title: Functional characterization of type-B response regulators in the Arabidopsis cytokinin response.
abstract: Cytokinins play critical roles in plant growth and development, with the transcriptional response to cytokinin being mediated by the type-B response regulators. In Arabidopsis (Arabidopsis thaliana), type-B response regulators (ARABIDOPSIS RESPONSE REGULATORS [ARRs]) form three subfamilies based on phylogenic analysis, with subfamily 1 having seven members and subfamilies 2 and 3 each having two members. Cytokinin responses are predominantly mediated by subfamily 1 members, with cytokinin-mediated effects on root growth and root meristem size correlating with type-B ARR expression levels. To determine which type-B ARRs can functionally substitute for the subfamily 1 members ARR1 or ARR12, we expressed different type-B ARRs from the ARR1 promoter and assayed their ability to rescue arr1 arr12 double mutant phenotypes. ARR1, as well as a subset of other subfamily 1 type-B ARRs, restore the cytokinin sensitivity to arr1 arr12. Expression of ARR10 from the ARR1 promoter results in cytokinin hypersensitivity and enhances shoot regeneration from callus tissue, correlating with enhanced stability of the ARR10 protein compared with the ARR1 protein. Examination of transfer DNA insertion mutants in subfamilies 2 and 3 revealed little effect on several well-characterized cytokinin responses. However, a member of subfamily 2, ARR21, restores cytokinin sensitivity to arr1 arr12 roots when expressed from the ARR1 promoter, indicating functional conservation of this divergent family member. Our results indicate that the type-B ARRs have diverged in function, such that some, but not all, can complement the arr1 arr12 mutant. In addition, our results indicate that type-B ARR expression profiles in the plant, along with posttranscriptional regulation, play significant roles in modulating their contribution to cytokinin signaling.
authors.affiliation: Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA.
authors.forename: Kristine
authors.initials: K
authors.lastname: Hill
authors.forename: Dennis E
authors.initials: DE
authors.lastname: Mathews
authors.forename: Hyo Jung
authors.initials: HJ
authors.lastname: Kim
authors.forename: Ian H
authors.initials: IH
authors.lastname: Street
authors.forename: Sarah L
authors.initials: SL
authors.lastname: Wildes
authors.forename: Yi-Hsuan
authors.initials: YH
authors.lastname: Chiang
authors.forename: Michael G
authors.initials: MG
authors.lastname: Mason
authors.forename: Jose M
authors.initials: JM
authors.lastname: Alonso
authors.forename: Joseph R
authors.initials: JR
authors.lastname: Ecker
authors.forename: Joseph J
authors.initials: JJ
authors.lastname: Kieber
authors.forename: G Eric
authors.initials: GE
authors.lastname: Schaller
mesh_terms: D017360:Arabidopsis / Q000187:drug effects / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D003583:Cytokinins / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant; D005810:Multigene Family; D016254:Mutagenesis, Insertional; D010641:Phenotype; D010937:Plant Growth Regulators / Q000378:metabolism*; D018517:Plant Roots / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D030821:Plants, Genetically Modified; D018749:RNA, Plant / Q000235:genetics; D012045:Regulatory Sequences, Nucleic Acid; D036226:Seedlings / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology*; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D019076:Transgenes
chemical_list: D029681:Arabidopsis Proteins; D003583:Cytokinins; D010937:Plant Growth Regulators; D018749:RNA, Plant; D014157:Transcription Factors