The Role of Phytohormones in Combating Biotic Stress

Plant hormones (phytohormones) are small chemical molecules that travel across the plant to stimulate responses against various adverse environmental conditions. These signalling molecules stimulate plant responses in their distal parts upon detecting various stresses. Thus, phytohormones are tuners of plant responses that play a significant role in supporting the plants to adapt to both abiotic and biotic stresses. Four hormones, jasmonic acid (JA), ethylene (ET), salicylic acid (SA), and abscisic acid (ABA) are commonly recognised for the substantial roles they play in regulating plant protection responses against different pathogens and pests. SA is largely involved in the stimulation of the defence response against hemi-biotrophic and biotrophic pathogens, while ET and JA are responsible for defence against herbivorous insects and necrotrophic pathogens. ABA positively controls the plant defence in the initial phases of infection by mediating the closure of stomata against pathogens and inducing the deposition of callose if the pathogen escapes the primary defence. Current studies revealed that other plant hormones, such as auxin, cytokinin, gibberellic acid, brassinosteroids, and peptide hormones, are also involved in plant defence signalling pathways but their role in plant defence is not fully understood. This chapter highlights the synthesis, signalling pathways and the role of phytohormones in moderating plant defence responses against different pathogens and pests.