ABSTRACT
Phaseolin genes are induced by unidentified factors at the onset of seed maturation in embryos of both Phaseolus and tobacco. We show that in tobacco, expression of a beta-phaseolin promoter-GUS (PHSbeta-uidA) mRNA and the corresponding GUS activity, could be induced by abscisic acid (ABA). The effect paralleled an increase in the amount of endogenous 12S globulin (Glb12S) mRNA. In contrast, ABA repressed the expression of isocitrate lyase (ll9) mRNA. The responses of PHSbeta-uidA and Glb12S to ABA declined markedly between 11 and 13 DAF, indicating that they are developmentally regulated. We also show evidence that the ABA response of PHSbeta-uidA can be modulated by the external concentrations of sucrose and Ca2+ ion. These compounds inhibited the response if added to the medium separately, in the concentration ranges of 80-200 mM for sucrose and 0.76-20 mM for CaCl2. However, the presence of both sucrose and CaCl2 restored the ABA response to 20-40% of the maximum value measured in sucrose- and CaCl2-free media. These results suggest that ABA induction of beta-phaseolin gene expression is modulated by developmental signals and by the external supply of sucrose and calcium to the embryos.
