The genetic and molecular dissection of abscisic acid biosynthesis and signal transduction in Arabidopsis

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 aba1, aba2 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 (abi1 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 ABI1 and ABI2 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 stomatal closure.