[Paul Verslues] Phosphoproteomics of Arabidopsis Highly ABA-Induced1 identifies AT-Hook Like10 phosphorylation required for stress growth regulation

The mechanisms plants use to detect and signal abiotic stress are not well understood but have major implications for plant productivity during periods of drought and other environmental stresses. Clade A Protein Phosphatases 2Cs (PP2Cs) are part of the core signaling pathway of the stress hormone Abscisic Acid which regulates many plant stress responses.  In previous research, the Verslues laboratory noted that the Clade A PP2C Highly ABA-Induced 1 (HAI1) has especially prominent effects on drought phenotypes such as growth and proline accumulation (Bhaskara et al., 2012).  To investigate how HAI1 affects drought response, phosphoproteomic analysis of the hai1-2 mutant was conducted for plants under unstressed conditions or after low water potential (drought) treatment (Wong et al., 2019 PNAS).  This identified more than 100 phosphopeptides of increased abundance in hai1-2, indicating that phosphorylation status of these sites could be regulated by HAI1 directly of via HAI1-regulated kinases.  Further experiments on At-Hook Like 10, a nuclear localized DNA-binding protein of unclear function, demonstrated that it could be directly dephosphorylated by HAI1.  The S314 phosphorylation site identified in our proteomics analysis was crucial for AHL10 regulation of growth and gene expression during low water potential stress and was also required for AHL10 complexes to form nuclear foci. By identifying HAI1-affected phosphoproteins and functionally important AHL10 phosphorylation site, these results elucidate HAI1 and AHL10 function and also demonstrate a mechanism plants use to balance maximal growth versus robust response to environmental stress.