A comparative light and electron microscopical study of compatible and incompatible interactions between Xanthomonas campestris pv. campestris and cabbage ( Brassica oleracea)

Leaves of a black-rot-susceptible cultivar (Golden Acre) and of a black-rot-resistant cultivar (Early Fuji) of cabbage ( Brassica oleracea) were inoculated with Xanthomonas campestris pv. campestris (10 8 cfu ml −1) via droplets formed through guttation at the hydathodes. Inoculated tissue was fixed after five days and examined by light and transmission electron microscopy. The bacteria were found within the xylem conducting elements, accompanied by a dense fibrillar material which appeared to occlude the entire lumen of the cells. In the compatible interaction, bacteria and fibrillar material filled only the xylem conducting elements while the surrounding mesophyll tissue remained essentially free from bacteria and was relatively undamaged. Bacteria and fibrillar material also filled xylem conducting elements in the incompatible interaction, but, in addition, many bacteria were present in the intercellular spaces. This was associated with severe degradation and deformation of mesophyll cell walls, plasmolysis, and swelling of chloroplasts. In the compatible interaction xylem conducting elements contained what appeared to be a combination of lysed bacteria and less intensely stained fibrillar material, whereas in the incompatible interaction lysed bacterial cells were not observed, and the fibrillar material did not appear to be degraded. The possible nature and origin of the plugging material in the xylem conducting elements is discussed. The density and distribution of bacteria within the host, and the nature of tissue, cellular and sub-cellular disruption are described in relation to the resistance reaction or black rot pathogenesis.