The first committed step in ABA synthesis is the oxidative cleavage of a 9-cisepoxy-carotenoid. The pathway of ABA synthesis had for many years been a point of contention because of difficulties in demonstrating this activity in vitro.
This problem was eventually resolved by the identification and characterizationof an ABA-deficient mutant in maize, viviparous 14 (vp14). Analysis of vp14 embryos showed that it had a normal complement of carotenoids, and protein
preparations from the mutant were able to convert xanthoxin to ABA (40).These biochemical experiments indicated that vp14 was most likely impaired in the cleavage reaction.