Ultrastructural localization of β-(1 → 4)-D-glucans in two pathogenic fungi and in their host tissues by means of an exoglucanase–gold complex

Abstract

An exoglucanase, purified from a cellulase produce by the fungus Trichoderma harzianum Rifai., was successfully bound to colloidal gold and used for ultrastructural detection of intracellular cellulosic β-(1 → 4) glucans. These saccharides were found to be present in great amount in the walls of Ophiostoma ulmi (Buism.) Nannf., the Dutch elm disease agent, whereas they were randomly distributed in the walls of Fusarium oxysporum Schlecht f. sp. radicis-lycopersici Jarvis and Shoemaker (FORL), the agent of tomato crown and root rot. In O. ulmi cell walls, the β-(1 → 4) glucans were predominantly concentrated over the central portion of the inner walls. In both colonized elm wood and infected tomato root tissues, the compound middle lamella and secondary walls of parenchyma cells, fibers (absent in tomato roots), and vessel members were always intensely labeled, but gold particles appeared somewhat irregularly distributed. In fibers having an S₃ gelatinous layer, the latter was always preferentially labeled. Penetration of O. ulmi in elm wood tissues resulted in the digestion of host wall areas free of labeling. Such areas were not observed in infected tomato tissues; instead, an accumulation of gold particles was noted along the fungal portion that was in close contact with the host wall. Results of this study confirm the potential value of gold-labeled exoglucanase and provide some new information about wall topochemistry during host-pathogen interactions.