Use of Phytohormones in Conferring Tolerance to Environmental Stress

Abstract

Due to sessile nature of plants, they face a variety of biotic and abiotic stresses during their life cycle. These stresses are responsible for disturbed cellular processes that adversely affect their growth and yield. To cope with these stresses, plants have developed various physiological mechanisms at cellular level that result in a change in morphology and help them to tolerate these environmental changes and respond to these changes with an optimal response. These responses of plants toward environmental stresses are both dynamic and complex that help them complete their life cycle rapidly under these stressful environmental conditions. The defense response of the plants to these stresses starts through variations in different molecular events with the involvement of different signaling molecules including the phytohormones. Phytohormones are small low-molecular-weight endogenous molecules that play important roles in different defense responses of plants against biotic as well as abiotic stresses. Along with their role in defense signaling, they also regulate various physiological, growth, and developmental processes in plants. These phytohormones include the auxins, cytokinins (CKs), gibberellins (GAs), ethylene (ET), jasmonic acid (JA), abscisic acid (ABA), salicylic acid (SA), and brassinosteroids (BRs) tocotrienols, triacontanols, and polyamines which are important ones that have ability to help plants to respond to different environmental stresses through their specific signaling properties. These signaling defense responses are the results of the interaction of various genes, helping the phytohormones to be involved in almost all cellular metabolic processes due to their specific modulations in the activities of these genes. All these phytohormones have their specific roles, e.g., auxin is involved in the regulation of differentiation and plant growth, cytokinin is responsible for cell division, gibberellin is responsible for stem elongation, seed germination, dormancy, senescence, and flower development, ethylene is involved in fruit ripening, and ABA has stress tolerance ability. The stress tolerance mechanism of plant is much complex that also has the involvement of other phytohormones, including the brassinosteroids (BRs), jasmonic acid (JA), salicylic acid (SA), polyamines (spermine, spermidine, putrescine, and thermospermine), tocotrienols, triacontanols (TRAI) as newly discovered ones. All the phytohormones help plants to survive under adverse environmental conditions with their specific roles in various growth, developmental, and physiological processes either through their endogenous accumulation or by exogenous application (foliar spray or seed priming) where the optimal concentration for the stress response is not sufficient. The exogenous use of these phytohormones has been increasing in crop plants with their economic value for obtaining the desired characters along with better production. The information given in the chapter will be helpful for plant growers and researchers to understand the mechanism of action of these phytohormones for better growth and production under changing environmental conditions.