In plants, disease resistance mediated by the gene-for-gene mechanism involves the recognition of specific effector molecules produced by the pathogen either directly or indirectly by the resistance-gene products. This recognition triggers a series of signals, thereby serving as a molecular switch in regulating defense mechanisms by the plants. To understand the mechanism of action of the barley stem rust resistance gene Rpg1, we investigated the fate of the RPG1 protein in response to infection with the stem rust fungus, Puccinia graminis f. sp. tritici. The investigations revealed that RPG1 disappears to undetectable limits only in the infected tissues in response to avirulent, but not virulent pathotypes. The RPG1 protein disappearance is rapid and appears to be due to specific protein degradation via the proteasome-mediated pathway as indicated by inhibition with the proteasomal inhibitor MG132, but not by other protease inhibitors.avirulence ͉ cultivar ͉ programmed cell death ͉ Puccinia graminis P lants have evolved diverse mechanisms to recognize pathogen attack and trigger defense responses. Pathogen recognition specificity is often determined by a pathogen avirulence (Avr) gene and its corresponding plant resistance (R) gene in a gene for gene manner (1). The R gene products may function directly or indirectly as receptors for the Avr gene products, providing detection of pathogen attack (2-6). Avr proteins secreted from the pathogen are recognized by the R proteins either in the intercellular spaces or after their transport into the plant cell. The Avr-R interactions lead to activation of defense responses and often result in the hypersensitive response (HR) (1), inhibiting the growth of the pathogen. In the absence of either the cognate R or Avr gene product, the pathogen colonizes the host and causes disease. Despite the cloning of several R genes and their corresponding Avr genes, direct physical interaction between matched Avr and R proteins has been shown only in a few cases. To exemplify, the Avr-Pita protein of the rice blast fungus Magnaporthe grisea encoding a metalloprotease is secreted with an N-terminal signal sequence. After delivery into the plant cell and removal of the proprotein sequence, the mature enzyme binds to the leucine-rich domain of the Pi-ta R protein and elicits the resistance response. This interaction was confirmed with the yeast two-hybrid system, with transient expression in rice seedling leaves of resistant or susceptible lines, by in vitro binding of the recombinantly synthesized Pi-ta protein to the Avr-Pita protein and by inactivation of either of the proteins through amino acid substitutions (7). Direct physical interaction has been demonstrated for the tomato Pto R protein and the AvrPto gene product (2, 4), the Arabidopsis RRS1 R and Ralstonia solanacearum PopP2 (8), and between the flax R gene L6 with the corresponding Avr-L6 of Melampsora lini (9). However, attempts with other R-Avr pairs have failed to establish a direct physical interaction. These observations ...

The dominant barley stem rust resistance gene Rpg1 confers resistance to many but not all pathotypes of the stem rust fungus Puccinia graminis f. sp. tritici (Pgt). Transformation of Rpg1 into susceptible cultivar Golden Promise rendered the transgenic plants resistant to Pgt pathotype MCC but not to Pgt pathotype QCC. Our objective was to identify genes that are induced/repressed during the early stages of pathogen infection to elucidate the molecular mechanisms and role of Rpg1 in defense. A messenger ribonucleic acid expression analysis using the 22K Barley1 GeneChip was conducted in all pair-wise combinations of two isolines (cv. Golden Promise and Rpg1 transgenic line G02-448F-3R) and two Pgt pathotypes (MCC and QCC) across six time points. Analysis showed that a total of 34 probe sets exhibited expression pattern differences between Golden Promise (susceptible) and G02-448F-3R (resistant) infected with Pgt-MCC. A total of 14 probe sets exhibited expression pattern differences between Pgt-MCC (avirulent) and Pgt-QCC (virulent) inoculated onto G02-448F-3R. These differentially expressed genes were activated during the early infection process, before the hypersensitive response or fungal growth inhibition occurred. Our analysis provides a list of candidate signaling components, which can be analyzed for function in Rpg1-mediated disease resistance.

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Fusarium head blight (FHB) is a threat to barley (Hordeum vulgare L.) production in many parts of the world. A number of barley accessions with partial resistance have been reported and used in mapping experiments to identify quantitative trait loci (QTL) associated with FHB resistance. Here, we present a set of barley germplasm that exhibits FHB resistance identified through screening a global collection of 23,255 wild (Hordeum vulgare ssp. spontaneum) and cultivated (Hordeum vulgare ssp. vulgare) accessions. Seventy-eight accessions were classified as resistant or moderately resistant. The collection of FHB resistant accessions consists of 5, 27, 46 of winter, wild and spring barley, respectively. The population structure and genetic relationships of the germplasm were investigated with 1,727 Diversity Array Technology (DArT) markers. Multiple clustering analyses suggest the presence of four subpopulations. Within cultivated barley, substructure is largely centered on spike morphology and growth habit. Analysis of molecular variance indicated highly significant genetic variance among clusters and within clusters, suggesting that the FHB resistant sources have broad genetic diversity. The haplotype diversity was characterized with DArT markers associated with the four FHB QTLs on chromosome 2H bin8, 10 and 13 and 6H bin7. In general, the wild barley accessions had distinct haplotypes from those of cultivated barley. The haplotype of the resistant source Chevron was the most prevalent in all four QTL regions, followed by those of the resistant sources Fredrickson and CIho4196. These resistant QTL haplotypes were rare in the susceptible cultivars and accessions grown in the upper Midwest USA. Some two- and six-rowed accessions were identified with high FHB resistance, but contained distinct haplotypes at FHB QTLs from known resistance sources. These germplasm warrant further genetic studies and possible incorporation into barley breeding programs.