Phospho‐site mapping,genetic and in planta activation studies reveal key aspects of the different phosphorylation mechanisms involved in activation of SnRK2s

Snf1‐related protein kinases 2 (SnRK2s) are major positive regulators of drought stress tolerance. The kinases of this family are activated by hyperosmotic stress, but only some of them are also responsive to abscisic acid (ABA). Moreover, genetic evidence has indicated the ABA‐independence of SnRK2 activation in the fast response to osmotic stress. Although phosphorylation was demonstrated to be crucial for the activation or activity of the kinases of both subgroups, different phosphorylation mechanisms were suggested. Here, using one kinase from each subgroup (SnRK2.6 and SnRK2.10), two phosphorylation sites within the activation loop were identified by mass spectrometry after immunoprecipitation from Arabidopsis cells treated by ABA or osmolarity. By site‐directed mutagenesis, the phosphorylation of only one of the two sites was shown to be necessary for the catalytic activity of the kinase, whereas both sites are necessary for the full activation of the two SnRK2s by hyperosmolarity or ABA. Phosphoprotein staining together with two‐dimensional PAGE followed by immunoblotting indicated distinct phosphorylation mechanisms of the two kinases. While SnRK2.6 seems to be activated through the independent phosphorylation of these two sites, a sequential process occurs in SnRK2.10, where phosphorylation of one serine is required for the phosphorylation of the other. In addition, a subgroup of protein phosphatases 2C which interact and participate in the regulation of SnRK2.6 do not interact with SnRK2.10. Taken together, our data bring evidence for the involvement of distinct phosphorylation mechanisms in the activation of SnRK2.6 and SnRK2.10, which may be conserved between the two subgroups of SnRK2s depending on their ABA‐responsiveness.