Production of extracellular matrices during development of infection structures by the downy mildew Peronospora parasitica

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 beta -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 beta -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.