**2. Phytohormones signaling roles**

### **2.1. Abscisic acid**

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‐ tive proteins [3, 11, 12].

## **2.2. Auxins**

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

## **2.3. Cytokinins**

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].

#### **2.4. Ethylene**

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 together to regulate plant growth and development [3].
