Abstract
Recent research has revealed that abscisic acid (ABA), synthesised in response to water stress, is an apo‐carotenoid. Two potential carotenoid precursors, 9′‐cis‐neoxanthin and 9‐cis‐violaxanthin, have been identified in light‐grown and etiolated leaves, and in roots of a variety of species. Experiments utilizing etiolated Phaseolus vulgaris leaves and deuterium oxide strongly suggest that 9′‐cis‐neoxanthin, synthesised from all‐trans‐violaxanthin, is the immediate pre‐cleavage precursor of ABA. The cleavage of 9′‐cis‐neoxanthin, performed by an inducible and specific dioxygenase, is likely to be the rate‐limiting step in ABA biosynthesis. Any apocarotenoids formed as by‐products of cleavage are probably rapidly degraded by lipoxygenase or related enzymes. After cleavage xanthoxin is converted via ABA‐aldehyde to ABA by constitutive enzymes in the cytosol.
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