Phosphorylation of OsRbohB by the CaM-dependent protein kinase OsDMI3 promotes H 2 O 2 production to potentiate ABA responses in rice.

In rice, the Ca/calmodulin (CaM)-dependent protein kinase OsDMI3 is an important positive regulator of abscisic acid (ABA) signaling. In the ABA signaling, HO is required for ABA-induced activation of OsDMI3, but the activated OsDMI3 can also increase the production of HO. However, how OsDMI3 regulates HO production in ABA signaling remains unknown. Here we show that OsRbohB is the main NADPH oxidase that is involved in ABA-induced HO production and ABA-mediated physiological responses. OsDMI3 directly interacts with and phosphorylates OsRbohB at Ser-191, and this Ser-191 phosphorylation is OsDMI3-specific 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 the tolerance of rice plants to water stress and oxidative stress. Moreover, we also show that the OsDMI3-OsRbohB pathway is regulated by the sucrose non-fermenting 1-related protein kinases SAPK8/9/10 in ABA signaling. Our results not only reveal an important regulatory mechanism that directly activates Rboh in ABA signaling, but also highlight the importance of this regulatory mechanism in ABA signaling.