Plant hormone abscisic acid (ABA) is a sesqui-terpenoid. It is biosynthesized from isopentenyl diphosphate, via b-carotene and cis-neoxanthine in plastids. The cis-neoxanthine is transported to cytoplasm. Its cleavage represents the rate-limiting step in ABA formation.
ABA consists of a six-member ring with double bond between C2' and C3', one methyl group at C2', keto group at C4' , and two methyl residues at C6'. Six-member side chain has terminal carboxyl group. According to the position of car-boxyl, cis- and trans-isomer can be distinguished, the former one exhibiting the biological activity. Due to the optically active C1', bearing a hydroxyl group, ABA may occur either as (S)/(+) isomer, which is the most physiologically active, or as much less active (R)/(-) isomer.
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