The plant growth regulator abscisic acid (ABA) is formed by the oxidative cleavage of an epoxy-carotenoid. The synthesis of other apocarotenoids, such as vitamin A in animals, may occur by a similar mechanism. In ABA biosynthesis, oxidative cleavage is the first committed reaction and is believed to be the key regulatory step. A new ABA-deficient mutant of maize has been identified and the corresponding gene, Vp14, has been cloned. The recombinant VP14 protein catalyzes the cleavage of 9-cis-epoxy-carotenoids to form C25 apo-aldehydes and xanthoxin, a precursor of ABA in higher plants.

Novel Arabidopsis mutants with lowered levels of endogenous abscisic acid (ABA) were isolated. These were selected in a screen for germination in the presence of the gibberellin biosynthesis inhibitor paclobutrazol. Another mutant was isolated in a screen for NaCl tolerance. The ABA-deficiency was caused by two monogenic, recessive mutations, aba2 and aba3, that were both located on chromosome 1. The mutants showed a phenotype that is known to be characteristic for ABA-deficiency: a reduced seed dormancy and excessive water loss, leading to a wilty phenotype. Double mutant analysis, combining different aba mutations, indicated the leaky nature of the mutations.

Abscisic acid (ABA), an apocarotenoid synthesized from cleavage of carotenoids, regulates seed maturation and stress responses in plants. The viviparous seed mutants of maize identify genes involved in synthesis and perception of ABA. Two alleles of a new mutant, viviparous14 (vp14), were identified by transposon mutagenesis. Mutant embryos had normal sensitivity to ABA, and detached leaves of mutant seedlings showed markedly higher rates of water loss than those of wild type. The ABA content of developing mutant embryos was 70% lower than that of wild type, indicating a defect in ABA biosynthesis. vp14 embryos were not deficient in epoxy-carotenoids, and extracts of vp14 embryos efficiently converted the carotenoid cleavage product, xanthoxin, to ABA, suggesting a lesion in the cleavage reaction. vp14 was cloned by transposon tagging. The VP14 protein sequence is similar to bacterial lignostilbene dioxygenases (LSD). LSD catalyzes a double-bond cleavage reaction that is closely analogous to the carotenoid cleavage reaction of ABA biosynthesis. Southern blots indicated a family of four to six related genes in maize. The Vp14 mRNA is expressed in embryos and roots and is strongly induced in leaves by water stress. A family of Vp14-related genes evidently controls the first committed step of ABA biosynthesis. These genes are likely to play a key role in the developmental and environmental control of ABA synthesis in plants.In plants, abscisic acid (ABA) is a key hormonal regulator of seed maturation (1) that mediates responses to a variety of stress conditions including water stress (2). Biochemical studies (3) and analyses of mutants that block ABA synthesis (4, 5) indicate that ABA is synthesized from oxidative cleavage of epoxy-carotenoids to produce xanthoxin, which is subsequently converted to ABA via the ABA-aldehyde intermediate (6).Biochemical studies suggest that cleavage of 9-cisxanthophylls is the key regulatory step in the ABA biosynthetic pathway (6). The carotenoid precursors are present in very high concentrations relative to ABA, indicating that synthesis of the 9-cis-xanthophylls is unlikely to regulate ABA synthesis in plant tissues (7). The enzyme activities required for conversion of xanthoxin to ABA are constitutively active in most plant tissues (6). However, protein synthesis and transcription inhibitors prevent induction of ABA synthesis in droughtstressed leaves, indicating that stress-induced ABA synthesis requires gene expression (reviewed in ref.3).ABA-deficient mutants are known in maize, Arabidopsis, and a few other species (3,5,(8)(9)(10)(11)(12). In maize, ABA-deficient mutants cause precocious seed germination (1). The Arabidopsis aba1 and N. plumbaginifolia aba2 mutants are deficient in the epoxy-carotenoid precursors of ABA (4, 11) and encode a zeaxanthin epoxidase (5). The steps downstream of the cleavage reaction, conversion of xanthoxin to ABA-aldehyde and oxidation of ABA-aldehyde to ABA, are defined in Arabidopsis by the aba2 and aba3 mutants, respectively (9).Other ...