Increased killing of Bacillus subtilis on the hair roots of transgenic T4 lysozyme-producing potatoes

Transgenic potato plants expressing the phage T4 lysozyme gene which are re sistant to the plant-pathogenic enterobacterium Erwinia carotovora subsp. c arotovora have been constructed. The agricultural growth of these potatoes might have harmful effects on soil microbiota as a result of T4 lysozyme re lease into the rhizosphere. To assess the bactericidal effect of roots, we have developed a novel method to associate the cells of Bacillus subtilis w ith hair roots of plants and to quantify the survival of cells directly on the root surface by appropriate staining and fluorescence microscopy. With this technique, we found that the roots of potato plants (Desiree and trans genic control lines) without T4 lysozyme gene display measurable killing ac tivity on root-adsorbed B. subtilis cells. Killing was largely independent of the plant age and growth of plants in greenhouse or field plots. Roots f rom potato lines expressing the T4 lysozyme gene always showed significantl y (1.5 to 3.5-fold) higher killing. It is concluded that T4 lysozyme is rel eased from the root epidermis cells and is active in the fluid film on the root surface. We discuss why strong negative effects of T4 lysozyme-produci ng potatoes on soil bacteria in field trials may not be observed. We propos e that the novel method presented here to study interactions of bacteria wi th roots can be applied not only to bacterial killing but also to interacti ons leading to growth-sustaining effects of plants on bacteria.