Amino Acids on the Move: Boosting Permeases for High-Quality Crops Under Reduced Nitrogen Supply

Nitrogen (N) availability is a major determinant of crop productivity; however, nitrogen use efficiency (NUE) remains relatively low in most agricultural systems. After uptake from the soil, inorganic N is assimilated into organic forms, primarily amino acids, which represent the principal long-distance transport form in most plants. The distribution of amino acids from source tissues to developing sink organs therefore plays a central role in plant growth, yield formation, and the nutritional quality of harvested organs. Amino acid transporters (AATs), also known as permeases, regulate the cellular and long-distance movement of amino acids and play a central role in nitrogen partitioning within the plant. These membrane proteins belong to the AAAP, APC, and UMAMIT transporter families and participate in multiple physiological processes, including amino acid uptake in roots, xylem and phloem transport, intracellular compartmentalization, and partitioning to reproductive tissues. Recent functional studies in both model plants and crop species demonstrate that manipulation of amino acid transporters can significantly influence biomass production, seed yield, grain protein content, and nitrogen use efficiency. In this review, we synthesize current knowledge on the structure, transport mechanisms, and physiological roles of plant amino acid transporters, with particular emphasis on their contribution to nitrogen partitioning and crop productivity. We also discuss emerging opportunities for exploiting amino acid transporters in crop breeding and biotechnology to enhance nitrogen utilization and improve the sustainability of agricultural systems.