VIVIPAROUS-5 ENCODES PHYTOENE DESATURASE, AN ENZYME ESSENTIAL FOR ABSCISIC-ACID (ABA) ACCUMULATION AND SEED DEVELOPMENT IN MAIZE

The role of synthesis in the regulation of abscisic acid accumulation was investigated in the developing maize seed. To do this, expression and regulation of the abscisic acid biosynthetic enzyme phytoene desat urase were examined. Comparison of the gene sequence encoding phytoene desaturase and its transcript in the wild-type and viviparous-5 mutan t showed that the mutant gene contains multiple insertions and deletio ns, resulting in the synthesis of a larger transcript. In addition, th e 55-kDa phytoene desaturase protein was not detectable in the vivipar ous-5 mutant, indicating that this phenotype results from a mutation a t the phytoene desaturase locus. Levels of phytoene desaturase transcr ipt and protein were compared to abscisic acid levels during developme nt to determine whether phytoene desaturase might regulate abscisic ac id accumulation. In the endosperm, transcript levels were initially hi gh and declined during late maturation and dormancy, while protein lev els remained high throughout development. In the embryo, transcript le vels were low and constant, while protein levels declined. Both tempor al and tissue-specific expression of phytoene desaturase were unrelate d to abscisic acid levels. An abscisic acid mutant (viviparous-2) defi cient in phytoene desaturation was used to determine whether the wild- type protein encoded by Viviparous-2 regulates phytoene desaturase. Ph ytoene desaturase transcript and protein levels were compared in wild- type and viviparous-2 mutant embryos and endosperm. Normalized levels of phytoene desaturase were similar in wild-type and mutant tissues, s uggesting that the wild-type Viviparous-2 protein does not regulate ph ytoene desaturase transcript or protein levels.