Pre-inoculation of Ri T-DNA transformed cucumber roots with the mycoparasite, Verticillium lecanii, induces host defense reactions against Pythium ultimuminfection

The ability of the mycoparasite, Verticillium lecanii(Zimm.) Vegas, to trigger plant defense reactions was investigated using Ri T-DNA transformed cucumber roots subsequently infected with the root pathogen,Pythium ultimum. The antagonistic potential of V. lecaniiagainst P. ultimumwas determined in dual culture tests. Cytological investigations of V. lecanii-inoculated cucumber roots revealed that the fungus grew actively at the root surface and colonized a small number of epidermal and cortical cells, without inducing extensive host cell damage. Direct cell wall penetration was seldom observed and Verticilliumingress into the root tissues proceeded via an intercellular route. In V. lecanii-free roots, the pathogen multiplied extensively through much of the tissues while inV. lecanii-treated roots, pathogen growth was restricted to the epidermis and the outer cortex. At the root surface,V. lecaniiinteracted with P. ultimumin a way similar to that observed in vitro. However, a novel finding concerned the structural alteration of P. ultimumhyphae that penetrated the root tissues. This was clearly shown by the frequent occurrence of empty fungal cells in intercellular spaces colonized by V. lecanii. Pythiumingress in the root tissues resulted in the deposition of an electron-opaque material frequently encircling pathogen hyphae and also accumulating in the noninfected xylem vessels, suggesting that the host plant was signalled to defend itself through the stimulation of defense mechanisms. The present observations provide the first convincing evidence that V. lecaniimay colonize root cells and trigger plant defense reactions in addition to being an efficient mycoparasite.