The class III peroxidase OsPrx20 is a key regulator of stress response and growth in rice.

In plants, reactive oxygen species (ROS) are maintained at strictly low intracellular levels, which is a prerequisite for their function as signaling molecules. Class III peroxidases serve as core regulators of intracellular ROS homeostasis when environmental stimuli trigger long-term responses, that cause ROS accumulation. Here, we identified a rice class III peroxidase, OsPrx20, which is a target of Ca²⁺/calmodulin-dependent protein kinase (OsDMI3) and is phosphorylated at its threonine (Thr)-244 site by OsDMI3, thereby positively regulating osmotic stress tolerance while negatively affecting blast resistance in rice. Overexpression of OsPrx20 enhances osmotic stress tolerance through intracellular ROS scavenging and simultaneously promotes panicle enlargement with increased grain size. Conversely, the loss of OsPrx20 potentiates immunity against rice blast infection by elevating intracellular ROS levels, which results in reduced plant height and smaller panicles. Crucially, abscisic acid (ABA)-dependent phosphorylation of OsPrx20 at Thr-244 by OsDMI3 promotes intracellular ROS scavenging in rice, thereby enhancing osmotic stress tolerance without compromising growth, though at the expense of blast resistance. In summary, this study uncovers an essential regulatory mechanism that directly activates OsPrx20 within ABA signaling, highlighting its multifunctional role across contrasting physiological processes.