Developmental phenotypes of Arabidopsis plants expressing phosphovariants of AtMYB75.

The Arabidopsis transcription factor Myeloblastosis protein 75 (MYB75, AT1G56650) is a well-established transcriptional activator of genes required for anthocyanin and flavonoid production, and a repressor of lignin and other secondary cell wall biosynthesis genes. MYB75 is itself tightly regulated at the transcriptional, translational and post-translational levels, including protein phosphorylation by Arabidopsis MAP kinases Examination of the behavior of different phosphovariant versions of MYB75 in vitro and in vivo revealed that overexpression of the MYB75T131E phosphovariant had a particularly marked effect on global changes in gene expression suggesting that phosphorylated MYB75 could be involved in a broader range of functions than previously recognized. Here, we describe a range of distinct developmental phenotypes observed among Arabidopsis lines expressing various phosphovariant forms of MYB75. Expression of either MYB75T131E or MYB75T131A phosphovariants, from the endogenous MYB75 promoter, in Arabidopsis myb75- mutants (Nossen background), resulted in severely impaired germination rates, and developmental arrest at early seedling stages. Arabidopsis plants overexpressing MYB75T131E from a strong constitutive Cauliflower mosaic virus (CaMV35S) promoter displayed slower development, with delayed bolting, flowering and onset of senescence. Conversely, MYB75T131A -overexpressing lines flowered and set seed earlier than either Col-0 WT controls or other MYB75-overexpressors (MYB75WT and MYB75T131E ). Histochemical analysis of mature stems also revealed ectopic vessel development in plants overexpressing MYB75; this phenotype was particularly prominent in the MYB75T131E phosphovariant. These data suggest that MYB75 plays a significant role in plant development, and that this aspect of MYB75 function is influenced by its phosphorylation status.