Submitted to: Annual Review of Phytopathology
Publication Type: Review Article
Publication Acceptance Date: 9/30/1995
Publication Date: N/A
Interpretive Summary: Except for eyespot resistance, most alien gene transfers for disease resistance are still not used in commercial wheats. resistance levels achieved in adapted lines are in some examples equal to that of the wild species, however, linked with these genes come undesirable traits that affect yield, end-use quality and other agronomic traits. Continued rounds of radiation or homoeologus pairing to promote recombination are still required to remove unwanted alien chromatin before breeders can incorporate these valuable genes into commercially acceptable cultivars. New approaches are also needed for the identification of resistant genotypes, especially when dealing with race nonspecific resistance and pathogens that are facultative parasites. In these cases, potentially valuable genotypes can be lost due to an inability to select resistant individuals because resistance is not complete and environmental effects may be large. Marker-based selections, such as the isozyme marker for eyespot resistance genes, enables selection of resistant individuals with certainty and eliminates susceptible genes. Developing markers still requires the identification of resistant genotypes based on a phenotypic response following inoculation with a pathogen. Use of a GUS-transformed strain of the eyespot fungus along with chromosome addition lines enabled us to map a new gene for resistance in a distant relative of wheat, which was not possible with visual disease evaluations. Ease of transformation of most fungi should allow use of reporter genes for detection of resistance in other host-pathogen systems.
Technical Abstract: No other cultivated plant is equal to wheat (Triticum spp.) in the breadth of knowledge of genomic structures and relationships, availability of wild germplasm, and global importance. The genus Triticum contains three ploidy levels and about 30 species. Most of these species have been investigated as sources of disease resistance genes and several have been used in successful transfers of resistance to domestic wheat. At least six genera from the tribe Triticeae have been used successfully as donors of disease resistance genes for domestic wheat. The amount of alien chromatin involved in these transfers varies from a single gene to chromosome arms or entire chromosomes. The main factor determining the amount of alien chromatin carried through generations of recombination is chromosome pairing. Recombination frequencies involving homologous chromosomes such as from T. tauschii (genome DD) and D genome chromosomes of wheat (T. aestivum, genomes AABBDD) can be very close to those observed for wheat chromosome to wheat chromosome exchanges. Conversely, transfers between chromosomes that do not normally pair are much more complex and usually result in translocations and a large amount of alien chromatin introgressed into the wheat genomes. Induced homoeologous pairing, spontaneous translocations, and irradiation to achieve gene transfer have been used by several wheat breeding groups. Most recent reviews outline the techniques and complexities involved in alien tranfers. Therefore, no attempt was made in this review to describe all alien resistance gene transfers in wheat or to outline the various techniques involved.