Role of ABA in integrating plant responses to drought and salt stresses

Since their early migration from aquatic environments to the land, plants have had to cope with periodic and unpredictable environmental stresses during growth and development. Surviving such stresses over a long evolutionary scale led plants to acquire mechanisms by which they can sensitively perceive incoming stresses and regulate their physiology accordingly. The plant hormone abscisic acid (ABA) plays a major role in plant responses to stress. Although rapid production of ABA in response to drought and salt stresses is essential to define ABA as a stress hormone, an equally rapid catabolism of ABA when such stresses are relieved is also essential in that role. Since ABA mediates so many stress responses, the initial perception of dehydration and the subsequent changes in gene expression that lead to rapid ABA biosynthesis constitute the most important stress signal transduction pathway among all the plant responses to stresses. Identification of the genes involved and understanding their roles during stress perception and physiological regulation has become an important and exciting research field in recent years. This review covers mainly our understanding of this aspect. ABA-induced changes in gene expression and their roles in physiological regulation are dealt with in less detail.