Transmission electron microscopy of Verticillium albo-atrum hyphae in xylem vessels of tomato plants

Vessels of tomato plants cvs Craigella and Potentate colonized by Verticillium albo-atrum hyphae were examined by transmission electron microscopy (TEM). Many hyphae appeared enveloped in an electron-dense deposit which was frequently layered, resulting in a sheath of concentric bands. The thickness of the deposit ranged from 0·03 to 0·5 μm, and was often associated with loosely connected, similar, amorphous material in the vessel lumina. The layered encapsulation of some hyphae was contigous with an identical banded deposit on the vessel wall coating the hyphae to the wall or connecting them by thick bands. Pit membranes of colonized vessels appeared swollen and bordered pits were occluded by deposits similar to the hyphal electron-dense sheath material. No evidence was found using TEM of localized cellular outgrowths connecting hyphae to vessels or each other. The possible implications of the hyphal deposits in pathogenecity and resistance are discussed briefly.