Use of Pathogen-Produced Toxins in Genetic Engineering of Plants and Pathogens

Abstract

Certain bacterial and fungal plant pathogens produce extracellular toxins that are known to be causally involved in disease. Some of these are required for pathogenicity (pathogenicity factors) whereas others contribute to virulence (virulence factors) of the producing organism. Both types of toxin are potentially useful in pest control. First, disease-resistant plants or cells can be efficiently selected both in vitro and in vivo. If a pathogenicity factor is used, a high level of resistance is expected; if a virulence factor is used an intermediate level of resistance is expected. Either level can be economically valuable. Toxins that have not been shown to be causally involved in disease are not expected to select any disease-resistance at all. Second, pathogen-produced toxins may provide an effective approach to pest control. For example, a gene or set of genes that controls production of a nonspecific toxin might be transferred to and expressed in a pathogen that is specific for a certain pest, such as a weed or an insect, making the pathogen capable of reducing the pest population to a low level. Before pathogens can be genetically engineered in this way, genes that control toxin production must be isolated and cloned. The isolation of most genes of this type will require complementation of toxinless recipient cells with a library of DNA fragments from a toxin-producing strain (these toxins are synthesized constitutively and no abundant mRNA is likely to be found). However, the genes in question will probably not function in easily transformable organisms such as E. coli or yeast. Therefore, genetic transformation systems must be developed for toxin-producing bacterial and fungal pathogens so that they can serve as hosts for the isolation of their own genes. Recent advances in several laboratories indicate that the technology for cloning pathologically important genes from bacteria is now available. Fungi are less tractable than bacteria, but a transformation system for the toxin-producing fungal pathogen Cochliobolus heterostrophus is under development. It is based on complementation of adenine-requiring fungal protoplasts with a cloned ADE gene from yeast. To date all transformants have aborted; efforts to construct a cloning vector that can be maintained indefinitely by Cochliobolus cells are in progress.