To establish a successful infection plant viruses have to overcome a defense system composed of several layers. This review will overview the various strategies plants employ to combat viral infections with main emphasis on the current status of single dominant resistance (R) genes identified against plant viruses and the corresponding avirulence (Avr) genes identified so far. The most common models to explain the mode of action of dominant R genes will be presented. Finally, in brief the hypersensitive response (HR) and extreme resistance (ER), and the functional and structural similarity of R genes to sensors of innate immunity in mammalian cell systems will be described.

/pdf/dominant-resistance-against-plant-viruses-4axyua0a21.pdf

Dominant resistance against plant viruses.

Barley Yellow Mosaic Virus disease caused by different strains of BaYMV and BaMMV is a major threat to winter barley cultivation in Europe. Pyramiding of resistance genes may be considered as a promising strategy to avoid the selection of new virus strains and to create more durable resistances. However, this goal cannot be achieved by phenotypic selection due to the lack of differentiating virus strains. For pyramiding of resistance genes rym4, rym5, rym9 and rym11, located on chromosomes 3H and 4H of barley two different strategies have been developed. These strategies are based on doubled haploid lines (DHs) and marker assisted selection procedures. On the one hand F1 derived DH-plants of single crosses were screened by molecular markers for genotypes being homozygous recessive for both resistance genes. These genotypes were crossed to lines carrying one resistance gene in common and an additional third gene, leading to a DH-population of which 25% carry three resistance genes, 50% have two resistance genes and 25% possess a single resistance gene homozygous recessively. Alternatively, F1 plants having one resistance gene in common were directly inter-crossed [e.g. (rym4 × rym9) × (rym4 × rym11)] and about 100 seeds were produced per combination. Within these complex cross progenies plants were identified by markers being homozygous at the common resistance locus and heterozygous at the others. From such plants, theoretically present at a frequency of 6.25%, DH-lines were produced, which were screened for the presence of genotypes carrying three or two recessive resistance genes in a homozygous state. Besides DH-plants carrying all possible two-gene combinations, 20 DH-plants out of 107 analysed carrying rym4, rym9, and rym11 and 27 out of 187 tested carrying rym5, rym9, and rym11 homozygously have been detected using the second strategy which is faster but needs co-dominant markers, because in contrast to the first strategy marker selection is carried out on heterozygous genotypes.

Strategies for Pyramiding Resistance Genes Against the Barley Yellow Mosaic Virus Complex (BaMMV, BaYMV, BaYMV-2)

SUMMARY Recent studies have shown that resistance in several dicotyledonous plants to viruses in the genus Potyvirus is controlled by recessive alleles of the plant translation initiation factor eIF4E or eIF(iso)4E genes. Here we provide evidence that the barley rym4 gene locus, controlling immunity to viruses in the genus Bymovirus, corresponds to eIF4E. A molecular marker based on the sequence of eIF4E was developed and used to demonstrate that eIF4E and rym4 map to the same genetic interval on chromosome 3HL in barley. Another genetic marker was developed that detects a polymorphism in the coding sequence of eIF4E and consistently distinguishes between rym4 and susceptible barley cultivars of diverse parentage. The eIF4E gene product from barley genotypes carrying rym4 and allelic rym5 and rym6 genes, originating from separate exotic germplasm, and a novel resistant allele that we identified through a reverse genetics approach all contained unique amino acid substitutions compared with the wild-type protein. Three-dimensional models of the barley eIF4E protein revealed that the polymorphic residues identified are all located at or near the mRNA cap-binding pocket, similarly to recent findings from studies on recessive potyvirus resistance in dicotyledonous plants. These new data complement our earlier observations that specific mutations in bymovirus VPg are responsible for overcoming rym4/5-controlled resistance. Because the potyviral VPg is known to interact with eIF4E in dicotyledonous plants, it appears that monocotyledonous and dicotyledonous plants have evolved a similar strategy to combat VPg-encoding viruses in the family Potyviridae.

Evidence that the recessive bymovirus resistance locus rym4 in barley corresponds to the eukaryotic translation initiation factor 4E gene

This review covers genetic resources from wild relatives; barriers and approaches to interspecific gene transfer; beneficial traits from wild relatives that contribute to the gene pool; biotechnological approaches to increase the utilization of wild relatives in crop improvement; and some results of usage of wild relatives in plant breeding

/pdf/enhancing-crop-gene-pools-with-beneficial-traits-using-wild-57dotoco83.pdf

Enhancing Crop Gene Pools with Beneficial Traits Using Wild Relatives

Soil-borne viruses affecting cereals: known for long but still a threat

Hordeum bulbosum represents the secondary gene pool of barley and constitutes a potential source of various disease resistances in barley breeding. Interspecific crosses of H. vulgare × H. bulbosum resulted in recombinant diploid-barley progeny with immunity to BaMMV after mechanical inoculation. Tests on fields contaminated with different viruses demonstrated that resistance was effective against all European viruses of the soil-borne virus complex (BaMMV, BaYMV-1, -2). Genetic analysis revealed that resistance was dominantly inherited. Marker analysis in a F5 mapping family was performed to map the introgression in the barley genome and to estimate its size after several rounds of recombination. RFLP anchor-marker alleles indicative of an H. bulbosum introgression were found to cover an interval 2.9 cM in length on chromosome 6HS. The soil-borne virus resistance locus harboured by this introgressed segment was designated Rym14
 Hb
 . For marker-assisted selection of Rym14
 Hb
 carriers, a diagnostic codominant STS marker was derived from an AFLP fragment amplified from leaf cDNA of homozygous-resistant genotypes inoculated with BaMMV.

Mapping of Rym14Hb, a gene introgressed from Hordeum bulbosum and conferring resistance to BaMMV and BaYMV in barley.

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Mapping of Rym14Hb, a gene introgressed from Hordeum bulbosum and conferring resistance to BaMMV and BaYMV in barley.