Amino acids on the move: boosting transporters for high-quality crops under reduced nitrogen supply

Nitrogen availability is one of the main factors limiting crop productivity, and the low nitrogen use efficiency (NUE) of crops imposes significant economic and environmental costs. Research on NUE has traditionally focused on improving nitrogen uptake and assimilation; however, the efficient redistribution of assimilated nitrogen within the plant is equally important for growth, yield, and crop quality. Amino acids are the main form in which reduced nitrogen is transported over long distances. Their movement between source and sink tissues depends on a diverse group of membrane transporters belonging to the Amino Acid/Auxin Permeases (AAAP), Amino Acid/Polyamine/Choline transporters (APC), and Usually Multiple Acids Move In and Out Transporters (UMAMIT) families. These transporters mediate amino acid uptake, export, vascular loading and unloading, intracellular transport, and redistribution to developing organs, thereby influencing plant growth, reproductive development and the composition of harvested products. Recent work in cereals, legumes and horticultural crops has shown that modifying amino acid transport can alter biomass production, grain filling, seed protein content, fruit quality, and NUE. In this review, we summarize current knowledge of plant amino acid transporters, from their molecular and physiological functions to their roles in crop yield, nutritional quality and NUE. We also discuss how advances in genome editing, exploitation of natural genetic variation, high-throughput phenotyping and systems biology may facilitate the development of crops with more efficient nitrogen use and improved agricultural sustainability.