A highly virulent form of the wheat stem rust pathogen, Puccinia graminis f. sp. tritici (Pgt) race TTKSK, is virulent on both wheat and barley presenting a major threat to world food security. The recessive and temperature sensitive rpg4 gene is the only effective source of resistance identified in barley (Hordeum vulgare L.) against Pgt race TTKSK. Efforts to positional clone rpg4 localized resistance to a small interval on barley chromosome 5HL, tightly linked to the rye stem rust (Puccinia graminis f. sp. secalis) resistance gene Rpg5. High-resolution genetic analysis and post transcriptional gene silencing of the genes at the rpg4/Rpg5 locus determined that three tightly linked genes (Rpg5, HvRga1 and HvAd... More
A highly virulent form of the wheat stem rust pathogen, Puccinia graminis f. sp. tritici (Pgt) race TTKSK, is virulent on both wheat and barley presenting a major threat to world food security. The recessive and temperature sensitive rpg4 gene is the only effective source of resistance identified in barley (Hordeum vulgare L.) against Pgt race TTKSK. Efforts to positional clone rpg4 localized resistance to a small interval on barley chromosome 5HL, tightly linked to the rye stem rust (Puccinia graminis f. sp. secalis) resistance gene Rpg5. High-resolution genetic analysis and post transcriptional gene silencing of the genes at the rpg4/Rpg5 locus determined that three tightly linked genes (Rpg5, HvRga1 and HvAdf3) are required together for rpg4-mediated wheat stem rust resistance. Alleles of the three genes were analyzed from a diverse set of fourteen domesticated barley lines (Hordeum vulgare) and eight wild barley accessions (Hordeum vulgare subsp. spontaneum) to characterize diversity that may determine incompatibility (resistance). The analysis determined that HvAdf3 and HvRga1 code for predicted functional proteins that do not appear to contain polymorphisms determining the compatible (susceptible) interactions with the wheat stem rust pathogen and were expressed at the transcriptional level from both resistant and susceptible barley lines. The HvAdf3 alleles shared 100% amino acid identity among all twenty-two genotypes examined. The Pgt race QCCJ susceptible barley lines with HvRga1 alleles containing the limited amino acid substitutions unique to the susceptible varieties also contained predicted non-functional rpg5 alleles. Thus, susceptibility in these lines is likely due to the non-functional RPG5 proteins. The Rpg5 allele analysis determined that nine of the thirteen Pgt race QCCJ susceptible barley lines contain alleles that either code for predicted truncated proteins as the result of a single nucleotide substitution resulting in a stop codon at amino acid 161, a single cytosine indel causing a frame shift and a stop codon at amino acid 217, or an indel that deleted the entire STPK domain. The three Pgt race QCCJ susceptible lines, Swiss landraces Hv489, Hv492 and Hv611 and the wild barley accession WBDC160, contain rpg5 alleles predicted to encode full-length proteins containing a non-synonomous nucleotide substitution that results in the amino acid substitution E1287A. This amino acid substitution present in the uncharacterized C-terminal domain is not found in any resistant line and may be important to elicit the resistance reaction. These data suggest that rpg4-mediated resistance against many wheat stem rust pathogen races including Pgt race TTKSK rely on the Rpg5 resistance gene, thus rpg4- and Rpg5-mediated resistance rely on a common R-gene and should not be considered completely distinct. The data also determined that Rpg5 gene specific molecular markers could be used to detect rpg4-mediated wheat stem rust resistance for marker-assisted selection.
