Network-based multiomics and transgenic validation reveal that OsPHR3 modulates phosphate-carbon metabolic trade-offs during rice seed development

Phosphate (Pi) allocation during the grain-filling stage is a major determinant of crop yield, supporting macromolecule synthesis, energy metabolism, and nutrient storage. However, its storage as phytic acid (PA) reduces nutritional quality by chelating essential minerals. Despite its importance, a comprehensive understanding of the molecular mechanisms integrating Pi transport, carbohydrate metabolism, and PA biosynthesis during seed development remains incomplete. To address this gap, we investigated stage-specific phosphate regulatory networks in rice by integrating transcriptomic, proteomic, and metabolomic approaches. Temporal expression profiling and gene coexpression network analyses of phosphate regulators and transporter genes revealed their distinct roles during early and mid-grain filling stages. PHOSPHATE STARVATION RESPONSE 3 (OsPHR3) emerged as a central regulatory hub, coordinating the balance of Pi, sugar, starch and phytate, along with other metabolites. Network-based multiomics integration further identified 126 genes involved in nutrient storage and stress tolerance, with myo-inositol-1-phosphate synthase (OsMIPS1) and starch synthase 3 (OsSSIII) as key genes. CRISPR/Cas9-generated osphr3 knockout lines confirmed the critical role of OsPHR3 in regulating these target genes. Mutants exhibited significantly reduced seed starch, PA, and total phosphorus contents, while scanning electron microscopy revealed aberrant starch granule morphology. Loss-of-function of OsPHR3 lowered PA levels by 19.46-22.50 %, with moderate trade-offs in yield-related traits. Although, OsPHR3 is known to contribute to nitrogen and phosphorus homeostasis, our findings establish it as a key regulator orchestrating a stage-specific phosphate-carbon allocation during seed development. These insights provide key targets for refining nutrient partitioning to achieve increased yields, reduced phytic acid, and enhanced phosphorus use efficiency for agricultural sustainability.