There are several steps in ABA biosynthesis preceding the cleavage reaction that are not well characterized. The epoxy-carotenoid precursor must have a 9-cis configuration to be cleaved by an NCED and for subsequent conversion to
ABA [cis-(+)-S-ABA]. The formation of these 9-cis isomers has not yet been established. An enzyme that catalyzes a similar reaction, the cis/trans isomerization of prolycopene to lycopene, has recently been identified (17).This isomerase appears to be necessary only in non-photosynthetic tissue. In light-grown tissue, photo-isomerization of lycopene is sufficient. It has not
been established whether the 9-cis isomerization of neoxanthin and violaxanthin is an enzymatic reaction.