**5. Plant-growth-promoting rhizobacteria (PGPR): an ecologically sustainable alternative to chemical fertilizers for agricultural production**

The rhizosphere is a well-defined ecological niche that consists of the volume of soil surrounding plant roots and is home to a wide range of microbial species [61, 62]. As a result of phytomicrobiome research, certain plant-microbe interactions that directly aid in plant nutrition are beginning to emerge [63]. Microbes have the power to positively influence plant growth and combat the majority of modern agriculture's challenges, making them a promising alternative for agricultural sustainability. The rhizomicrobiome is indispensable for agriculture because of the extensive diversity of root exudates and plant cell debris that attract diverse and unique patterns of microbial colonization. Fertilizer requirements are often lower in soils with dynamic microbial ecologies and rich organic matter than in traditionally treated soils [64].

Despite the fact that the rhizosphere is home to a diverse range of microbes, including bacteria, fungi, algae, protozoa, and actinomycetes, bacterial colonies are predominant [65, 66]. The bacterial community in the soil, in particular, has the potential to proliferate quickly and use a wide variety of nutrient sources. A group of natural soil bacterial flora that resides in the rhizosphere and grows in, on, or around plant roots [67] and has a beneficial effect on the plant's overall health is referred to as PGPR [68]. PGPR is a nonpathogenic, beneficial bacterium that promotes plant growth by modifying hormone levels and nutritional requirements, as well as reducing stress-related damage [69]. Nutrient absorption is thought to be increased as a result of the increased root surface area mediated by PGPR. Besides, they mineralize organic contaminants and are employed in polluted soil bioremediation [70]. When compared to other microorganisms, PGPR has unique characteristics, such as the ability to synthesize growth regulators, nitrogen fixation, phosphorus solubilization, siderophore generation, nutrients, and mineral solubilization, demonstrating their exceptional tendency in stimulating plant growth [71]. They are also environmentally friendly and ensure that nutrients from natural sources are available at all times. In addition to stimulating plant growth through their active mechanisms, the bacterial

*Plant-Bacterial Symbiosis: An Ecologically Sustainable Agriculture Production Alternative… DOI: http://dx.doi.org/10.5772/intechopen.104838*

colonies in the rhizosphere have a considerable influence on suppressing phytopathogenic microorganisms. Beneficial rhizobacteria can emit antibiotics and other chemicals that are effective at inhibiting pathogens [72].

The fundamental impacts that rhizosphere bacteria have on plants have evolved into an important mechanism for protecting plant health in an environmentally sustainable manner [73]. They participate in a variety of biotic activities in the soil ecosystem to keep it active and productive for farming systems [74]. Furthermore, in recent times, PGPR has garnered much attention for its potential to substitute agrochemicals for plant growth and yield through multiple processes, including decomposition of organic matter, recycling of essential elements, soil structure formation, production of numerous plant growth regulators, degrading organic pollutants, stimulation of root growth, and solubilization of mineral nutrients, which are important for soil health [75]. It is cost-effective and environmentally beneficial to replace chemical fertilizers with PGPR, as well as to identify the most effective soil and crop management approaches in an attempt to develop more sustainable farming and soil conservation fertility [76]. The employment of phytomicrobiome representatives as a long-term disease prevention and nutrient supplement method in farming production might help to reduce the negative impacts of pesticide usage [77]. The inoculated plant's biocontrol and induction of disease resistance, biological N2 fixation, phosphate solubilization, and/or phytohormone synthesis are all potential explanations for PGPR's growth-promoting actions [78].

PGPR has both direct and indirect modes of action as a biofertilizer and a biopesticide.

#### **6. The effect of PGPR on plant nutrient supplementation**

#### **6.1 PGPR as biofertilizers**

One of the most prevalent ways for increasing agricultural production is to improve soil fertility. PGPR promotes soil fertilization through the biofixation and biosolubilization processes (**Figure 2**).

#### *6.1.1 Biofixation of atmospheric nitrogen*

Nitrogen (N) is found in all forms of life and is one of the most significant mineral nutrients for plant growth as it is a crucial component for various physiological activities in plants, including photosynthesis, nucleic acids, and protein synthesis [80]. Unfortunately, due to the low degree of reactivity, no plant species are capable of directly converting atmospheric dinitrogen into ammonia and using it for growth, hence making the plants dependent on biological nitrogen fixation (BNF). Nitrogen fertilizer, as being the most effective approach to nitrogen supplementation, has been an integral part of modern crop production and agricultural systems; yet, their continued and undesirable use is contaminating the climate. Though carbon dioxide (CO2) is widely regarded as the primary cause of climate change, nitrous oxide (N2O), which has a 265-fold higher heat-trapping efficiency than CO2 [81], is indeed a significant contribution. PGPR in this regard is a potential alternative to minimize the fertilizer requirements to a certain degree as the majority of the plant microbial community can either directly fix atmospheric nitrogen through legume-rhizobium interaction or indirectly by helping nitrogen fixers via their secretion [82].

**Figure 2.** *PGPR's mechanism of action [79].*

Worldwide, total N fixation is estimated to be ∼175Tg, with symbiotic nitrogen fixation in legumes accounting for ∼ 80 Tg by fixing 20–200 kg N year-1, while the remaining nearly half (∼88 Tg) is industrially fixed during the production of N fertilizers [83]. The most prominent symbiotic nitrogen fixer is Rhizobium [84], whereas Azospirillum, Acetobacterdiazotrophicus, Azotobacter, Herbaspirillum, Cyanobacteria, Bacillus, Paenibacillus, Gluconacetobacter, and Azoarcus, etc., represent the free-living N fixers [85].

Symbiotic nitrogen fixation: A mutualistic association between a microorganism and a plant is known as symbiotic nitrogen-fixing. The N-fixing symbiosis between rhizobia and legumes is the most well-studied and utilized beneficial plant-bacteria interaction. In this interaction, legumes supply rhizobia with reduced carbon and a protected, anaerobic environment that is necessary for nitrogenase activity, while rhizobia feed legumes with biologically accessible nitrogen. The bacteria enter the root first, causing the growth of nodulation, which converts atmospheric nitrogen into usable forms (primarily NH3) [86]. Rhizobia can fix up to 200 kg of nitrogen ha − 1 by establishing symbiotic relationships with more than 70% of leguminous plants, thus making it available to plants.

Free-living nitrogen-fixing: Several nitrogen-fixing microorganisms do not interact in a symbiotic manner. These microorganisms are free-living and rely on plant leftovers or their own photosynthesis to exist. Although free-living nitrogen fixers do not enter the plant's tissues, a tight interaction is developed where these bacteria reside close enough to the root that the atmospheric nitrogen fixed by the bacteria is taken up by the plant, resulting in greater nitrogen absorption. Besides, other bacteria that do not fix nitrogen have been demonstrated to boost nitrogen uptake in plants, resulting in increased nitrogen use efficiency [87], most likely due to increased root development, which allows plants to reach more soil [63]. Evidence of PGPR involvement in the plant N budget has been identified for various plants, particularly sugarcane [88].

Rhizobial N-fixation is an integrated approach for disease control, growth stimulation, as well as providing and maintaining the nitrogen level in agricultural soils

*Plant-Bacterial Symbiosis: An Ecologically Sustainable Agriculture Production Alternative… DOI: http://dx.doi.org/10.5772/intechopen.104838*

around the world, thus minimizing the need for extensive N-fertilizer application and limiting the soil and environmental challenges associated with it.

#### *6.1.2 Phosphate solubilization*

Phosphorus (P) is the most significant vital element in plant nutrition (N), alongside nitrogen [89]. It is involved in a number of major metabolic activities in plants, including macromolecular biosynthesis, photosynthesis, respiration, energy transfer, and signal transduction [90]. Although most soils hold a significant amount of phosphorus, which builds over time as a consequence of fertilizer treatments, plants have access to only a small portion of it. Despite the fact that P is abundant in both organic and inorganic forms in the soil, only 0.1% of it is available to plants because 95–99% of phosphate is either insoluble, immobilized, or precipitated [91]. Plants can absorb mono and dibasic phosphate on their own, but organic and insoluble phosphate must be mineralized or solubilized by microorganisms [92]. Phosphate anions are highly reactive and, depending on the soil quality, can be trapped by precipitation with cations including Mg2+, Ca2+, Al3+, and Fe3+. Plants cannot absorb phosphorus in these forms because it is highly insoluble. As a result, plants only get a small percentage of the total.

When deficient, phosphorus-based fertilizers are typically used to replenish soil P, which is readily available to plants. Supplementing P with commercial fertilizers, however, is not an ideal option due to their high cost and sometimes inaccessibility to plants since they are easily lost from the soil and subsequently mix with local streams, contaminating both terrestrial and aquatic environments [93]. Therefore, phosphorus solubilization, in addition to nitrogen fixation, is also vital biologically. Phosphate solubilization is among the most profound consequences of PGPR on plant nutrition. Persistent plant growth, PGPR, plays a major role in solubilizing phosphorus [94]. The potential of various bacterial species to solubilize insoluble inorganic phosphate compounds such as dicalcium phosphate, tricalcium phosphate, rock phosphate, and hydroxyapatite has been documented by many researchers. Phosphate can be dissolved in insoluble forms by a variety of PGPR, including Pseudomonas, Bacillus, and Rhizobium. PGPR solubilizes P by employing a number of mechanisms, including the synthesis of organic acids and extracellular enzymes, to make use of inaccessible forms of P, hence assisting in the availability of P for plant absorption. Miller et al. [95] pointed out two processes—acidification of the external medium via the release of low molecular weight organic acids (such as gluconic acid) that chelate phosphate-bound cations and the formation of phosphatases/phytases that hydrolyze organic forms of phosphate compounds. Phosphorus solubilizing bacteria (PSB) has been shown to lower the recommended P dose by approximately 25% [96] and is even more efficacious when combined with other PGPRs or mycorrhizal fungi, reducing the P supplementation to 50%. As a result, the risk of P runoff and eutrophication is mitigated [97].

#### *6.1.3 Solubilization of potassium*

Potassium (K) deficit has become a severe crop production bottleneck. Plants with insufficient potassium have poor root development, low seed production, a slow growth rate, and a decreased yield. Soluble potassium concentrations in soil are typically low; over 90% of the potassium in the soil is in the form of insoluble rocks and silicate minerals [98]. Several microbes, particularly fungal and bacterial genera, have close connections with plants and can release potassium in accessible form from potassium-bearing minerals in soils through the synthesis and secretion of organic

acids [99–101]. Setiawati and Mutmainnah [101] synthesize organic acids produced by soil microorganisms, such as acetate, ferulic acid, oxalate, coumaric acid, and citrate, which significantly increase mineral dissolution rates and proton production by acidifying the soil rhizosphere and resulting in mineral K solubilization. As a result, using potassium-solubilizing PGPR as a biofertilizer in agricultural production can reduce agrochemical use while also encouraging environmentally friendly crop production.

#### *6.1.4 Iron sequestration by siderophore production*

Iron (Fe) is a major bulk mineral abundantly available on Earth, yet it is inaccessible in the soil for plants, owing to the fact that Fe3+ (ferric ion), the most common form of Fe found in nature, is hardly soluble [102]. PGPRs are the right fit to address this issue as they produce siderophores, which are tiny organic compounds that increase Fe absorption capability when it is scarce. Since PGPR can form siderophores, they are a valuable asset in supplying the plant with the necessary iron. Siderophores released by PGPRs boost plant growth and development via facilitating access to Fe in the soil surrounding the roots [103]. Plant growth can be stimulated directly by siderophore-producing bacteria, which improves plant Fe intake, or indirectly by suppressing the activities of plant pathogens in the rhizosphere, which limits their Fe availability [104]. Pathogen suppression is induced by the synthesis of siderophores, which decrease pathogen survival by chelating available Fe and therefore restricting pathogen survival [105]. In the presence of other metals, such as nickel and cadmium, a robust siderophore, such as the ferric-siderophore complex, is crucial for Fe uptake by plants [106]. Siderophores alleviate stress on the plants caused by potentially hazardous metals, such as Al, Cd, Cu, Pb, and Zn, found in polluted soil via forming stable compounds with them [107]. This phenomenon is beneficial for reducing plant stress induced by potentially harmful metals found in contaminated soils. Furthermore, siderophore-expressing rhizobacteria could be a potential alternative to chemical fertilizers by concurrently addressing salt-stress effects and Fe limitation in saline soils.

#### *6.1.5 Exopolysaccharide synthesis or biofilm formation*

One of the many advantages of rhizobacteria in encouraging plant growth and controlling plant diseases is their ability to synthesize polysaccharides. Multifunctional polysaccharides, for instance, structural polysaccharides, intracellular polysaccharides, and extracellular polysaccharides, are synthesized by specific bacteria. Exopolysaccharide production is critical for biofilm development, and root colonization can influence microbial interactions with root appendages. The colonization of plant roots by EPS-producing bacteria aids in the separation of free and insoluble phosphorus in soils, circulating critical nutrients to the plant for appropriate growth and development, as well as protecting it against disease attacks. EPS-producing bacteria have a variety of roles in plant-microbe interactions, including protection against desiccation, stress [108], adherence to surfaces, plant invasion, and plant defense response [109]. Plant exopolysaccharides produced by plant-growth-enhancing rhizobacteria are critical in stimulating plant growth because they act as an active signal molecule during beneficial interactions and generate a defense response during the infection phase [110]. Some plant-growth-promoting rhizobacteria that produce exopolysaccharides can also bind cations, including Na+, implying that they may play a role in limiting the amount of Na + available for plant uptake and thereby reducing salt stress [111].

*Plant-Bacterial Symbiosis: An Ecologically Sustainable Agriculture Production Alternative… DOI: http://dx.doi.org/10.5772/intechopen.104838*

#### **6.2 Production of biostimulants by PGPR**

Phytohormones, commonly known as plant growth regulators, are organic chemicals that, at low levels (less than 1 mM), promote, inhibit, or modify plant growth and development [112]. Phytohormones are categorized based on where they act. Botanists recognize five main kinds of phytohormones: Auxins, Gibberellins, Ethylene, Cytokinins, Ethylene, and Abscisic acid.

Phytohormones stimulate root cell proliferation by overproducing lateral roots and root hairs, resulting in increased nutrition and water intake [113]. This is crucial for regulating nutrient uptake depending on soil composition and environmental circumstances. Slower primary root development and a spike in the proportion and length of lateral roots and root hairs are the most common effects.

Phytohormones play an important role in regulating developmental processes and signaling networks that are involved in plant adaptation to a variety of biotic and abiotic stressors [114]. Abiotic stressors, however, disrupt plant growth by altering endogenous levels of phytohormones [115]. Surprisingly, some bacteria, such as PGPR, may stimulate plants to produce phytohormones.

A diverse spectrum of rhizospheric microorganisms is capable of producing growth hormones that can promote cell proliferation in the root architecture by inducing an increase in nutrition and water intake by encouraging root hair growth, thus regulating overall plant growth and development, as well as activating pathogen defensive responses [116]. Important biological rhizobacteria can adjust to their surroundings and develop stress tolerance by repairing plant roots. The production of growth metabolites by PGPRs may help provide water stress resistance in host root colonization, resulting in higher optimal crop output.

Auxin is a critical molecule that regulates most plant functions, either directly or indirectly, and indole-3-acetic acid (IAA) is the most abundant and physiologically potent phytohormone that regulates gene expression by upregulating and downregulating it [116, 117]. More than 80% of rhizospheric bacteria have been known to be capable of synthesizing and releasing auxins. IAA produced by PGPR regulates a wide range of processes in plant development and growth, including cell division, differentiation, organogenesis, tropic responses, primary root elongation, and the formation of lateral roots [118]. As a result of the increased root surface area and length mediated by bacterial IAA, plants have better access to soil nutrients. Under salinity stress circumstances, the secretion of IAA by PGPR may have a key function in managing and regulating IAA concentrations in the root system, resulting in improved plant salinity stress responses [119]. Besides, microbe-induced IAA can boost root and shoot biomass output in water-stressed situations [120].

Gibberellins (GA) are another type of phytohormone produced by rhizobacteria. Different activities in higher plants, such as seed germination, root and leaf meristem size, cell division and stem elongation, floral induction, fruiting, and the flowering process, growth of the hypocotyl and stem, are all mediated by GA [121]. However, shoot elongation is by far the most significant physiological function of GA [122], which modifies the morphology of plants.

Cytokinins are a type of growth regulators that are responsible for seed germination, production of shoots, vascular cambium sensitivity, the proliferation of root hairs, improvement of cell division and root development, interactions of plants with pathogens, and nutrient mobilization and assimilation [123, 124], but suppress root elongation and lateral root formation [125, 126]. They are especially important for the cell cycle's progression. Cytokinin, either alone or in combination with other

phytohormones like abscisic acid and auxin, can help salt-stressed plants grow faster while also improving resistance by altering the expression of genes [127]. PGPRs, such as Bradyrhizobiumjaponicum, Azospirillumbrasilense, Pseudomonas fluorescens, Arthrobactergiacomelloi, Paenibacilluspolymyxa, and Bacillus licheniformis, have been demonstrated to produce cytokinin (particularly zeatin) [69]. Cytokininproducing PGPRs act as biocontrol agents against a variety of pathogens [128].

PGPR has been proven in various investigations to be effective in both creating and regulating the amounts of ABA and gibberellic acid in plants. Gibberellins promote primary root elongation and lateral root development. Several PGPR, including Azotobacterspp, Azospirillumspp, Achromobacterxylosoxidans, Gluconobacterdiazotrophicus, Acinetobactercalcoaceticus, Bacillus spp., and Rhizobia spp., have been found to produce gibberellin [129].

The role of ABA under drought stress, for example, is well-known. Under conditions of water deficit, increased ABA levels cause stomata to shut, limiting water loss. This hormone, on the other hand, offers a variety of benefits during lateral root development [129]. Inoculation with Azospirillumbrasilense Sp245 increased ABA content in Arabidopsis, especially when grown under osmotic stress [130].

In addition to their roles in plant RSA, these two hormones are involved in plant defense mechanisms. As a result, PGPR, which produces these hormones, may affect the hormonal balance involved in plant defense, including the jasmonate and salicylic acid pathways [131].

#### **7. PGPR and abiotic stress tolerance**

As climate change conditions worsen, extreme environmental conditions that can cause significant annual losses in total crop output are now more prevalent worldwide [132, 133]. Many biotic and abiotic stresses are causing havoc on the sector, resulting in enormous plant productivity losses all around the world. Stress factors comprise nutrient shortages, heavy metal contamination, high wind, extreme temperatures, salinity, drought, illnesses, plant invasions, pests, salt, and soil erosion [69].

As a result of climate change, abiotic stresses, such as drought and high temperatures, have risen in frequency and intensity, resulting in 70% losses in major staple food crops, posing a danger to global food security [134]. Drought and high soil salinity, as well as their downstream impacts, such as osmotic, oxidative, and ionic stress, are regarded as important hindrances to long-term agriculture production [135]. Stressed plants suffer from internal metabolism disruption due to metabolic enzyme inhibition, substrate scarcity, excessive need for different chemicals, or a combo of the following. To endure unfavorable conditions, metabolic reconfiguration is required to comply with the demand for anti-stress compounds, such as suitable solutes, antioxidants, and proteins [136].

Agricultural breeding practices have tried to produce species that are more productive in unfavorable environments for ages. However, crop breeding for abiotic stress resistance has been impeded by a lack of reliable and consistent traits. Tolerance to stress is influenced by a number of genes working together. Furthermore, using agrochemicals to address biotic stresses and nutritional deficits contributes to environmental degradation, has a negative influence on the biogeochemical cycle system, and puts people at increased risk. The potential repercussions of the aforementioned stresses are significant, necessitating the development of robust, cost-effective, and environmentally acceptable methods to mitigate these stresses' harmful effects

#### *Plant-Bacterial Symbiosis: An Ecologically Sustainable Agriculture Production Alternative… DOI: http://dx.doi.org/10.5772/intechopen.104838*

on plants. As a result, there has been a spike in interest in environmentally friendly and organic agriculture techniques. Plant growth stimulants have been utilized in bio-fertilization, root revitalization, rhizoremediation, disease resistance, and other modes of microbial revival [137].

The efficient approach of PGPR can alleviate stresses that cause serious damage to crop yield, hence, the application of PGPR and/or their byproducts, which can help plants successfully resist extreme environmental circumstances, is one of the most eco-friendly ways [138]. Some PGPR genera, for instance, P. fluorescens, produce the enzymes 1-aminocyclopropane-1-carboxylate (ACC) deaminase and hydroxyacetophenone monooxygenase, which break down the ethylene precursor ACC to a-ketobutyrate and ammonia, thereby protecting plants from abiotic stressors [139]. The most destructive factors that reduce agricultural productivity are salinity and drought [140]. Furthermore, greater ethylene levels in the plant lead to premature fatuity symptoms, including leaf yellowing, abscission, and desiccation/necrosis [141]. PGPR is essential to minimize ethylene concentrations in plants, which in turn reduces stress.

During dry spells, turgor pressure and water potential have a significant impact on plant functionality. Drought stress results in substantial losses in agricultural output and the flow of nutrients, such as sulfates, nitrates, calcium, silica, and magnesium, as well as a reduction in photosynthesis activity [142]. To achieve sustainable agricultural productivity, bacterial colonies in the rhizosphere and endorhizosphere stimulate the plant to withstand drought [143]. PGPR releases osmolytes, which function in tandem with those obtained from plants to keep plants healthy and improve their growth and development, as well as withstand drought-related stress and excessive salt levels in the soil [144]. According to research findings, inoculating plants growing in dry and semi-arid areas with beneficial plant-growth-promoting rhizobacteria (PGPR), which enhances plant abiotic stress tolerance with an osmotic component, could improve drought tolerance and water utilization efficiency. PGPR-induced root development, nutrient uptake efficiency, and systemic tolerance have been proposed as biochemical changes in plants that result in increased abiotic stress tolerance (IST) [78].

#### **8. Plant biotic stress, pesticide use, and PGPR as biopesticides**

Rise in global temperature and fluctuations in precipitation as a result of climate change have resulted in unprecedented crop pests and illnesses in various parts of the world [82]. Biotic agents, such as pathogenic bacteria, viruses, fungi, nematodes, protists, weeds, insects, and arachnids, are a prevalent concern in crop production and a long-term danger to sustainable agriculture and ecosystem stability around the world [145]. These species can induce biotic stress in their hosts by interfering with normal metabolism, injuring their plant hosts, reducing plant vigor, limiting plant development, and/or inducing plant mortality. Biotic stress has an impact on co-evolution, ecosystem nutrient cycling, population dynamics, horticulture plant health, and natural habitat ecology [146]. They also result in pre- and post-harvest damage to agricultural crops [147].

According to the FAO, pests are estimated to be responsible for up to 40% of global agricultural production losses each year. Plant diseases cost the world economy more than \$220 billion per year while invading insects cost at least \$70 billion [148].

Pesticides are chemical compounds that are used to prevent or control pests. However, these are poisonous compounds that pollute soil, watercourses, and plant life. The inappropriate application and overuse of such chemicals have triggered

numerous problems (e.g., the emergence of resistance in target organisms, food contamination, and environmental pollution) [149]. Pesticide use causes morphological, physiological, biochemical, and molecular changes in plants that can have a detrimental effect on the plant's development and growth, leaving chemical residues in numerous plant tissues, as well as insect resistance to pesticides [150, 151]. Besides, pesticides cause oxidative stress in plants, hinder physiological and biochemical pathways, cause toxicity, obstruct photosynthesis, and reduce crop yield. The overgeneration of reactive oxygen species has a negative effect on non-targeted plants. Reactive oxygen species are highly reactive in nature, causing oxidative damage to lipids, nucleic acids, proteins, carbohydrates, and DNA in plants, as well as disruptions in other biochemical and physiological cell processes [152].

The rising number and intensity of pesticide consumption have presented a significant obstacle to the pests being targeted, leading them to disseminate to dynamic habitats and/or adjust to the changing settings [153]. Resistance is currently the greatest serious impediment to the effective use of pesticides. Many pest species have developed resistance to pesticides as a result of their use around the World [154].

Pesticides' impact on non-target species has been a source of debate and worry around the world for decades. Pesticides' adverse impacts on non-target arthropods have been well documented [155]. Natural insect adversaries, such as parasitoids and predators, are tragically the most vulnerable to insecticides and suffer the most harm [156]. Natural enemies that ordinarily keep small pests in check are sometimes harmed, which can lead to subsequent pest outbreaks.

Not just that, pesticide use may have a negative impact on the earthworm population. Earthworms contribute to the improvement and maintenance of soil structure by producing channels in the soil that allow for aeration and drainage. In agricultural settings, they are regarded as a key indicator of soil quality [157]. Earthworms are harmed by a wide range of agricultural practices, with indiscriminate pesticide usage being one of the most serious [158]. Yasmin and D'Souza [159] found that pesticides have a dose-dependent effect on earthworm reproduction and proliferation.

Moreover, pesticide usage has the potential to destroy biodiversity. Degraded pesticides interface with the soil as well as its inhabitants, affecting microbial diversity, biochemical processes, and enzyme activity [160]. Any change in the activity of soil microorganisms as a result of pesticide application disrupts the ecological environment, resulting in a loss of soil quality. In crops cultivated on soils excessively exposed to chemical pesticides, nutrient loss and disease incidence are widespread [161], which is unfavorable from the perspective of agricultural soil management for food and nutritional security.

Exogenous pesticide residues may also alter the efficacy of beneficial root-colonizing microbes, such as fungi, bacteria, algae, and arbuscular mycorrhiza (AM), in soil by affecting their growth, and metabolic activity, among other things [162].

Furthermore, pesticides are widely distributed when they are transported across long distances by air or water [163]. Several pesticides have a prolonged half-life (up to years) in the environment; for example, the half-life of HCH in water is determined to be 191 days [164], hence posing a threat to aquatic creatures.

The mode of pesticides' action is hazardous not just to the target organisms but also to non-target creatures, such as humans. The physicochemical parameters of the active ingredient are known to influence pesticide diffusion into plant tissue. As a result, pesticides with a systemic effect are absorbed by the roots or leaves and transported throughout the plant, as a result, they pose a major health risk to anyone who consumes them [165]. Pesticides' negative impacts on human health

*Plant-Bacterial Symbiosis: An Ecologically Sustainable Agriculture Production Alternative… DOI: http://dx.doi.org/10.5772/intechopen.104838*

have begun to emerge as a result of their toxicity, longevity in the environment, and tendency to penetrate the food chain. Based on the side effects, chemical pesticides employed in crop protection to limit the damage caused by pathogens and pests in agricultural areas pose significant long-term risks and challenges to life forms. Pesticides can penetrate the human body through immediate exposure to chemicals, contaminated water, or polluted air, as well as through food, particularly fruits and vegetables. Pesticide exposure can cause both acute and chronic disorders. Humans develop chronic sickness after being exposed to sub-lethal levels of pesticides for extended periods of time [166]. They are believed to stimulate cancer [167] and fetal malformations [168], and they are nonbiodegradable [169]. Encountering pesticides with genetic makeup, resulting in DNA damage and chromosomal abnormalities, is one of the primary pathways that lead to chronic disorders, such as cancer [170]. Pesticides can also cause oxidative stress by modifying the amounts of antioxidant enzymes, including glutathione reductase, superoxide dismutase, and catalase, which increase reactive oxygen species (ROS) [171]. Pesticide-induced oxidative stress has been linked to a number of health concerns, including Parkinson's disease and glucose homeostasis disruption [170].

Given the pervasive harmful effects of pesticides on plants, soil, the environment, and human health, an environmentally friendly replacement is required, making PGPR a viable option.

#### **8.1 Biopesticides/biocontrol agents using PGPR**

Biocontrol agents are bacteria that suppress the occurrence or severity of plant diseases, whereas antagonists are bacteria that have antagonistic behavior toward a pathogen. PGPR can be used as a biocontrol agent (**Figure 3**) to protect plants from pathogens, such as viruses, bacteria, insects, and fungi [173].

When compared to chemical pesticides, PGPR has unique benefits, including being harmless to mankind and nature, dissolving more quickly in soil, and having a lesser possibility of pathogen resistance development [174]. Because plants, unlike animals, are unable to use avoidance and escape as stress-relieving strategies, their

**Figure 3.** *PGPR as biocontrol agent [172].*

#### *Revisiting Plant Biostimulants*

existence has been marked by the establishment of extraordinarily favorable partnerships with their more mobile partners, microbes. PGPR and its interactions with plants are economically harnessed [175], and they hold considerable promise for longterm agricultural sustainability. Plants that have been inoculated by immersing their roots or seeds in PGPR cultures overnight have been shown to be extremely resistant to many forms of biotic stress [176].

#### *8.1.1 Antibiotic synthesis*

Antibiotic synthesis is one of the most robust and well-studied biocontrol mechanisms of PGPR against phytopathogens during the last two decades [177]. Antibiotics are low-molecular-weight toxins that have the ability to kill or inhibit the growth of other bacteria. The Bacillus genus and Rhizobacteria are the most significant for antibiotic synthesis [178]. Antibacterial and antifungal antibiotics are produced by Bacillus amyloliquefaciens and B. subtilis, including subtilin, bacilysin, and emicobacillin [179].

#### *8.1.2 Induced systemic resistance*

Induced systemic resistance (ISR) is a physiological condition of increased defensive capacity triggered by a specific environmental stimulation. Conrath et al. [180] define ISR as "an enhanced defensive ability of plants in response to specific pathogens stimulated by beneficial microorganisms present in the rhizosphere," a scenario wherein the interaction of certain microorganisms with roots results in plant tolerance to pathogenic bacteria, fungi, and viruses. ISR can also be induced by certain environmental cues that cause upregulation of plants' innate defenses in response to the biotic assault, allowing plants to respond faster and stronger to subsequent pathogen attacks [181]. Following the pathogenic invasion, signals are produced, and a defense mechanism is activated via the vascular system. Among the defense mechanisms produced by ISR in plants are cell wall reinforcement [182], production of secondary metabolites, and accumulation of defense-related enzymes, such as chitinases, glucanases, peroxidase, phenylalanine ammonia-lyase, and polyphenol oxidase, lipoxygenase, SOD, CAT, and APX along with some proteinase inhibitors [183].

ISR is not unique to a particular pathogen but can benefit a plant by evading a variety of diseases. Various plants develop systemic resistance to a wide range of plant diseases and a variety of environmental stresses when primed with PGPR [184]. ISR is among the pathways through which PGPR might minimize the onset of various plant diseases by modifying the physical and biochemical attributes of host plants and thereby boosting plant growth [185]. After applying plant-growth-promoting rhizobacteria, diseases of fungal, bacterial, and viral origin, as well as damage caused by insects and nematodes, can be decreased [186].

Non-pathogenic microorganisms promote ISR, which starts in the roots and extends to the shoots [187]. ISR stimulates plant defense mechanisms and shields unexposed regions of plants against future pathogenic attacks by microbes and insects. The signaling of ethylene and jasmonic acid in the plant is involved in induced systemic resistance, and these hormones increase the host plant's defense responses against a range of plant diseases [188]. Lipopolysaccharides (LPS), siderophores, homoserine lactones, 2, 4-diacetylphloroglucinol, cyclic lipopeptides, and volatiles like acetoin and 2, 3-butanediol are only a few of the bacterial components that cause induced systemic resistance [189].

*Plant-Bacterial Symbiosis: An Ecologically Sustainable Agriculture Production Alternative… DOI: http://dx.doi.org/10.5772/intechopen.104838*

#### *8.1.3 Production of protective enzymes*

Plant-growth-promoting rhizobacteria use another mechanism to promote growth—enzymatic activity, producing compounds that inhibit phytopathogenic agents [190]. Rhizobacterial strains that promote plant growth can secrete enzymes, including ACC-deaminase, phosphatases, chitinases, 1,3-glucanase, proteases, dehydrogenases, and lipases, among others [94, 191]. They excrete cell wall hydrolases, which are used to break down cell walls, neutralize infections, assault pathogens, and cause hyperparasitic activity [192]. Plant-growth-promoting rhizobacteria suppress pathogenic fungi, such as *Phytophthora sp, Botrytis cinerea, Fusarium oxysporum, Sclerotium rolfsii*, *Pythium ultimum*, and *Rhizoctonia solani* by the activation of such enzymes [193, 194], hence defending the plant against various biotic and abiotic stresses. Because 1,4-N-acetylglucosamine and chitin make up the majority of fungal cell wall constituents, bacteria that generate 1,3-glucanase and chitinase restrict their proliferation. Inoculation of plants with arbuscular mycorrhiza has also been shown to increase plant growth. *Trichoderma* strains have been employed as biological control agents and plant growth boosters in the past [195].

#### *8.1.4 Production of volatile organic compounds (VOCs)*

In recent years, microbial volatile organic compounds (mVOC) have been shown to play an important role in microorganism–plant interactions [196–198]. VOCs are produced by a wide range of soil microorganisms. Bacillus bacteria are the most common microbes that produce antimicrobial MVOCs. Bacterial volatiles have a key function in encouraging plant growth by regulating phytohormone synthesis and metabolism.

They can also promote plant health by acting as antibacterial, nematicidal, oomyceticidal, and antifungal agents, as well as eliciting plant immunity via the salicylic acid (SA) and jasmonic acid (JA) pathways [199]. These molecules have the potential to increase plant growth and development and induce systemic resistance (ISR) against pathogenic organisms, resulting in improved agricultural well-being [200]. Through the SA-signaling pathway, acetoin from the bacteria B. subtilis produces systemic resistance in *A. thaliana* against *P. syringae* [201].

Depending on the species, the quantity and composition of VOCs varies [202]. 2, 3-Butanediol is a volatile organic compound (VOC) generated by a variety of microorganisms that, among other things, can activate plant resistance against pathogens. This mVOC generated by *B. subtilis* and *B. amyloliquefaciens* is capable of generating a systemic resistance in A. thaliana mediated by the ethylene (ET)-signaling pathway against *Erwinia carotovora* subsp. *carotovora* [196]. The same MVOC from *Enterobacter aerogenes* was engaged in the establishment of plant tolerance against *Setosphaeria turcica,* a fungus that causes Northern corn leaf blight [203].

#### *8.1.5 Hydrogen cyanide (HCN) production*

The antagonistic activity of PGPR also results in the production of volatile compounds. HCN, a well-studied biocontrol agent, commonly known as prussic acid, is a broad-spectrum volatile secondary metabolite generated by numerous rhizobacteria and is crucial for the biological control of several infectious microorganisms in the soil. Most metalloenzymes are inhibited by their cyanide ion, particularly coppercontaining cytochrome c oxidases [204]. HCN-producing Pseudomonas strains are

employed in the biological control of tomato bacterial canker [205]. For instance, the inhibition of Macrophomina phaseolina and Meloidogyne javanica caused sunflower charcoal rot and tomato root-knot diseases and has been related to bacterial strains secreting HCN [206]. The inhibitory activity process starts in the mitochondria, where HCN inhibits electron transport, reducing energy supply to the cell and finally causing pathogenic organisms to die.

#### *8.1.6 Aminocyclopropane-1-carboxylate (ACC) deaminase production*

Plants generate a lot of "stress ethylene" (ET) after the onset of a disease or stress. Much of the growth inhibition that happens as a result of environmental stress is due to the plant's response to elevated levels of stress ethylene, which aggravates the stressor's response. Likewise, ethylene production inhibitors can considerably reduce the intensity of various environmental stressors. The production of defense enzymes, including 1-aminocyclopropane-1-carboxylate (ACC) deaminase, has also been linked to PGPM's ability to protect against biotic stress [207]. Numerous results suggest that seed inoculation with bacterial endophytes increases plant defense. This is because bacteria produce the enzyme 1-aminocyclopropane-1-carboxylate (ACC), which can cleave ET into ketobutyrate and ammonia, lowering the presence of this enzyme linked to plant stress and physiological impairment [208]. As a result, if ACC deaminase-containing bacteria can reduce plant ethylene levels, treating plants with these organisms may give some defense against the stress inhibitory effects. The synthesis of ACC-deaminase by *Paenibacillus lentimorbus* B-30488 (B-30488) is assumed to be the pathway whereby *P. lentimorbus* B-30488 (B-30488) negates Scelerotium rolfsii in tomato [207]. Hence, the usage of PGPR is appropriate for reducing the environmental stress that crop plants face.

### **9. Conclusion**

To meet the ever-increasing nutritional demand of the rapidly increasing world population, chemical fertilizers must be employed. However, unintended and excessive use has a variety of negative repercussions on the natural environment resulting in soil degradation, global warming, and climate change, necessitating the search for environmentally sound alternatives. PGPR, in this regard, is a realistic choice for agricultural production that does not deplete natural resources. Plants and microbial communities in the soil have evolved a variety of biotic connections, ranging from commensalism to mutualism. Plant-PGPR collaboration is an important aspect of this web of interactions, promoting the growth and health of a variety of plants. PGPR has recently received a lot of attention for its potential to replace agrochemicals for plant growth and yield through a variety of processes, including decomposition of organic matter, recycling of essential elements, formation of soil structure, production of numerous plant growth regulators, fixation of atmospheric nitrogen, degradation of organic pollutants, stimulation of root growth, solubilization of phosphorus, production of siderophore, and solubilization of mineral nutrients, all of which are important for soil and plant health. Furthermore, they are cost-efficient and environmentally sustainable and assure that nutrients from natural sources are always accessible. Besides, bacterial colonies in the rhizosphere have a considerable impact on phytopathogenic microorganism reduction, in addition to boosting plant growth through active processes, hence the use of phytomicrobiome representatives

*Plant-Bacterial Symbiosis: An Ecologically Sustainable Agriculture Production Alternative… DOI: http://dx.doi.org/10.5772/intechopen.104838*

in farming production as long-term disease prevention and nutrient supplement strategy could also help to mitigate the detrimental effects of pesticide use.

As a nutshell, in the face of global climate change, PGPR could be a more environmentally friendly option than chemical fertilizers.
