Genetic engineering of plants to enhance resistance to fungal pathogens - a review of progress and future prospects

Recent applications of techniques in plant molecular biology and biotechnol ogy to the study of host-pathogen interactions have resulted in the identif ication and cloning of numerous genes involved in the defense responses of plants following pathogen infection. These include: genes that express prot eins, peptides, or antimicrobial compounds that are directly toxic to patho gens or that reduce their growth in situ; gene products that directly inhib it pathogen virulence products or enhance plant structural defense genes, t hat directly or indirectly activate general plant defense responses; and re sistance genes involved in the hypersensitive response and in the interacti ons with avirulence factors. The introduction and expression of these genes , as well as of antimicrobial genes from nonplant sources, in a range of tr ansgenic plant species have shown that the development of fungal pathogens can be significantly reduced. The extent of disease reduction varies with t he strategy employed as well as with the characteristics of the fungal path ogen, and disease control has never been complete. Manipulation of salicyli c acid, ethylene, and cytokinin levels in transgenic plants have provided s ome interesting results with regard to enhanced disease tolerance or suscep tibility. The complex interactions among the expressed gene product, plant species, and fungal pathogen indicate that the response of transgenic plant s cannot be readily predicted. Combinations of defense gene products have s hown considerably more promise in reducing disease than single-transgene in troductions. The use of tissue-specific or pathogen-inducible promoters, an d the engineered expression of resistance genes, synthetic antimicrobial pe ptides, and elicitor molecules that induce defense responses have the poten tial to provide commercially useful broad-spectrum disease resistance in th e not-too-distant future. The issues and challenges that will need to be ad dressed prior to the widespread utilization of these transgenic plants are highlighted.