The molecular regulation of rice grain size: Pathways and prospects for precision breeding

Rice grain size is a pivotal agronomic trait that directly influences yield and quality, making it a prime target for breeding programs aimed at ensuring global food security. This review systematically synthesizes current knowledge on the molecular and genetic mechanisms governing grain size regulation in rice, focusing on key pathways such as G-protein signaling, MAPK cascades, ubiquitin-proteasome systems, and transcriptional regulation, as well as phytohormone networks (brassinosteroids, auxin, cytokinin, and gibberellin), and epigenetic-mediated signaling. We highlight the roles of major quantitative trait loci (QTLs) and genes in modulating cell proliferation and expansion during grain development. Emerging technologies such as CRISPR/Cas9 based genome editing and multi-omics approaches are discussed as powerful tools for dissecting these complex regulatory networks and accelerating precision breeding. By integrating fundamental discoveries with applied strategies, this review provides insights into optimizing grain size to enhance both yield and quality, and addressing key challenges in future molecular breeding efforts.