Regulation of ABA and Water Stress Responsive Genes in Maize Embryos

The maize rab genes are expressed in different plant parts in response to abscisic acid and osmotic stress. The pattern of expression of rab mRNAs and corresponding proteins was studied during maize embryogenesis and in vegetative tissues of wild type and viviparous mutants. ABA induces the synthesis of the mRNAs and accumulation of the proteins in embryo tissues. However, in vegetative tissues subjected to water stress or ABA treatment the levels of mRNA are high but rab proteins are absent or poorly accumulated. A chimaeric gene containing the rab 17 promoter and the reporter gene CAT is correctly regulated in transgenic tobacco, although lower levels of the enzyme activity have been noticed in vegetative tissues. All these data suggest that more than one mechanism determines the level of expression of the maize rab genes.