Ultrastructure of the Colletotrichum trifolii-Medicago sativa pathosystem. II. Post-penetration events

Light and electron microscopy were used to compare the development of compatible with incompatible interactions in the Colletotrichum trifolii-Medicago sativa pathosystem. The host responses to infection by races 1 or 2 of the pathogen were similar. Differential host reactions became evident only after the fungal penetration peg entered the epidermal cell wall. On both resistant and susceptible plants, papillae and wall thickenings formed in the initially infected epidermal cells. Although histochemically different, the papillae failed to block pathogen ingress in either case. In both resistant and susceptible plants there was an apparent increase in the cytoplasmic: vacuolar ratio; however, the difference was more dramatic in the latter where the central vacuole almost disappeared. Penetration of the host plasmalemma resulted in the rapid death of the epidermal cell in all tested alfalfa plants. In incompatible interactions the massive deposition of phenolic and related compounds in and around the necrotic cell resulted, simultaneously, in necrosis of the fungal infection vesicle within and protection of adjacent plant tissues outside the target cell. In compatible interactions this did not occur. The cytoplasm of the target cell and its neighbours was completely degraded by autolysis while the fungus continued to proliferate into adjacent degenerate epidermal and cortical tissues.