Nitrogen nutrition and the role of root-shoot nitrogen signalling particularly in symbiotic systems

To obtain and concentrate reduced N from the environment, plants have evolved a diverse array of adaptations to utilize soil, biotic and atmospheric N. In symbiotic N(2)-fixing systems the potential for oversupply exists and regulation of activity to match demand is crucial. N status in plants is likely to be most strongly sensed in the shoot and signals translocated to the roots may involve phloem transported amino compounds or very low concentrations of specific signal molecules. The mechanism for sensing N status in plant cells is not understood at the molecular level although it may be expected to be similar in all plants. Mechanisms for the regulation of symbiotic N(2) fixation may be very different in the different symbiotic types. Rhizobia, Frankia and cyanobacteria are all symbiotic with different species of plants and the provision of O(2), carbohydrate or other nutrients may control symbiotic activity to varying extents in the different symbioses.