Multiple routes communicating nitrogen availability from roots to shoots: a signal transduction pathway mediated by cytokinin.

In higher plants, inorganic nitrogen has crucial effects on growth and development, providing cellular components and modulating gene expression. To date, not only nitrogen assimilatory genes but also a substantial number of genes with other functions have been shown to be selectively regulated by the availability of nitrogen. In terms of the communicating substance(s) between root and shoot, accumulating evidence suggests that nitrate itself is the primary signal molecule triggering the activation of transcription of nitrate assimilation and related genes. On the other hand, some of the genes involved in photosynthesis, cell cycling and translation machinery are also regulated, at least in part, by nitrate and other nitrogen sources and, in some cases, the effect can be mimicked by cytokinin treatment. Spatial and temporal studies on the accumulation levels and the translocation of cytokinin in response to nitrate replenishment in maize showed subsequent accumulation of various cytokinin species in the roots, xylem sap and leaves. In Arabidopsis thaliana, trans-zeatin riboside-5'-monophosphate and/or trans-zeatin riboside also accumulated in the roots in response to nitrate resupply. These studies suggest that cytokinin metabolism and translocation could be commonly modulated by nitrogen availability in higher plants. Thus, in addition to nitrate, cytokinin could be another root-to-shoot signal communicating nitrogen availability.