Identification of endogenous gibberellins in petunia flowers: Induction of anthocyanin biosynthetic gene expression and the antagonistic effect of abscisic acid

The elongation and pigmentation of corollas of Petunia hybrida requires the presence of anthers. The ability of exogenous gibberellic acid (GA₃) to substitute for the anthers suggests a role for endogenous GAs. Here we report the identification of endogenous GAs in corollas and in anthers and show that both tissues contain detectable levels of GA₁, GA₄, and GA₉, of which GA₄ is the most abundant. These GAs stimulate corolla pigmentation, chalcone synthase (chs) mRNA accumulation, and chs transcription in an in vitro flower bud culture system. Methyl ester derivatives of GA₃ and GA₄ were not active but did not inhibit the bioactive GAs. Even though it is unknown whether abscisic acid (ABA) is involved in corolla maturation, ABA inhibited pigmentation of intact flowers, overruling the effect of the anthers. In detached flower buds it was shown that ABA prevented activation of the chs promoter by GA₃. The synthesis of anthocyanin pigments requires the coordinate expression of at least 15 structural genes. Expression of early biosynthetic genes and of late biosynthetic genes are regulated by different transcriptional activators. GA induces both classes of genes with similar kinetics, indicating that GA acts relatively early in the signaling pathway.