GENETIC DISSECTION OF THE ABSCISIC-ACID BIOSYNTHETIC-PATHWAY IN MAIZE(ZEA-MAYS L.) EMBRYOS

Metabolites in the ABA biosynthetic pathway were identified by analyzi ng xanthophyll profiles generated in embryos of wild-type and viviparo us mutants of Zea mays L. The level of lutein differed greatly in vp8 embryos depending on whether or not elongated shoots were included in the sample. Lutein was the major xanthophyll in embryos when elongated shoots were included. However, lutein was a minor xanthophyll in vp8 embryos without elongated shoots and in wild-type embryos. Lutein Nas produced mainly in the elongated shoots of viviparous mutants. No lute in epoxides were detected in mutant or wild type embryos. These result s show that lutein is not a precursor Of ABA in maize embryos. Accumul ation of zeaxanthin combined with a reduction of epoxy-xanthophylls in vp10 suggest that this defect impairs thr epoxidation of zeaxanthin. Mutant vp-92EP8 accumulated all-trans-violaxanthin, while vp-92GN24 ac cumulated all-trans-violaxanthin and all-trans-neoxanthin. These resul ts imply that all-trans-neoxanthin is an intermediate between all-tran s-violaxanthin and 9'-cis-neoxanthin. Under this model, vp-92EP8 preve nts the conversion of all-trans-violaxanthin to all-trans-neoxanthin, while vp-92GN24 prevents the conversion of all-trans-neoxanthin to 9'- cis-neoxanthin. Accumulation of all-trans-violaxanthin, all-trans-neox anthin and 9'-cis-neoxanthin combined with the reduction of zeaxanthin was shown in vp8. A block in the oxidative cleavage step.