Figure 1.
The biosynthesis and metabolism pathways of abscisic acid (ABA) in higher plant. The C40 zeaxanthin is first converted into all-trans-violaxanthin under the catalysis of zeaxanthin epoxidase (ZEP). Then, all-trans-violaxanthin can be converted to 9’-cis-violaxanthin or 9’-cis-neoxanthin in two pathways under the catalysis of neoxanthin synthase (NSY) and the unknown isomerase. The enzyme 9-cis-cyclocarotenoid dioxygenase (NCED) cleaves both 9’-cis-neoxanthin and 9’-cis-violaxanthin into xanthoxin. Eventually, xanthoxin is mainly converted into abscisic aldehyde and further forms ABA, which are catalyzed by a short-chain alcohol dehydrogenase (ABA2) and abscisic aldehyde oxidase (AAO/ABA3) respectively. ABA metabolism involves two main pathways: hydroxylation and glycosylation, which are catalyzed by the cytochrome P450 monooxygenase (CYP707A) and ABA glucosyltransferase (GT) respectively. Red letters represent enzymes, and blue letters represent cellular components divided by dashed line. ABA-GE, ABA-glucose ester; BG, β-glucosidase; VDE, violaxanthin de-epoxidase.