ULTRASTRUCTURE OF INTERACTIONS BETWEEN CASSAVA AND XANTHOMONAS-CAMPESTRIS PV MANIHOTIS - CYTOCHEMISTRY OF CELLULOSE AND PECTIN DEGRADATION IN A SUSCEPTIBLE CULTIVAR

A cytochemical investigation was carried out on the development of an aggressive strain of Xanthomonas campestris pv. manihotis, responsible for the cassava bacterial blight, to gain better insights into molecu lar and cellular mechanisms involved in leaf cell wall degradation by this pathogen. The use of anti-pectin monoclonal antibodies revealed t hat the plant middle lamellae were damaged during the infection proces s, from the epiphytic stage on the leaf surface to invasion of vascula r bundles. In parallel, application of a beta-1,4-exoglucanase-gold pr obe to healthy and infected tissues indicated that primary and seconda ry cell walls were also altered. Quantitation of gold labeling confirm ed that pectin was more severely degraded than cellulose. Accumulation of pectinlike compounds was also detected in occluded infected vessel s. Bacterial-surrounding sheaths, which were routinely seen during pat hogenesis early after leaf inoculation, had a dense or loosened fibril lar appearance and were differentiated from the pathogen cell wall. Cl ose association occurred between extracellular fibrils and leaf cell w alls, both at early and advanced stages of wall degradation. Bacterial extracellular sheaths were often seen deep in host cell walls, someti mes enclosing residual plant cell wall fragments. Our cytochemical dat a demonstrated that cell wall degradation of cassava by Xanthomonas ca mpestris pv, manihotis plays an important role in host tissue coloniza tion. It is also suggested that bacterial extracellular sheaths are in volved in plant cell surface degradation.