Nitrogen Use Efficiency in Crops: Transporters, Metabolism, and Systemic Signaling

Nitrogen is a critical macronutrient limiting crop productivity, yet soil degradation and low nitrogen use efficiency (NUE) constrain sustainable agriculture. Improving NUE by elucidating transporter regulation, enzyme kinetics, and N remobilization offers opportunities to enhance crop performance. While significant progress has been made in characterizing individual regulatory components, substantial knowledge gaps persist regarding the systems-level coordination of N signaling, the dynamic interactions between the plant and the rhizosphere microbiomes, and the challenges of utilizing current knowledge to implement robust NUE improvements under variable field conditions. Addressing these fundamental questions is essential for developing sustainable strategies to optimize N resource management and minimize agriculture's environmental footprint. This review describes the complex molecular mechanisms governing N acquisition, assimilation, and remobilization in plants. We focus on the functional diversity and regulation of key transporters, including NRT1/NPF, NRT2, CLC, SLAC/SLAH, AMT1/AMT2 families, and the central role of enzymes such as nitrate reductase, nitrite reductase, and the GS/GOGAT cycle. It also focuses on how N availability acts as a critical signaling molecule, affecting systemic changes in root architecture, photosynthesis, and overall carbon allocation that ultimately determine crop yield.