ROP11 GTPase Negatively Regulates ABA Signaling by Protecting ABI1 Phosphatase Activity from Inhibition by the ABA Receptor RCAR1/PYL9 in <i>Arabidopsis</i>

Abstract The phytohormone abscisic acid (ABA) regulates many key processes in plants, such as seed germination, seedling growth, and abiotic stress tolerance. In recent years, a minimal set of core components of a major ABA signaling pathway has been discovered. These components include a RCAR/PYR/PYL family of ABA receptors, a group of PP2C phosphatases, and three SnRK2 kinases. However, how the interactions between the receptors and their targets are regulated by other proteins remains largely unknown. In a companion paper published in this issue, we showed that ROP11, a member of the plant‐specific Rho‐like small GTPase family, negatively regulates multiple ABA responses in Arabidopsis . The current work demonstrated that the constitutively active ROP11 (CA‐ROP11) can modulate the RCAR1/PYL9‐mediated ABA signaling pathway based on reconstitution assays in Arabidopsis thaliana protoplasts. Furthermore, using luciferase complementation imaging, yeast two‐hybrid assays, co‐immunoprecipitation assays in Nicotiana benthamiana and bimolecular fluorescence complementation assays, we demonstrated that CA‐ROP11 directly interacts with ABI1, a signaling component downstream of RCAR1/PYL9. Finally, we provided biochemical evidence that CA‐ROP11 protects ABI1 phosphatase activity from inhibition by RCAR1/PYL9 and thus negatively regulates ABA signaling in plant cells. A model of how ROP11 acts to negatively regulate ABA signaling is presented.