The RCAR12-CAP1-OST1 module controls ABA-mediated stomatal closure in Arabidopsis.

Phytohormone abscisic acid (ABA) induces the stomatal closure in plants under drought stress, a process requiring actin reorganization. Yet how ABA perception couples with cytoskeletal dynamics in stomatal closure remains unclear. In this study, we report that Arabidopsis cyclase-associated protein 1 (CAP1) functions as a negative regulator of ABA-induced drought responses through modulating actin network organization in Arabidopsis. Further analyses demonstrate that CAP1 interacts with RCAR12 suppresses actin filaments (F-actin) disassembly, whereas ABA disrupts the CAP1-RCAR12 interaction, thus recovering CAP1's depolymerization activity. Further, ABA-activated OST1 (OPEN STOMATA 1) phosphorylates CAP1 in Arabidopsis. OST1-mediated phosphorylation of CAP1 attenuates CAP1 binding to ADF4 and inhibits the ADF4-CAP1 complex. This inhibition leads to F-actin stabilization, which in turn maintains stomata in a closed state. This study demonstrates that CAP1 orchestrates ABA-induced stomatal closure under drought stress in Arabidopsis. Specifically, CAP1 acts by coupling ABA signaling to dynamic reorganization of actin cytoskeleton.