Cysteine protease RD21A regulated by E3 ligase SINAT4 is required for drought-induced resistance to Pseudomonas syringae in Arabidopsis

Plants can be simultaneously exposed to multiple stresses. The interplay of abiotic and biotic stresses may result in synergistic or antagonistic effects on plant development and health. Temporary drought stress can stimulate plant immunity; however, the molecular mechanism of drought-induced immunity is largely unknown. In this study, we demonstrate that cysteine protease RD21A is required for drought-induced immunity. Temporarily drought-treated wild type Arabidopsis plants become more sensitive to bacterial pathogen associated molecular pattern (PAMP), flg22, for triggering the stomatal closure, which results in increased resistance to Pseudomonas syringae pv. tomato DC3000 (Pst-DC3000). Knocking out rd21a inhibits the flg22-triggered stomatal closure and compromises the drought-induced immunity. Ubiquitin E3 ligase SINAT4 interacts with RD21A and promotes its degradation in vivo. The overexpression of SINAT4 also consistently compromised the drought-induced immunity to Pst-DC3000. A bacterial type III effector AvrRxo1 interacts with both SINAT4 and RD21A, enhancing SINAT4 activity, promoting the degradation of RD21A in vivo. Therefore, RD21A could be a positive regulator of drought-induced immunity, which can be targeted by pathogen virulence effectors during pathogenesis.