Isolation and functional characterization of a floral tissue-specific R2R3 MYB regulator from tobacco

Abstract

Tobacco is a commonly used heterologous system for studying combinatorial regulation of the flavonoid biosynthetic pathway by the bHLH-MYB transcription factor (TF) complex in plants. However, little is known about the endogenous tobacco bHLH and MYB TFs involved in the pathway. Ectopic expression in tobacco of heterologous bHLH TF genes, such as maize Lc, leads to increased anthocyanin production in the reproductive tissues, suggesting the presence of a reproductive tissue-specific MYB TF that interacts with the Lc-like bHLH TFs. We isolated a gene (NtAn2) encoding a R2R3 MYB TF from developing tobacco flowers. NtAn2 shares high sequence homology with other known flavonoid-related MYB TFs and is mostly expressed in developing flowers. Constitutive ectopic expression of NtAn2 induces whole-plant anthocyanin production in tobacco and Arabidopsis. In transgenic tobacco and Arabidopsis expressing NtAn2, both subsets of early and late flavonoid pathway genes are up-regulated. Suppression of NtAn2 by RNAi in tobacco resulted in a white-flowered phenotype and the inhibition of the late pathway genes. Yeast two-hybrid assays demonstrated that NtAn2 can interact with five heterologous bHLH TFs known to induce anthocyanin synthesis in other species including maize, perilla, snapdragon and Arabidopsis. Bimolecular fluorescent complementation using split YFP demonstrated that NtAn2 interacts with Lc in tobacco cells and that the complex is localized to nuclei. Transient co-expression of NtAn2 and Lc or Arabidopsis TT8 in tobacco protoplasts activated the promoters of two key flavonoid pathway genes, chalcone synthase and dihydroflavonol reductase. These results suggest that NtAn2 is a key gene controlling anthocyanin production in reproductive tissues of tobacco.