title: SCARN a Novel Class of SCAR Protein That Is Required for Root-Hair Infection during Legume Nodulation.
abstract: Rhizobial infection of legume root hairs requires a rearrangement of the actin cytoskeleton to enable the establishment of plant-made infection structures called infection threads. In the SCAR/WAVE (Suppressor of cAMP receptor defect/WASP family verpolin homologous protein) actin regulatory complex, the conserved N-terminal domains of SCAR proteins interact with other components of the SCAR/WAVE complex. The conserved C-terminal domains of SCAR proteins bind to and activate the actin-related protein 2/3 (ARP2/3) complex, which can bind to actin filaments catalyzing new actin filament formation by nucleating actin branching. We have identified, SCARN (SCAR-Nodulation), a gene required for root hair infection of Lotus japonicus by Mesorhizobium loti. Although the SCARN protein is related to Arabidopsis thaliana SCAR2 and SCAR4, it belongs to a distinct legume-sub clade. We identified other SCARN-like proteins in legumes and phylogeny analyses suggested that SCARN may have arisen from a gene duplication and acquired specialized functions in root nodule symbiosis. Mutation of SCARN reduced formation of infection-threads and their extension into the root cortex and slightly reduced root-hair length. Surprisingly two of the scarn mutants showed constitutive branching of root hairs in uninoculated plants. However we observed no effect of scarn mutations on trichome development or on the early actin cytoskeletal accumulation that is normally seen in root hair tips shortly after M. loti inoculation, distinguishing them from other symbiosis mutations affecting actin nucleation. The C-terminal domain of SCARN binds to ARPC3 and ectopic expression of the N-terminal SCAR-homology domain (but not the full length protein) inhibited nodulation. In addition, we found that SCARN expression is enhanced by M. loti in epidermal cells and that this is directly regulated by the NODULE INCEPTION (NIN) transcription factor.
authors.affiliation: National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China.
authors.forename: Liping
authors.initials: L
authors.lastname: Qiu
authors.affiliation: National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China.
authors.forename: Jie-Shun
authors.initials: JS
authors.lastname: Lin
authors.affiliation: National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China.
authors.forename: Ji
authors.initials: J
authors.lastname: Xu
authors.affiliation: Kazusa DNA Research Institute, Kisarazu, Japan; Graduate School of Life Sciences, Tohoku University, Sendai, Japan.
authors.forename: Shusei
authors.initials: S
authors.lastname: Sato
authors.affiliation: University of Munich LMU, Faculty of Biology, Martinsried, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Parniske
authors.affiliation: John Innes Centre, Norwich, United Kingdom.
authors.forename: Trevor L
authors.initials: TL
authors.lastname: Wang
authors.affiliation: John Innes Centre, Norwich, United Kingdom.
authors.forename: J Allan
authors.initials: JA
authors.lastname: Downie
authors.affiliation: National Key Laboratory of Plant Molecular Genetics, Institute of Plant Physiology and Ecology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China.
authors.forename: Fang
authors.initials: F
authors.lastname: Xie
mesh_terms: D051376:Actin-Related Protein 2-3 Complex / Q000235:genetics; D000199:Actins / Q000235:genetics; D017360:Arabidopsis / Q000235:genetics; D007887:Fabaceae / Q000235:genetics / Q000382:microbiology; D018506:Gene Expression Regulation, Plant; D000070116:Lotus / Q000235:genetics* / Q000382:microbiology; D059128:Mesorhizobium / Q000472:pathogenicity; D010935:Plant Diseases / Q000235:genetics* / Q000382:microbiology; D010940:Plant Proteins / Q000096:biosynthesis / Q000235:genetics*; D055170:Plant Root Nodulation / Q000235:genetics*; D018517:Plant Roots / Q000235:genetics / Q000382:microbiology; D013559:Symbiosis / Q000235:genetics
chemical_list: D051376:Actin-Related Protein 2-3 Complex; D000199:Actins; C403406:NIN protein, Lotus japonicus; D010940:Plant Proteins