Antifungal effect of bean endochitinase on Rhizoctonia solani: ultrastructural changes and cytochemical aspects of chitin breakdown

Abstract

A chitinase, purified to homogeneity from ethylene-treated bean leaves, was applied to actively growing mycelial cells of Rhizoctonia solani to evaluate a potential antifungal activity. Light microscopic investigations at 30-min intervals following enzyme exposure revealed the induction of morphological changes such as swelling of hyphal tips and hyphal distortions. More precise information concerning fungal cell alteration was obtained by ultrastructural observation and cytochemical detection of chitin distribution in fungal cell walls. Chitin breakdown was found to be an early event preceding wall disruption and cytoplasm leakage. The large amounts of chitin present in the walls of control R. solani cells and the rapid chitin hydrolysis upon chitinase treatment lead us to suggest that this polysaccharide is one of the main components of this fungal cell wall and is readily accessible to chitinase, especially in the apical zone. By 60 min after enzyme treatment, labeled molecules were observed in the vicinity of some fungal cells, suggesting the release of chitin oligosaccharides from fungal cell walls. The antifungal activity of the bean chitinase on cells of R. solani grown in culture is discussed in relation to the potential of genetically modified transgenic plants to resist attack by R. solani through an antimicrobial activity in planta.Key words: gold labeling, wheat germ agglutinin, cytochemistry, Rhizoctonia solani, bean endochitinase.