Update on Abscisic Acid Biosynthesis Regulation Regulation of Abscisic Acid Biosynthesis

Plant growth and development are regulated by internal signals and by external environmental conditions. One important regulator that coordinates growth and development with responses to the environment is the sesquiterpenoid hormone abscisic acid (ABA). ABA plays important roles in many cellular processes including seed development, dormancy, germination, vegetative growth, and environmental stress responses. These diverse functions of ABA involve complex regulatory mechanisms that control its production, degradation, signal perception, and transduction. Because of the key role of ABA in plant stress responses, understanding these regulatory mechanisms will help devise rational strategies to breed or genetically engineer crop plants with increased tolerance to adverse environmental conditions. Since the discovery of ABA in the early 1960s, much effort has been devoted to understanding how ABA is synthesized. Through genetic and biochemical studies, the pathway for ABA biosynthesis in higher plants is now understood in great detail. Recently, all the major genes for the enzymes in the biosynthesis pathway have been identified (Schwartz et al., 2003). The new challenge is to understand how these biosynthesis genes, and the biosynthetic pathway as a whole, are regulated. Although much remains to be learned about the regulatory mechanism, evidence thus far indicates that ABA biosynthesis is subject to complex regulation during plant development and in response to environmental stresses. In this Update, we first present a brief overview of the functions of ABA and the biosynthesis pathway. We then focus on the regulation of ABA production and attempt to provide some future directions in ABA biosynthesis studies.