R2R3 MYB transcription factor PtrMYB192 regulates flowering time in Arabidopsis by activating FLOWERING LOCUS C

R2R3 MYB transcription factors regulate multiple aspects of plant growth and development. Here we report the identification of PtrMYB192, a Populus R2R3 MYB transcription factor, as a negative regulator of flowering time. By using quantitative RT-PCR, we found that PtrMYB192, but not its closely homologous gene PtrMYB028, is highly expressed in mature leaves in Populus. Heterologously expression of PtrMYB192 under control of 35S promoter in Arabidopsis resulted in late flowering phenotypes under both long and short day conditions, indicating that PtrMYB192 controls flowering time independent of the photoperiod pathway. Domain swapping experiment showed that neither PtrMYB028DB-192AD nor PtrMYB192DB-028AD affected flowering time when heterologously expressed in Arabidopsis. However, when recruit to the promoter of a GAL4-GUS reporter gene by a GAL4 DNA binding domain in Arabidopsis protoplasts, both of PtrMYB028DB-192AD and PtrMYB192DB-028AD activated the reporter gene. Quantitative RT-PCR results showed an elevated expression of the floral repressor gene FLOWERING LOCUS C (FLC), but not the flowering-promoting gene CONSTANS (CO) in PtrMYB192 transgenic plants. Taken together, these results suggest that PtrMYB192 is a transcription activator that negatively regulating flowering time in Arabidopsis by activating FLC and possible other genes, and that both R2R3 DNA binding domain and activation domain maybe required for its full function.