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

Hyperosmolarity caused by drought, high salinity, or cold stress inhibits plant growth and crop productivity. 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 plants. 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 nonionic hyperosmolarity by reversible condensation. B4-RAFs recruit and cocondense with subclass-I SnRK2s to phosphorylate and turn on SnRK2s, evading the noncondensable inhibitory A-clade PP2C phosphatases. This straightforward osmosensing and relaying module can be fully reconstituted in Escherichia coli by coexpressing 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 cocondensation, forming a signal hub with SnRK2s to orchestrate adaptive responses in plants, and represent an evolutionarily conserved osmosensing mechanism across kingdoms.