Bacterial polysaccharides suppress induced innate immunity by calcium chelation

Aslam, S.N., Newman, M.-A. , Erbs, G., Morrissey, K.L., Chinchilla, D. , Boller, T. , Jensen, T.T. , De Castro, C. , Ierano, T. , Molinaro, A. , Jackson, R.W., Knight, M.R. and Cooper, R.M. (2008) Bacterial polysaccharides suppress induced innate immunity by calcium chelation. Current Biology, 18 (14). pp. 1078-83. ISSN 1053-1144 doi: 10.1016/j.cub.2008.06.061

Abstract/Summary

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.