**2.5. Gibberellins**

**2. Phytohormones signaling roles**

Abscisic acid (ABA), termed stress hormone, plays an important role in plant leaves abscis‐ sion and abiotic stresses tolerance [3]. ABA also has important roles in various plant devel‐ opmental and physiological processes such as seed dormancy, embryo morphogenesis, stomatal opening, cell turgor maintenance, and biosynthesis of lipids and storage proteins [3, 5]. ABA regulates protein‐encoding genes [6]. ABA enables plants to survive under severe environmental factors [7] and water‐deficit conditions [8]. ABA is also important for root growth and architectural modifications under nitrogen deficiency [9] and drought stress [10]. Furthermore, ABA is involved in the biosynthesis of dehydrins, osmoprotectants and protec‐

2 Phytohormones - Signaling Mechanisms and Crosstalk in Plant Development and Stress Responses

Some pathways for auxin (IAA) biosynthesis in plants have been reported so far including one tryptophan‐independent and four tryptophan‐dependent pathways [3, 13]. IAA plays an important role in plant growth and development as well as in regulating growth under stress factors [14]. IAA plays essential roles in plant adaptation to salinity [15] and heavy metal stresses [16]. Furthermore, auxins induce the transcription of the primary auxin response genes which are identified in various plants such as rice, *Arabidopsis* and soybean [3, 17].

Cytokinins (CKs) regulate plant growth and development [3, 19]. They are also involved in abiotic stresses [20] such as salinity [21] and drought [19]. They are also important for vari‐ ous crop traits such as productivity and enhanced stress tolerance [3, 22]. CKs also release seeds from dormancy [18] and are considered as abscisic acid antagonists [23]. Decreased CK content promotes apical dominance, which assists in the adaptation to drought stress [3, 20].

Ethylene (ET) is a gaseous phytohormone regulating plant growth and developmental processes, including flower senescence, fruit ripening, and petal and leaf abscission, as well as regulating stress responses [3, 24, 25]. Ethylene biosynthesis begins from methionine via S‐adenosyl‐l‐methionine and the cyclic amino acid ACC. ACC synthase converts S‐adenosyl‐l‐methionine to ACC, whereas ACC oxidase catalyzes the conversion of ACC to ET. Various abiotic stresses affect endogenous ethylene levels in plant species. Higher ET concentrations promote stress tolerance [26]. Ethylene may combine with other hormones such as jasmonates and salicylic acid and plays crucial roles in regulating plant defense against biotic stress factors [3, 1]. Ethylene and abscisic acid may act

Auxin also regulates crosstalk between biotic sand abiotic stresses [18].

together to regulate plant growth and development [3].

**2.1. Abscisic acid**

tive proteins [3, 11, 12].

**2.2. Auxins**

**2.3. Cytokinins**

**2.4. Ethylene**

Gibberellins (GAs) are carboxylic acids that may regulate plant growth and development [27]. They positively regulate leaf expansion, seed germination, stem elongation, flower develop‐ ment and trichome initiation [3, 28]. They also play important role in abiotic stress toler‐ ance [29] such as osmotic stress. GAs may interact with other hormones and regulate various developmental processes [30]. These interactions may involve both negative and positive regulatory roles [3, 30].

## **2.6. Brassinosteroids**

Brassinosteroids (BRs) comprise polyhydroxy steroidal phytohormones which regulate plant growth and developmental processes including root and stem growth, and flower initiation and development [3]. BRs were first isolated from *Brassica napus*. Brassinolide, 24‐epibrassino‐ lide, and 28‐homobrassinolide are the most bioactive BRs widely used in physiological stud‐ ies [31]. They are found in flower buds, pollen, fruits, vascular cambium, seeds, leaves, roots, and shoots [32]. BRs also play important roles in abiotic stress responses such as chilling, high temperature, soil salinity, drought, light, flooding, and organic pollutants [3].

## **2.7. Jasmonates**

Jasmonates (JAs) are multifunctional phytohormones derived from the membrane fatty acids metabolism and are widely distributed in several plant species [3]. JAs play crucial roles in growth and developmental processes such as fruiting, flowering, senescence and secondary metabolism [3, 33]. JAs are also involved in biotic and abiotic stress responses such as salin‐ ity, drought, irradiation and low temperature [3, 34]. Exogenous concentrations of methyl jasmonate (MeJA) minimize salinity stress symptoms [35]. Additionally, endogenous levels of JA are induced in roots under salinity stress [36]. JA levels also reduce heavy metal stress through inducing the antioxidant machinery [3, 37]. MeJA accumulates phytochelatins, con‐ ferring tolerance against Cu and Cd stress [38].

## **2.8. Salicylic acid**

Salicylic acid (SA) is a phenolic compound which regulates the expression of pathogenesis‐ associated proteins [39]. SA plays an important role in plant growth and development, as well as in biotic and abiotic stress responses [3, 40]. SA has two biosynthesis pathways: the major isochorismate (IC) pathway and the phenylalanine ammonia‐lyase (PAL) pathway. Low lev‐ els of SA promote the plant antioxidant capacity [3]. However, the high SA levels may result in cell death [41]. SA comprises genes encoding chaperones, antioxidants, heat shock pro‐ teins, and secondary metabolite biosynthetic genes such as cinnamyl alcohol dehydrogenase, sinapyl alcohol dehydrogenase and cytochrome P450 [3, 41]. SA may also combine with ABA to regulate drought response [39]. However, the SA mechanism in abiotic stress tolerance remains mainly unknown and still needs more investigations.

### **2.9. Strigolactones**

Strigolactones (SLs) are carotenoid‐derived compounds, produced in small quantities in roots, or synthesized in several plant species [3, 42]. SLs play an important role in root archi‐ tecture and development [43]. They induce nodulation during interaction processes [44] and may be used for inducing the parasitic plants seed germination [45]. SLs are also involved in biotic and abiotic responses [3].
