Regulation of Abscisic Acid Biosynthesis

Abstract

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. Under non-stressful conditions, ABA in plant cells is maintained at low levels. Some low levels of ABA may be required for normal plant growth, as evidenced by reduced vigor observed in ABA-deficient mutant plants that can be restored to the wild-type level of growth by exogenous ABA (Finkelstein and Rock, 2002). Because all ABA-deficient mutants still have certain basal levels of ABA that are not dramatically lower than those in the wild type under normal growth conditions, it is difficult to uncover the cellular processes that require a very small amount of ABA. As a consequence, our knowledge of ABA functions has been gained mainly from observations with ABA at elevated levels, either from endogenous or exogenous sources. ABA levels can increase dramatically during seed maturation and in response to environmental stresses. Thus, ABA functions have been most extensively studied in these two processes. During seed development, ABA is known to initiate the following programs: embryo maturation, synthesis of storage reserves and late embryogenesis-abundant (LEA) proteins, and initiation of seed dormancy, although ABA is not the sole regulator of these processes. In particular, the induction of LEA protein synthesis to preserve the viability of embryos in the extremely dry condition of seeds is related to the role of ABA in promoting synthesis of LEA-like proteins in vegetative tissues to tolerate dehydration stress. Embryos from ABA antibody-expressing plants lose their viability as a result of desiccation intolerance (Phillips et al., 1997). In vegetative tissues, ABA levels increase when plants encounter adverse environmental conditions such as drought, salt, and to a lesser extent, low temperatures. Although a higher level of exogenous ABA inhibits plant growth under non-stressful conditions, an increased ABA content is beneficial for plants under environmental stress as a result of ABA-induced changes at the cellular and whole-plant levels. ABA promotes the closure of stomata to minimize transpirational water loss. It also mitigates stress damage through the activation of many stress-responsive genes that encode enzymes for the biosynthesis of compatible osmolytes and LEA-like proteins, which collectively increase plant stress tolerance (Hasegawa et al., 2000; Bray, 2002; Finkelstein et al., 2002). In addition, ABA has been shown to offset the inhibitory effect of stress-induced ethylene on plant growth (Sharp, 2002). Plant mutants defective in ABA biosynthesis are more susceptible to the environmental stresses and have been isolated in stress sensitivity screens (Xiong et al., 2002b). Importantly, manipulating ABA levels by changing the expression of key ABA biosynthetic genes provides an effective means to increase plant stress resistance. ABA-deficient mutants have been instrumental for revealing the pathway of ABA biosynthesis. By virtue of their precocious germination of seeds and the wilty appearance of the plants, mutants defective in ABA biosynthesis were isolated from a number of plant species including maize (Zea mays), tomato (Lycopersicon esculentum), tobacco (Nicotiana tabacum), potato (Solanum tuberosum), barley (Hordeum vulgare), and Arabidopsis. Before the molecular identities of the