Rust Resistance Breeding Research Articles

Identification and genetic mapping of a recessive gene for resistance to stripe rust in wheat line LM168-1

Stripe rust (or yellow rust), caused by the fungus Puccinia striiformis f. sp. tritici (Pst), is one of the most important foliar diseases of wheat. Characterization and utilization of novel resistant genes is the most effective, economic and environmentally friendly approach to controlling the disease. Wheat line LM168-1, which was derived from a cross between common wheat Chuannong 16 and Milan, has good adult-plant resistance to stripe rust, based on field tests over several years. To elucidate the genetic basis of resistance, LM168-1 was crossed with susceptible variety SY95-71. Parents and F1, F2, BC1 and F2:3 progenies were tested in 2009–2011 in a field inoculated with the predominant races of Pst in China. The genetic analysis showed that resistance to stripe rust in LM168-1 was controlled by a single recessive gene, temporarily designated yrLM168. Simple sequence repeat (SSR), resistance gene analog polymorphism (RGAP) and target region amplification polymorphism (TRAP) techniques were used to identify molecular markers linked to the resistance locus. Finally, a linkage group consisting of two SSR, four RGAP and five TRAP markers was constructed for yrLM168 with 102 F2 plants. The closest markers R1 and R2 flanked the resistance gene locus at 2.4 and 2.4 cM, respectively. Furthermore, two SSR markers Xwmc59 and Xwmc145 assigned the gene to chromosome 6A. Because yrLM168 confers high-level resistance to the predominant races of Pst in China, it should be useful in stripe rust resistance breeding programs. The closely linked markers can be used for rapidly transferring yrLM168 to wheat breeding populations.

Variation for Resistance to White Rust (Puccinia horiana) among Ajania and Chrysanthemum Species

White rust (causative pathogen Puccinia horiana) is a destructive disease of commercial chrysanthemum crops. A panel of 19 accessions of commercial chrysanthemum near-relatives (four Ajania species, 11 Chrysanthemum species including five accessions of Chrysanthemum indicum) were screened for their reaction to white rust infection in separate greenhouse trials carried out at two independent sites in eastern China, one in 2010 and the other in 2012. The reaction of the accessions to artificial inoculation ranged from immune to highly susceptible. Accessions of Chrysanthemum indicum, C. yoshinaganthum, C. makinoi var. wakasaense, C. nankingense, C. vestitum, C. lavandulifolium, C. crassum, and Ajania tripinnatisecta were immune, and strong resistance was present in C. japonense, C. × shimotomaii, and A. przewalskii. Most of the accessions behaved similarly in the two trials, but two of the C. indicum accessions produced inconsistent results, each being highly resistant in one trial but susceptible in the other. Because wide crosses are relatively easy to achieve in the chrysanthemum complex, these immune and highly resistant accessions represent promising germplasm for white rust resistance breeding.

RLP1.1, a novel wheat receptor-like protein gene, is involved in the defence response against Puccinia striiformis f. sp. tritici

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most serious diseases of wheat; therefore, exploring effective resistance-related genes is critical for breeding and studying resistance mechanisms. However, only a few stripe rust resistance genes and defence-related genes have been cloned. Moreover, transgenic wheat with enhanced stripe rust resistance has rarely been reported. Receptor-like proteins (RLPs) are known to be involved in defence and developmental pathways. In this research, a novel RLP gene TaRLP1.1 was characterized as an important stripe rust defence gene. TaRLP1.1 was screened by GeneChip and was found to be induced by Pst specifically in the resistant variety. Knock down of TaRLP1.1 in the stripe rust-resistant plants resulted in increased susceptibility to Pst, and phenolic autofluorogen accumulation at the pathogen–host interaction sites, usually correlated with the hypersensitive response, was decreased dramatically. However, when the TaRLP1.1 gene was transformed into the susceptible wheat variety Yangmai158, the transgenic plants showed highly increased resistance to Pst, and the hypersensitive response was enhanced at the infection sites. Meanwhile, the expression of pathogenesis-related genes decreased in the TaRLP1.1-silenced plants and increased in the TaRLP1.1-overexpressing plants. Thus, it was proposed that TaRLP1.1 greatly contributed to the hypersensitive response during the pathogen–host interaction. Along with the functional analysis, an evolutionary study of the TaRLP1 family was performed. Characterization of TaRLP1.1 may facilitate breeding for stripe rust resistance and better understanding of the evolution of the RLP genes in wheat.

Introgression of stem rust resistance genes SrTA10187 and SrTA10171 from Aegilops tauschii to wheat

Aegilops tauschii, the diploid progenitor of the wheat D genome, is a readily accessible germplasm pool for wheat breeding as genes can be transferred to elite wheat cultivars through direct hybridization followed by backcrossing. Gene transfer and genetic mapping can be integrated by developing mapping populationsduring backcrossing. Using direct crossing, two genes for resistance to the African stem rust fungus race TTKSK (Ug99), were transferred from the Ae. tauschii accessions TA10187 and TA10171 to an elite hard winter wheat line, KS05HW14. BC2 mapping populations were created concurrently with developing advanced backcross lines carrying rust resistance. Bulked segregant analysis on the BC2 populations identified marker loci on 6DS and 7DS linked to stem rust resistance genes transferred from TA10187 and TA10171, respectively. Linkage maps were developed for both genes and closely linked markers reported in this study will be useful for selection and pyramiding with other Ug99-effective stem rust resistance genes. The Ae. tauschii-derived resistance genes were temporarily designated SrTA10187 and SrTA10171 and will serve as valuable resources for stem rust resistance breeding.

Identification and mapping in spring wheat of genetic factors controlling stem rust resistance and the study of their epistatic interactions across multiple environments

Stem rust (Puccinia graminis f. sp. tritici) is responsible for major production losses in hexaploid wheat (Triticum aestivum L.) around the world. The spread of stem rust race Ug99 and variants is a threat to worldwide wheat production and efforts are ongoing to identify and incorporate resistance. The objectives of this research were to identify quantitative trait loci (QTL) and to study their epistatic interactions for stem rust resistance in a population derived from the Canadian wheat cultivars AC Cadillac and Carberry. A doubled haploid (DH) population was developed and genotyped with DArT® and SSR markers. The parents and DH lines were phenotyped for stem rust severity and infection response to Ug99 and variant races in 2009, 2010 and 2011 in field rust nurseries near Njoro, Kenya, and to North American races in 2011 and 2012 near Swift Current, SK, Canada. Seedling infection type to race TTKSK was assessed in a bio-containment facility in 2009 and 2012 near Morden, MB. Eight QTL for stem rust resistance and three QTL for pseudo-black chaff on nine wheat chromosomes were identified. The phenotypic variance (PV) explained by the stem rust resistance QTL ranged from 2.4 to 48.8 %. AC Cadillac contributed stem rust resistance QTL on chromosomes 2B, 3B, 5B, 6D, 7B and 7D. Carberry contributed resistance QTL on 4B and 5A. Epistatic interactions were observed between loci on 4B and 5B, 4B and 7B, 6D and 3B, 6D and 5B, and 6D and 7B. The stem rust resistance locus on 6D interacted synergistically with 5B to improve the disease resistance through both crossover and non-crossover interactions depending on the environment. Results from this study will assist in planning breeding for stem rust resistance by maximizing QTL main effects and epistatic interactions.

Molecular identification of a new wheat-Thinopyrum intermediumssp.trichophorumaddition line for resistance to stripe rust

Stripe rust, caused by Puccinia striiformis f. sp. tritici, was one of the most disaster foliar diseases for wheat-growing areas of the world. Thinopyrum intermedium has provided novel resistance genes to multi-fungal disease, and new wheat-Th. intermedium derivatives for stripe rust resistance still need to develop for wheat breeding. Wheat line X484-3 was selected from a cross between wheat line MY11 and wheat-Th. intermedium ssp. trichophorum partial amphiploid TE-1508, and was characterized by genomic in situ hybridization (GISH) and functional molecular markers. Chromosome counting revealed that the X484-3 was 2n = 44 and GISH analysis using Pseudoroegneria spicata genomic DNAas a probe demonstrated that X484-3 contained a pair of St-chromosomes from Th. intermedium donor parents. The functional molecular markers confirmed that introduced St-chromosomes belonging to linkage group 7, indicating that line X484-3 was a 7St addition line. The resistance observation displayed that the introduced Th. intermedium ssp. trichophorum derived chromosomes 7St were responsible for the stripe rust resistances at adult plant. The identified wheat-Th. intermedium chromosome 7St addition line X484-3 can be used as a donor in wheat breeding for stripe rust resistance.

Simultaneous transfer, introgression, and genomic localization of genes for resistance to stem rust race TTKSK (Ug99) from Aegilops tauschii to wheat

Wheat production is currently threatened by widely virulent races of the wheat stem rust fungus, Puccinia graminis f. sp. tritici, that are part of the TTKSK (also known as 'Ug99') race group. The diploid D genome donor species Aegilops tauschii (2n = 2x = 14, DD) is a readily accessible source of resistance to TTKSK and its derivatives that can be transferred to hexaploid wheat, Triticum aestivum (2n = 6x = 42, AABBDD). To expedite transfer of TTKSK resistance from Ae. tauschii, a direct hybridization approach was undertaken that integrates gene transfer, mapping, and introgression into one process. Direct crossing of Ae. tauschii accessions with an elite wheat breeding line combines the steps of gene transfer and introgression while development of mapping populations during gene transfer enables the identification of closely linked markers. Direct crosses were made using TTKSK-resistant Ae. tauschii accessions TA1662 and PI 603225 as males and a stem rust-susceptible T. aestivum breeding line, KS05HW14, as a female. Embryo rescue enabled recovery of F1 (ABDD) plants that were backcrossed as females to the hexaploid recurrent parent. Stem rust-resistant BC1F1 plants from each Ae. tauschii donor source were used as males to generate BC2F1 mapping populations. Bulked segregant analysis of BC2F1 genotypes was performed using 70 SSR loci distributed across the D genome. Using this approach, stem rust resistance genes from both accessions were located on chromosome arm 1DS and mapped using SSR and EST-STS markers. An allelism test indicated the stem rust resistance gene transferred from PI 603225 is Sr33. Race specificity suggests the stem rust resistance gene transferred from TA1662 is unique and this gene has been temporarily designated SrTA1662. Stem rust resistance genes derived from TA1662 and PI 603225 have been made available with selectable molecular markers in genetic backgrounds suitable for stem rust resistance breeding.

Response of vetches (Vicia spp.) to specialized forms of Uromyces vicia-fabae and to Uromyces pisi

The genus Vicia includes many species of agricultural interest, such as faba bean and the various vetches that may be infected by rust, with little understanding of the specificity of these interactions. This work contributes to the clarification of host range of Uromyces viciae-fabae and Uromyces pisi , and confirms host specialization within U. viciae-fabae . The differential response of Vicia faba , Vicia sativa and Lens culinaris checks confirms the existence of specialized isolates of U. viciae-fabae . Host range of these host specialized forms of U. viciae-fabae was not so clear cut when we test other species of Vicia . The most specialized form was U. viciae-fabae ex V. faba that was able to infect profusely only faba bean ( V. faba ), all other Vicia species being highly resistant. Conversely, susceptibility against U. viciae-fabae ex V. sativa was very common in Vicia spp., particularly in those belonging to the subgenus Vicia , although not in V. faba . Still, susceptibility could be identified in a number of accessions of section Cracca such as Vicia cracca , Vicia monantha , Vicia sicula or Vicia villosa . Most Vicia accessions were highly resistant to U. viciae-fabae ex L. culinaris , but a number of accessions could be severely rusted, particularly in species of the subgenus Vicilla but also in some of section Peregrinae of subgenus Vicia. Similarly, most Vicia accessions were highly resistant to U. pisi but a number of accessions could be severely rusted, particularly in species of the subgenus Vicilla and in sections Hypechusa and Peregrinae of subgenus Vicia . This work also identified sources of resistance to the various rusts that can be used in rust resistance breeding. In spite of the susceptibility against various rusts, interesting levels of resistance were identified in crops and related species being based either on hypersensitive response or on reduced severity in spite of a compatible interaction making resistance breeding feasible. ► This work contributes to the clarification of host range of Uromyces viciae-fabae and Uromyces pisi . ► Existence of host specialized isolates of U. viciae-fabae . ► U. viciae-fabae ex V. faba is very specialized being able to infect profusely only faba bean. ► Sources of resistance have been identified to the various rusts that can be used in rust resistance breeding.

Combining ability for resistance to soybean rust in F2 and F3 soybean populations

The impact of soybean rust (Phakopsora pachyrhizi) on soybean yields has been extensively studied. However, few studies have evaluated early generation segregating material under field conditions for soybean rust resistance to facilitate selection. The objective of this study was to estimate combining abilities for soybean rust resistance in the F2 and F3 populations at MUARIK and F3 populations simultaneously across five locations. Combining ability for soybean rust resistance was estimated from a half diallel cross of eight soybean using disease severity and sporulation rates as indices for resistance. A consistent contribution of additive gene action was observed at MUARIK across F2 and F3 despite high environment contribution to both severity and sporulation rate. The simultaneous evaluation of F3 populations in five diverse locations produced similar results with significant GCA effects for both traits. There were, however, greater genotypic effects to soybean rust severity and sporulation across the five test environments, although genetic systems of severity and sporulation rate acted independently. Additive and additive×additive epistatic gene effects were the most common form of GCA controlling resistance. Specific combining ability did not always contribute to soybean rust resistance. The positive correlation between parental severity, sporulation rate performance and GCA estimates suggested that selection of parents for soybean rust resistance breeding can be based on parental performance. Parental line UG 5 was the most outstanding producing the greatest number of resistant populations. This study underscores the importance of additive gene effects in the control of soybean rust severity and sporulation rate.

Advanced backcross quantitative trait locus analysis in winter wheat: Dissection of stripe rust seedling resistance and identification of favorable exotic alleles originated from a primary hexaploid wheat (Triticum turgidum ssp. dicoccoides × Aegilops tauschii)

Stripe rust (Puccinia striiformis W.) causes a range of disease symptoms in hexaploid wheat. We have utilized the AB-QTL (advanced backcross quantitative trait locus) strategy for the genetic dissection of complex disease resistance against stripe rust. An advanced backcross population designated Z86 was made by crossing the winter wheat cultivar Zentos (Triticum aestivum L.) and the primary (exotic) synthetic wheat accession Syn86L (T. turgidum ssp. dicoccoides × Aegilops tauschii). The population Z86, containing 150 BC2F3 lines, was inoculated with the stripe rust isolate R108E141. The disease symptoms were subjected to QTL analysis by using a genetic map based on 118 simple sequence repeat markers. This analysis revealed six QTL effects that were located on chromosomes 1B, 2B, 6B, 7B, 1D and 4D. At four loci, the exotic alleles were associated to increased resistance against stripe rust. The strongest effect, QYrs.Z86-1B, was detected on the short arm of chromosome 1B. Here, the introgression of the exotic allele resulted in 86% enhancement of resistance which explained 37.2% of the genetic variance (R2). The second favorable effect of an exotic allele was detected on chromosome 1D at QYrs.Z86-1D, which accounted for 72% increase in resistance and explained 18.4% of the R2. Each of the exotic allele at QTL QYrs.Z86-6B and QYrs.Z86-7B accounted for around 60% enhancement of resistance against stripe rust. At QTL QYrs.Z86-2B and QYrs.Z86-4D, the relative performance of the exotic alleles was inferior due to the pre-eminence of the elite alleles which ranged from 67 to 72%. In addition, QTL analysis revealed four QTL by marker interaction effects. In most cases, the interaction between the elite and exotic alleles brought up resistance in the mixed background of BC2F3 lines. The data presented here provide valuable new genetic resources to be used for stripe rust resistance breeding as well as to isolate new alleles of exotic origin.

Discovery and molecular mapping of a new gene conferring resistance to stem rust, Sr53, derived from Aegilops geniculata and characterization of spontaneous translocation stocks with reduced alien chromatin

This study reports the discovery and molecular mapping of a resistance gene effective against stem rust races RKQQC and TTKSK (Ug99) derived from Aegilops geniculata (2n = 4x = 28, U(g)U(g)M(g)M(g)). Two populations from the crosses TA5599 (T5DL-5M(g)L·5M(g)S)/TA3809 (ph1b mutant in Chinese Spring background) and TA5599/Lakin were developed and used for genetic mapping to identify markers linked to the resistance gene. Further molecular and cytogenetic characterization resulted in the identification of nine spontaneous recombinants with shortened Ae. geniculata segments. Three of the wheat-Ae. geniculata recombinants (U6154-124, U6154-128, and U6200-113) are interstitial translocations (T5DS·5DL-5M(g)L-5DL), with 20-30% proximal segments of 5M(g)L translocated to 5DL; the other six are recombinants (T5DL-5M(g)L·5M(g)S) have shortened segments of 5M(g)L with fraction lengths (FL) of 0.32-0.45 compared with FL 0.55 for the 5M(g)L segment in the original translocation donor, TA5599. Recombinants U6200-64, U6200-117, and U6154-124 carry the stem rust resistance gene Sr53 with the same infection type as TA5599, the resistance gene donor. All recombinants were confirmed to be genetically compensating on the basis of genomic in situ hybridization and molecular marker analysis with chromosome 5D- and 5M(g)-specific SSR/STS-PCR markers. These recombinants between wheat and Ae. geniculata will provide another source for wheat stem rust resistance breeding and for physical mapping of the resistance locus and crossover hot spots between wheat chromosome 5D and chromosome 5M(g)L of Ae. geniculata.

Molecular mapping of the rust resistance gene R 4 to a large NBS-LRR cluster on linkage group 13 of sunflower

Rust is a serious fungal disease in the sunflower growing areas worldwide with increasing importance in North America in recent years. Several genes conferring resistance to rust have been identified in sunflower, but few of them have been genetically mapped and linked to molecular markers. The rust resistance gene R ( 4 ) in the germplasm line HA-R3 was derived from an Argentinean open-pollinated variety and is still one of most effective genes. The objectives of this study were to determine the chromosome location of the R ( 4 ) gene and the allelic relationship of R ( 4 ) with the R ( adv ) rust resistance gene. A total of 63 DNA markers previously mapped to linkage group (LG)13 were used to screen for polymorphisms between two parental lines HA 89 and HA-R3. A genetic map of LG 13 was constructed with 21 markers, resulting in a total map length of 93.8cM and an average distance of 4.5cM between markers. Two markers, ZVG61 and ORS581, flanked the R ( 4 ) gene at 2.1 and 0.8cM, respectively, and were located on the lower end of LG 13 within a large NBS-LRR cluster identified previously. The PCR pattern generated by primer pair ZVG61 was unique in the HA-R3 line, compared to lines HA-R1, HA-R4, and HA-R5, which carry other R ( 4 ) alleles. A SCAR marker linked to the rust resistance gene R ( adv ) mapped to LG 13 at 13.9cM from the R ( 4 ) locus, indicating that R ( adv ) is not an allele of the R ( 4 ) locus. The markers tightly linked to the R ( 4 ) gene will facilitate gene pyramiding for rust resistance breeding of sunflower.

Molecular characterization of a wheat -Thinopyrum ponticum partial amphiploid and its derived substitution line for resistance to stripe rust

Stripe rust (caused by Puccinia striiformis) occurs annually in most wheat-growing areas of the world. Thinopyrum ponticum has provided novel rust resistance genes to protect wheat from this fungal disease. Wheat - Th. ponticum partial amphiploid line 7430 and a substitution line X005 developed from crosses between wheat and 7430 were resistant to stripe rust isolates from China. Genomic in situ hybridization (GISH) analysis using Pseudoroegneria spicata genomic DNA as a probe demonstrated that the partial amphiploid line 7430 contained ten J(s) and six J genome chromosomes, and line X005 had a pair of J(s)-chromosomes. Giemsa-C banding further revealed that both lines 7430 and X005 were absent of wheat chromosomes 6B. The EST based PCR confirmed that the introduced J(s) chromosomes belonging to linkage group 6, indicating that line X005 was a 6J(s)/6B substitution line. Both resistance observation and sequence characterized amplified region (SCAR) markers displayed that the introduced chromosomes 6J(s) were responsible for the stripe rust resistances. Therefore, lines 7430 and X005 can be used as a donor in wheat breeding for stripe rust resistance.

Characterization of wheat yellow rust resistance geneYr17using EST-SSR and rice syntenic region

Wheat yellow rust resistance gene Yr17 was originated from the wheat-Aegilops ventricosa introgression, and still effective on the adult plant in Southern China. The previous studies located the gene Yr17 on the translocation of 2NS-2AS using the molecular and cytological markers. In the present study, we screened new PCR-based markers to map the gene Yr17 region from the investigation of a segregating 120 F2 population. All markers including four EST-PCR markers, a SCAR (sequence characterized amplified region) and a PLUG (PCR based landmark unique gene) marker specific to Yr17 gene were mapped on the chromosome 2AS, and located on the chromosomal deletion bin 2AS5-0.8–1.00 region. Based on the wheat-rice collinearity, we found that the sequences of the Yr17 gene linked markers were comparatively matched at rice chromosome 4 and chromosome 7. However, the identified closely linked genomic sequence of Yr17 gene is most likely collinear with genomic region of rice chromosome 4. The newly produced PCR based markers closely linked to Yr17 gene will be useful for the marker-assisted selection in wheat breeding for rust resistance.

Inheritance of stripe rust resistance to predominant Chinese races in six spring wheat cultivars from the pacific Northwest of the United States

Stripe rust, caused by Puccinia striiformis Westend. f. sp. tritici Eriks., is a major disease that causes substantial losses to wheat production worldwide. The utilization of effective resistance genes in wheat cultivars is the preferred control of the disease. To study the inheritance of all-stage resistance in spring wheat cultivars Louise, WA008016, Express, Solano, Alturas and Zak from the Pacific Northwest (PNW) of the United States, the six cultivars were crossed with the Chinese susceptible variety Taichung 29. Single-spore isolates of CYR32 and CYR33, the predominant Chinese races of P. striiformis f. sp. tritici, were used to evaluate F1, F2 and BC1 generations for stripe rust resistance under controlled greenhouse conditions. Genetic analysis determined that Louise had one dominant resistance gene to CYR32, temporarily designated as YrLou. WA008016 had two dominant and one recessive resistance genes to CYR32, temporarily designated as YrWA1, YrWA2 and YrWA3, respectively. Express had a single recessive gene that conferred resistance to CYR32, temporarily designated as YrExp3. The two independent dominant genes in Solano conferring resistance to CYR32 were temporarily designated as YrSol1 and YrSol2. Alturas had two recessive genes for resistance to CYR32, temporarily designated as YrAlt1 and YrAlt2. Zak has one dominant gene for resistance to CYR33, temporarily designated as YrZak1. These six cultivars can be important resistance sources in Chinese wheat stripe rust resistance breeding.

Development of a PCR assay and marker-assisted transfer of leaf rust resistance gene Lr58 into adapted winter wheats

Leaf rust resistance gene Lr58 derived from Aegilops triuncialis L. was transferred to the hard red winter wheat (HRWW) cultivars Jagger and Overley by standard backcrossing and marker-assisted selection (MAS). A co-dominant PCR-based sequence tagged site (STS) marker was developed based on the sequence information of the RFLP marker (XksuH16) diagnostically detecting the alien segment in T2BS·2BL-2tL(0.95). STS marker Xncw-Lr58-1 was used to select backcross F1 plants with rust resistance. The co-dominant marker polymorphism detected by primer pair NCW-Lr58-1 efficiently identified the homozygous BC3F2 plants with rust resistance gene Lr58. The STS marker Xncw-Lr58-1 showed consistent diagnostic polymorphism between the resistant source and the wheat cultivars selected by the US Wheat Coordinated Agricultural Project. The utility and compatibility of the STS marker in MAS programs involving robust genotyping platforms was demonstrated in both agarose-based and capillary-based platforms. Screening backcross derivatives carrying Lr58 with various rust races at seedling stage suggested the transferred rust resistance in adapted winter wheats is stable in both cultivar backgrounds. Lr58 in adapted winter wheat backgrounds could be used in combination with other resistance genes in wheat rust resistance breeding.

Legume breeding for rust resistance: lessons to learn from the model Medicago truncatula

Rusts are major biotic constraints of legumes worldwide. Breeding for rust resistance is regarded as the most cost efficient method for rust control. However, in contrast to common bean for which complete monogenic resistance exists and is efficiently used, most of the rust resistance reactions described so far in cool season food legumes are incomplete and of complex inheritance. Incomplete resistance has been described in faba bean, pea, chickpea and lentil and several of their associated QTLs have been mapped. However, the relatively large distance between the QTLs and their associated molecular markers hampers their efficient use for marker assisted selection. Their large genome size drastically hampers the development of genomic resource and limits the saturation of their genetic maps. The use of model plants such as the model legume Medicago truncatula may circumvent this drawback. The important genetic and genomic resources and tools available for this model legume can considerably speed up the discovery and validation of new genes and QTLs in resistance to legume pathogens. Here, the potential of M. truncatula as a model to study rust resistance in legumes, and to transfer rust resistance genes to cool season grain legumes is reviewed.

Molecular Screening and Resistance Evaluation of American Wheat Cultivars to Chinese Stripe Rust Races

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the major diseases of wheat in China. In order to asses the resistance levels and existing Yr genes among 59 wheat cultivars (lines) from the Pacific Northwest (PNW) of the United States, to provide resistance resources for genetic improvement of wheat stripe rust resistance in China, 59 wheat cultivars (lines) from PNW of the United States were infected by 3 mixed races of predominant Chinese stripe rust races CRY31, CRY32, and CRY33 to evaluate their resistance at seedling and adult plant stages, and screened with molecular markers tightly linked to currently effective all-stage resistance genes Yr10, Yr15 and adult plant resistance genes Yr18, Yr39. Of 59 American cultivars (lines), five cultivars (lines), Expresso, 02W50076, ACS52610, WA008012, and WA00801833, had all-stage resistance, showing resistance to mixed races of CRY31, CRY32, and CRY33 at both seedling and adult plant stages. 33 cultivars (lines) had adult plant resistance, only showing resistance to stripe rust at adult stage. Based on the molecular screening, none of the 59 PNW cultivars (lines) had the polymorphic bands of linked markers to Yr10. There were 12, 33 and 29 cultivars (lines) which had polymorphic bands of linked markers to Yr15, Yr18 and Yr39, accounting for 20, 55 and 49% of the 59 PNW cultivars (lines), respectively. All these results suggested that Yr15, Yr18 and Yr39 were widespread among PNW cultivars (cultivars) and could be utilized in Chinese wheat stripe rust resistance breeding.

Biology and Genetics of Crown Rust Disease in Ryegrasses

ABSTRACTThe most serious foliar disease of perennial ryegrass (Lolium perenne L.) is crown rust (caused by Puccinia coronata Corda f.sp. lolii Brown). Progress in resistance breeding using recurrent selection has been slow, due to lack of genetic knowledge. Puccinia coronata is a basidiomycete fungus with a complex life‐cycle involving both asexual and sexual reproductive modes. Pathotype variation may exist, but confirmation is complicated by the outbreeding nature of the host grass and consequent intrapopulation diversity. Qualitative and quantitative resistance mechanisms may hence be due to either major resistance genes responding to mixed pathogen populations, or minor (quantitative) resistance genes. Advances in molecular biology have permitted analysis of both host and pathogen genetics. Development of pathogen‐derived simple sequence repeat (SSR) genetic markers allowed detailed analysis of genetic variation, as well as generation of homogenized inoculum for detailed trait‐dissection studies. Multiple genomic regions were identified as containing quantitative trait loci (QTLs) for pathogen resistance, and specific single nucleotide polymorphism (SNP) markers for defense response (DR) and disease resistance (R) genes have been evaluated for QTL co‐location. The current knowledge status of the crown rust pathogen and genetics of host resistance is reviewed, as well as future prospects for facilitated rust resistance breeding.

A QTL study on late leaf spot and rust revealed one major QTL for molecular breeding for rust resistance in groundnut (Arachis hypogaea L.)

Late leaf spot (LLS) and rust are two major foliar diseases of groundnut (Arachis hypogaea L.) that often occur together leading to 50–70% yield loss in the crop. A total of 268 recombinant inbred lines of a mapping population TAG 24 × GPBD 4 segregating for LLS and rust were used to undertake quantitative trait locus (QTL) analysis. Phenotyping of the population was carried out under artificial disease epiphytotics. Positive correlations between different stages, high to very high heritability and independent nature of inheritance between both the diseases were observed. Parental genotypes were screened with 1,089 simple sequence repeat (SSR) markers, of which 67 (6.15%) were found polymorphic. Segregation data obtained for these markers facilitated development of partial linkage map (14 linkage groups) with 56 SSR loci. Composite interval mapping (CIM) undertaken on genotyping and phenotyping data yielded 11 QTLs for LLS (explaining 1.70–6.50% phenotypic variation) in three environments and 12 QTLs for rust (explaining 1.70–55.20% phenotypic variation). Interestingly a major QTL associated with rust (QTLrust01), contributing 6.90–55.20% variation, was identified by both CIM and single marker analysis (SMA). A candidate SSR marker (IPAHM 103) linked with this QTL was validated using a wide range of resistant/susceptible breeding lines as well as progeny lines of another mapping population (TG 26 × GPBD 4). Therefore, this marker should be useful for introgressing the major QTL for rust in desired lines/varieties of groundnut through marker-assisted backcrossing.Electronic supplementary materialThe online version of this article (doi:10.1007/s00122-010-1366-x) contains supplementary material, which is available to authorized users.