Plant Physiol. Biochem., 1998, 36 (l-2), 83-89 The genetic and molecular dissection of abscisic acid biosynthesis and signal transduction in Arabidopsis Maarten Koornneef ‘*, Karen M. Leon-Kloosterziel I, Steven H. Schwartz 2 and Jan A.D. Zeevaart ’ ’ Department of Genetics, Wageningen Agricultural University, Dreijenlaan 2,6703 HA, Wageningen, The Netherlands. * MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, MI 48824-1312, USA. * Author to whom correspondence should be addressed (Fax + 31 317 483146; E-mail maarten.koornneef@botgen.el.wau.nl) Abstract The role of the plant hormone abscisic acid (ABA) has been studied in Arabidopsis by using mutants affected in either the biosynthesis or the mode of action of this hormone. Mutants have been isolated mainly by altered germination characteristics and seedling growth. The biochemical lesions of the abal, a&2 and aba3 mutants have been identified and the zeaxanthin epoxidase gene encoded by ABA1 has been cloned by homology with a Nicotiana plumbaginifolia gene blocked at the same biosynthetic step. ABA-insensitive mutants have either a phenotype affecting several ABA processes (abil and abi2) and therefore were suggested to encode early steps in ABA signal transduction, or they affect specific steps (e.g. abi3, abi4, abi5). The ABII and ABZ2 genes encode protein phosphatase 2C enzymes and ABI3 a transcription factor with seed-specific expression. The ABA hypersensitive era1 mutant is impaired in a farnesyl transferase. The various mutants have been used to analyse the role of ABA in seed development and seed germination, stress tolerance, and stomata1 closure. 0 Elsevier, Paris. Key words Abscisic acid, gibberellins, seed dormancy, stress responses, stomata1 closure, Arabidopsis thaliana. Abbreviations ABA, abscisic acid; GA, gibberellin. Introduction The plant hormone abscisic acid (ABA) plays a major role in such diverse aspects of plant growth and development as stomata1 closure, seed germination and the adaptation to environmental stresses. Some of the ABA responses are rapid and involve the moditi- cation of ion fluxes, whereas others are long term and involve changes in gene expression. The role of ABA has been analysed in the past by monitoring the effects of the application of either ABA itself or by applying inhibitors of carotenoid biosynthesis and by relating changes in ABA content with physiological and devel- opmental changes. Complications with exogenous applications of chemical compounds to modify endog- enous ABA levels are numerous and include problems of uptake, rapid metabolism and the effects of inhibi- tors on other pathways apart from ABA. The use of mutants defective in either ABA biosynthesis or ABA action has been an effective alternative for this spray- ing approach. Furthermore, ABA deficient mutants have become essential tools for the study of ABA bio- synthesis and for the cloning of the respective genes, which may allow the genetic modification of ABA levels in plants. Mutants affected in ABA responsiveness are assumed to be defective in ABA signal transduction, and the cloning of the respective genes will allow the identification of such signal transduction steps. In Arubidopsis, a large number of mutants has been iso- lated thus far. However, ABA mutants, especially those affected in biosynthesis, are also available in a number of other species including, Nicotiana plum- baginifolia, Zea mays, Lycopersicon esculentum and Pisum sativum (Taylor, 1991). The similarities among the mutant phenotypes are a strong indication that the effects of ABA are similar in most plant species, allowing the extrapolation of results from Arabidopsis to other plant species. The present review elaborates on earlier reviews by Finkelstein and Zeevaart (1994) Plant Physiol. Biochem., 098 I -9428/98/l -2/O Elsevier, Paris