His-Asp phosphorelay signaling: a communication avenue between plants and their environment

His-Asp phosphorelay systems have been recently discovered in plants and have emerged as some of the most important signaling systems. The phosphorelay systems in plants include components with sensor (His-protein kinase) domains, His-containing phosphotransfer (HPt) domains, and receiver (response regulator) domains. Recent studies implicate phosphorelay systems in sensing and propagating signals from a wide variety of external and/or internal stimuli such as ethylene, cytokinin, and osmolarity. In maize and Arabidopsis, some response regulators are up-regulated by both cytokinins and nitrate. These findings imply that the His-Asp phosphorelay may operate in an inorganic nitrogen-signaling pathway mediated by cytokinin in plants.