OsOSCA2.4 regulates post-Golgi trafficking of storage proteins by modulating Ca2+ homeostasis in rice endosperm

Seed storage proteins (SSPs), which accumulate specifically during seed development, constitute a major source of plant-derived protein in the human diet. Despite their critical role in determining crop quality, the molecular mechanisms underlying the intracellular trafficking of SSPs remain poorly understood. Here, we characterize the rice glutelin precursor accumulation16 (gpa16) mutant, which exhibits defective dense vesicle (DV)-mediated post-Golgi trafficking of proglutelins, resulting in their overaccumulation in the apoplast. GPA16 encodes OsOSCA2.4, a member of the reduced hyperosmolality-induced Ca2+ increase channel (OSCA) family. OsOSCA2.4 localizes to post-Golgi compartments including the trans-Golgi network (TGN) and prevacuolar compartment (PVC) in vegetative tissue, and DVs in the endosperm. The OsOSCA2.4(P397L) variant disrupts the formation of the Rab5a molecular module, a process influenced by intracellular Ca2+ homeostasis. Ca2+ imaging assays indicate that OsOSCA2.4 functions as a regulator of Ca2+ homeostasis of PVCs/DVs, the primary sites of Rab5a module activity. Genetic analyses reveal functional redundancy between OsOSCA2.4 and OsOSCA4.1 in modulating post-Golgi trafficking of proglutelins. Collectively, our findings identify OSCA-mediated Ca2+ homeostasis as a previously unrecognized regulatory layer governing post-Golgi trafficking of SSPs and uncover an unexpected role for OSCA proteins in vesicular trafficking in eukaryotes.