Phosphorylation of OsRbohB by the protein kinase OsDMI3 promotes H2O2 production to potentiate ABA responses in rice

Abstract

In rice, the Ca²⁺/calmodulin-dependent protein kinase OsDMI3 is an important positive regulator of abscisic acid (ABA) signaling. In ABA signaling, H₂O₂ is required for ABA-induced activation of OsDMI3, which in turn increase H₂O₂ production. However, how OsDMI3 regulates H₂O₂ production in ABA signaling remains unknown. Here we show that OsRbohB is the main NADPH oxidase involved in ABA-induced H₂O₂ 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 H₂O₂ 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 H₂O₂ 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 H₂O₂ production but also uncover the importance of this regulatory mechanism in ABA signaling.

Keywords: Ca(2+)/calmodulin-dependent protein kinase; NADPH oxidases; abscisic acid; protein phosphorylation; rice.