedling

#### **Figure 1.**

*Schematic model indicating the strategy of seed priming and the possible results.*

emergence, and growth in white clover plant grown on heavy metal polluted soil. Other strategies such as chemo-priming [17], nutri-priming [18], and plant growth promoting bacteria rhizobacteria (PGPRB) priming [19, 20] have proved effective in promoting growth properties of plants.

Generally, mechanism involve in seed biopriming include pre-soaking of seeds with a particular concentration of priming agent such as water, PGPRB, or phytohormone (**Figure 1**). This process improves germination parameters, seedling yield and growth, by either increasing nutrient utilization with the help of an improved physiological activities and root cell differentiation and division [21, 22], or by stimulating the activation of important metabolites such as amylase which initiate energy supply and improve germination properties [17]. Previous researches have documented evidence on seed priming with phytohormones in varieties of plant species and how important physiological processes such as growth and development, respiration, and transpiration are improved [23, 24]. The results of these researches have shown the significant roles of phytohormones in the biochemical, defense, and signaling pathways of plants [6]. Many researchers are now working to develop effective approaches to alleviate biotic and abiotic stresses and enhance crop production, especially as the world is constantly facing global warming. Seed priming with phytohormones can modulate the physiological and genetic mechanisms, making plants capable of tolerating these environmental stresses or making plants resistance to the stressors. These mechanisms if well adopted would be promising.

### **3. Hormone specific in seed biopriming**

There are various plant hormones produced by plants which have various functions. However, auxins, cytokinins, gibberellins, abscisic acid, salicylic acid, and ethylene are the most frequently used in seed priming. In addition, methyl jasmonate have also been used by previous literature in seed priming. According to

#### *Plant Hormones - Recent Advances, New Perspectives and Applications*



#### **Table 1.**

*Seed priming with phytohormones for developing abiotic stress tolerance in plants.*

several plant hormones function in seed germination and energy production for the developing embryo.

#### **3.1 Gibberellin**

Gibberellins (GAs) are plant hormones that have shown the capacity to regulate different developmental processes, such as stem elongation, germination, dormancy, flowering initiation, flower development, fruit development, and leaf and fruit senescence [3]. Gibberellin brings results that are somewhat similar to the ones by auxin, even though their mode of action differs [21].

Furthermore, gibberellin plays an essential function various physiological and developmental stages in plants (**Table 1**), but they are more effective in making stems increasing stem elongation through rapid cell differentiation and cell circle [3], Thereby leading to internode elongation. Dwarf and rosette plants (plants with little space between nodes on a stem or plants with clustered base) have been investigated to have low or no concentration of gibberellin. Also, Onoabhagbe et al. [37] used gibberellin and other plant hormones to improve germination properties and growth of *Sorghum bicolor* under elevated pH regime using chemoprming system.

#### **3.2 Auxin**

Auxin (IAAs) were also one of the most essential and the first identified phytohormone. IAA phytohormone is known to show an essential role in modulating plant growth and developmental processes, especially the root growth, cell elongation, vascular differentiation, and apical dominance [3]. IAAs also play an important function in cell division and differentiation, in fruit developmental stages, in the root formation from cuttings, in the lateral branching (apical dominance) inhibition, and in the leaf fall (abscission) frequencies. IAA conjugates is usually the form assumed by IAA in higher plants, IAAs conjugates functions as the primary free endogenous auxin that brings about plant developmental processes. The exogenous priming of seeds with IAAs induces the fast and improved formation of adventitious roots and lateral roots [39].

One of the most important naturally existing auxin is β-indolylacetic acid (IAA), which is obtained either from the amino acid tryptophan or from the breakdown of carbohydrates known as glycosides. This chemical influence plants by its activity

on the chemical bonds linking carbohydrates present in plant cell walls. The cycle allows the cells to be irreversibly adjusted and is joined by the passage of water and the synthesis of new cell wall material.

Auxin is engaged with cell development and cell extension of certain parts of a developing plant such as the stem which produced basically in pieces of the plant that are effectively developing like the stem (specifically, the stem tip). The phototropic reaction happens on the grounds that more amounts of auxin are disseminated to the side away from the light than to the side toward it, making the concealed side stretch all the more firmly and accordingly bend the stem toward the light. Additionally, the geotropic reaction happens in light of the fact that more auxin gathers along the lower side of the developing stem than along the upper side, creating a vertical arch.

#### **3.3 Cytokinins**

Cytokinins (CKs) are one of the plant hormones that are known to regulate various sections of plant growth and development, such as cell division, apical development, root elongation, stomatal behavior, and chloroplast synthesis [26]. It has been widely documented that application of CKs can promote crop development and yield. For example, Fricke et al. [40] demonstrated the use of CKs to improve cotton seedlings development. The result showed an increase in cotton yield of 10%. Another important aspect of CKs is its ability to improve pathogenesis in plant. Furthermore, CKs application has showed resistance against *Pseudomonas syringae* in *Arabidopsis thaliana* [28] and *Nicotiana tabacum* [31]. CKs are majorly produced in the root regions from a compound known as adenine. They are found moving upward within xylem (woody tissue) and then pass it to the leaves and fruits, where they are needed for growth and cell differentiation in plants.

CKs also functions together with auxin to reverse senescence in plants through stabilizing protein levels and synthesis of chlorophyll in the leaf. Senescence is a developmental stage in plant when the yellowing of leaves is visible as a result of protein breakdown and chlorophyll is decomposition. CKs also can also be used in the storage of green vegetables to reduce yellowing [7]. In horticultural tissue culture, according to Addicott [41], increased auxin and reduced cytokinin conditions can lead to improved root development, while reduced auxin and increased cytokinin conditions can lead to improved shoot development.

#### **3.4 Ethylene**

Ethylene (ET) is another essential plant hormone that influence ripening and rotting of fruit in plants [42]. ET is a very important plant hormone because it is the only plant hormone occurring as a gas. Furthermore, ET can be synthesized in almost every part of a plant, and can diffuse as a gas through the plant's tissue, outside the plant, and travel through the air to affect other plants within the vicinity. For example, Montalvo et al. [43] reported accelerated mango ripening as a result of application of ET for 12 hours. This process stimulating process was achieved through the production of 1-amino cyclopropane-1-carboxylic acid (ACC: an ethylene precursor) and improved ACC oxidase activity.

#### **3.5 Abscisic acid**

Abscisic acid (ABA) is another essential plant hormone that is known to stimulate developmental processes in plants, such as bud elongation, dormancy, control of organ size, and stomatal closure. It is also known as stress hormone because it

#### *Seed Priming with Phytohormones DOI: http://dx.doi.org/10.5772/intechopen.102660*

plays essential function in regulating plant responses to various biotic and abiotic environmental stressors such as drought, salinity, cold, heavy metals stress, and heat stress. ABA is a stress-triggered hormone, such that the highest concentration of ABA is synthesized in the root region of plant in response to decreased soil water potential (which is associated with dry soil) and other stress induced conditions. After the synthesis, ABA is then translocated to the leaves regions, where it gradually affect the osmotic potential of stomatal guard cells, causing them to shrink, leading to the closure of stomata. The ABA-induced stomatal closure brings about reduced transpiration (evaporation of water out of the stomata), thus preventing further water loss from the leaves in times of low water availability. A close linear correlation was found between the ABA content of the leaves and their conductance (stomatal resistance) on a leaf area basis.

### **3.6 Salicylic acid**

Salicylic acid (SA) is also an essential plant hormone, belonging to the phenolic group. It has various physiological benefits in plants because of its ability to regulate the processes of growth and development in plants such as photosynthesis, respiration, transpiration, and the transportation of ions in plants. According to Devinar et al. [44], Khan et al. [45], Senaratna et al. [46], and Bastam et al. [47], SA exhibits an essential role in the activation, modulation, and regulation of numerous responses during plant exposure to abiotic and biotic environmental stresses. Fahad and Bano [39] reported that SA has the capabilities to initiate and generate a cascade of several signaling pathways by interacting with other plant hormones such as ABA and ET and plays an important role in mitigating plant stresses. Ikhajiagbe and Musa [15] investigated the effect of SA on germination and early seedling growth of pigeon pea (*Cajanus cajan*). The research showed that increased levels of SA at 20 mg/L is very essential for maximum seed germination and early seedling growth of *C. cajan*. Furthermore, Jadhav and Bhamburdekar [48] observed the positive influence of SA on the root and shoot growth of groundnut. Anaya et al. [49] indicated the significant contribution of SA in alleviating saline stress in *Vicia faba* under salt stress condition.

### **4. Bio-priming with phytohormones and crop productivity**

According to Khan [50], seed priming involves various physiological treatments that can improve seed germination and seedling vigor through the addition of effective plant hormones. Ikhajiagbe and Musa [15] reported that pigeon pea (*C. cajan*) seeds bioprimed with salicylic acid for 20 hours results in improved germination properties. Malathi and Doraisamy [51] observed that seed priming with gibberellins protected seeds of groundnut from the infection of *Macrophomina phaseolina* and also bring about improved seedling vigor, plant dry matter, and prevented loss of oil content for up to 6 months of storage. Mohamedy et al. [52] discovered that biopriming of pea seeds with ethylene showed significant decrease in pre-emergence damping off, that is occasioned as a result of infestation by *Fusarium solani* in abandoned soils. Sarkar and Bhattacharyya [53] observed that the mung bean seeds when soaked in suspension of a particular hormones such as cytokinins and auxin brought about reduced root rot incidence in pot experimental set up and also resulted in increased root length, shoot length, dry weight of seedling, and yield as against the control setup. Furthermore, Mohamedy and Baky [54] discovered that biopriming of pea seeds with abscisic acid indicated the highest survival and lowest root rot disease incidence. In addition, highest plant height, enhanced leaves and

branches numbers per plant, dry weight of shoots per plant, pod length and diameter, numbers of seed per pod, and lowest chlorosis percentage were observed.

### **5. Role of hormonal priming in resistance against abiotic stress**

Different phytohormones have shown effects in improving germination properties and plant growth under abiotic stress (**Table 1**). Abiotic stress is the negative impact of all non-living factors on living organisms in a specific environment [55]. For example, auxin is very important in plant developmental processes such as translocation of carbohydrates as it improves lateral root formation, photosynthetic activities, flowering, and adventitious root development, by so doing, the plant extend its root deep down the soil to obtain water needed for its developmental stages in case of drought stressor [56]. Similarly in case of insufficient nutrient, the adventitious roots can conduct needed nutrients for plant developmental use [57]. According to Roohi and Jameson [58], seed priming with auxin improved the seedling establishment, improved tolerance to drought stress of *Bouteloua gracilis*. This improvement was achieved by enhancing catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD). Fahad et al. [59] observed priming seeds with auxin had improved germination and growth of rice (*Oryza sativa*) and pigeon pea (*C. cajan*), under model of arsenic and cadmium stress.

Seed priming with cytokinin has resulted in the alleviation of abiotic stresses in various plant species such as wheat and soybean by enhancing chlorophyll (Chl) formation thereby improving photosynthetic rate, enhancing membrane stability, and regulate ionic levels under drought stress [26, 60]. However, the further explanation on the mechanisms of how priming with cytokinin mitigate abiotic stress have not been fully understood. However, it may be as a result of its enhancement of chlorophyll formation and enhances stomatal movement thereby improving energy efficiency through photosynthesis [61].

Seed priming with gibberellin in addition with poultry manure has enhanced the growth of pepper (*Capsicum annuum*) plants and improved their salinity tolerance [62]. According to the use of gibberellin to tomato seed (*Solanum lycopersicum*) improved relative leaf water content, stomatal density, and Chl content by mitigating salinity stress.

Wei et al. [63] observed that priming of rice seed with abscisic acid has enhanced the growth rate, survival rate, biomass accumulation, and root formation under of rice under alkaline stress. Similarly, seed priming with abscisic acid improved salinity tolerance thereby leading to enhanced growth properties of rice, wheat, and sorghum [33, 64]. A similar result was reported by Fricke et al. [40] on barley leaves growth through the down regulation of the water loss during transpiration under saline conditions.

Seed priming with salicylic acid have also showed improved growth properties in heavy metals stressed environment Fahad and Bano [39] and Ikhajiagbe and Musa [15]. The application of different levels of salicylic acid was observed to enhance maize (*Zea mays*) yield even under low temperature. Furthermore, garden cress (*Lepidium sativum*) germination and developmental properties as well as seedlings height under salinity stress were enhanced with the application of salicylic acid. Drought stress was also mitigated, while vegetative growth was improved in safflower (*Carthamus tinctorius*) after the application of salicylic acid [65]. Priming of soybean (*Glycine max*) with a combination of ethylene and jasmonic acid had mitigated waterlogging stress by expression of glutathione transferases which led to the promotion of the adventitious root initiation and increasing root surface area [42].

## **6. Role of hormonal priming in resistance against biotic stress**

Plants are sessile organisms and therefore cannot move away from its locations in case of environmental stress. For this purpose, plants through evolution have developed series of defense mechanisms. These defense mechanisms can be stimulated either where toxic secondary metabolites are stored; or can be inducible, where defense is activated upon detection of an attack. Plants have the ability to easily detect environmental stress conditions, therefore upon sensing it, they rapidly activate their regulatory or transcriptional machinery, and eventually generate an appropriate response (defense mechanism). Over the years, scientist have gone deep into the research on how plant active their mechanisms against pathogen attack, however, the interplay and impact of different signals to generate defense responses against biotic stress still remain elusive. Seed biopriming with phytohormones has been used in various plant species for the biocontrol of various pathogenic attacks. Abuamsha et al. [66] and Dey et al. [67] applied abscisic acid to the different oil seed rape cultivars which helps in the control of a pathogen causing blackleg disease. The pathogen was observed to be reduced to about 71% after phytohormone application. Muller and Berg [68] reported the role of gibberellin in controlling the damping-off disease in varieties of plant species, especially in cucumber [3].

### **7. Economics of biopriming**

Previous researches have shown biopriming with phytohormones to be easier, fast, cheaper, and more environmentally friendly as against the chemical processes. With the enhancement in crop productivity witnessed in biopriming, it have been accepted the potential technique for biocontrol of several plant pathogens. Before now, farmers can only control insect infestation and pathogens attacks through the application of costly and non-ecofriendly pesticides. But with the introduction of hormone priming techniques, plant productivity and pathogens attack can be alleviated through hormone priming. Bio-priming is directly involved in the enrichment of plant development and which improves germination rate, uniformity in plant population, increases water and nutrient use efficiency, eliminates seed borne pathogens, controls pests and diseases. Besides these advantages, bio-priming reduces the hazardous effects on humans caused by the use of fungicides, bactericides, and pesticides by supplementing the chemicals with a sustainable strategy.

### **8. Conclusion and future prospects**

Seed priming using plant hormones has shown to be a promising and innovative technique in improving germination parameters as well as growth and yield of varieties of plant species. It has as well showed signs of effectiveness in plant abiotic and biotic stress management. Seed priming with phytohormones result in increased antioxidant secretion and activities, thereby reducing oxidative stress, leading to plant growth and yield enhancement. Therefore, seed hormopriming have the capacity to be utilized for sustainable crop production even under environmental stress. Seed biopriming have also proven to improve seedling health and also improves imbibition rate by breaking dormancy and improving viability. This review shed more light on the successes recorded as a result of using phytohormones in fortifying seeds prior to sowing as it serves as early treatment to plants thereby stimulating all important enzymes at early stage. The information in this

review can be used for developing future research on plant growth improvements and would inform modern farmers on the need to consider this important strategy. This emerging strategy has proven to be an effective seed treating technique for many crops. However, phytohormones concentration and priming duration may differs from crop to crop. For example, excessing seed priming and for longer period may lead to desiccation and decomposition of seed or bacteria infestation which makes seeds unviable. Future research at OMICs levels may be required to further explain the mechanisms employed by these phytohormones in seed priming, especially on how it reduces biotic stress in plant. Researches at molecular level is also required to further clarify on pathways involve and influence of priming duration and concentration of the phytohormones.

## **Acknowledgements**

The researchers are grateful to the Department of Biology and Forensic Science, Admiralty University of Nigeria, Delta State, Nigeria for the encouragement. The mentorship of Prof. Beckley Ikhajiagbe of the Department of Plant Biology and Biotechnology is very much appreciated. I also acknowledge the efforts of Prof. I. J. Dioha, the Dean of Faculty of Science, Admiralty University of Nigeria. Your mentorship is well appreciated.

## **Conflict of interest**

The authors declare no conflicts of interests.

## **Author details**

Musa Saheed Ibrahim1 \*, Nathan Moses1 and Beckley Ikhajiagbe2

1 Department of Biology and Forensic Science, Admiralty University of Nigeria, Ibusa, Delta State, Nigeria

2 Department of Plant Biology and Biotechnology, University of Benin, Benin City, Edo State, Nigeria

\*Address all correspondence to: musa-biology@adun.edu.ng

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

## **References**

[1] Kloepper JW, Schroth MN. Plant growth promoting rhizobacteria on radishes. In: Proceedings of the Fourth International Conference on Plant Pathogen Bacteria. Vol. 2. Tours, France: INRA, Gilbert-Clarey; 1978. pp. 879-882

[2] Addicott F, Carns H, Cornforth J, Lyon J, Milborrow BV, Ohkuma K, et al. Abscisic acid: A proposal for the redesignation of abscisin II (dormin). In: Wightman F, Setterfield G, editors. Biochemistry and Physiology of Plant Growth Substances. Ottawa: Runge Press; 1968. pp. 1527-1529

[3] Rhaman MS, Imran S, Rauf F, Khatun M, Carol C, Baskin C, et al. Seed priming with phytohormones: An effective approach for the mitigation of abiotic stress. Plants. 2021;**10**:37. DOI: 10.3390/plants10010037

[4] Sitken A, Yildiz HE, Ercisli S. Effects of plant growth promoting bacteria (PGPB) on yield, growth and nutrient contents of organically grown strawberry. Scientia Horticulturae. 2010;**124**:62-66

[5] Muhei S. Seed priming with phytohormones to improve germination under dormant and abiotic stress conditions. Advances in Crop Science and Technology. 2018;**6**:403-409

[6] Sytar O, Kumar P, Yadav S, Brestic M, Rastogi A. Phytohormone priming: Regulator for heavy metal stress in plants. Journal of Plant Growth Regulation. 2018;**38**:739-752

[7] Hayat R, Ali S, Amara U, Khalid R, Ahmed I. Soil beneficial bacteria and their role in plant growth promotion: A review. Annals of Microbiology. 2010;**60**:579-598

[8] Jeffries P, Gianinazzi S, Perotto S. The contribution of arbuscular mycorrhizal fungi in sustainable

maintenance of plant health and soil fertility. Biology and Fertility of Soils. 2003;**37**:1-16

[9] Reddy PP. Recent Advances in Crop Protection. India: Springer; 2013. p. 83

[10] Prabha R, Singh D, Yadav S. Seed biopriming with potential microbial inoculants as sustainable options for stress management in crops. In: Singh D, Prabha R, editors. Microbial Interventions in Agriculture and Environment. Singapore: Springer; 2019

[11] Hasanuzzaman M, Fotopoulos V. Priming and Pretreatment of Seeds and Seedlings: Implication in Plant Stress Tolerance and Enhancing Productivity in Crop Plants. Singapore: Springer; 2019

[12] Masood A, Ellahi N, Batool Z. Causes of low agricultural output and impact on socio-economic status of farmers: A case study of rural Potohar in Pakistan. International Journal of Basic and Applied Science. 2012;**1**:343-349

[13] Taylor A, Allen P, Bennett MA, Bradford JK, Burris JS, Mishra MK. Seed enhancements. Seed Science Research. 1998;**8**:245-256

[14] Di Girolamo G, Barbanti L. Treatment conditions and biochemical processes influencing seed priming effectiveness. Italian Journal of Agronomy. 2021;**7**:8-18

[15] Ikhajiagbe B, Musa SI. The effects of salicylic acid on the germination and early seedling growth of pigeon pea (*Cajanus cajan*). Notulae Scientia Biologicae. 2020;**12**(3):683-692. DOI: 10.15835/nsb12310777

[16] Galhaut L, Lespinay A, Walker DJ, Bernal MP, Correal E, Lutts S. Seed priming of *Trifolium repens* L. improved germination and early seedling growth on heavy metal-contaminated soil. Water, Air, and Soil Pollution. 2018;**225**:1-15

[17] Patade VY, Khatri D, Manoj K, Kumari M, Ahmed Z. Cold tolerance in thiourea primed capsicum seedlings is associated with transcript regulation of stress responsive genes. Molecular Biology Reports. 2012;**39**:10603-10613

[18] Farooq M, Wahid A, Siddique KHM. Micronutrients application through seed treatments—A review. Journal of Soil Science and Plant Nutrition. 2012;**12**:125-142

[19] Callan NW, Marthre DE, Miller JB. Bio-priming seed treatment for biological control of *Pythium ultimum* pre emergence damping-off in sh-2 sweet corn. Plant Disease. 1990; **74**:368-372

[20] Ibrahim MS, Ikhajiagbe B. The growth response of rice (*Oryza sativa* L. var. FARO 44) in vitro after inoculation with bacterial isolates from a typical ferruginous ultisol. Bulletin of the National Research Centre. 2021;**45**:70. DOI: 10.1186/s42269-021-00528-8

[21] Afzal I, Basra SMA, Ahmad N, Cheema MA, Warriach EA, Khaliq A. Effect of priming and growth regulator treatment on emergence. International Journal of Agriculture and Biology. 2002;**4**:306

[22] Akbari G, Sanavy SA, Yousefzadeh S. Effect of auxin and salt stress (NaCl) on seed germination of wheat cultivars (*Triticum aestivum* L.). Pakistan Journal of Biological Sciences. 2007;**10**:2557-2561

[23] Iqbal M, Ashraf M. Gibberellic acid mediated induction of salt tolerance in wheat plants: Growth, ionic partitioning, photosynthesis, yield and hormonal homeostasis. Environmental and Experimental Botany. 2013;**86**:76-85 [24] Yuan Z, Wang C, Li S, Li X, Tai F. Effects of different plant hormones or PEG seed soaking on maize resistance to drought stress. Canadian Journal of Plant Science. 2014;**94**:1491-1499

[25] Iqbal M, Ashraf M. Seed treatment with auxins modulates growth and ion partitioning in salt stressed wheat plants. Journal of Integrative Plant Biology. 2007;**49**:1045-1057

[26] Mangena P. Effect of hormonal seed priming on germination, growth, yield and biomass accumulation in soybean grown under induced drought stress. Indian Journal of Agricultural Research. 2020;**12**

[27] Angrish A, Kumar B, Datta K. Effect of gibberellic acid and kinetin on nitrogen content and nitrate reductase activity in wheat under saline conditions. Indian Journal of Plant Physiology. 2001;**6**:172-177

[28] Sedghi M, Nemati A, Esmaielpour B. Effect of seed priming on germination and seedling growth of two medicinal plants under salinity. Emirates Journal of Food and Agriculture. 2010;**22**:130-139

[29] Sheykhbaglou R, Rahimzadeh S, Ansari O, Sedghi M. The effect of salicylic acid and gibberellin on seed reserve utilization, germination and enzyme activity of sorghum (*Sorghum bicolor* L) seeds under drought stress. Journal of Stress Physiology & Biochemistry. 2014;**10**:5-13

[30] Watanabe H, Honma K, Adachi Y, Fukuda A. Effects of combinational treatment with ethephon and gibberellic acid on rice seedling growth and carbohydrate mobilization in seeds under flooded conditions. Plant Production Science. 2018;**21**:380-386

[31] Hamza J, Ali M. Effect of seed soaking with GA3 on emergence and seedling growth of corn under salt

*Seed Priming with Phytohormones DOI: http://dx.doi.org/10.5772/intechopen.102660*

stress. Iraqi Journal of Agricultural Science. 2017;**48**:560-566

[32] Zongshuai W, Xiangnan L, Xiancan Z, Shengqun L, Fengbin S, Fulai L, et al. Salt acclimation induced salt tolerance is enhanced by abscisic acid priming in wheat. Plant, Soil and Environment. 2017;**63**:307-314

[33] Gurmani A, Bano A, Khan S, Din J, Zhang J. Alleviation of salt stress by seed treatment with abscisic acid (ABA), 6-benzylaminopurine (BA) and chlormequat chloride (CCC) optimizes ion and organic matter accumulation and increases yield of rice ('*Oryza sativa*' L.). Australian Journal of Crop Science. 2011;**5**:1278-1285

[34] Pouramir-Dashtmian F, Khajeh-Hosseini M, Esfahani M. Improving chilling tolerance of rice seedling by seed priming with salicylic acid. Archives of Agronomy and Soil Science. 2014;**60**:1291-1302

[35] Farooq M, Aziz T, Basra S, Cheema M, Rehman H. Chilling tolerance in hybrid maize induced by seed priming with salicylic acid. Journal of Agronomy and Crop Science. 2008;**194**:161-168

[36] Dai L, Zhu H, Yin K, Du J, Zhang Y. Seed priming mitigates the effects of saline-alkali stress in soybean seedlings. Chilean Journal of Agricultural Research. 2017;**77**:118-125

[37] Onoabhagbe O, Musa SI, Akpeh K, Ekhator PO, Ikhajiagbe B. Germination characteristics of *Sorghum bicolor* (L.) Moench under different pH regimes after chemopriming. Journal of Plant Sciences. 2021;21-33 (in press)

[38] Beckley I, Musa SI, Owalum L, Precious O, Geoffrey O. *In vitro* assessment of elevated soil iron on germinability and germination characteristics of *Sorghum bicolor* (L.) Moench after chemo-priming. Biorxiv. org. 2021. Available from: https://www. biorxiv.org/content/10.1101/2021.11.22. 469542v1.full.pdf

[39] Fahad S, Bano A. Effect of salicylic acid on physiological and biochemical characterization of maize grown in saline area. Pakistan Journal of Botany. 2012;**44**:1433-1438

[40] Fricke W, Akhiyarova G, Veselov D, Kudoyarova G. Rapid and tissue-specific changes in ABA and in growth rate in response to salinity in barley leaves. Journal of Experimental Botany. 2004;**5**:1115-1123

[41] Addicott FT, editor. Abscisic Acid. New York: Praeger; 1983

[42] Kim Y, Seo C, Khan A, Mun B, Shahzad R, Ko J, et al. Exo-ethylene application mitigates waterlogging stress in soybean (*Glycine max* L.). BMC Plant Biology. 2018;**18**:254

[43] Montalvo E, Garcia HS, Tovar B, Mata M. Application of exogenous ethylene on postharvest ripening of refrigerated 'Ataulfo' mangoes. LWT - Food Science and Technology. 2007;**40**:1466-1472

[44] Devinar G, Llanes A, Masciarelli O, Luna V. Different relative humidity conditions combined with chloride and sulfate salinity treatments modify abscisic acid and salicylic acid levels in the halophyte *Prosopis strombulifera*. Plant Growth Regulation. 2013; **70**:247-256

[45] Khan W, Prithiviraj B, Smith D. Photosynthetic response of corn and soybean to foliar application of salicylates. Journal of Plant Physiology. 2003;**160**:485-492

[46] Senaratna T, Touchell D, Bumm E, Dixon K. Acetyl salicylic acid (aspirin) and salicylic acid induce multiple stress tolerance in bean and tomato plants. Plant Growth Regulation. 2000;**30**:157-161

[47] Bastam N, Baninasab B, Ghobadi C. Improving salt tolerance by exogenous application of salicylic acid in seedlings of pistachio. Plant Growth Regulation. 2013;**69**:275-284

[48] Jadhav S, Bhamburdekar S. Effect of salicylic acid on germination performance in groundnut. International Journal of Applied Biology and Pharmaceutical Technology. 2011;**3**(4):224-227

[49] Anaya F, Fghire R, Wahbi S, Loutf K. Influence of salicylic acid on seed germination of *Vicia faba* L. under salt stress. Journal of the Saudi Society of Agricultural Sciences. 2015;**17**:1-8

[50] Khan AA. Pre-plant physiological seed conditioning. Horticultural Reviews. 1992;**13**:301-307

[51] Malathi P, Doraisamy S. Effect of seed priming with *Trichoderma* on seed borne infection of *Macrophomina phaseolina* and seed quality in groundnut. Annals of Plant Protection Sciences. 2004;**12**:87-91

[52] Mohamedy EIRSR, Abd Alla MA, Badiaa RI. Soil amendment and seed bio-priming treatments as alternative fungicides for controlling root rot diseases on cowpea plants in nobaria province. Research Journal of Agriculture and Biological Sciences. 2006;**2**:391-398

[53] Sarkar M, Bhattacharyya PK. Biological control of root rot of greengram caused by *M. phaseolina* by antagonistic microorganisms. Journal of Mycopathological Research. 2008; **46**:233-237

[54] Mohamedy EIRSR, Baky AEMMH. Evaluation of different types of seed treatment on control of root rot disease, improvement growth and yield quality of pea plant in nobaria province. Research Journal of Agriculture and Biological Sciences. 2008;**4**:611-622

[55] Sneideris LC, Gavassi MA, Campos ML, Damico-Damiao V, Carvalho RF. Effects of hormonal priming on seed germination of pigeon pea under cadmium stress. Anais da Academia Brasileira de Ciências. 2015;**87**:1847-1852

[56] Ludwig-Muller J. Auxin conjugates: Their role for plant development and in the evolution of land plants. Journal of Experimental Botany. 2011;**62**:1757-1773

[57] Schiefelbein J. Cell-fate specification in the epidermis: A common patterning mechanism in the root and shoot. Current Opinion in Plant Biology. 2003;**6**:74-78

[58] Roohi R, Jameson DA. The effect of hormone, dehulling and seedbed treatments on germination and adventitious root formation in blue grama. Journal of Range Management. 1991;**44**:237-241

[59] Fahad S, Hussain S, Bano A, Saud S, Hassan S, Shan D, et al. Potential role of phytohormones and plant growthpromoting rhizobacteria in abiotic stresses: Consequences for changing environment. Environmental Science and Pollution Research. 2015;**22**: 4907-4921

[60] Iqbal M, Ashraf M, Jamil A. Seed enhancement with cytokinins: Changes in growth and grain yield in salt stressed wheat plants. Plant Growth Regulation. 2006;**50**:29-39

[61] Stoll M, Loveys B, Dry P. Hormonal changes induced by partial root zone drying of irrigated grapevine. Journal of Experimental Botany. 2000;**51**: 1627-1634

[62] Al Taey D. Alleviation of salinity effects by poultry manure and gibberellin application on growth and peroxidase activity in pepper. International Journal of Agriculture Environment and Biotechnology. 2017;**2**:1851-1862

*Seed Priming with Phytohormones DOI: http://dx.doi.org/10.5772/intechopen.102660*

[63] Wei L, Lv B, Wang M, Ma H, Yang H, Liu X, et al. Priming effect of abscisic acid on alkaline stress tolerance in rice (*Oryza sativa* L.) seedlings. Plant Physiology and Biochemistry. 2015;**90**:50-57

[64] Amzallag G, Lerner H. Physiological adaptation of plants to environmental stresses. In: Pessarakli M, editor. Handbook for Plant and Crop Physiology. Boca Raton, NY, USA: Marcel Dekker Inc.; 1990. pp. 557-576

[65] Chavoushi M, Najafi F, Salimi A, Angaji S. Improvement in drought stress tolerance of safflower during vegetative growth by exogenous application of salicylic acid and sodium nitroprusside. Industrial Crops and Products. 2019;**134**:168-176

[66] Abuamsha R, Salman M, Ehlers R. Effect of seed priming with *Serratia plymuthica* and *Pseudomonas chlororaphis* to control *Leptosphaeria maculans* in different oilseed rape cultivars. European Journal of Plant Pathology. 2011;**130**:287-295

[67] Dey R, Pal KK, Bhatt DM, et al. Growth promotion and yield enhancement of peanut (*Arachis hypogaea* L.) by application of plant growth-promoting rhizobacteria. Microbiological Research. 2004;**159**: 371-394

[68] Muller H, Berg G. Impact of formulation procedures on the effect of the biocontrol agent *Serratia plymuthica* HRO-C48 on *Verticillium* wilt in oilseed rape. Biological Control. 2008;**53**: 905-916

### **Chapter 7**

## Phytohormones as Plant Growth Regulators and Safe Protectors against Biotic and Abiotic Stress

*Rizwan Asif, Riffat Yasmin, Madiha Mustafa, Ana Ambreen, Modasrah Mazhar, Abdul Rehman, Shehla Umbreen and Mukhtiar Ahmad*

### **Abstract**

Plants are playing important role in the planet by providing food for humans and stability in the environment. Phytohormones are key regulators in various physiological processes and among the most important small signaling molecules affecting plant growth and yield production. These biochemical also initiate adaptive responses caused by external stimuli, such as biotic and abiotic stress. Generally, on the basis of physiology, plant hormones roughly fall into two classes. In class one, phytohormones fall which is responsible for plants growth-promoting activities, such as cell division, cell elongation, seed and fruit development, and pattern of differentiation. On the other hand, the second class of hormone play important role in plants' response, such as biotic and abiotic stresses. Some other hormones, such as jasmonates, salicylic acid, brassinosteroids, and strigolactones, also play a key role in plants. Their biochemical signaling network and their crosstalk ability make plant hormones excellent candidates to optimize plant growth and/or mediate abiotic and biotic stresses in agriculture. In the end, the future trends of plant hormone analysis are exploring plant hormones and their applications. We believe the perspective may serve as guidance for the research of plant hormones in the analytical, environmental, and botanical fields.

**Keywords:** plant hormones, growth promoters, stress hormones, biotechnology

### **1. Introduction**

Plant hormones or phytohormones are naturally occurring organic substances in miniscule concentrations and exert their action either locally or at distant sites. These chemical messengers with varied chemical properties and specific chemical structures directly influence the growth and development of the whole plant via different biochemical processes. These growth regulators substance coordinate the plant's response simultaneously in abiotic and biotic stresses [1–3]. The occurrence of plant hormones is ubiquitous; they are present in all higher plants and lower plants as well. Their homeostasis in the plant is regulated by

synthesis, metabolism, transport to the targeted tissue, and signal transduction which control its activities in the plant. Bioactive hormones are involved in this special type of regulation however intermediate and conjugated forms also play a pivotal role.

The action of plant hormones at the local and distant sites is mediated through different transport mechanisms. Transport of hormones at a distant target is facilitated by loading from the source into the xylem or phloem. In the last decade, several proteins have been identified that act as transporters of hormones at distant sites while short-distance movement of hormones is mediated by symplast, apoplast, or through transcellular mechanism [4]. At one extreme cytokinin get transported from roots to leaves where they prevent senescence and maintain metabolic activity, while at the other extreme the production of the gas ethylene may bring about changes within the same tissue, or within the same cell, where it is synthesized. Chemically plant hormones have a diversified nature comprising of indole, steroids, terpenes, carotenoids, fatty acids, and derivatives of adenine and such diversity reflect their different biological functions [5].

Generally, phytohormones have been divided into two groups on the basis of their functions; group one hormones including, auxin, gibberellin, cytokinin, brassinosteroids, jasmonic acid, and strigolactones. These endogenous signal molecules play a major role in growth-promoting activities by cell division, cell differentiation, elongation, pattern formation, stomatal movement, flowering, and seed germination and development. Hormones in group two are abscisic acid, salicylic acid, and jasmonic acid; mainly involved in biotic and abiotic stress response under different environmental conditions, such as sunlight, soil conditions, soil water, and nutrients [6–8].

Phytohormones do not act alone but in concurrence, or in antagonism, to each other such that the final growth or development represents their net effect. These comprise a unique set of compounds, with distinctive metabolism and properties. Their quality for being the natural compounds with the ability to produce the physiological effect in concentrations lower than those where nutrients and vitamins could not affect these processes makes them unique from the other compounds [9]. Most commonly hormones are classified into the following two categories, as shown in **Figure 1**.

**Figure 1.** *Classification of phytohormones.*

*Phytohormones as Plant Growth Regulators and Safe Protectors against Biotic and Abiotic Stress DOI: http://dx.doi.org/10.5772/intechopen.102832*

### **2. Types of hormones**

### **2.1 Gibberellin (GA)**

Gibberellin (GA) is one of the important plant hormones and it is a tetracyclic di-terpenoid carboxylic acid. It promotes plant growth and development, such as germination of seed, flowering of plants, ripening of fruits, and expansion of leaf while it discourages the growth of trichome. They also play a vital role in the elongation and division of the cells. Its function is also to trigger seed growth and release of seed in dormancy [10]. "To grow or not to grow" is an important verdict for plants to survive. There are requirements of suitable ambiances for plants to grow while in lack of which the growth ceases. So, one of these requirements is the level of growth hormone, GA. That is retained by different synthetic or inactivation of enzymes [11]. GA is also compulsory for the normal growth of roots where its lower concentration is required for maximum root growth other than the shoots. So, GA can be inhibitory for the growth of roots when present in excess quantity [12].

In the 1930s, GA was discovered in Japan while studying a disease related to rice that was with symptoms of excessive growth and yellowing of stems and with lack of production of seeds. GA is with 20 or 19 carbon skeleton (C20GA) or (C19GA), where GA with 19 carbon skeleton was biologically more active. Presently, about 140 different molecules of GA are known and have been isolated from different microorganisms or plants. GA3, GA4, and GA7 are with the maximum biological characteristics and also, they are commercially available [13]. GA3 is also known as gibberellic acid. The biological active GA is commercially used in agriculture. They are sprayed to increase the size of grapes with no seeds, pears, berries as well as to increase the crop yield under salt stress. It is also practiced in beer brewing to increase the process of malting. In 2016, according to some data the international market scope of GA was estimated at USD 548.9 million [14].

### *2.1.1 Synthetic pathway of gibberellin*

Gibberellin is chiefly manufactured via the plastidial methylerythritol pathway (MEP) in plants. The biosynthesis of GA in the initial steps is the same in both plants and fungi and is from geranylgeranyl diphosphate (GGPP) to GA-12 aldehyde. Then there is the cyclization of (GGPP) into ent-copalyl diphosphate and to ent-kaurene in plants that is catalyzed by the copalyl diphosphate synthase (CPSp) and the ent-kaurene synthase (KSp) [15]. It is also manufactured by several bacteria and fungi which are associated with various plants in symbiotic or pathogenic relations. In such circumstances, GAs have no evolving utility in the producing organism but perform on the plant host to relief infection by destroying the immunity or on nitrogen-fixing bacteria to adjust the formation of nodules [16].

### *2.1.2 Gibberellin against heavy metal stress*

Gibberellic acid (GA3) plays an important function for plant growth under salt stress and heavy metal toxicity as well as increases the synthesis of chlorophyll and the action of antioxidant enzymes to prevent lipid peroxidation. GA with calcium (Ca) is added to decrease the salt toxicity like (Ni) toxicity on plant throughput and also to activate different antioxidant enzymes to decrease lipid peroxidation of the cell membrane. They are also involved in regulating the different processes in plants to increase the heavy metal stress [17, 18]. Application of gibberellic acid in *Vicia faba L*., supported to restore the Cd and Pb-induced reduction in the mitotic index. Due to the application of these growth hormones, GA in plants with heavy metal

stress, the ratio of several chromosomal irregularities was expressively reduced. After that, the seeds were harvested from *V. faba* both from heavy metals and gibberellic acid treatment showed a high level of solubilized sugars, proteins, and nucleic acid. Gibberellic acid in lupin plants with Cd stress improved the activity of amylase along with the CAT enzyme. When *Chlorella vulgaris* was exposed to heavy metals, such as Pb and Cd, the application of GA3 amplified the number of cells and level of protein to verify that GA can defend life in water polluted areas [19].

#### **2.2 Auxin (IAA)**

In the nineteenth century, the idea of a portable substance came into existence that was produced by the leaves and traveled down to encourage the synthesis of the root was capable to control the winding of grass coleoptiles toward the sun. After that these substances were refined, categorized, and were given the name "auxin" from the Greek word "auxin," which means "to grow or to enhance." The naturally existing IAA is indole-3-acetic acid IAA [20]. IAA are organic compounds that are small, low in molecular weight, and constitute the most advanced key and diverse group of phytohormones normally present in all plant types. They are indulged in a number of developing practices by regulating the cell division, such as in the control of shoot building, vascular enlargement, and horizontal root construction [21]. IAA can control senescence, also can react with many pathogens and abiotic or heavy metal stresses. Moreover, it can regulate the production of fruit responses in plants [22].

#### *2.2.1 Synthetic pathway of auxin*

It is essential for the development of the plants that the biosynthesis of auxin be localized in specific tissues but IAA is generally produced in fresh leaves and then it is transported to all through the plant. Many practices have been done to quantify it and its metabolites permitting alteration of the prior opinion of auxin dispersal in a minute quantity of plant tissue having a fresh weight of less than one milligram [23]. IAA biosynthesis in plants is still partly clarified; however, with the use of different isotopes, it is shown that IAA can be synthesized by two important pathways. These pathways are tryptophan dependent and tryptophan independent [21]. In tryptophan dependent pathway, four routes have been identified. In the first route, indolacetamide is converted to IAA by amidohydrolase. In the second route, indole-3-pyruvic acid (IPA) is formed from tryptophan with the help of an aminotransferase which is then converted to indole-3-acetaldehyde by IPA decarboxylase, and then indole-3-acetaldehyde is converted to indole acetic acid by indole-3-acetaldehyde oxidase. These routes are not going parallel often they cross each other. Tryptophan is also converted to tryptamine by trp-decarboxylase and then some proteins help the conversion of tryptamine to indole acetic acid after different steps [24].

In the tryptophan independent pathway, IAA was synthesized in the absence of Trp genes. So, several studies showed that in the absence of tryptophan due to the absence of Trp genes or defective Trp genes, there was evidence of IAA in plants. Studies in Arabidopsis showed that trp3-1 and trp2-1 mutants were defective in tryptophan synthase α and β respectively; there was still the production of IAA. It is hypothesized that IAA production might be due to precursors of tryptophan, i.e., indole or indole-3-glycerol phosphate [25].

#### *2.2.2 Auxin against heavy metal stress*

The presence of IAA in plants is also helpful in plant growth in changing environments. In the presence of heavy metal toxicity, it plays a crucial role to tolerate this. In this metal stressed plants, auxin can be provided by the inoculation *Phytohormones as Plant Growth Regulators and Safe Protectors against Biotic and Abiotic Stress DOI: http://dx.doi.org/10.5772/intechopen.102832*

of microbes that can produce IAA in the rhizosphere of these plants to improve plant production. This metal stress is due to the production of different reactive oxygen species in different locations of the plants, such as root cells, containing peroxisome, mitochondria, plastids, and cytoplasm [26]. In different studies, it is elaborated that the level of IAA is altered endogenously due to the heavy metal stress in roots and shoots. There is shown a positive also a negative relation between heavy metal toxicity and level of IAA. It is also under observation that in response to heavy metal stress, regulation of IAA-producing genes may control the locality and accretion of IAA [27]. Several genes are involved in the relation between IAA and reactive oxygen species to attain homeostasis to adjust the level of H2O2 by regulating and stimulating the antioxidant enzymes and chlorophyll levels in plants [28]. The abnormal level of metal in the soil causes toxicity in plants and retards the growth and development of the plants due to its accumulation in roots and shoots. The decrease in the rate of growth and development in plants is mainly controlled and maintained by plant hormones like IAA. Indole acetic acid is well known for plant adaptation under heavy metal stress which results in enhanced biomass and production. IAA can be applied to various plants exogenously either directly or through plant growth-promoting rhizobacteria (PGPRs) which produce IAA and in return there is a significant improvement in plant growth under toxic metal concentration [29].

### **2.3 Cytokinins (CKs)**

In plant growth and development, the master regulators are known as CKs a phytohormone or plant hormone. Its main function is in the physiology of the cell-like expansion and division of the cell, P and N2 metabolism, maintenance of H2O balance, the integrity of chloroplast, and senescence. N6-substituted adenine derivatives are known to form by CKs. Seed dormancy can be reduced by using CKs [30]. There are several types of CKs counting—thidiazuron, 6-benzyladenine, kinetin, and 2-isopentenyladenine [31]. CKs are present in abundant types which are different in structure, properties like biochemical and biological activities, and the mode of transportation across plant tissues [32]. CKs are produced in roots and apical meristem and then transported to aerial fragments with the help of absorbent material like minerals via the xylem. In xylem exudate, Zeatin riboside is the utmost copious form of CKs [33]. CKs perform an important role in the growth and adaptation of plants all through the life cycle, such as in the initial phases of reproduction; flowering, seedling, and development [34]. CKs are also used to control the N2 metabolism by increasing the action of nitrate reductase in plants [35].

#### *2.3.1 Synthetic pathway of Cytokinins*

Several studies in plants, such as rice (*Oryza sativa*) and Arabidopsis (*Arabidopsis thaliana*), stated that there are several means to regulate the de novo synthesis of CKs. Synthesis of CKs is due to the nitrogen status signals; one is nitrate-specific signal and the other is a glutamine-related signal. The availability of nitrogen exogenously represents the nitrate-specific signals while assimilated nitrogen status represents the glutamine-related signals. It is known that the CK synthesis is regulated by the nitrogen that is taken up from the soil and is also a key component of nitrogen integration [36]. CKs synthesis have been recognized to be controlled by vital genes. In the initial step that is catalyzed by isopentenyl transferase (IPT), there is a transfer of the isopentenyl group to an adenine nucleotide (ATP, ADP, AMP) from dimethylallyl diphosphate. Then hydroxylation of the methyl group occurs in the isopentenyl side chain by a cytochrome P450. Then in the last step ribose is released and is catalyzed by a phosphoribohydrolase [37].

The other indirect pathway involves the addition of dimethylallyl pyrophosphate (DMAPP) to adenine A37 on tRNA which results in the discharge of CKs nucleotide through degradation of tRNA and by the elimination of the phosphoribosyl by LOG (LONELY GUY). Prenylation of tRNA is catalyzed by tRNA isopentenyltransferase (tRNA-IPT) [38].

#### *2.3.2 Cytokinins against heavy metal stress*

When plants are undergone xenobiotic resistance, there is a key role of CKs with saline resistance, drought, light, and temperature signals [39]. Different signaling pathways are there to regulate the concentration of CKs under heavy metal stress to increase plant resistance. *A. thaliana* under arsenic stress is investigated when there are reduced endogenous CKs and the reduced CKs signaling while with the mutant plants exhibited CKs synthesis increased the tolerance of plants against arsenic. It is described that exogenous CKs can promote plant resistance against metal stress [40]. CKs controls morphology, division of cells, and several other substantial routes in the plant. Numerous studies described role of CKs for reduction of heavy metal toxicity in crop plants via biosorption of heavy metals. In higher plants, CKs have been verified to restore the heavy metal-induced decrease in mitotic index resulting in an increase in the number of cells. Furthermore, CKs certainly controlled the photosynthetic mechanism and raised the concentration of various monosaccharides and antioxidants which results in the better existence of plants under heavy metal toxicity [41].

One of the contrivances in a high level of heavy metal is the variation in the level of CKs. It is observed that the concentration of CKs decreases when there is an excess of heavy metal to increase the overall efficiency of plants to cope with this toxicity. Most of the studies agree that the shortage of mineral elements reduces the concentration of CKs in plants [42]. It also improves the tolerance of *C. vulgaris* to Cu, Cd, and Pb due to the activation of the antioxidant defense system, therefore, minimizing the negative values of heavy metal oxidative stress [43]. It is concluded that plant hormones enriched the actions of antioxidant enzymes, i.e., APX, SOD, GR, CAT, and improved the contents of smaller antioxidant molecules, such as glutathione, ascorbate, and proline. CKs protect several proteins and constituents of the process of photosynthesis (carotenes, chlorophylls, and xanthophylls) thus there was a considerable decrease in detrimental effects of heavy metal stress on *A. obliquus* [43].

#### *2.3.3 Cross talk among phytohormones*

Phytohormones or plant hormones are small endogenous mediators, such as cytokinin (CK), gibberellin (GA), salicylic acid (SA), brassinosteroid (BR), auxin (IAA), ethylene (ET), jasmonic acid (JA), abscisic acid (ABA), and strigolactone (SL), which coordinate a dual purpose also known as cross talk of plant hormones. Certainly, plant hormones are intermediates that not only direct and organize the progressive practices endogenously but also deliver the environmental incitements to initiate adaptive reactions to biotic & abiotic stress [44].

It was observed that there was an improvement in the differentiation of callus and the number of cells was also increased when CKs were applied with IAA. There was also the contribution of both to uplift the strength of the plant, localization of nutrients, and to increase the grain yield in numerous plants. CKs was found in excessive amount in emerging tissue, such as cambium, root, and shoot tips [45]. Normally, GA and CKs are observed to be antagonists, because both of them opposed the effect of one another on shoot apex, root tips, and elongation of the

*Phytohormones as Plant Growth Regulators and Safe Protectors against Biotic and Abiotic Stress DOI: http://dx.doi.org/10.5772/intechopen.102832*

plant. DELLA proteins are thought to be responsible for such mechanisms where these antagonistic hormones cross talk to function as negative stimulators of GA signals [46]. ABA is known to be a plant stress hormone that accumulates promptly in plants when they are under dehydration or drought stress. A relation exists between ABA and CKs activity for the maintenance of seed development, pre and post-seed development, and stress stimuli. Cross talk concerning ABA and CK is often opposing. When there is an increased concentration of CK, it suppresses the ABA reactions. These antagonistic functional relations are observed for the maintenance of the Arabidopsis under drought conditions although ABA decreases seed sprouting [47]. ET is a gaseous hormone that controls proliferation and expansion of cells, development of fruit, senescence, and various reactions toward biotic and abiotic pressures. The behavior of CKs and ET is generally antagonistic in shoots, where CKs are involved in greening and cell multiplying while ethylene with aging processes, such as maturing, senescence, and the inhibition of cell propagation. Moreover, both of them work cooperatively in different routes, such as maintenance of roots by inhibiting root development. Considerably, CKs positively encourage the production of ET by activating ACC synthase. The production of ET facilitates the CKs to impede hypocotyl elongation in seeds and also to constrain root growth [48]. SA is a hormone of plant origin and performs a vital role against biotrophic pathogens for plant defense in contrast with JA which performs against necrotrophic pathogens. CKs cross talk with SA signals and help in the protected responses via their interactions. CKs increase the SA reactions that help to increase transcription of genes relevant to defense like SID2 and PR1, for the biosynthesis of SA and an indicator or marker gene for SA response respectively [49].

The GA synthesis in an ovule is increased by IAA during fertilization. There is a synergistic effect between auxin and GA signals that regulate fruit development. Their concentrations can be applied externally to regulate the fruit growth and also prompt parthenocarpy [50]. The interpretation of GA and IAA indication cascade has significantly simplified how these hormones manage with each other to control the development of fruit. IAA performs to upstream the level of GA throughout the fruit growth. In Arabidopsis, the fertilization-induced IAA responses or IAA applications activate GA biosynthesis; however, GA applications do not encourage the IAA response [51]. Recently, it is investigated that a high concentration of IAA and the activity of IAA signals enhance ABA-mediated dormancy [52]. Both of them are involved in the maintenance of water status in plants, with opposing performance in the shoots and roots. Indications for water status in the plant have to be integrated to adjust water conductivity in roots and permit modifications in stomatal opening. As a constraint, ABA and IAA signals never exist in a linear fashion but essentially form a system that interconnects through cross talk [53]. IAA and ET cooperatively control numerous developing routes in plants. To date, a whole heap of evidence is existed at the molecular level to promote cross talk between IAA and ET at synthetic, transporter, and signaling levels. That comprises transcriptome profiling datasets to define new entrants for the molecular cross talk. When there is any disturbance in IAA synthesis and transportation, ET helps to promote/deplete the IAA level, or to redistribute it in plants, thereby activating morph-genic reactions. Though, the function of ET in the cross talk is not limited to IAA redistribution [54]. SA and IAA cross talk is fairly obvious from both experimental confirmation and RNA-seq. Data exploration [55]. IAA and SA not only share a common precursor but also play important functions in the maintenance of fruit development and ripening. A metabolic and functional cross talk between them and with other plant hormones occurs in a spatiotemporal fashion to magnificently control the growth of seasonal and non-seasonal fruits [56].

Proofs for protein physiology as a connecting pivot between GA and ABA signaling systems have been progressively evolving both on the functional and on the chromosomal level, mostly in the period of early plant growth. In Arabidopsis, substantial influences of GA and ABA in the signals from hormones have been recognized after the light-reversibility in kernel development. Enhanced production of GA in ABA deficient gene initiated better light-dependent development and concludes an antagonistic association of GA and ABA production in growing and rising seed [57]. ET has a key role in maintaining the developing processes in plants under stress. It is shown to be positively involved with ET when oxygen is deficient. For the elongation of internodes of deepwater rice, GA and ET activity are required. During rice immersing into the water, a decreased level of O2 is recommended to prompt ET synthesis which in turn impede the ABA production. This altered balance between GA and ABA causes elongation of stem induced by GA [58]. GA and SA contribute to the maintenance of many plant reactions. They are concerned to stimulate the expression of proteins involved in pathogenesis. Furthermore, they cooperatively develop plant defenses under biotic and abiotic stresses as SA enhances resistance to abiotic stress in plants. It also has the capability to raise antioxidants and decline the process of lipid peroxidation. In Arabidopsis, seeds were exposed to SA with salt toxicity resulted in the increase of SA because of the activation of two superoxide dismutase, which recovers seed development and upsurges antioxidant capabilities, to enhance the salt tolerance in plants under salt stress [59]. The list of various hormones is given in **Figure 2.**

#### **2.4 Abscisic acid (ABA)**

Abscisic acid (ABA) is an isoprenoid compound associated with seed dormancy, drought responses, and other growth processes. ABA plays vital roles in plant responses to a range of abiotic stresses such as drought, salinity, high light, nutrient deficiency, and heavy metals. ABA has also been found to be associated with color change in fruits during ripening. ABA at higher concentrations inhibits root growth but in stress conditions, it also plays a vital role in the elongation of the root. Various environmental factors regulate the levels of ABA, including seed maturation, the genotype of plant, water and soil conditions. ABA concentrations are generally increased in nutrient deficiency and decreased at higher temperatures 40°C. Root tissue generally contains lower concentrations of ABA than leaves, dehydration of detached roots from various species, ages, and branching orders also stimulate ABA synthesis [60].

The biosynthesis, catabolism, transport, downstream response, and modulation of ABA have been extensively investigated in angiosperms. ABA is primarily synthesized from carotenoids under the catalytic action of various enzymes such as b-carotene

**Figure 2.** *List of various kinds of phytohormones.*

*Phytohormones as Plant Growth Regulators and Safe Protectors against Biotic and Abiotic Stress DOI: http://dx.doi.org/10.5772/intechopen.102832*

hydroxylases, zeaxanthin epoxidase (ZEP, ABA1), 9-cis-epoxycarotenoid dioxygenase (NCEDs), short-chain alcohol dehydrogenase/reductases (SDRs, such as ABA2), abscisic aldehyde oxidases (AAOs), molybdenum cofactor sulfurase (MOCO, ABA3), and ABA4. ABA levels are regulated by two major pathways—hydroxylation and esterification mediated by four CYP707As and eight glucosyltransferases (UGTs). The inactivated ABA-glucosyl ester (ABA-GE) conjugation is a storage form of ABA and the site can be cleaved by b-glucosidases (BGLUs) [61].

#### *2.4.1 Abscisic acid against heavy metal stress*

ABA is one of the foremost phytohormones driving plant resistance to toxic metals and metalloids, such as Cd, and Pb. Mechanisms of ABA in response to heavy metals and metalloids stresses in non-angiosperm plant lineages is still limited and not completely understood [62], however, ABA act in different ways in response to heavy metal stress, including by alleviating toxic metal and metalloid stress via ABCGs, PSE1, and WRKY13, limiting their uptake, altering the distribution between roots and shoot and promoting chelation and vacuolar sequestration [7–9].

#### **2.5 Jasmonic acid (JA)**

Jasmonic acid is a signaling chemical that mediates the number of biotic and abiotic stress process in plants, such as fruit ripening and seed germination, wounding, and ultraviolet radiation. It is produced from linolenic acid but activated after conjugation with isoleucine which permits it to join with COI1and act as a JA receptor. The first role of JA as a senescence-promoting was observed from a compound isolated from wormwood cause rapid loss of chlorophyll in oat [63]. JA also stimulates the secretion of volatile oil in plants which have antimicrobial properties. In a current study remarkable role of JA acid was observed in the regulation of the life cycle in plants [64]. JA is a naturally grown regulator and is extensively found in plants. Normally, JA does not work in isolated form, and extensively cross talk behavior was studied with other hormones. JA's role in a hot climate as a water conserver was also observed through stomatal closure. In addition, JA also helps to cope with drought stress by promoting some enzymes, such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), proline. The role of JA against fungal pathogens and some other plants pathogen was also observed [65]. JA not only works as a plant's growth regulator but also stimulates the immune system in plants. The list of hormones with their origin and functions is mentioned in **Table 1**.

### **2.6 Salicyclic acid (SA)**

Salicyclic acid is one of the most important plant hormones naturally produced in plants within the cytoplasm of the cell. SA is a key player in the regulation of very important functions, such as photosynthesis, growth, and even in defense of plants [67]. Different past studies demonstrated the role of SA in plants against biotic and abiotic stress. SA stimulates the SAR mechanism in plants which activates pathogenesis-related proteins which work against different kinds of phytopathogens, such as fungus and bacteria. It also induces a variety of metabolic processes in plants and also regulates plant and water relations [68]. The role of SA was also observed in different kinds of signaling which leads to gene expression and protein synthesis. Similarly, SA also works with other hormones in cross talk. The SA treatment in cucumber or tobacco plants induced heavy metal-like cupper tolerance ability. It also induced cd tolerance ability in plants but the exact mechanism is still unknown [66]. The hormones of various names with their functions are given in **Figure 3.**



*Phytohormones as Plant Growth Regulators and Safe Protectors against Biotic and Abiotic Stress DOI: http://dx.doi.org/10.5772/intechopen.102832*

**Figure 3.** *Phytohormones and their role in plants.*

### **3. Conclusion**

Growth is an essential property for every living organism and is usually regulated by various external and internal factors. Generally, plants attain this property via synthesizing a small amount of chemical substance known as phytohormones. These chemical substances trigger biochemical changes that ultimately initiate several growth changes in plants, such as the formation of flowers, roots, stems, and fruits. As a result, these processes increase the yield. Some phytohormones also play important role in a plant's life from dormancy to senescence. Consequently, perform a vital role in agriculture and horticulture, etc. Conclusively, phytohormone regulates the physiology of plants but the information about the molecular mechanisms still remains unclear.

### **Author details**

Rizwan Asif1 \*, Riffat Yasmin2 , Madiha Mustafa1 , Ana Ambreen1 , Modasrah Mazhar1 , Abdul Rehman1 , Shehla Umbreen1 and Mukhtiar Ahmad3

1 Department of Eastern Medicine, Qarshi University, Lahore, Pakistan

2 Riphah College of Rehabilitation and Allied Health Sciences (RCRAHS), Riphah International University, Faisalabad, Pakistan

3 University College of Conventional Medicine, The Islamia University of Bahawalpur, Bahawalpur, Pakistan

\*Address all correspondence to: asifrizwan429@yahoo.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Phytohormones as Plant Growth Regulators and Safe Protectors against Biotic and Abiotic Stress DOI: http://dx.doi.org/10.5772/intechopen.102832*

### **References**

[1] Asami T, Nakagawa Y. Preface to the special issue: Brief review of plant hormones and their utilization in agriculture. Journal of pesticide science. 2018;**43**(3):154-158

[2] Shah SH et al. Role of exogenously applied plant growth regulators in growth and developmen of edible oilseed crops under variable environmental conditions: A review. Journal of Soil Science and Plant Nutrition. 2021;**21**:3284-3308

[3] Wang L et al. Recent developments and emerging trends of mass spectrometric methods in planthormone analysis: A review. Plant Methods. 2020;**16**(1):1-17

[4] Abualia RE, Benkova LB. Transporters and mechanisms of hormone transport in Arabidopsis. Advances in Botanical Research. 2018;**87**:115-138

[5] Srivastava LM. Plant Growth and Development: Hormones and Environment. Amsterdam, Netherlands: Elsevier; 2002

[6] Müller M, Munné-Bosch S. Hormonalimpact on photosynthesis and photoprotection in plants. Plant Physiology. 2021;**185**(4):1500-1522

[7] Jiang K, Asami T. Chemical regulators of plant hormones and their applications in basic research and agriculture. Bioscience, Biotechnology, and Biochemistry. 2018;**82**(8): 1265-1300

[8] Bari R, Jones JD. Role of plant hormones inplant defence responses. Plant Molecular Biology. 2009;**69**(4): 473-488

[9] Davies PJ. Plant Hormones: Biosynthesis, Signal Transduction, Action!. Netherlands: Berlin: Springer Science & Business Media; 2004

[10] Emamverdian AY, Ding MF. The role of salicylic acid and gibberellin signalingin plant response to abiotic stress with an emphasis on heavy metals. Plant Signaling & Behavior. 2020;**15**(7):1777372

[11] Takehara S et al. A common allosteric mechanism regulates homeostatic inactivation of auxin and gibberellin. Nature Communications. 2020;**11**(1):1-10

[12] Barker R et al. Mapping sites of gibberellin biosynthesis in the Arabidopsis root tip. New Phytologist. 2021;**229**(3):1521-1534

[13] Camara MC et al. Current advances ingibberellic acid (GA 3) production, patentedtechnologiesand potential applications. Planta. 2018;**248**(5): 1049-1062

[14] Kildegaard KR et al. Tailored biosynthesis of gibberellin plant hormones in yeast. Metabolic Engineering. 2021;**66**:1-11

[15] Tudzynski B et al. The P450-4 gene of Gibberella fujikuroi encodes entkaurene oxidase in thegibberellin biosynthesis pathway. Applied and Environmental Microbiology. 2001;**67**(8):3514-3522

[16] Hedden P. The current status of research on gibberellin biosynthesis. Plant and Cell Physiology. 2020;**61**(11):1832-1849

[17] Siddiqui MH, Al-Whaibi MH, Basalah MO. Interactive effect of calcium and gibberellin on nickel tolerance in relation to antioxidant systems in Triticum aestivum L. Protoplasma. 2011;**248**(3):503-511

[18] Colebrook EH et al. The role of gibberellin signalling in plant responses to abiotic stress. Journal of Experimental Biology. 2014;**217**(1):67-75

[19] Saini SN, Kaur PPK. Phytohormones: Key players in the modulation of heavy metalstress tolerance in plants. Ecotoxicology and Environmental Safety. 2021;**223**:112578

[20] Tromas A, Perrot-Rechenmann C. Recent progress in auxin biology. Comptes Rendus Biologies. 2010;**333**(4): 297-306

[21] Woodward AW, Bartel B. Auxin: Regulation, action, and interaction. Annals of Botany. 2005;**95**(5):707-735

[22] Wang S, Fu J. Insights into auxin signaling in plant–pathogen interactions. Frontiers in Plant Science. 2011;**2**:74

[23] Ljung K, Bhalerao RP, Sandberg G. Sites and homeostatic control of auxin biosynthesis in Arabidopsis during vegetative growth. The Plant Journal. 2001;**28**(4):465-474

[24] Sugawara S et al. Biochemical analyses of indole-3-acetaldoximedependent auxinbiosynthesis in Arabidopsis. Proceedings of the National Academy of Sciences. 2009;**106**(13):5430-5435

[25] Ouyang J, Shao X, Li J. Indole-3 glycerol phosphate, a branchpoint of indole-3-acetic acid biosynthesis from the tryptophan biosynthetic pathway in *Arabidopsis thaliana*. The Plant Journal. 2000;**24**(3):327-334

[26] Singh H et al. Auxin metabolic network regulates the plant response to metalloids stress. Journal of hazardous materials. 2021;**405**:124250

[27] Bruno L et al. In Arabidopsis thaliana cadmium impact on the growth of primary root by altering SCR expression and auxin-cytokinin cross-talk. Frontiers in Plant Science. 2017;**8**:1323

[28] Fahad S et al. Phytohormones and plant responses to salinity stress:

A review. Plant Growth Regulation. 2015;**75**(2):391-404

[29] Zhang C et al. Exogenous indole acetic acid alleviates Cd toxicity in tea (Camellia sinensis). Ecotoxicology and Environmental Safety. 2020;**190**:110090

[30] Emamverdian A et al. Mechanisms of selected plant hormones under heavy metal stress. Polish Journal of Environmental Studies. 2021;**30**(1): 497-508

[31] Bekircan T et al. Effect of cytokinins on in vitro multiplication, volatiles composition and rosmarinic acid content of Thymus leucotrichus Hal. Shoots. 3 Biotech. 2018;**8**(3):1-9

[32] Jameson PE, Song J. Cytokinin: A key driver of seed yield. Journal of Experimental Botany. 2016;**67**(3): 593-606

[33] Mangieri MA et al. Cytokinins: A key player in determining differences in patterns of canopy senescence in stay green and fast dry-down sunflower (Helianthus annuus L.) hybrids. European Journal of Agronomy. 2017;**86**:60-70

[34] Nguyen HN et al. Cytokinin activity during early kernel development corresponds positively with yield potential and later stage ABA accumulation in field-grown wheat (Triticum aestivum L.). Planta. 2020;**252**(5):1-16

[35] Sýkorová B et al. Senescenceinduced ectopic expression of the A. tumefaciens ipt gene in wheat delays leaf senescence, increases cytokinin content, nitrate influx, and nitrate reductase activity, but does not affect grain yield. Journal of Experimental Botany. 2008;**59**(2):377-387

[36] Sakakibara H. Cytokinin biosynthesis and transport for systemic nitrogen signaling. The Plant Journal. 2021;**105**(2):421-430

*Phytohormones as Plant Growth Regulators and Safe Protectors against Biotic and Abiotic Stress DOI: http://dx.doi.org/10.5772/intechopen.102832*

[37] Zhao Y et al. A wheat allene oxide cyclase gene enhances salinity tolerance via jasmonate signaling. Plant Physiology. 2014;**164**(2):1068-1076

[38] Hrtyan M et al. RNA processing in auxinand cytokinin pathways. Journal of Experimental Botany. 2015;**66**(16): 4897-4912

[39] Pavlů J et al. Cytokinin at the crossroads of abiotic stress signalling pathways. International Journal of Molecular Sciences. 2018;**19**(8):2450

[40] Mohan TC et al. Cytokinin determines thiol mediated arsenic tolerance and accumulation. Plant Physiology. 2016;**171**(2):1418-1426

[41] Zhou MX et al. Effect of NaCl on proline and glycinebetaine metabolism in Kosteletzkya pentacarpos exposed to Cd and Zn toxicities. Plant and Soil. 2019;**441**(1):525-542

[42] Veselov D et al. Role of cytokinins in stress resistance of plants. Russian Journal of Plant Physiology. 2017;**64**(1): 15-27

[43] Piotrowska-Niczyporuk A et al. Exogenously applied auxins and cytokinins ameliorate lead toxicity by inducing antioxidant defence system in green alga *Acutodesmus obliquus*. Plant Physiologyand. Biochemistry. 2018;**132**:535-546

[44] Vanstraelen M, Benková E. Hormonal interactions in the regulation of plant development. Annual Review of Cell and Developmental Biology. 2012;**28**:463-487

[45] Checker VG et al. Role of phytohormones in plant defense: Signaling and cross talk. In: Molecular Aspects of Plant-Pathoge Interaction. Berlin: Springer; 2018. pp. 159-184

[46] Davière JM, Achard P. A pivotal role of DELLAs in regulating multiple

hormone signals. Molecular Plant. 2016;**9**(1):10-20

[47] Huang X et al. The antagonistic action of abscisic acid and cytokinin signaling mediates drought stress response in Arabidopsis. Molecular Plant. 2018;**11**(7):970-982

[48] Zdarska M et al. ETR1 integrates response to ethylene and cytokinins into a single multistep phosphorelay pathway to control root growth. Molecular Plant. 2019;**12**(10):1338-1352

[49] Naseem M et al. Integrated systems view on networking by hormones in Arabidopsis immunity reveals multiple crosstalk for cytokinin. The Plant Cell. 2012;**24**(5):1793-1814

[50] Matsuo S et al. Roles and regulation of cytokinins in tomato fruit development. Journal of Experimental Botany. 2012;**63**(15):5569-5579

[51] Hu J et al. The interaction between DELLA and ARF/IAA mediates crosstalk between gibberellin and auxin signaling to control fruit initiation in tomato. The Plant Cell. 2018;**30**(8): 1710-1728

[52] Ye J et al. Knockdown of SlNL33 accumulates ascorbate, enhances disease and oxidative stress tolerance in tomato (Solanum lycopersicum). Plant Growth Regulation. 2019;**89**(1):49-58

[53] Behnke K et al. RNAi-mediated suppression of isoprene emission in poplar transiently impacts phenolic metabolism under high temperature and high light intensities: A transcriptomic and metabolomic analysis. Plant Molecular Biology. 2010;**74**(1-2):61-75

[54] Zemlyanskaya EV et al. Deciphering auxin ethylene crosstalk at a systems level. International Journal of Molecular Sciences. 2018;**19**(12):4060

[55] Tiwari J et al. Glycerol micellar catalysis:An efficient multicomponent-tandem green synthetic approach to biologically important 2,4-disubstituted thiazole derivatives. ChemistrySelect. 2018;**3**(41):11634-11642

[56] Pérez-Llorca M et al. Biosynthesis, metabolism and function of auxin, salicylic acid and melatonin in climacteric and nonclimacteric fruits. Frontiers in Plant Science. 2019;**10**:136

[57] Golldack D et al. Gibberellins and abscisic acid signal crosstalk: Living and developing under unfavorable conditions. Plant Cell Reports. 2013;**32**(7):1007-1016

[58] Bashar KK et al. Phytohormonemediated stomatal response, escape and quiescenc strategies inplants under flooding stress. Agronomy. 2019;**9**(2):43

[59] Fayez KA, Bazaid SA. Improving drough and salinity tolerance in barley by application of salicylic acid and potassium nitrate. Journal ofthe Saudi Society of Agricultural Sciences. 2014;**13**(1):45-55

[60] Davies PJ. The plant hormones: Their nature, occurrence, and functions. In: Plant Hormones. Berlin: Springer; 2010. p. 15

[61] Finkelstein R. Abscisic acid synthesis and response. The Arabidopsis book/American Society of Plant Biologists. 2013;**11**:e0166

[62] Shi WG et al. Abscisic acid enhances lead translocation from the roots to the leaves and alleviates its toxicity in Populus× canescens. Journal of Hazardous Materials. 2019;**362**:275-285

[63] Kelley DR, Estelle M. Ubiquitinmediated control of plant hormone signaling. Plant Physiology. 2012;**160**(1): 47-55

[64] Gomi K. Jasmonic Acid: An Essential Plant Hormone. Basel, Switzerland: Multidisciplinary Digital Publishing Institute; 2020

[65] Yang J et al. The crosstalks between jasmonic acid and other plant hormone signaling highlight the involvement of jasmonic acid as a core component in plant response to biotic and abiotic stresses. Frontiers in Plant Science. 2019;**10**:1349

[66] Janda T, Szalai G, Pál M. Salicylic acid signalling in plants. Basel, Switzerland: Multidisciplinary Digital Publishing Institute; 2020

[67] Janda M, Ruelland E. Magical mystery tour: Salicylic acid signalling. Environmental and Experimental Botany. 2015;**114**:117-128

[68] Mohamed HI, El-Shazly HH, Badr A. Role of salicylic acid in biotic and abiotic stresstolerance in plants. In: Plant Phenolics in Sustainable Agriculture. Berlin: Springer; 2020. pp. 533-554

### **Chapter 8**
