Phosphorylation of S11 in PHR1 negatively controls its transcriptional activity

Plant responses to phosphate starvation (-Pi) are very well characterized at the biochemical and molecular levels. The expression of thousands of genes is modified under this stress condition, depending on the action of Phosphate starvation response 1 (PHR1). Existing data indicate that neither the PHR1 transcript nor the quantity or localization of its protein increase during nutrient stress, raising the question of how its activity is regulated. Here, we present data showing that SnRK1 kinase is able to phosphorylate some phosphate starvation response proteins (PSRs), including PHR1. Based on a model of the three-dimensional structure of the catalytic subunit SnRK1α1, docking simulations predicted the binding modes of peptides from PHT1;8, PHO1 and PHR1 with SnRK1. PHR1 recombinant protein interacted in vitro with the catalytic subunits SnRK1α1 and SnRK1α2. A BiFC assay corroborated the in vivo interaction between PHR1 and SnRK1α1 in the cytoplasm and nucleus. Analysis of phosphorylated residues suggested the presence of one phosphorylated site containing the SnRK1 motif at S11, and mutation in this residue disrupted the incorporation of ³² P, suggesting that it is a major phosphorylation site. Electrophoretic mobility shift assay results indicated that the binding of PHR1 to P1BS motifs was not influenced by phosphorylation. Importantly, transient expression assays in Arabidopsis protoplasts showed a decrease in PHR1 activity in contrast with the S11A mutant, suggesting a role for Ser11 as a negative regulatory phosphorylation site. Taken together, these findings suggest that phosphorylation of PHR1 at Ser11 is a mechanism to control the PHR1-mediated adaptive response to -Pi.