Transformation technologies available for enhancing fungal resistance in wheat

The range of sources of disease resistance genes for a given plant species encompasses particular alleles at one or more loci in its primary gene pool, alleles whose origin might be more or less removed from that pool (thus requiring wide crosses for their introgression) and, finally, completely foreign genes from unrelated species that would require the lese of transformation techniques for their transfer. In the case of helminthosporium diseases of wheat, sources of resistance genes have been identified both in hexaploid wheat itself and in its wild relatives. Breeding efforts using both sources have yielded satisfactory products and the prospects for further resistance improvement by combining diverse sources are good since resistance appears to be controlled by only one to three or four genes. With the publication of comprehensive molecular marker maps of the wheat genome over the last four years, and the availability of new marker techniques for mapping the complex wheat genome more effectively and efficiently, it should be possible to more precisely dissect the genetic factors contributing to helminthosporium disease resistance in wheat. However, molecular markers should prove most effective in identifying the genes in alien sources of resistance and in accelerating their transfer to modern wheats. Ultimately, agronomically viable levels of durable resistance in crop plants against a relatively broad range of fungi might be achieved using transformation approaches. Such an approach could include insertion and expression of genes encoding inhibitors of fungal enzymes or known antifungal proteins such as chitinases and Beta1-3 glucanases. These hydrolytic enzymes catalyze the degradation of chitin and Beta1-3 glucan and, since these compounds are abundantly present in the cell wall of many filamentous fungi, they are thought to be capable of inhibiting fungal growth in planta. Other genes that could be used include ribosome inhibiting proteins (RIP), which inhibit protein synthesis in target cells, and osmotin, which forms pores in fungal membranes. Insertion of such genes into wheat, when combined with the corresponding plant resistance genes, provides plant breeders with additional sources of resistance.