Surface characteristics of necrotrophic secondary hyphae produced by the bean anthracnose fungus, Colletotrichum lindemuthianum

During infection of bean (Phaseolus vulgaris), the hemibiotrophic anthracnose pathogen, Colletotrichum lindemuthianum, initially produces biotrophic primary hyphae that are large-diameter and entirely intracellular, followed by necrotrophic secondary hyphae that are narrower and either intercellular or intracellular. In the present study, transmission electron microscopy of infected tissues prepared by high-pressure freezing and freeze-substitution showed that secondary hyphae have much thinner cell walls (25–40 nm) than primary hyphae (100–130 nm) and are not surrounded by an extracellular matrix. Immunofluorescence labelling with a panel of monoclonal antibodies showed that glycoproteins which are present on conidia, germ-tubes, appressoria, primary hyphae and mycelium grown in vitro are absent from the surface of secondary hyphae. Chitin, detected with the lectin wheat germ agglutinin, was the only surface component shared by secondary hyphae and the other fungal cell types. The results suggest that the fungal cell surface becomes modified during necrotrophic growth, with none of the glycoproteins associated with earlier stages of the infection process being produced.