VIEW ARTICLE

Cytology and Histology

Ultrastructure and Cytochemistry of Pectin and Cellulose Degradation in Tobacco Roots Infected by Phytophthora parasitica var. nicotianae. Nicole Benhamou, Département de phytologie, Faculté des sciences de l’agriculture et de l’alimentation, Université Laval, Sainte-Foy, Québec, Canada, G1K 7P4; François Côté, Département de phytologie, Faculté des sciences de l’agriculture et de l’alimentation, Université Laval, Sainte-Foy, Québec, Canada, G1K 7P4. Phytopathology 82:468-478. Accepted for publication 12 November 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/Phyto-82-468.

Root colonization of tobacco (Nicotiana tabacum L. ‘Xanthi-nc’) by the pathogen Phytophthora parasitica var. nicotianae was studied by electron microscopy. Gold-complexed probes were used for the localization of pectin and cellulose. The epidermis was colonized from 24 to 48 h after inoculation, and the cortex appeared entirely colonized by the time the pathogen reached the paratracheal parenchyma (96 h after inoculation). The fungus did not penetrate the xylem vessels. Colonization of the root tissues was associated with changes, including alteration of primary cell walls and breakdown of middle lamella matrices. The pattern of pectin distribution, as revealed by the Aplysia gonad lectin-gold complex, indicated that the fungus produced pectic enzymes that diffused extracellularly. The release of pectic fragments and the accumulation of pectin molecules at specific sites such as the intercellular spaces were frequently detected. Cellulose was localized with an exoglucanase-gold complex, which showed that the fungus could produce cellulases but to a lesser extent than pectinases. Various host reactions were observed during pathogen ingress into the root; these reactions included the filling of intercellular spaces with a pectin-rich fibrillar material, the plugging of sieve pores and plasmodesmata with an amorphous material, and the occlusion of noninvaded xylem vessels by a coating material. Although highly susceptible to P. p. nicotianae, N. tabacum ‘Xanthi-nc’ appears capable of using a complex array of defense reactions. The possibility of sensitizing this cultivar to respond more rapidly to pathogen attack is raised.