Gibberellin Dose-Response Curves and the Characterization of Dwarf Mutants of Barley

Dose-response curves relating gibberellin (GA) concentration to the maximal leaf-elongation rate ($\text{LER}_{\text{max}}$) defined three classes of recessive dwarf mutants in the barley (Hordeum vulgare L.) 'Himalaya.' The first class responded to low (10-8-10-6 M) [GA3] (as did the wild type). These grd (GA-responsive dwarf) mutants are likely to be GA-biosynthesis mutants. The second class of mutant, gse (GA sensitivity), differed principally in GA sensitivity, requiring approximately 100-fold higher [GA3] for both leaf elongation and α-amylase production by aleurone. This novel class may have impaired recognition between the components that are involved in GA signaling. The third class of mutant showed no effect of GA3 on the $\text{LER}_{\text{max}}$. When further dwarfed by treatment with a GA-biosynthesis inhibitor, mutants in this class did respond to GA3, although the $\text{LER}_{\text{max}}$ never exceeded that of the untreated dwarf. These mutants, called elo (elongation), appeared to be defective in the specific processes that are required for elongation rather than in GA signaling. When sln1 (slender1) was introduced into these different genetic backgrounds, sln was epistatic to grd and gse but hypostatic to elo. Because the rapid leaf elongation typical of sln was observed in the grd and gse backgrounds, we inferred that rapid leaf elongation is the default state and suggest that GA action is mediated through the activity of the product of the Sln gene.