**4. Abiotic stress and phytohormones**

Abiotic stress like salt, low temperature, heat, drought, UV-radiation, elevated CO2, ozone, and heavy metals stress, are the major factors that reduce the growth of plants. There are various adaptations and ways by which plants protect them from the abiotic stress, and one such strategy is phytohormones produced by them. Phytohormones play a major role in some of the important functions in plants in how to respond to external environmental changes [39].

Abscisic acid (ABA), also called as the plant stress hormones as it is the most studied plant hormones under abiotic stress and has the important role in stress tolerance too. ABA activates specific signalling molecules which helps in activating the genes by signal transduction and activates set of genes through abscisic acid-responsive elements (ABRE) and Dehydration Responsive elements (DRE) pathways.in response to ABS LEA and dehydration proteins are also induced [40].

Auxin's primary role is to play an important role in plant growth and development, but auxin also plays an important role in abiotic stress tolerance. Functional genomics studies based upon the transcriptome analysis proves the upregulated genes related to auxins under abiotic stress conditions [41, 42]. Auxin level has been altered and PIN proteins play an important role in all kinds of abiotic stress like salt, drought, heat, heavy metal etc. As auxin plays a role in root architecture, so it helps in abiotic stress response by altering root physiology [43].

Cytokinin has a main role in growth and development like Auxin, but also it helps in stress signalling, which was proved by the transcriptomics studies. At stress forms cytokinin levels increase and decrease accordingly [44]. It was studied that the cytokinin levels initially drop down in less stress conditions but gradually increase as the stress increases. Lots of studies have been done and still more studies need to be done to clarify the statements [45].

Gibberellic acid, which is also called as germinating hormones also plays a role in abiotic stress tolerance. Studies has shown its role in all kinds of stress [46]. Experiments have shown its role in heat stress tolerance in Tomatoes [47], and chilling stress tolerance by increasing antioxidant activity in sprouts has also been proved [48]. Studies in *Arabidopsis thaliana* have shown that GA also helps in mitigating heavy metal stress via modulating antioxidants [49].

Salicylic acid (SA) serves as a key hormone in plant innate immunity, including resistance in both local and systemic tissue upon biotic attacks, hypersensitive responses, and cell death. Salicylic acid plays an important role in the growth and development of the plant for important physiological roles such as increasing the plant's response to stress conditions (biotic and abiotic) by increasing the resistance of the plant to System Acquired Resistance (SAR) by stimulating or changing the internal paper dissection endogenous signalling to withstand a large number of stresses [50]. Salicylic acid acts as a stimulant or transmitter of the cell to withstand environmental stress conditions such as drought, cold, heat, stress of heavy elements, and conditions of ammonia tension and also increases the plant's ability to withstand salt stress salt particularly harmful sodium chloride compound NaCl. This phenolic acid hormone plays important role in the regulation of plant growth, fruit ripening and development. It is involved in pathogenesis-related protein expression. It also has the ability to bind conjugate with some amino acids such as proline and arginine, which increase the plant's effectiveness in resisting environmental stresses and at the same time maintain systemic acquired resistance [51].

Salicylic acid and Jasmonic acid, hormones which are known for the biotic stress hormones also play a role in abiotic stress tolerance. In wheat application of SA and JA has shown to increase the germination during the drought stress [52]. Even exogenous application of SA helps in salt stress response in cucumbers [53] and reports have shown their role in heavy metal toxicity tolerance too [54].

Strigolactones are also very important hormones in plant microbe relations but also play a role in different stress. Moreover Strigolactones play a role in stress adaptation by cross talking with other plant hormones and playing a regulatory role [55].

### **5. Biotic stress and phytohormones**

As soon as plat sense a stress response lots of signalling pathway is triggered in the plant. One of the most important event is alteration in calcium levels in plants, which in turn helps in signalling via PIP and activating the kinases enzymes. Calcium ions

#### *Types and Function of Phytohormone and Their Role in Stress DOI: http://dx.doi.org/10.5772/intechopen.109325*

bins to calmodulin or calcium-dependent protein kinases (CDPKs), or phosphatases which can either phosphorylate/dephosphorylate specific transcription factors, and regulate the gene expression [56]. Although all plant hormones play a role in stress response both abiotic and biotic as they cross talk among each other and help in combating stress in coordinated manner, but few hormones are well studied and are like font line warriors in biotic stress tolerance, like abscisic acid, ethylene, salicylic acid, jasmonates.

Abscisic acid positively regulates or negatively regulates defence responsive genes depending upon the type of pathogen plant is facing and type of plant. It exerts its effect by upregulating some of the gene families like ABA-responsive elements (ABRE) and other transcription factors like MYC, MYB and NAC families [57]. As the herbivore attacks the plant, their secretions increase the ABA level in the plant. Research has been done in plants with the herbivore attack and mutant analysis too and it proves to be correct. Transcriptome analysis also prove the same [58]. ABA is also involved in plant antiviral defence, as infection with few viruses studied has shown to have increased accumulation of ABA in plants [59].

Salicylic acid, Jasmonic acid and Ethylene play an important role in biotic stress response. These are the key hormones which play an important role in plant defence against the pathogens and pests. Increased Salicylic acid levels protect the whole plants with the help of upregulation of Pathogenesis related genes (PR genes) [60]. Jasmonic acid helps in production of secondary metabolites like tannins, total phenols, total flavonoids, and lignin upon insect attack and helps in insect resistance in plants [61].

All these hormones and signalling pathways interact among each other to help plants fight against the stress. Plant adaptations and its development stages and cross talk of these biotic and abiotic stresses with help of different transcription factors help a plant to survive in harsh conditions.
