Influence of the production of antibacterial and antifungal proteins by transgenic plants on the saprophytic soil microflora

To obtain durable and broad-spectrum resistance against plant pathogens, plants are transformed with genes coding for antimicrobial proteins from plant, animal or microbial origin. An obvious concern is that increased levels of these antimicrobial compounds affect not only the target pathogen, but also beneficial micro-organisms such as mycorrhizae, rhizobia and other micro-organisms involved in plant health, litter decomposition and nutrient cycling. This literature study focuses on effects of these transgenic plants on the non-target saprophytic soil microflora. Transgenic plants that constitutively express proteins with potential antifungal and/or antibacterial activity, can reduce activities of specific soil-borne plant pathogens in the rhizosphere. Reports on non-target effects on the saprophytic soil microflora are scarce and incomplete, and mainly focused on mycorrhizal symbiosis. Constitutive expression of antifungal pathogenesis-related proteins in tobacco in most cases did not affect root colonization by the mycorrhizal fungus Glomus mosseae. However, increased levels of a class II tobacco P-l,3-glucanase reduced the colonization potential, indicating that non-target effects can occur. Concerning other members of the plant-beneficial rhizosphere microflora, it can be assumed that they will come into contact with the transgenic product. By natural wounding, senescence and sloughing-off of root cells, at least some of the antimicrobial protein(s) will be released in the rhizosphere. Despite proteolytic activity of the rhizosphere microflora, part of the protein can remain active due to protective adsorption to clay minerals or humic components.