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Keywords:

  • Peronospora parasitica;
  • conidia;
  • appressoria;
  • extracellular matrix;
  • adhesion;
  • infection structures;
  • lectins;
  • β-1;
  • 3-glucans

Abstract

  • • 
    Differences are shown here in the structure and composition of the extracellular matrices surrounding conidia, germ-tubes and appressoria of the downy mildew Peronospora parasitica, which is a serious pathogen of several cultivated Brassica spp.
  • • 
    The extracellular matrices of germlings growing in vitro (glass and polycarbonate substrata) were investigated using freeze-substitution transmission electron microscopy, lectin cytochemistry and immunogold labelling.
  • • 
    The cell surface carbohydrates present on P. parasitica conidia differed markedly from those on germ-tubes and appressoria. The conidial cell wall comprised an inner electron-lucent layer containing β-1,3-glucans, and an outer electron-opaque layer containing carbohydrates recognized by Bauhinia purpurea agglutinin.
  • • 
    Germ-tubes and appressoria release two types of extracellular matrix: a fibrillar matrix containing β-1,3-glucans is confined to the germling-substratum interface; a second matrix containing protein spreads beyond the contact interface as a thin film. The tenacious adherence of both types of matrix to the substratum after mechanical removal of the germlings suggests that they may contribute to germling attachment.