GA-sensitive dwarf1-1D (gsd1-1D) Defines a New Mutation that Controls Endogenous GA Levels in Arabidopsis

Gibberellin (GA) regulates diverse plant growth and developmental processes. A number of GA inactivation mechanisms and the associated enzyme-encoding genes and transcriptional regulators have been identified in plants. Identification of a novel dominant GA-sensitive mutant of Arabidopsis, GA-sensitive dwarf1-1D (gsd1-1D), exhibiting phenotypes typical of GA-deficient mutants, including semidwarfism and smaller and dark green leaves, has been reported previously. Unlike the severe GA-deficient mutants such as ga1-3, gsd1-1D seeds displayed a normal GA-sensitive germination phenotype. To gain insights into how the gsd1-1D mutation affects GA metabolism, we performed dose-response studies with several GA biosynthetic intermediates. The results implied that the gsd1-1D mutation preferentially affects the activity of the non-13-hydroxylated bioactive GA, GA₄. Our gene expression analysis indicated upregulation of GA2ox1 in the gsd1-1D mutant relative to that in the wild-type and GA-deficient ga3ox1/ga3ox2 mutant; however, this does not appear to be the major cause for the phenotype exhibited by gsd1-1D as the basal level of GA2ox1 expression is low. Measurement of endogenous levels of GAs showed a substantial decrease in the amounts of non-13-hydroxylated GA precursors and GA₄ and their respective catabolites in gsd1-1D. Although to a lesser extent, the levels of some 13-hydroxylated GA precursors and bioactive GA₁ were also reduced by the gsd1-1D mutation. Taken together, our results suggest that GSD1 is involved in the GA inactivation process and thereby regulates the levels of bioactive GAs in Arabidopsis.