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Background  Flavonoid pathway is spatially and temporally controlled during plant development and the transcriptional regulation of the structural genes is mostly orchestrated by a ternary protein complex that involves three classes of transcription factors (R2-R3-MYB, bHLH and WDR). In grapevine ( Vitis vinifera  L.), several MYB transcription factors have been identified but the interactions with their putative bHLH partners to regulate specific branches of the flavonoid pathway are still poorly understood.    Results  In this work, we describe the effects of a single amino acid substitution (R69L) located in the R2 domain of VvMYB5b and predicted to affect the formation of a salt bridge within the protein. The activity of the mutated protein (name VvMYB5b L , the native protein being referred as VvMYB5b R ) was assessed in different  in vivo  systems: yeast, grape cell suspensions, and tobacco. In the first two systems, VvMYB5b L  exhibited a modified trans-activation capability. Moreover, using yeast two-hybrid assay, we demonstrated that modification of VvMYB5b transcriptional properties impaired its ability to correctly interact with VvMYC1, a grape bHLH protein. These results were further substantiated by overexpression of  VvMYB5b R   and  VvMYB5b L   genes in tobacco. Flowers from  35S::VvMYB5b L   transgenic plants showed a distinct phenotype in comparison with  35S::VvMYB5b R   and the control plants. Finally, significant differences in transcript abundance of flavonoid metabolism genes were observed along with variations in pigments accumulation.    Conclusions  Taken together, our findings indicate that VvMYB5b L  is still able to bind DNA but the structural consequences linked to the mutation affect the capacity of the protein to activate the transcription of some flavonoid genes by modifying the interaction with its co-partner(s). In addition, this study underlines the importance of an internal salt bridge for protein conformation and thus for the establishment of protein-protein interactions between MYB and bHLH transcription factors. Mechanisms underlying these interactions are discussed and a model is proposed to explain the transcriptional activity of VvMYB5 L  observed in the tobacco model.

bmc plant biology

A single amino acid change within the R2 domain of the VvMYB5b transcription factor modulates affinity for protein partners and target promoters selectivity