PuCBL9‐PuCIPK11 Modulates Stomatal Microfilament Reorganisation in a Phosphorylation‐Dependent Manner to Enhance Drought Tolerance in Poplar

Drought stress severely constrains the growth and survival of forest trees. Plant stomata serve as portals for water transpiration and gas exchange, where adjustments in stomatal aperture critically influence drought resistance in trees. Although guard cell actin cytoskeleton dynamics are essential for stomatal movement, their regulation in trees remains poorly understood. This study characterises PuCIPK11 in Populus ussuriensis Kom., demonstrating its role in enhancing drought resistance via stomatal closure. PuCIPK11 predominantly expressed in guard cells and induced by drought and ABA, and its overexpression promotes microfilaments (MFs) depolymerisation and longitudinal rearrangement, accelerating stomatal closure. Under drought stress, PuCIPK11 is recruited to the plasma membrane by the Ca²⁺ sensor PuCBL9, where it undergoes conformational activation through the formation of a PuCBL9‐PuCIPK11 module. Furthermore, PuCIPK11 phosphorylates PuCBL9 at Ser201, a process enhanced by ABA that strengthens their interaction. Functional validation using pucbl9 knockout mutants in a PuCIPK11 ‐overexpression background confirmed that PuCIPK11 acts in concert with PuCBL9 to modulate MFs dynamics and drought tolerance. This study provides mechanistic insights into the phosphorylation‐dependent regulation of stomatal cytoskeletal dynamics and identifies the PuCBL9‐PuCIPK11 module as a key regulator of drought resistance in poplar, offering potential targets for genetic improvement of poplar.