Gibberellin Dose-Response Regulation of GA4 Gene Transcript Levels in Arabidopsis

Abstract

The gibberellins (GAs) are a complex family of diterpenoid compounds, some of which are potent endogenous regulators of plant growth. As part of a feedback control of endogenous GA levels, active GAs negatively regulate the abundance of mRNA transcripts encoding GA biosynthesis enzymes. For example, Arabidopsis GA4 gene transcripts encode GA 3β-hydroxylase, an enzyme that catalyzes the conversion of inactive to active GAs. Here we show that active GAs regulateGA4 transcript abundance in a dose-dependent manner, and that down-regulation of GA4 transcript abundance is effected by GA₄ (the product of 3β-hydroxylation) but not by its immediate precursor GA₉ (the substrate). Comparison of several different GA structures showed that GAs active in promoting hypocotyl elongation were also active in regulating GA4transcript abundance, suggesting that similar GA:receptor and subsequent signal transduction processes control these two responses. It is interesting that these activities were not restricted to 3β-hydroxylated GAs, being also exhibited by structures that were not 3β-hydroxylated but that had another electronegative group at C-3. We also show that GA-mediated control of GA4 transcript abundance is disrupted in the GA-response mutants gaiand spy-5. These observations define a sensitive homeostatic mechanism whereby plants may regulate their endogenous GA levels.