Phytochrome regulation and differential expression of gibberellin 3 beta-hydroxylase genes in germinating Arabidopsis seeds

Despite extensive studies on the roles of phytochrome in photostimulated se ed germination, the mechanisms downstream of the photoreceptor that promote germination are largely unknown. Previous studies have indicated that ligh t-induced germination of Arabidopsis seeds is mediated by the hormone gibbe rellin (GA). Using RNA gel blot analyses, we studied the regulation of two Arabidopsis genes, GA4 and GA4H (for GA4 homolog), both of which encode GA 3 beta-hydroxylases that catalyze the final biosynthetic step to produce bi oactive GAs. The newly isolated GA4H gene was expressed predominantly durin g seed germination. We show that expression of both GA4 and GA4H genes in i mbibed seeds was induced within 1 hr after a brief red (R) light treatment. In the phytochrome B-deficient phyB-1 mutant, GA4H expression was not indu ced by R light, but GA4 expression still was, indicating that R light-induc ed GA4 and GA4H expression is mediated by different phytochromes. In contra st to the GA4 gene, the GA4H gene was not regulated by the feedback inhibit ion mechanism in germinating seeds. Our data demonstrate that expression of GA 3 beta-hydroxylase genes is elevated by R light, which may result in an increase in biosynthesis of active GAs to promote seed germination. Furthe rmore, our results suggest that each GA 3 beta-hydroxylase gene prays a uni que physiological role during light-induced seed germination.