Sulfur Homeostasis in Rice as a Dynamic Regulatory Network: Functional Genomics, Metabolic Crosstalk, and...

Sulfur (S) homeostasis in rice (Oryza sativa L.) depends on coordinated sulfate acquisition, transport, assimilation, and allocation into cysteine, methionine, glutathione, and other sulfur-containing compounds. Although these processes influence growth, redox regulation, detoxification, grain quality, and immunity, the functional evidence supporting individual sulfur-related genes in rice remains uneven. This review critically evaluates sulfur metabolism through an evidence-graded functional genomics framework, distinguishing direct validation in rice from expression-based inference, heterologous assays, and mechanisms extrapolated from Arabidopsis. Particular emphasis is placed on sulfate transporter families, sulfur assimilation enzymes, the cysteine synthase complex, glutathione-dependent pathways, and micro-RNA-mediated regulation. The miR395-OsAPS1-OsSULTR2;1/2;2 modules are highlighted as a key regulatory system that coordinates sulfate activation and vascular redistribution. Its contribution to resistance against Xanthomonas oryzae demonstrates that sulfur-dependent immunity may arise from direct pathogen sensitivity to accumulated inorganic sulfate rather than exclusively from glutathione-mediated redox buffering. Rice functional genomics studies also reveal important metabolic trade-offs: enhanced sulfur flux can improve detoxification or stress resistance but may impose costs on carbon and nitrogen use, growth, reproductive development, or grain composition. We therefore propose that sulfur metabolism should be viewed as a dynamic resource allocation network rather than a linear assimilation pathway. Future progress will require reciprocal gain- and loss-of-function analyses, target-specific rescue, multiplex genome editing, isotope-assisted flux measurements, spatially resolved multi-omics, and field validation across contrasting sulfur and nitrogen regimes. Integrating these approaches will help identify regulatory variants that improve sulfur-use efficiency, stress resilience, immunity, and grain quality without compromising yield stability.