Functional redundancy of R2R3-MYB transcription factors involved in anthocyanin biosynthesis is manifested in anther pigmentation in petunia

Abstract

Accumulation of anthocyanin provides pigmentation in plant tissues. In petunia, gene expression profiles that lead to anthocyanin production have been extensively characterized in terms of pigmentation in flower petals. Anthers are also pigmented, but the transcriptional control of the genes for anthocyanin biosynthesis in anthers has not been fully characterized. Here we addressed this issue by analyzing the expression of structural genes and genes encoding transcription factors (TFs) of the pathway. Ectopic expression of the PURPLE HAZE (PHZ) gene encoding an R2R3-MYB activator induced pigmentation in anthers. The pigmentation was accompanied by an increase in mRNA levels of AN1, MYB27 and MYBx among the genes encoding TFs. Among the structural genes, mRNA levels of four late biosynthetic genes (LBGs) were higher in the transformants than in the wild type. Analyses of gene expression profile using commercial varieties indicated that mRNA levels of MYB27, MYBx and LBGs and of AN4, responsible for anther pigmentation, were higher in pigmented anthers than in nonpigmented. Differences in the gene expression profile between pigmented anthers induced by ectopic PHZ expression and their nonpigmented control and those between pigmented anthers and nonpigmented anthers of existing varieties were thus remarkably similar. These observations suggest that a high level of expression of the LBGs is characteristic of pigmented anthers and that ectopic PHZ expression in the an4⁻ genetic background induced changes in the transcriptional network toward the state established in pigmented anthers, which is intrinsically brought about by the function of AN4.