Classification and characterization of the viviparous mutants of maize (Zea mays L.)
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S1 nuclease protection assays were used to examine 26 previously undefined, carotenoid-containing, viviparous mutants of maize. Embryo extracts of 4 of the mutants were completely devoid of Vp1 mRNA, identifying them as class I mutants. The remainder were tentatively classified as class III mutants, 18 of which were further analyzed using HPLC. By comparing the profile of xanthophylls present in mutant embryos to that in normal embryos from the same ear, it was possible to separate the mutants into groups that appear to share a common metabolic block. These procedures can greatly reduce the time and labor required for allelism tests using conventional crossing. Zeaxanthin comprised a higher proportion of the xanthophylls extracted from embryos from vp10 and 4 vp10-like mutants than from non-mutant seeds segregating on the same ears. Viviparous mutants in this group had relatively lower levels of epoxy-xanthophylls, especially violaxanthin. These mutations thus appear to impair the epoxidation of zeaxanthin. In contrast, zeaxanthin comprised a smaller proportion of the xanthophylls extracted from embryos of vp8 and 9 vp8-like mutants than in their normal counterparts. In these mutants, the relative levels of 9'-cis-neoxanthin and neoxanthin were increased. These mutations appear to affect an oxidative cleavage step late in the ABA biosynthesis pathway. The decrease in zeaxanthin in these mutants suggests that one of the accumulated compounds may serve as a feedback inhibitor for zeaxanthin synthesis. Embryos homozygous for the vp-92EP8 mutation accumulated violaxanthin, while two mutants, vp-92GN24 and vp-90GS8 accumulated violaxanthin and neoxanthin. These results are compatible with a pathway where neoxanthin is an intermediate between violaxanthin and 9'-cis-neoxanthin. Under this model, vp-92EP8 limits the isomerization of violaxanthin to neoxanthin, while vp-92GN24 and vp-90GS8 limit the isomerization of neoxanthin to 9'-cis-neoxanthin. The xanthophyll composition in embryos homozygous for one mutation was almost identical to that in normal embryos, and in three mutants small decreases in zeaxanthin were offset by an increased percentage of 9'-cis-neoxanthin. These mutants may be blocked in ABA synthesis at a point or points beyond xanthoxin.
