Unlocking associative nitrogen fixation in Poaceae: molecular mechanisms, regulatory constraints, and pathways to...

Biological nitrogen fixation (BNF) is a sustainable and environmentally friendly approach to nitrogen assimilation that extends beyond leguminous plants, where efficient nitrogen fixation occurs. Poaceae crops can also obtain nitrogen through associative nitrogen fixation (ANF) mediated by endophytic and rhizospheric diazotrophs, which typically contributes 10-30 kg N ha^-1 yr.^-1 to cereal crops, i.e., wheat, maize, rice, though certain tropical grasses, such as sugarcane may achieve up to 40-50 kg N ha^-1 yr.^-1 under optimal conditions. This review synthesizes recent advancements of ANF research across Poaceae crops, providing comprehensive analysis of their functional mechanisms. It explores the underlying mechanisms regulating ANF and assesses key factors influencing nitrogenase activity, including oxygen sensitivity, external nitrogen availability, and energy constraints. Furthermore, it discusses advanced strategies, such as genetic engineering of nitrogen-fixing microbes, host plant metabolic modifications, and optimized agronomic strategies to enhance nitrogen fixation efficacy. This review establishes a robust framework for sustainable crop production and long-term agricultural productivity by integrating molecular, physiological, and ecological perspectives. The major challenges of climate change, regional fertilizer restrictions, and achieving sustainable agricultural intensification collectively underscore the urgent need for biologically based nitrogen management strategies in cereal cropping systems.