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 extracel lular matrices surrounding conidia, germ-tubes and appressoria of the downy mildew Peronospora parasitica, which is a serious pathogen of several cult ivated Brassica spp. The extracellular matrices of germlings growing in vitro (glass and polycar bonate substrata) were investigated using freeze-substitution transmission electron microscopy, lectin cytochemistry and immunogold labelling. The cell surface carbohydrates present on P. parasitica conidia differed ma rkedly from those on germ-tubes and appressoria. The conidial cell wall com prised 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 fib rillar matrix containing beta -1,3-glucans is confined to the germling-subs tratum interface; a second matrix containing protein spreads beyond the con tact interface as a thin film. The tenacious adherence of both types of mat rix to the substratum after mechanical removal of the germlings suggests th at they may contribute to germling attachment.