Transporters for amino acids in plant cells: some functions and many unknowns.

Membrane proteins are essential to move amino acids in or out of plant cells as well as between organelles. While many putative amino acid transporters have been identified, function in nitrogen movement in plants has only been shown for a few proteins. Those studies demonstrate that import systems are fundamental in partitioning of amino acids at cellular and whole plant level. Physiological data further suggest that amino acid transporters are key-regulators in plant metabolism and that their activities affect growth and development. By contrast, knowledge on the molecular mechanisms of cellular export processes as well as on intracellular transport of amino acids is scarce. Similarly, little is known about the regulation of amino acid transporter function and involvement of the transporters in amino acid signaling. Future studies need to identify the missing components to elucidate the importance of amino acid transport processes for whole plant physiology and productivity.