The way out and in: phloem loading and unloading of amino acids.

Amino acids represent the major transport form of reduced nitrogen in plants. Long-distance transport of amino acids occurs in the xylem and the phloem. However, the phloem is the main transport route for bulk flow of the organic nitrogen from source leaves to sink tissues. Phloem loading in leaves of most annual plant species follows an apoplasmic transport path and requires the coordinated activity of transport protein mediating cellular export or import of amino acids. Phloem unloading of amino acids is generally a symplasmic process but apoplasmic transport is additionally required for efficient post-phloem nitrogen transport. In this review we summarize the current data on the physiology of amino acid phloem loading and unloading, and the molecular players involved. We discuss the implications of amino acid transporters in nitrogen signaling and highlight the necessity to investigate the coordination of symplasmic and apoplasmic transport processes.