title: Identification of five B-type response regulators as members of a multistep phosphorelay system interacting with histidine-containing phosphotransfer partners of Populus osmosensor.
abstract: BACKGROUND
  In plants, the multistep phosphorelay signaling pathway mediates responses to environmental factors and plant hormones. This system is composed of three successive partners: hybrid Histidine-aspartate Kinases (HKs), Histidine-containing Phosphotransfer proteins (HPts), and Response Regulators (RRs). Among the third partners, B-type RR family members are the final output elements of the pathway; they act as transcription factors and clearly play a pivotal role in the early response to cytokinin in Arabidopsis. While interactions studies between partners belonging to the multistep phosphorelay system are mainly focused on protagonists involved in cytokinin or ethylene pathways, very few reports are available concerning partners of osmotic stress signaling pathway.
  
  
  RESULTS
  In Populus, we identified eight B-type RR proteins, RR12-16, 19, 21 and 22 in the Dorskamp genotype. To assess HPt/B-type RR interactions and consequently determine potential third partners in the osmosensing multistep phosphorelay system, we performed global yeast two-hybrid (Y2H) assays in combination with Bimolecular Fluorescence Complementation (BiFC) assays in plant cells. We found that all B-type RRs are able to interact with HPt predominant partners (HPt2, 7 and 9) of HK1, which is putatively involved in the osmosensing pathway. However, different profiles of interaction are observed depending on the studied HPt. HPt/RR interactions displayed a nuclear localization, while the nuclear and cytosolic localization of HPt and nuclear localization of RR proteins were validated. Although the nuclear localization of HPt/RR interaction was expected, this work constitutes the first evidence of such an interaction in plants. Furthermore, the pertinence of this partnership is reinforced by highlighting a co-expression of B-type RR transcripts and the other partners (HK1 and HPts) belonging to a potential osmosensing pathway.
  
  
  CONCLUSIONS
  Based on the interaction studies between identified B-type RR and HPt proteins, and the co-expression analysis of transcripts of these potential partners in poplar organs, our results favor the model that RR12, 13, 14, 16 and 19 are able to interact with the main partners of HK1, HPt2, 7 and 9, and this HPt/RR interaction occurs within the nucleus. On the whole, the five B-type RRs of interest could be third protagonists putatively involved in the osmosensing signaling pathway in Populus.
authors.affiliation: Université d'Orléans, UFR-Faculté des Sciences, UPRES EA 1207, Laboratoire de Biologie des Ligneux et des Grandes Cultures (LBLGC), BP 6759, Orléans, F-45067, France.
authors.forename: Lucie
authors.initials: L
authors.lastname: Bertheau
authors.forename: Françoise
authors.initials: F
authors.lastname: Chefdor
authors.forename: Grégory
authors.initials: G
authors.lastname: Guirimand
authors.forename: Vincent
authors.initials: V
authors.lastname: Courdavault
authors.forename: Christiane
authors.initials: C
authors.lastname: Depierreux
authors.forename: Domenico
authors.initials: D
authors.lastname: Morabito
authors.forename: Franck
authors.initials: F
authors.lastname: Brignolas
authors.forename: François
authors.initials: F
authors.lastname: Héricourt
authors.forename: Sabine
authors.initials: S
authors.lastname: Carpin
mesh_terms: D002467:Cell Nucleus / Q000378:metabolism; D018076:DNA, Complementary / Q000235:genetics; D018744:DNA, Plant / Q000235:genetics; D006639:Histidine / Q000378:metabolism*; D000071677:Histidine Kinase; D005810:Multigene Family; D010802:Phylogeny; D010937:Plant Growth Regulators; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D032107:Populus / Q000235:genetics* / Q000502:physiology; D011494:Protein Kinases / Q000378:metabolism; D015398:Signal Transduction; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D059467:Transcriptome; D020798:Two-Hybrid System Techniques; D014867:Water / Q000502:physiology
chemical_list: D018076:DNA, Complementary; D018744:DNA, Plant; D010937:Plant Growth Regulators; D010940:Plant Proteins; D014157:Transcription Factors; D014867:Water; D006639:Histidine; D011494:Protein Kinases; D000071677:Histidine Kinase