Biological impacts of phosphomimic AtMYB75.

The phosphorylation status of MYB75 at T-131 affects protein stability, flavonoid profiles, and patterns of gene expression. The Arabidopsis transcription factor Myeloblastosis protein 75 (MYB75, AT1G56650) is known to act as a positive transcriptional regulator of genes required for flavonoid and anthocyanin biosynthesis. MYB75 was also shown to negatively regulate lignin and other secondary cell wall biosynthetic genes (Bhargava et al. in Plant Physiol 154(3):1428-1438, 2010). While transcriptional regulation of MYB75 has been described in numerous publications, little is known about post-translational control of MYB75 protein function. In a recent publication, light-induced activation of a MAP kinase (MPK4, AT4G01370) in Arabidopsis was reported to lead to MYB75 phosphorylation at two canonical MPK target sites, threonines, T-126 and T-131. This double phosphorylation event positively influenced MYB75 protein stability (Li et al. in Plant Cell 28(11):2866-2883, 2016). We have examined this phenomenon through use of phosphomutant forms of MYB75 and found that MYB75 is phosphorylated primarily at T-131, and that the phosphorylation of MYB75 recombinant protein in vitro can be catalyzed by multiple MAP kinases, including MPK3 (AT3G45640), MPK6 (AT2G43790), MPK4 and MPK11 (AT1G01560). We also demonstrate that MYB75 can bind to a large number of Arabidopsis MPK's in vitro, suggesting it could be a target of multiple signalling pathways. The impact of MYB75 phosphorylation at T-131 on the function of this transcription factor, in terms of localization, stability, and protein-protein interactions with known binding partners was examined in transgenic lines expressing phosphomimic and phosphonull versions of MYB75, to capture the behaviour of permanently phosphorylated and unphosphorylated MYB75 protein, respectively. In addition, we describe how ectopic over-expression of different phosphovariant forms of MYB75 (MYB75, MYB75, and MYB75) affects flavonoid biochemical profiles and global changes of gene expression in the corresponding transgenic Arabidopsis plants.