ULTRASTRUCTURE OF INTERACTIONS BETWEEN CASSAVA AND XANTHOMONAS-CAMPESTRIS PV MANIHOTIS - CYTOCHEMISTRY OF CELLULOSE AND PECTIN DEGRADATION IN A SUSCEPTIBLE CULTIVAR
B. Boher et al., ULTRASTRUCTURE OF INTERACTIONS BETWEEN CASSAVA AND XANTHOMONAS-CAMPESTRIS PV MANIHOTIS - CYTOCHEMISTRY OF CELLULOSE AND PECTIN DEGRADATION IN A SUSCEPTIBLE CULTIVAR, Phytopathology, 85(7), 1995, pp. 777-788
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.