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 molecular and cellular mechanisms involved in leaf cell wall degradation by this pathogen. The use of anti-pectin monoclonal antibodies revealed that the plant middle lamellae were damaged during the infection process, from the epiphytic stage on the leaf surface to invasion of vascular bundles. In parallel, application of a beta-1,4-exoglucanase-gold probe to healthy and infected tissues indicated that primary and secondary cell walls were also altered. Quantitation of gold labeling confirmed that pectin was more severely degraded than cellulose. Accumulation of pectinlike compounds was also detected in occluded infected vessels. Bacterial-surrounding sheaths, which were routinely seen during pathogenesis early after leaf inoculation, had a dense or loosened fibrillar appearance and were differentiated from the pathogen cell wall. Close association occurred between extracellular fibrils and leaf cell walls, both at early and advanced stages of wall degradation. Bacterial extracellular sheaths were often seen deep in host cell walls, sometimes enclosing residual plant cell wall fragments. Our cytochemical data demonstrated that cell wall degradation of cassava by Xanthomonas campestris pv. manihotis plays an important role in host tissue colonization. It is also suggested that bacterial extracellular sheaths are involved in plant cell surface degradation eng