title: Bacterial polysaccharides suppress induced innate immunity by calcium chelation.
abstract: Bacterial pathogens and symbionts must suppress or negate host innate immunity. However, pathogens release conserved oligomeric and polymeric molecules or MAMPs (Microbial Associated Molecular Patterns), which elicit host defenses [1], [2] and [3]. Extracellular polysaccharides (EPSs) are key virulence factors in plant and animal pathogenesis, but their precise function in establishing basic compatibility remains unclear [4], [5], [6] and [7]. Here, we show that EPSs suppress MAMP-induced signaling in plants through their polyanionic nature [4] and consequent ability to chelate divalent calcium ions [8]. In plants, Ca2+ ion influx to the cytosol from the apoplast (where bacteria multiply [4], [5] and [9]) is a prerequisite for activation of myriad defenses by MAMPs [10]. We show that EPSs from diverse plant and animal pathogens and symbionts bind calcium. EPS-defective mutants or pure MAMPs, such as the flagellin peptide flg22, elicit calcium influx, expression of host defense genes, and downstream resistance. Furthermore, EPSs, produced by wild-type strains or purified, suppress induced responses but do not block flg22-receptor binding in Arabidopsis cells. EPS production was confirmed in planta, and the amounts in bacterial biofilms greatly exceed those required for binding of apoplastic calcium. These data reveal a novel, fundamental role for bacterial EPS in disease establishment, encouraging novel control strategies.
authors.affiliation: Department of Biology & Biochemistry, University of Bath, Bath BA2 7AY, UK.
authors.forename: Shazia N
authors.initials: SN
authors.lastname: Aslam
authors.forename: Mari-Anne
authors.initials: MA
authors.lastname: Newman
authors.forename: Gitte
authors.initials: G
authors.lastname: Erbs
authors.forename: Kate L
authors.initials: KL
authors.lastname: Morrissey
authors.forename: Delphine
authors.initials: D
authors.lastname: Chinchilla
authors.forename: Thomas
authors.initials: T
authors.lastname: Boller
authors.forename: Tina Tandrup
authors.initials: TT
authors.lastname: Jensen
authors.forename: Cristina
authors.initials: C
authors.lastname: De Castro
authors.forename: Teresa
authors.initials: T
authors.lastname: Ierano
authors.forename: Antonio
authors.initials: A
authors.lastname: Molinaro
authors.forename: Robert W
authors.initials: RW
authors.lastname: Jackson
authors.forename: Marc R
authors.initials: MR
authors.lastname: Knight
authors.forename: Richard M
authors.initials: RM
authors.lastname: Cooper
mesh_terms: D017360:Arabidopsis / Q000276:immunology / Q000382:microbiology; D001419:Bacteria / Q000472:pathogenicity; D020013:Calcium Signaling / Q000187:drug effects / Q000276:immunology; D007113:Immunity, Innate / Q000187:drug effects; D010935:Plant Diseases / Q000276:immunology / Q000382:microbiology; D010944:Plants / Q000187:drug effects / Q000276:immunology* / Q000382:microbiology*; D011135:Polysaccharides, Bacterial / Q000633:toxicity*; D014774:Virulence / Q000276:immunology; D016959:Xanthomonas campestris / Q000472:pathogenicity
chemical_list: D011135:Polysaccharides, Bacterial