SnRK2.5-mediated phosphorylation of PIN2 links osmotic stress signaling with auxin dependent root adaptive growth in Arabidopsis

The spatiotemporal regulation of polar auxin transport, mediated by PIN-FORMED (PIN) efflux carriers, enables plants to coordinate developmental programs with environmental cues. Here we identify SnRK2.5, an abscisic acid (ABA)-independent member of the SNF1-related protein kinase family, as a key regulator linking osmotic stress signaling to the modulation of auxin transport in Arabidopsis. Osmotic stress-activated SnRK2.5 directly phosphorylates PIN2 at Ser237 and Ser259. Genetic and cell biological analyses demonstrate that these phosphorylation events govern PIN2 vesicular trafficking, vacuolar targeting, and transport activity. Mutating these phosphorylation sites impairs PIN2-dependent auxin redistribution, thereby compromising root tropic responses and reducing osmotic stress tolerance. Our findings reveal a regulatory mechanism whereby SnRK2.5-mediated phosphorylation of PIN2 dynamically adjusts auxin flux to optimize plant growth in response to water availability, uncovering a critical adaptive strategy in plants.