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Stem rust (incited by Puccinia graminis f. sp. tritici) is a devastating disease of wheat and barley in many production areas. The widely virulent African P. graminis f. sp. tritici race TTKSK is of particular concern, because most cultivars are susceptible. To prepare for the possible arrival of race TTKSK in North America, we crossed a range of barley germplasm—representing different growth habits and end uses—with donors of stem rust resistance genes Rpg1 and rpg4/Rpg5. The former confers resistance to prevalent races of P. graminis f. sp. tritici in North America, and the latter confers resistance to TTKSK and other closely related races from Africa. We produced doubled haploids from these crosses and determined their allele type at the Rpg loci and haplotype at 7,864 single-nucleotide polymorphism loci. The doubled haploids were phenotyped for TTKSK resistance at the seedling stage. Integration of genotype and phenotype data revealed that (i) Rpg1 was not associated with TTKSK resistance, (ii) rpg4/Rpg5 was necessary but was not sufficient for resistance, and (iii) specific haplotypes at two quantitative trait loci were required for rpg4/Rpg5 to confer resistance to TTKSK. To confirm whether lines found resistant to TTKSK at the seedling resistance were also resistant at the adult plant stage, a subset of doubled haploids was evaluated in Kenya. Additionally, adult plant resistance to leaf rust and stripe rust (incited by Puccinia hordei and Puccinia striiformis f. sp. hordei, respectively) was also assessed on the doubled haploids in field trials at three locations in the United States over a 2-year period. Doubled haploids were identified with adult plant resistance to all three rusts, and this germplasm is available to the research and breeding communities.

We identified Rph24 as a locus in barley (Hordeum vulgare L.) controlling adult plant resistance (APR) to leaf rust, caused by Puccinia hordei. The locus was previously reported as a quantitative trait locus in barley line ND24260-1 and named qRphND. We crossed ND24260-1 to the leaf-rust-susceptible standard Gus and determined inheritance patterns in the progeny. For the comparative marker frequency analysis (MFA), resistant and susceptible tails of the F2 were genotyped with Diversity Arrays Technology genotyping-by-sequencing (DArT-Seq) markers. The Rph24 locus was positioned at 55.5 centimorgans on chromosome 6H on the DArT-Seq consensus map. Evaluation of F2:3 families confirmed that a single locus from ND24260-1 conferred partial resistance. The haploblock strongly associated with the Rph24 locus was used to estimate the allele frequency in a collection of 282 international barley cultivars. Rph24 was frequently paired with APR locus Rph20 in cultivars displaying high levels of APR to leaf rust. The markers identified in this study for Rph24 should be useful for marker-assisted selection.

Partial resistance to leaf rust (Puccinia hordei G. H. Otth) in barley is a quantitative resistance that is not based on hypersensitivity. This resistance hampers haustorium formation, resulting in a long latency period in greenhouse tests. The three most consistent quantitative trait loci (QTL) uncovered in the L94 × ‘Vada’ mapping population were introgressed by marker-assisted backcrossing into the susceptible L94 background to obtain near-isogenic lines (NIL). We also developed the reciprocal Vada-NIL for the susceptibility alleles of those QTL. The QTL Rphq2 affected latency period of P. hordei more than the QTL Rphq3 and Rphq4. The NIL confirmed the contribution of Rphq2 to partial resistance by prolonging the latency period by 28 h on L94-Rphq2 and shortening the latency period by 23 h on Vada-rphq2. On the basis of flanking restriction fragment length polymorphism-based markers, Rphq2 appeared to be located near the telomeric end of the long arm of chromosome 2H, in a physical region of high recombination, making it the target QTL for map-based cloning. Microscopic observations on the NIL confirmed the nonhypersensitive nature of the resistance conferred by Rphq2. A high-resolution genetic map of the Rphq2 region was constructed using a population of 38 subNIL with overlapping L94 introgressions in Vada background across the region. Rphq2 mapped approximately 2 centimorgans (cM) proximal from the MlLa locus. By bulked segregant analysis and use of synteny with rice, we developed additional markers and fine-mapped Rphq2 to a genetic interval of 0.11 cM that corresponds to a stretch of sequence of, at most, 70 kb in rice. Analysis of this rice sequence revealed predicted genes encoding two proteins with unknown function, retrotransposon proteins, peroxidase proteins, and a protein similar to a mitogen-activated protein kinase kinase kinase (MAP3K). Possible homologs of those peroxidases and MAP3K in barley are candidates for the gene that contributes to partial resistance to P. hordei.

Prats, E., Martinez, F., Rojas-Molina, M. M., and Rubiales, D. 2007. Differential effects of phenylalanine ammonia lyase, cinnamyl alcohol dehydrogenase, and energetic metabolism inhibition on resistance of appropriate host and nonhost cereal–rust interactions. Phytopathology 97:1578-1583. Effects of phenylpropanoid and energetic metabolism inhibition on resistance were studied during appropriate host and nonhost cereal-rust interactions. In the appropriate barley–Puccinia hordei interaction, phenylalanine ammonia lyase (PAL) and cinnamyl alcohol dehydrogenase (CAD) inhibition reduced penetration resistance in two genotypes, suggesting a role for phenolics and lignins in resistance. Interestingly, penetration resistance of the barley genotype 17.5.16 was not affected by phenylpropanoid biosynthesis but penetration resistance was almost completely inhibited by D-mannose, which reduces the energy available in plant host cells. This suggests a parallel in the cellular basis of penetration resistance between 17.5.16 rust and mlo barleys powdery mildew interaction. Results revealed differing patterns of programmed cell death (PCD) in appropriate versus nonhost rust interactions. PAL and CAD inhibitors reduced PCD (hypersensitivity) in appropriate interactions. Conversely, they had no effect in PCD of wheat to P. hordei; whereas Dmannose dramatically reduced nonhost resistance and allowed colony establishment. The differential effects of inhibitors in the expression of the different resistances and the commonalities with the cereal-powdery mildew interaction is analyzed and discussed. Puccinia hordei Otth. is a biotrophic fungal pathogen that causes leaf rust of barley (Hordeum vulgare L.), which is a worldwide problem. The fungus enters the host leaf through stomata to form intercellular hyphae that differentiates haustorial mother cells (HMCs) upon contact with a host cell surface within the lamina. A penetration peg then attempts to penetrate host mesophyll cells and form an intracellular nutrient-absorbing haustorium (29). Attempted penetration may trigger different host cell defense responses that can act as both prepenetration to prevent haustorium formation or postpenetration to arrest further fungal

Annual surveys of pathogenic variability in the leaf rust pathogen of barley, Puccinia hordei, from 1992 to 2001 revealed a significant shift in the composition of populations across Austra-lia. Virulence for the resistance gene Rph12, first detected in a single pathotype, 4610P+, in Tasmania in 1991, was subsequently detected in 1993 in South Australia, Victoria, and southern New South Wales. By the end of 2001, eight pathotypes with virulence for Rph12 had been isolated, and virulence for this gene was present in all Australian barley growing regions. Virulence was not detected for the resistance genes Rph3, Rph7, Rph11, or Rph14. The distribution and spread of the pathotypes detected, their possible origins, pathogenicity on several uncharacterized seedling resistance sources, and implications for resistance breeding are discussed.

Cultivated barley, Hordeum vulgare L., is considered to be a nonhost or intermediate host species for the wheat leaf rust fungus Puccinia triticina. Here, we have investigated, at the microscopic and molecular levels, the reaction of barley cultivars to wheat leaf rust infection. In the nonhost resistant cultivar Cebada Capa, abortion of fungal growth occurred at both pre- and posthaustorial stages, suggesting that defense genes are expressed throughout the development of the inappropriate fungus during the nonhost resistance reaction. In the two barley lines L94 and Bowman, a low level of prehaustorial resistance to P. triticina was observed and susceptibility was comparable to that of wheat control plants. Suppression subtractive hybridization was used to identify genes that are differentially expressed during the nonhost resistance reaction in Cebada Capa as well as during the successful establishment of the inappropriate wheat leaf rust fungus in L94. Northern analysis indicated that two candidate genes, including a barley ortholog of the rice resistance gene Xa21, are putatively involved in non-host and non-race-specific resistance reactions. In addition, a new gene that is specifically induced during the successful development of the inappropriate fungus P. triticina in barley has been identified.

Leaf rust, caused by Puccinia hordei, is an important disease of barley in many parts of the world. In the eastern United States, this disease was effectively controlled for over 20 years through the deployment of cultivars carrying the resistance gene Rph7. Isolates of P. hordei with virulence for Rph7 appeared in this region in the early 1990s rendering barley cultivars with this gene vulnerable to leaf rust infection. From a preliminary evaluation test, 13 accessions from diverse geographic locations possessed resistance to P. hordei isolate VA90-34, which has virulence for genes Rph1, 2, 4, 6, 7, 8, and 11. Each of these 13 accessions was crossed with susceptible cvs. Moore or Larker to characterize gene number and gene action for resistance to P. hordei. Additionally, the 13 accessions were intercrossed and crossed to host differential lines possessing genes Rph3, Rph5, and Rph9 to determine allelic relationships of resistance genes. Seedlings of F1, F2, and BC1F1 populations were evaluated in the greenhouse for their reaction to P. hordei isolate VA90-34. Leaf rust resistance in six of the accessions including Collo sib, CR270.3.2, Deir Alla 105, Giza 119, Gloria, and Lenka is governed by a single dominant gene located at or near the Rph3 locus. All accessions for which the gene Rph3 was postulated to govern leaf rust resistance, except for Deir Alla 105, likely possess an allele different than Rph3.c found in Estate based on the differential reaction to isolates of P. hordei. The resistance gene in Grit and Donan is located at or near the Rph9 locus. Alleles at both the Rph3 and Rph9 loci confer resistance in Femina and Dorina. In addition to Rph3, Caroline and CR366.13.2 likely possess a second unknown recessive gene for leaf rust resistance. Resistance in Carre 180 is governed by a recessive gene that is different from all other genes considered in this study. Identification of both known and unique genes conferring leaf rust resistance in the barley germplasm included in this study provides breeding programs with the knowledge and opportunity to assess currently used sources of leaf rust resistance and to incorporate new sources of resistance into their programs.

Leaf rust resistance, derived from the barley cultivar Cebada Capa, has been effective in the southeastern United States since 1950, when it was first used in the Virginia barley breeding program. In 1990, races of Puccinia hordei virulent to barleys that possess the resistance gene Rph7 were identified for the first time in North America. This research assessed the potential impact of Rph7-virulent races of leaf rust on grain yield and quality in winter barley. Natural epidemics of leaf rust occurred in cultivar trials at Painter and Warsaw, Virginia, in 1991 and 1992. Mean leaf rust severities for barley lines observed over three environments ranged from 10% for the moderately resistant line VA 90-42-45 to 76% for the susceptible cultivar Barsoy

The barley (Hordeum vulgare) accession Cebada Capa has been used extensively as a source of resistance to the leaf rust pathogen (Puccinia hordei) in the Virginia barley breeding program, Cebada Capa possesses the resistance gene Rph7; and in the mid-Atlantic region of the United States, cultivars derived from it have remained resistant to P. hordei since 1968. In 1990, outbreaks of leaf rust were observed in Virginia on cultivars carrying Rph7. Isolates of P. hordei collected from these cultivars were found to possess virulence on Cebada Capa in greenhouse and growth chamber experiments. In addition to Virginia, P. hordei isolates with Rph7 virulence were identified in Pennsylvania and California

Studies were undertaken to map the chromosome location of leaf rustresistance genes Rph3, Rph7, and Rph12 in the barley cultivars Estate, Cebada Capa, and Triumph using various morphological markers. Crosses were made between genotypes resistant to Puccinia hordei and leaf rust susceptible morphological marker stocks. F 2 plants were evaluated for reaction (infection type) to P. hordei, and presence of morphological marker characters. Segregation data indicated the position of the Rph3 allele in Estate barley on the long arm of chromosome 1 []

There were large differences among Hordeum chilense lines in appressorium formation by the wheat and barley leaf rust fungi. The percentage of germlings producing appressoria over stomata differed from there to more than 10-fold (depending on genotype and growth stage). The differences with cultivated wheat and barley and among lines of H. chilense were most pronounced in mature plants. The low appressorium formation may be considered a case of avoidance; it was due to poor stoma recognition not to poor spore germination (...)

(...) Histological examination of barley leaves 6 days after inoculation with P. hordei in three experimental series revealed a negative association between germ tube length of sporelings and the chance of successful establishment of a colony. This effect was found in a susceptible, a hypersensitive, and two partially resistant barley lines. Within these lines there was a significant negative correlation between germ tube length and size of the established colony. It is suggested that the formation of a long germ tube, necessary to reach a stoma, decreases the amount of energy available to the sporelings to infect the host

Luke, H. H., Barnett, R. D., and Pfahler, P. L. 1987. Xenoparasite-nonhost reactions in Puccinia-Gramineae pathosystems. Phytopathology 77:1488-1491. Histological studies of six species of Gramineae inoculated with five leaf of a nonhost plant inoculated with a pure race of a xenoparasite. species of Puccinia were conducted to determine the kinds of defense Nonhost reaction types were clear cut, independent, and easily systems involved in xenoparasite-nonhost interactions. Although all distinguished one from the other. The nonhost had specific and nonspecific nonhost types had active (physiological) defense mechanisms, two resistance to most of the xenoparasites, indicating that these two types of exhibited quasi exclusion, but the few xenoparasites that penetrated resistance had a common origin. Another histological response was stimulated a physiological response similar to specific resistance. characterized by fluorescent-collapsed cells that did not retard radial Histological responses associated with cessation of fungal growth included growth of the hyphae. This reaction reduced haustorial mother cell fluorescent-collapsed mesophyll cells (specific resistance), fluorescent- formation and inhibited development of macroscopic symptoms in the noncollapsed (nonspecific resistance), and a nonfluorescent-noncollapsed nonhost. The nonhosts used in our study had five defense mechanisms, four type (unknown). These three defense mechanisms were observed in a single physiological and one quasi exclusion type.