The viviparous12 maize mutant is deficient in abscisic acid, carotenoids, and chlorophyll synthesis

The carotenoid/viviparous maize (Zea mays L.) mutant vp12 is a single locus mutation that resutts in lemoncoloured endosperms, viviparous embryos and albino seedlings. This work presents the first molecular and biochemical analysis of vp12. Levels of ABA were measured during embryo development and also in isolated organs under water deficit stress. ABA levels were lower in developing embryos of mutants than in non-mutant siblings at all stages analysed. In addition, under water deficit, mutant organs accumulated less ABA than corresponding non-mutant sibling organs. Furthermore, immature mutant embryos accumulated transcripts for several ABA or water deficit-responsive genes, Em, glbl, glb2, rab17, and vp1. These results indicated that vp12 is deficient in ABA accumulation, but not in the ABA signal transduction pathway. Analysis of carotenoid extracts showed that mutant endosperms accumulated lower amounts of coloured precursors than non-mutant endosperms. The expression of key enzymes in the carotenoid biosynthesis pathway was also analysed in vp12 endosperms. Western analysis indicated that phytoene synthase (PSY) was present at equal levels in normal and mutant endosperms. In addition, phytoene desaturase (PDS) transcript levels were similar in non-mutant and mutant tissues. Transcripts for geranylgeranyl pyrophosphate synthase (GGPPS), on the other hand, accumulated at lower levels in mutant endosperms than in non-mutant ones. However, Southern analysis of genomic DNA from normal and mutant tissues indicated that the gene encoding GGPPS is unlikely to be directly affected in vp12. Finally, vp12 seedlings grown under dim-light conditions produced white leaves, showing that vp12 is deficient in chlorophyll as well as carotenoid synthesis.