Abstract
The hydroxylation at the 8'-position of abscisic acid (ABA) has been known as the key step of ABA catabolism, and this reaction is catalyzed by cytochromes P450 (CYPs or P450s). Here, we demonstrate CYP707A genes as the P450 responsible for the 8'-hydroxylation of (+)-ABA. CYP707A cDNAs were cloned from Arabidopsis. The insect cells expressing CYP707A3 metabolized (+)-ABA to phaseic acid (PA), the isomerized form of 8'-hydroxy-ABA. The micorsomes from the insect cells exhibited a Km value for (+)-ABA of 1.3 μM and a kcat value of 15 min-1. The solubilized CYP707A3 protein bound (+)-ABA (Ks = 3.5 μM), but did not bind (-)-ABA. Analyses of the reaction products confirmed that CYP707A3 does not isomerize 8'-hydroxy-ABA to PA. The transcripts of CYP707A genes increased in response to salt, osmotic and dehydration stresses as well as ABA. These results establish that CYP707A genes play a key role in regulating the ABA level.
