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

The ability of the mycoparasite, Verticillium lecanii (Zimm.) Vegas, to tri gger plant defense reactions was investigated using Ri T-DNA transformed cu cumber roots subsequently infected with the root pathogen. Pythium ultimum. The antagonistic potential of V. lecanii against P. ultimum was 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 corticol cells. without induc ing extensive host cell damage. Direct cell wall penetration was seldom obs erved and Verticillium ingress into the root tissues proceeded via an inter cellular route. In V. lecanii-free roots. the pathogen multiplied extensive ly through much of the tissues while in V. lecanii-treated roots, pathogen growth was restricted to the epidermis and the outer cortex. At the root su rface. V. lecanii interacted with P. ultimum in a way similar to that obser ved in vitro. However, a novel finding concerned the structural alteration of P. ultimum hyphae that penetrated the root tissues. This was clearly sho wn by the frequent occurrence of empty fungal cells in intercellular spaces colonized by V lecanii. Pythium ingress in the root tissues resulted in th e deposition of an electron-opaque material frequently encircling pathogen hyphae and also accumulating in the noninfected xylem vessels, suggesting t hat the host plant was signalled to defend itself through the The present o bservations provide the first convincing evidence that stimulation of defen se mechanisms. V. lecanii may colonize root cells and trigger plant defense reactions in addition to being an efficient (C) 2001 Academic Press.