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.

A survey of the initial infection phases of Fusarium oxysporum Schlecht. f. sp. radicis-lycopersici Jarvis and Shoemaker in tomato roots demonstrated that the epidermis was colonized from 12 to 24 h after inoculation. Until 96 h the pathogen was usually limited to the outer cortical area, where fungal cells were found to be either intercellular or intracellular. Host cell wall thickenings and papilla formation were noticeable in the cortical cells but totally absent in the endodermis and the vascular stele. In the cortical area, cytoplasm and walls of affected host cells were mostly disintegrated when the whole root tissues were colonized about 144 h after inoculation. Between 96 and 120 h, hyphae were visible in the endodermis, and 24 h later the vascular stele was colonized. In the latter area, parenchyma cells generally reacted as in the inner cortical area and invasion of vessels proceeded directly through middle lamella and pit membranes. When colonized, vessels contained fibrillogranular material interspersed with bubbles and an osmiophilic coating material. This coating material lined the vessel secondary wall and pit cavities and appeared thicker in the more occluded vessels. The possible implications of these observations on symptoms expression in this and similar plant diseases are discussed.

Aplysia gonad lectin, a polygalacturonic acid-binding lectin isolated from the sea mollusc Aplysia depilans, was complexed to colloidal gold and used for localizing polygalacturonic-acidcontaining molecules in tomato root tissues infected with Fusarium oxysporum f. sp. radicis-lycopersici (FORL). Colonization of host tissues by FORL was associated with striking wall modifications including disruption and even loss of middle lamellae. According to the labeling pattern observed in host wall areas adjacent to fungal penetration channels, it is likely that FORL pectolytic enzymes act through localized wall degradation. The release of polygalacturonic acid-rich wall fragments and the accumulation of polygalacturonic acid-containing molecules in some altered phloem cells were frequently observed and considered to be specific host reactions to fungal attack. The heavy deposition of such molecules at strategic sites such as wall oppositions and intercellular spaces provides support to their implication in the plant defense system. The possible interrelation between polygalacturonic acid-containing molecules and other polymers such as lignin and phenolic compounds remains to be investigated further. The role of these molecules in host-pathogen interactions is discussed in relation to plant defense.

A cytochemical technique for the ultrastructural localization of chitin in tomato root cells infected by Fusarium oxysporum f. sp. radicis-lycopersici is reported. Chitinase was complexed to colloidal gold and thin sections were incubated with the enzyme-gold complex. This technique yielded a more uniform distribution of gold particles over the fungus wall, compared to that obtained with the lectin-gold technique. Both techniques revealed no labelling of the fungus cytoplasm, except for organelles resembling Woronin bodies. No significant labelling of either healthy or infected root cells was seen except for the secondary walls of vessels and, occasionally, that of adjoining parenchyma cells. The importance of this technique in studying the development of the pathogen within host cells is discussed.

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