Molecular and biochemical characterization of thevp12 mutant of maize (Zea mays L.)

The mutant vp12 is a single locus mutation that conditions lemon endosperm, viviparous embryos and white seedlings. Previous genetic analysis demonstrated that vp12 is not allelic to other viviparous mutants of maize. In this work, the objectives are to characterize vp12 at both molecular and biochemical levels. First, levels of the hormone ABA were measured at several stages during embryo development and also in isolated organs, under drought stress. In mutant embryos, ABA levels were lower than in normal ones, at all stages analyzed. Under drought stress, mutant organs from both 3-day and 9-day old seedlings accumulated less ABA than corresponding normal organs. However, the hormone levels in some stressed mutant organs, such as roots and leaves, are higher than in non-stressed organs. These results suggest that the ability of ABA synthesis is not completely blocked in vp12. Expression of several ABA-responsive genes, such as Em, glb1, glb2 and rab17, was detected in RNA extracts from mutant immature embryos, despite the lower ABA levels observed in those. In addition, transcripts corresponding to vp1, a transcriptional activator for embryo ABA-mediated responses, were also observed in mutant embryos. Therefore, it can be suggested that the ABA signal transduction pathway is not impaired in vp12 embryos.At the carotenoid level, analysis of carotenoid extracts from normal and lemon endosperms showed that no colored precursors are accumulated in vp12 endosperms. Also, HPLC analysis indicated that phytoene, the first non-colored carotenoid precursor, is not present in mutant endosperms. In order to verify the presence of enzymes from the carotenoid biosynthetic pathway, their expression was analyzed in vp12 endosperms. Phytoene synthase accumulation was detected using a corresponding maize polyclonal antibody. Transcripts corresponding to two other enzymes, phytoene desaturase (PDS) and geranylgeranyl pyrophosphate synthase (GGPPS), were detected by RT-PCR. PDS transcripts levels were similar in both normal and vp12 immature endosperm RNA. However, GGPPS transcripts accumulated at lower levels in mutant immature endosperms than normal ones. These results, combined with the profile of carotenoid accumulation, suggest that the block in the carotenoid pathway in vp12 occurs in the GGPP synthesis.U of I OnlyETDs are only available to UIUC Users without author permissio