Gibberellic acid regulates chalcone synthase gene transcription in the corolla of Petunia hybrids

The pigmentation of Petunia hybrida corollas is regulated by gibberellic acid (GA₃). It controls the increase of flavonoid enzyme levels and their corresponding mRNAs. We have used an in vitro culture system for corollas to study the regulatory role of GA₃ in the expression of flavonoid genes. By determining steady-state mRNA levels, we show that the accumulation of chalcone synthase (chs) mRNA in young corollas is dependent on the presence of both sucrose and GA₃ in the culture medium. Whereas sucrose had a general metabolic effect on gene expression, the stimulatory role of GA₃ was specific. Analysis of nascent transcripts in isolated corolla nuclei showed that changes in steady-state chs mRNA levels correlated very well with changes in the transcription rate. We therefore conclude that GA₃ controls the expression of chs at the transcriptional level. Preculturing the corollas in sucrose medium without GA₃ resulted in a lower chs mRNA level. The expression could be reinduced by the addition of GA₃. The hormone is thus required for the induction but also for the maintenance of chs transcription. The delayed reinduction of chs expression, the lag time in the kinetics of chs mRNA accumulation, and the inhibitory effect of cycloheximide on the action of GA₃ suggest that GA₃ controls chs transcription in an indirect manner. Our data support a model in which GA₃ induces the production of a regulatory protein such as a receptor or a transacting factor that is directly involved in chs transcription.