ULTRASTRUCTURE AND COMPOSITION OF THE CELL-SURFACES OF INFECTION STRUCTURES FORMED BY THE FUNGAL PLANT PATHOGEN COLLETOTRICHUM-LINDEMUTHIANUM

A variety of microscopical techniques and molecular probes have been u sed to study the ultrastructure and composition of the cell surfaces o f the conidia (i.e. spores) and infection structures produced by the h emibiotrophic fungal plant pathogen Colletotrichum lindemuthianum. The fungal conidium germinates to produce a germ-tube, the tip of which s wells to produce a domed, melanized appressorium which adheres firmly to the plant surface. Penetration of the cuticle and cell wall is foll owed by the development of a biotrophic intracellular hypha, which is surrounded by an invagination of the host plasma membrane. Freeze-subs titution of C. lindemuthianum germlings showed that conidia are coated with a dense layer of fibrillar material, This 'spore coat' contains irregularly shaped pores. giving it a reticular appearance. Negative s taining of germlings revealed the presence of numerous long, flexuous fibres or fimbriae, protruding from the surfaces of germ-tubes and app ressoria. Colloidal gold was used to visualize fungal extracellular pr oteins. The colloidal gold stained a fibrillar sheath around germ-tube s, whereas appressoria were surrounded by a halo, comprising an inner unstained region and a stained perimeter, The carbohydrate composition of the cell surfaces of the conidia and infection structures was stud ied by labelling cells with rhodamine-and fluorescein-conjugated lecti ns. The results showed that the extracellular matrices of germ-tubes a nd appressoria are very similar in composition, but differ from those of conidia and intracellular hyphae, Monoclonal antibodies have been p repared to germlings and infection structures of C. lindemuthianum and their use has provided further evidence that the extracellular matric es around germ-tubes and appressoria have several glycoproteins in com mon. The results also show that the cell surface of C. lindemuthianum becomes specialized during biotrophic development inside host cells.