Nitrate-specific and cytokinin-mediated nitrogen signaling pathways in plants.

Inorganic nitrogen is a limiting factor in plant growth and development. Plants constantly sense changes in nitrogen availability and respond appropriately by modulating gene expression. Plants employ multiple routes for the long-distance signaling and communication of nitrogen status. One of these depends on nitrate itself (nitrate-specific signaling), while another uses cytokinin as a messenger. Recent studies suggest that nitrate-specific signaling functions predominantly in the context of the synthesis of amino acids and nucleic acids. This pathway includes the control of the expression of a wide variety of genes. On the other hand, cytokinin-mediated signaling is related mainly to the control of nitrogen partitioning and development. Nitrogen-dependent cytokinin accumulation and the involvement of His-Asp phosphorelay systems are characteristic of this pathway. The coordination of both regulatory pathways seems to be crucially important for the integration of nitrogen signals at the whole plant level.