Regulation of Stomatal Responses to Pathogen and Drought Stress by the F-Box Protein AtSKIP5.

E3 ubiquitin ligases are major components of the ubiquitination cascade and contribute to the stomatal responses to pathogen and drought stress in plants. The F-box SKP1-Interacting Partners (AtSKIPs) proteins are members of the SCF E3 ubiquitin ligase complexes; however, whether they have any involvement in stomatal movement remains unclear. Here, based on tissue expression profiling, we found that the AtSKIP5 protein was highly expressed in guard cells. Mutation of AtSKIP5 rendered plants more susceptible to Pseudomonas syringae pv. tomato (Pst) DC3000 and resulted in a significant impairment in stomatal closure after flg22 and Pst DC3000 treatment. Consistently, lines overexpressing AtSKIP5 were more resistant to Pst DC3000 infection and exhibited more rapid stomatal closure than did other lines. However, the AtSKIP5-overexpressing lines and Col-0 line were similarly resistant to Pst^- (coronatine-deficient mutant) infection and did not exhibit stomatal reopening when exposed to Pst DC3000, a Pst^- strain, or a Pst^- strain accompanied by coronatine (COR) treatment. These results suggest that AtSKIP5-mediated resistance to Pst DC3000 is by controlling stomatal immunity via positive regulation of flg22-triggered stomatal closure and suppression of COR-mediated stomatal reopening. Furthermore, apoplastic immunity was compromised in the skip5 mutants, as evidenced by lower MAPK phosphorylation levels, less reactive oxygen species (ROS) production, and callose deposition induced by flg22, shifting the response in the pathogenic direction. In addition, the skip5 mutants evidenced an impairment in stomatal closure induced by abscisic acid (ABA), and a lower survival rate and greater water loss under drought stress, suggesting that AtSKIP5 serves as a positive regulator of drought tolerance via ABA-induced stomatal closure. Our results provide new insights into the importance of the stomatal responses to pathogen and drought stresses that are modulated by AtSKIP5 in Arabidopsis.