title: Cytokinins regulate a bidirectional phosphorelay network in Arabidopsis.
abstract: The cytokinin class of plant hormones plays key roles in regulating diverse developmental and physiological processes. Arabidopsis perceives cytokinins with three related and partially redundant receptor histidine kinases (HKs): CRE1 (the same protein as WOL and AHK4), AHK2, and AHK3 (CRE-family receptors). It is suggested that binding of cytokinins induces autophosphorylation of these HKs and subsequent transfer of the phosphoryl group to a histidine phosphotransfer protein (HPt) and then to a response regulator (RR), ultimately regulating downstream signaling events. Here we demonstrate that, in vitro and in a yeast system, CRE1 is not only a kinase that phosphorylates HPts in the presence of cytokinin but is also a phosphatase that dephosphorylates HPts in the absence of cytokinin. To explore the roles of these activities in planta, we replaced CRE1 with mutant versions of the gene or with AHK2. Replacing CRE1 with CRE1(T278I), which lacks cytokinin binding activity and is locked in the phosphatase form, decreased cytokinin sensitivity. Conversely, replacing CRE1 with AHK2, which favors kinase activity, increased cytokinin sensitivity. These results indicate that in the presence of cytokinins, cytokinin receptors feed phosphate to phosphorelay-integrating HPt proteins. In the absence of cytokinins, CRE1 removes phosphate from HPt proteins, decreasing the system phosphoload.
authors.affiliation: Plant Molecular Biology Laboratory, Institute of Biotechnology, University of Helsinki, Post office box 56, FIN-00014 Helsinki, Finland.
authors.forename: Ari Pekka
authors.initials: AP
authors.lastname: Mähönen
authors.forename: Masayuki
authors.initials: M
authors.lastname: Higuchi
authors.forename: Kirsi
authors.initials: K
authors.lastname: Törmäkangas
authors.forename: Kaori
authors.initials: K
authors.lastname: Miyawaki
authors.forename: Melissa S
authors.initials: MS
authors.lastname: Pischke
authors.forename: Michael R
authors.initials: MR
authors.lastname: Sussman
authors.forename: Ykä
authors.initials: Y
authors.lastname: Helariutta
authors.forename: Tatsuo
authors.initials: T
authors.lastname: Kakimoto
mesh_terms: D017360:Arabidopsis / Q000166:cytology / Q000201:enzymology* / Q000235:genetics; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D003583:Cytokinins / Q000502:physiology*; D000071677:Histidine Kinase; D008954:Models, Biological; D009154:Mutation; D010766:Phosphorylation; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D011956:Receptors, Cell Surface / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D015398:Signal Transduction; D015003:Yeasts / Q000235:genetics
chemical_list: D029681:Arabidopsis Proteins; D003583:Cytokinins; D011956:Receptors, Cell Surface; D011494:Protein Kinases; D000071677:Histidine Kinase; C000605492:AHK2 protein, Arabidopsis; C000605493:AHK3 protein, Arabidopsis; C421783:WOL protein, Arabidopsis