Abscisic acid, a principal regulator of plant abiotic stress responses

Abstract

Plants are routinely troubled by various abiotic stresses such as high salinity, dehydration, and low temperature during their life span. These abiotic stresses have detrimental effect on plant development, longevity, and productivity. Plants have evolved with a sessile nature, and unlike animals, they cannot move away from adverse growth conditions. Rather, they are destined to combat these stress conditions in their stationary habitat. Therefore, plants have devised an adaptive mechanism that entails the activation of several signal transduction pathways, leading to diverse molecular, cellular, and physiological changes (Singh et al., 2016, 2018). Most signal transduction pathways triggered in response to biotic or abiotic stresses are mediated by one or more plant hormones. Therefore, plant hormones are a crucial player in regulating plants’ response to various environmental cues (Iqbal et al., 2017; Khan et al., 2015a,b; Khan and Khan, 2014; Kazan, 2015; Per et al., 2018). Generally, phytohormones like salicylic acid (SA), jasmonic acid (JAs), and ethylene (ET) are implicated in plant response to pathogens, wounding, and other biotic stresses, whereas gibberellins (GAs), auxins (IAAs), brassinosteroids (BRs), and cytokinins are known to regulate plant development. However, recent advancements in plant stress related research have shown that all plant hormones could control multiple plant processes and are involved in crosstalk of signaling pathways. For example, SA, JA, and ET, apart from biotic stresses, are also involved in plant development and responses to abiotic stresses. Similarly, auxins and GA are crucial in abiotic and biotic stress responses (Colebrook et al., 2014; Kazan, 2013; Khan and Khan, 2013; Santino et al., 2013). Abscisic acid (ABA) is the key hormone that primarily regulates plants’ responses to various abiotic stresses; however, like other phytohormones ABA is also known to regulate plants’ response to biotic stress and development (Singh et al., 2016). The discovery of the vital phytohormone ABA dates way back to the 1960s. Several independent and convergent experiments carried out by various research groups led to the discovery of ABA (Cracker and Abeles, 1969). However, the earliest and most convincing was the discovery of ABA in cotton, where it was involved in fruit abscission and dormancy (Li et al., 2017). As time and research progressed, newer functions of ABA were unveiled, including adaptation to various stresses, stomatal closure, sugar accumulation, seed development, etc. Due to its crucial role in abiotic and biotic stresses, ABA is known as a “stress hormone”. Plenty of research on ABA accumulated ample information on its biosynthesis, storage, catabolism, site of action, and its possible targets. During the last decade, the ABA receptors and their crystal structures have been elucidated (Ma et al., 2009; Park et al., 2009). This information has provided a clear cut paradigm of the ABA signal transduction pathway. Moreover, recruiting the combinations of different key players such as PP2C phosphatase and SnRK2 kinases has helped to understand the signal transduction pathway. Recent studies have provided newer insights into the functional roles of the ABA signaling cascade in various aspects of plant growth and development. In this chapter, we discuss different facets of ABA in plants, including its biosynthesis, catabolism, ABA signaling pathway and various signaling components, and the role of ABA in abiotic stresses and plant development.