ABA biosynthesis pathway

IPP and its isomer DMAPP are produced in plastids from glyceraldehyde 3-phosphate and pyruvate via the MEP pathway. In the cytosol, IPP derives from acetyl-CoA via the MVA pathway; the dashed arrow indicates that IPP can be translocated from the cytosol to the plastid. The first step of the MEP pathway produces DXP by DXP synthase and the last
step, leading to IPP and DMAPP, is catalyzed by HDR. Then two successive reactions are necessary to form the first carotenoid, phytoene, catalyzed by GGPS and PSY. Desaturation and isomerization reactions (not shown) lead to lycopene, which is then converted to zeaxanthin, by LCYB (lycopene cyclase) and BCH (-carotene hydroxylase). Its conversion into all-trans-violaxanthin is catalyzed by ZEP. Under high light a reverse reaction occurs in chloroplasts catalyzed by VDE (violaxanthin de-epoxidase). Genes encoding enzymes for the synthesis of cis-isomers of violaxanthin and neoxanthin have not been characterized. Carotenoid cleavage is catalyzed by a family of NCED to form xanthoxin, which is converted into ABA by ABA2 and AAO3. Sulfuration of AAO3 molybdenumcofactor by ABA3 is necessary for enzyme activity