Coordinated regulation of amino acid metabolism and transport in cereal nitrogen remobilization for grain filling.

Improving nitrogen use efficiency (NUE) in cereals is crucial for sustainable agriculture and global food security. Nitrogen remobilization from senescing leaves to developing grains can contribute up to 80% of grain nitrogen content in cereals, though this varies with genotype and environment, making it a critical determinant of both yield and protein quality. This review synthesizes current evidence to highlight that remobilization efficiency emerges not from isolated processes but from the precise coordination between source leaves and sink grains. Specifically, we discuss how efficient nitrogen remobilization is determined by the coordinated regulation of amino acid interconversion pathways in source leaves-which transform proteolysis products into transport-optimized compounds like glutamine and asparagine-and the spatiotemporally controlled expression of specific amino acid permeases in developing sinks, which govern nitrogen uptake capacity. By synthesizing recent insights into metabolic reprogramming during senescence and transporter expression in grains, we provide a comprehensive overview of how systemic source-sink synchronization optimizes nitrogen flux for grain filling. Building on this synthesis, we hypothesize that engineering the spatiotemporal coordination of amino acid permease expression will yield greater NUE improvements than modulating any single component in isolation.