Regulation of nitrate uptake at the whole-tree level: interaction between nitrogen compounds, cytokinins and carbon metabolism.

Pedospheric nitrate uptake is closely integrated with the nitrogen (N) status and demand of the whole tree. Signaling substances communicating the N demand of the shoot to the roots are required in an integrated regulatory system. Besides phloem mobility, such signal compounds must have the potential to repress or increase nitrate uptake either at the transcriptional or post-transcriptional level. Amino compounds cycling within the tree are involved in the regulation of nitrate uptake. In many tree species, inorganic N is generally assimilated in roots, and amino acids--the direct products of N assimilation--are transported in the xylem to the sites of N demand. If the quantity of amino acids transported to the above-ground parts of the tree exceeds shoot N demand, some amino compounds are reallocated to the roots by phloem transport. Particular amino compounds exert transcriptional and post-transcriptional control over nitrate uptake by roots. Induction of nitrate transporters is mediated by nitrate or nitrite, or both, and possibly also by cytokinins, which cycle within the tree and act as both root-to-shoot and shoot-to-root signals. This review focuses on tree-specific requirements for N regulation and signaling, as well as the link between carbon metabolism and nitrate uptake.