Rapid sensing and relaying of cellular hyperosmotic-stress signals via RAF–SnRK2 core condensates

Summary Hyperosmolarity caused by drought, high salinity, or cold stress inhibits plant growth and crop productivity1,2. A conserved protein-kinase cascade of cytosolic B-RAFs and SnRK2s is rapidly activated upon osmotic stresses to initiate downstream adaptive responses, which represents one of the fastest known responses to osmotic stress in plants3–8. How the kinase cascade is activated by osmotic stress is unknown. Here, we show that Arabidopsis B4 subgroup RAFs have intrinsically disordered regions and directly sense both ionic and non-ionic hyperosmolarity by reversible condensation. B4-RAFs recruit and co-condense with subclass-I SnRK2s to phosphorylate and turn on SnRK2s, evading the non-condensable inhibitory A-clade PP2C phosphatases. This straightforward osmosensing and relaying module can be fully reconstituted in E. coli by co-expressing three components or in solution in a test tube using recombinant proteins. Our findings identify B-RAFs as the chief cellular osmosensors that detect low water potential by co-condensation, forming a signal hub with SnRK2s to orchestrate adaptive responses in plants, and represent an evolutionarily conserved osmosensing mechanism across kingdoms. Competing Interest Statement The authors have declared no competing interest. Footnotes ↵5 Lead contact