Abstract
Abscisic acid (ABA) controls many aspects of physiological processes including seed dormancy and drought tolerance. These hormonal action of ABA depends on the endogenous level of ABA which are controlled by the precise balance between biosynthesis and catabolism. Although much is known about its biosynthesis, ABA catabolism is poorly understood. ABA is mainly catabolized into phaseic acid through 8'-hydroxylation. The enzyme ABA 8'-hydroxylase was shown to be a member of cytochrome P450 (P450), however, its gene had not been identified. Based on the phylogenetic analysis, DNA microarray analysis, and comparison with the rice genome, the candidate genes for this enzyme were narrowed down from among 272 Arabidopsis P450 genes. Functional expression of these genes revealed that members of CYP707A family, CYP707A1 through CYP707A4,code for the target ABA 8'-hydroxylase. Expression analysis of these genes showed that CYP707A2 is responsible for rapid decrease in ABA level during seed imbibition. During drought stress, all four genes were upregulated, and upon rehydration, a significant increase in mRNA level was observed which correlated well with the change in the level of ABA.
