AHL10 phosphorylation determines RRP6L1 chromatin association and growth suppression during water stress.

Phosphorylation of AHL10, one of the AT-hook family of plant-specific DNA binding proteins, is critical for growth suppression during moderate severity drought (low water potential, {psi}w) stress. To understand how AHL10 phosphorylation determines drought response, we identified putative AHL10 interacting proteins and further characterized interaction with RRP6L1, a protein involved in epigenetic regulation. RRP6L1 and AHL10 mutants, as well as ahl10-1rrp6l1-2, had similar phenotype of increased growth maintenance during low {psi}w. Chromatin precipitation demonstrated that RRP6L1 chromatin association increased during low {psi}w stress and was dependent upon AHL10 phosphorylation. Transcriptome analyses showed that AHL10 and RRP6L1 have concordant effects on expression of stress- and development-related genes. Together these results indicate that stress signaling can act via AHL10 phosphorylation to control the chromatin association of the key regulatory protein RRP6L1. AHL10 and RRP6L1 interaction in meristem cells is part of a mechanism to down-regulate growth during low {psi}w stress. Interestingly, loss of AHL13, which is homologous to AHL10 and phosphorylated at similar C-terminal site, blocked the enhanced growth maintenance of ahl10-1. Thus, AHL10 and AHL13, despite their close homology, are not redundant but rather have distinct roles, likely related to the formation of AHL hetero-complexes. Summary StatementPhosphorylation of Arabidopsis thaliana AHL10 is important to control growth during drought stress, a function which is distinct from the closely related AHL13. One way that phosphorylation determines AHL10 function is by altering its ability to mediate chromatin recruitment of RRP6L1.