Mechanisms of Phyllosphere Microbial Regulation of Nitrogen Use in Rice.

Nitrogen (N) fertilizers are key drivers of high yields in rice (Oryza sativa L.), yet increasing N inputs often deliver diminishing improvements in N use efficiency (NUE) and can exacerbate N losses via volatilization, leaching, and runoffs, with associated environmental impacts. The rice phyllosphere microbiome, which consists of microbes inhabiting leaf surfaces and internal tissues, has the potential to influence foliar nutrient turnover, N metabolism, and stress responses and may therefore influence internal NUE (IEN) and, under reduced fertilizer-N input, potentially affect agronomic efficiency of applied N (AEN). Here, we summarize the major steps of rice N metabolism (uptake, assimilation, transport, redistribution, and remobilization) and discuss microbial pathways that could modulate N use, including foliar N transformations, microbially mediated N inputs, and indirect effects through hormonal regulation, stress buffering, and microbe-microbe interactions. Key knowledge gaps remain in quantification of underlying mechanistic processes, field robustness across seasons and sites, and genotype/microbiome matching under reduced-N management.