Differential activation of anthocyanin biosynthesis in Arabidopsis and tobacco over-expressing an R2R3 MYB from Chinese bayberry

MrMYB1, an R2R3 MYB transcription factor (TF) gene associated with anthocyanin biosynthesis in Chinese bayberry (Myrica rubra Sieb. and Zucc.), was introduced into Arabidopsis and tobacco (Nicotiana tabacum) under the control of the CaMV 35S promoter. Overexpression of MrMYB1 induced anthocyanin accumulation in all tissues of Arabidopsis as well as in petals, ovaries and young seeds of tobacco, but not in tobacco leaves. The anthocyanin biosynthetic pathway, including chalcone synthase (CHS), dihydroflavonol 4-reductase (DFR) and anthocyanidin synthase (ANS), and the basic helix-loop-helix (bHLH) transcriptional partner TRANSPARENT TESTA8 (TT8), were up-regulated significantly in MrMYB1-overexpressing Arabidopsis. In MrMYB1-overexpressing tobacco petals, ovaries and young seeds, anthocyanin biosynthetic genes and bHLH partners NtAn1a and NtAn1b, were up-regulated. In contrast, high expression of MrMYB1 in transgenic tobacco leaves did not induce the expression of anthocyanin biosynthesis. Unlike in petals and ovaries, the foliar transcript level of NtAn1a and NtAn1b was extremely low and not stimulated by MrMYB1 transformation. These results show that higher expression of an endogenous bHLH partner, either intrinsically or stimulated by exogenous gene transformation, is required for anthocyanin production in plant tissues, and the different abundance in endogenous bHLH transcript accounts for differential accumulation of anthocyanin in Arabidopsis and tobacco leaves. These findings demonstrate that higher levels of expression of an endogenous bHLH partner, either intrinsically or following genetic transformation, are required for anthocyanin production in plant tissues. Moreover, differences in levels of endogenous bHLH transcripts account for observed differential accumulation of anthocyanin in leaves of Arabidopsis and tobacco.