The Arabidopsis sensor His-kinase, AHK4, can respond to cytokinins

His-to-Asp (His→Asp) phosphorelay mechanisms are presumably involved in propagation of certain environmental stimuli, including phytohormones, in Arabidopsis thaliana. In addition to the previously characterized His-kinases, namely, the ETR1 family of ethylene receptors, CKI1 cytokinin-sensor, and ATHK1 osomo-sensor, this higher plant has three more His-kinases (named AHK2, AHK3, and AHK4). By employing the well-known His→Asp phosphorelay systems in both the fission yeast and Escherichia coli, evidence is presented showing that the AHK4 His-kinase has an ability to serve as a cytokinin-responsive environmental sensor. Taking advantage of this AHK4-dependent His→Asp phosphorelay system in E. coli, a phosphorelay interaction between the Arabidopsis His-kinase and histidine-containing phosphotransmitters (AHPs) was also demonstrated for the first time.