Phosphorylation of OsRbohB by the protein kinase OsDMI3 promotes HO production to potentiate ABA responses in rice.

In rice, the Ca/calmodulin-dependent protein kinase OsDMI3 is an important positive regulator of abscisic acid (ABA) signaling. In ABA signaling, HO is required for ABA-induced activation of OsDMI3, which in turn increase HO production. However, how OsDMI3 regulates HO production in ABA signaling remains unknown. Here we show that OsRbohB is the main NADPH oxidase involved in ABA-induced HO production and ABA-mediated physiological responses. OsDMI3 directly interacts with and phosphorylates OsRbohB at Ser-191, which is OsDMI3-mediated site-specific phosphorylation in ABA signaling. Further analyses revealed that OsDMI3-mediated OsRbohB Ser-191 phosphorylation positively regulates the activity of NADPH oxidase and the production of HO in ABA signaling, thereby enhancing the sensitivity of seed germination and root growth to ABA and plant tolerance to water stress and oxidative stress. Moreover, we discovered that the OsDMI3-mediated OsRbohB phosphorylation and HO production is dependent on the sucrose non-fermenting 1-related protein kinases SAPK8/9/10, which phosphorylate OsRbohB at Ser-140 in ABA signaling. Taken together, these results not only reveal an important regulatory mechanism that directly activates Rboh for ABA-induced HO production but also uncover the importance of this regulatory mechanism in ABA signaling.