Ultrastructure of Lesion Formation in Rhizoctonia-Infected Bean Hypocotyls

Abstract

Hypocotyls of P. vulgaris ''Red Kidney'' were inoculated with R. solani a fungus known to produce endopolygalacturonase and other enzymes capable of degrading cell wall polymers. Young, intermediate and mature lesions together with the tissue adjacent to lesions were examined by EM. Hyphae within infection cushions were surrounded by a moderately dense material, tentatively described as mucilaginous. Penetration did not involve swelling or changes in staining properties of the cuticle. Epidermal and cortex cell walls near the pathogen were swollen and their cytoplasm was collapsed. At a distance of 2-4 cells from the pathogen, the cytoplasm accumulated toward the infection. Walls of these cells were moderately swollen and the middle lamella was densely stained and granulated. The plasmalemma was retracted from the walls and other subtle cytoplasmic changes were evident. Cytoplasmic changes also were found in cells with densely stained walls showing no obvious swelling. Changes in fungal ultrastructure were uncommon except in mature lesions. The results suggest that changes in host structure and physiology associated with this disease are not caused solely by wall degradation and the consequent osmotic bursting of protoplasts.