title: Two direct targets of cytokinin signaling regulate symbiotic nodulation in Medicago truncatula.
abstract: Cytokinin regulates many aspects of plant development, and in legume crops, this phytohormone is necessary and sufficient for symbiotic nodule organogenesis, allowing them to fix atmospheric nitrogen. To identify direct links between cytokinins and nodule organogenesis, we determined a consensus sequence bound in vitro by a transcription factor (TF) acting in cytokinin signaling, the nodule-enhanced Medicago truncatula Mt RR1 response regulator (RR). Among genes rapidly regulated by cytokinins and containing this so-called RR binding site (RRBS) in their promoters, we found the nodulation-related Type-A RR Mt RR4 and the Nodulation Signaling Pathway 2 (NSP2) TF. Site-directed mutagenesis revealed that RRBS cis-elements in the RR4 and NSP2 promoters are essential for expression during nodule development and for cytokinin induction. Furthermore, a microRNA targeting NSP2 (miR171 h) is also rapidly induced by cytokinins and then shows an expression pattern anticorrelated with NSP2. Other primary targets regulated by cytokinins depending on the Cytokinin Response1 (CRE1) receptor were a cytokinin oxidase/dehydrogenase (CKX1) and a basic Helix-Loop-Helix TF (bHLH476). RNA interference constructs as well as insertion of a Tnt1 retrotransposon in the bHLH gene led to reduced nodulation. Hence, we identified two TFs, NSP2 and bHLH476, as direct cytokinin targets acting at the convergence of phytohormonal and symbiotic cues.
authors.affiliation: Institut des Sciences du Végétal, Centre National de la Recherche Scientifique, F-91198 Gif-sur-Yvette cedex, France.
authors.forename: Federico
authors.initials: F
authors.lastname: Ariel
authors.forename: Marianne
authors.initials: M
authors.lastname: Brault-Hernandez
authors.forename: Carole
authors.initials: C
authors.lastname: Laffont
authors.forename: Emeline
authors.initials: E
authors.lastname: Huault
authors.forename: Mathias
authors.initials: M
authors.lastname: Brault
authors.forename: Julie
authors.initials: J
authors.lastname: Plet
authors.forename: Michael
authors.initials: M
authors.lastname: Moison
authors.forename: Sandrine
authors.initials: S
authors.lastname: Blanchet
authors.forename: Jean Laurent
authors.initials: JL
authors.lastname: Ichanté
authors.forename: Mireille
authors.initials: M
authors.lastname: Chabaud
authors.forename: Sébastien
authors.initials: S
authors.lastname: Carrere
authors.forename: Martin
authors.initials: M
authors.lastname: Crespi
authors.forename: Raquel L
authors.initials: RL
authors.lastname: Chan
authors.forename: Florian
authors.initials: F
authors.lastname: Frugier
mesh_terms: D000595:Amino Acid Sequence; D016384:Consensus Sequence; D003583:Cytokinins / Q000494:pharmacology*; D020869:Gene Expression Profiling; D018506:Gene Expression Regulation, Plant; D046913:Medicago truncatula / Q000187:drug effects / Q000235:genetics / Q000382:microbiology / Q000502:physiology*; D035683:MicroRNAs / Q000235:genetics; D008969:Molecular Sequence Data; D016254:Mutagenesis, Insertional; D016297:Mutagenesis, Site-Directed; D009586:Nitrogen Fixation; D020411:Oligonucleotide Array Sequence Analysis; D010802:Phylogeny; D010937:Plant Growth Regulators / Q000494:pharmacology*; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism; D055170:Plant Root Nodulation / Q000235:genetics*; D011401:Promoter Regions, Genetic / Q000235:genetics; D053204:Root Nodules, Plant / Q000187:drug effects / Q000235:genetics / Q000382:microbiology / Q000502:physiology; D036226:Seedlings / Q000187:drug effects / Q000235:genetics / Q000382:microbiology / Q000502:physiology; D016415:Sequence Alignment; D015398:Signal Transduction; D016962:Sinorhizobium meliloti / Q000502:physiology*; D013559:Symbiosis; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism*; D059467:Transcriptome
chemical_list: D003583:Cytokinins; D035683:MicroRNAs; D010937:Plant Growth Regulators; D010940:Plant Proteins; D014157:Transcription Factors