A non-canonical MPK3-ACTPK1-RbcS signalling module fine-tunes photosynthesis in rice

Mitogen-activated protein kinase (MAPK) cascades orchestrate diverse physiological processes, yet their role in photosynthetic regulation has remained largely unexplored. Here, we uncovers a non-canonical MAPK signalling module, MITOGEN ACTIVATED PROTEIN KINASE 3 – ACT DOMAIN CONTAINING PROTEIN KINASE 1 – RUBISCO SMALL SUBUNIT (MPK3-ACTPK1-RbcS), that directly links MAPK signalling to photosynthesis optimization. ACTPK1, a dual-specificity cytosolic kinase from Group C of Raf-like MAPKKKs, is inhibited by MPK3 through direct phosphorylation. We demonstrated that ACTPK1 targets the RbcS, phosphorylating it at Thr12 within its signal peptide, thereby stabilizing its preprotein and enhancing its chloroplast import efficiency. Genetic analysis confirms that MPK3-ACTPK1-RbcS functions in a linear pathway to fine-tune photosynthesis under ambient conditions. Notably, mpk3 knockouts boost carbon fixation by 18-21%. This non-canonical MAPK module redefines cytoplasmic-chloroplast signalling integration, offering a transformative framework for enhancing crop productivity and global food security through targeted genetic interventions.