title: The Arabidopsis thaliana response regulator ARR22 is a putative AHP phospho-histidine phosphatase expressed in the chalaza of developing seeds.
abstract: BACKGROUND
  The Arabidopsis response regulator 22 (ARR22) is one of two members of a recently defined novel group of two-component system (TCS) elements. TCSs are stimulus perception and response modules of prokaryotic origin, which signal by a His-to-Asp phosphorelay mechanism. In plants, TCS regulators are involved in hormone response pathways, such as those for cytokinin and ethylene. While the functions of the other TCS elements in Arabidopsis, such as histidine kinases (AHKs), histidine-containing phosphotransfer proteins (AHPs) and A-type and B-type ARRs are becoming evident, the role of ARR22 is poorly understood.
  
  
  RESULTS
  We present evidence that ARR22 is a preferentially cytoplasmic protein, exclusively expressed in the chalaza of developing seeds. ARR22 specifically interacts with AHP2, AHP3 and AHP5 in yeast and living plant cells. Two new loss-of-function alleles, arr22-2 and arr22-3, were isolated and characterized. With respect to their morphology and metabolite status, no significant difference in the developing seeds of the arr22 mutants was observed compared to wild type. The genetic complementation of the arr22 mutants with a genomic ARR22 fragment resulted in plants (arr22/gARR22) with a pleiotropic phenotype of different penetrance. This phenotype was not observed when the phosphorylatable Asp74 of ARR22 was changed to either a dominant-active Glu or a dominant-inactive Asn. The phenotype of the arr22/gARR22 plants was comparable to that of multiple ahk, ahp and B-type arr mutants.
  
  
  CONCLUSIONS
  Our results favor the model that ARR22 acts as a phospho-histidine phosphatase on specific AHPs in the cytoplasm of Arabidopsis chalaza cells. The lack of any aberrant morphological and metabolite phenotype in the seeds of the arr22 mutants indicates that ARR22 is probably primarily responsible for the fine tuning of specific branches of chalaza-based TCS signalling. Even when slightly mis-expressed, ARR22 interferes with hormone homeostasis in non-chalaza tissues. Our data indicate that the chromatin status might play a crucial role in maintaining the chalaza-restricted expression of ARR22.
authors.affiliation: Zentrum für Molekularbiologie der Pflanzen/Pflanzenphysiologie, Universität Tübingen, Auf der Morgenstelle 1, D-72076 Tübingen, Germany. jakub.horak@zmbp.uni-tuebingen.de
authors.forename: Jakub
authors.initials: J
authors.lastname: Horák
authors.forename: Christopher
authors.initials: C
authors.lastname: Grefen
authors.forename: Kenneth W
authors.initials: KW
authors.lastname: Berendzen
authors.forename: Achim
authors.initials: A
authors.lastname: Hahn
authors.forename: York-Dieter
authors.initials: YD
authors.lastname: Stierhof
authors.forename: Bettina
authors.initials: B
authors.lastname: Stadelhofer
authors.forename: Mark
authors.initials: M
authors.lastname: Stahl
authors.forename: Csaba
authors.initials: C
authors.lastname: Koncz
authors.forename: Klaus
authors.initials: K
authors.lastname: Harter
mesh_terms: D017360:Arabidopsis / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000235:genetics* / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D005656:Fungal Proteins / Q000378:metabolism; D018507:Gene Expression Regulation, Developmental; D018506:Gene Expression Regulation, Plant; D005816:Genetic Complementation Test; D010641:Phenotype; D010766:Phosphorylation; D010770:Phosphotransferases / Q000235:genetics / Q000378:metabolism; D030821:Plants, Genetically Modified / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism; D011485:Protein Binding; D012639:Seeds / Q000235:genetics* / Q000254:growth & development / Q000378:metabolism; D015398:Signal Transduction / Q000235:genetics / Q000502:physiology
chemical_list: C117967:AHP2 protein, Arabidopsis; C117968:AHP3 protein, Arabidopsis; C493632:ARR22 protein, Arabidopsis; D029681:Arabidopsis Proteins; D005656:Fungal Proteins; D010770:Phosphotransferases