A comprehensive germplasm evaluation study of wheat accessions conserved in the Indian National Genebank was conducted to identify sources of rust and spot blotch resistance. Genebank accessions comprising three species of wheat–Triticum aestivum, T. durum and T. dicoccum were screened sequentially at multiple disease hotspots, during the 2011–14 crop seasons, carrying only resistant accessions to the next step of evaluation. Wheat accessions which were found to be resistant in the field were then assayed for seedling resistance and profiled using molecular markers. In the primary evaluation, 19,460 accessions were screened at Wellington (Tamil Nadu), a hotspot for wheat rusts. We identified 4925 accessions to be resistant and these were further evaluated at Gurdaspur (Punjab), a hotspot for stripe rust and at Cooch Behar (West Bengal), a hotspot for spot blotch. The second round evaluation identified 498 accessions potentially resistant to multiple rusts and 868 accessions potentially resistant to spot blotch. Evaluation of rust resistant accessions for seedling resistance against seven virulent pathotypes of three rusts under artificial epiphytotic conditions identified 137 accessions potentially resistant to multiple rusts. Molecular analysis to identify different combinations of genetic loci imparting resistance to leaf rust, stem rust, stripe rust and spot blotch using linked molecular markers, identified 45 wheat accessions containing known resistance genes against all three rusts as well as a QTL for spot blotch resistance. The resistant germplasm accessions, particularly against stripe rust, identified in this study can be excellent potential candidates to be employed for breeding resistance into the background of high yielding wheat cultivars through conventional or molecular breeding approaches, and are expected to contribute toward food security at national and global levels.
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Stem rust, caused by Puccinia graminis f. sp. tritici, is a destructive disease of wheat that can be controlled by deploying effective stem rust resistance (Sr) genes. Highly virulent races of P. graminis f. sp. tritici in Africa have been detected and characterized. These include race TRTTF and the Ug99 group of races such as TTKSK. Several Canadian and U.S. spring wheat cultivars, including the widely grown Canadian cultivar ‘Harvest’, are resistant to TRTTF. However, the genetic basis of resistance to TRTTF in Canadian and U.S. spring wheat cultivars is unknown. The objectives of this study were to determine the number of Sr genes involved in TRTTF resistance in Harvest, genetically map the resistance with DNA markers, and use markers to assess the distribution of that resistance in a panel of Canadian cultivars. A doubled haploid (DH) population was produced from the cross LMPG-6S/Harvest. The DH population was tested with race TRTTF at the seedling stage. Of 92 DH progeny evaluated, 46 were resistant and 46 were susceptible which perfectly fit a 1:1 ratio indicating a single Sr gene was responsible for conferring resistance to TRTTF in Harvest. Mapping with single nucleotide polymorphism (SNP) and simple sequence repeat (SSR) markers placed the resistance gene distally on the chromosome 6AS genetic map, which corresponded to the location reported for Sr8. SSR marker gwm459 and 30 cosegregating SNP markers showed the closest linkage, mapping 2.2 cM proximal to the Sr gene. Gene Sr8a confers resistance to TRTTF and may account for the resistance in Harvest. Testing a panel of Canadian wheat cultivars with four SNP markers closely linked to resistance to TRTTF suggested that the resistance present in Harvest is present in many Canadian cultivars. Two of these SNP markers were also predictive of TRTTF resistance in a panel of 241 spring wheat lines from the United States, Canada, and Mexico.
Leaf rust (LR) caused by Puccinia triticina, is among the most important diseases of wheat (Triticum aestivum L.) crops globally. Deployment of cultivars incorporating genetic resistance, such as adult plant resistance (APR) or all-stage resistance, is considered the most sustainable control method. APR is preferred for durability because it places lower selection pressure on the pathogen and is often polygenic. In the search for new sources of APR, here we explored a diversity panel sourced from the N. I. Vavilov Institute of Plant Genetic Resources. Based on DNA marker screening, 83 of the 300 lines were deemed to carry known APR genes; namely, Lr34, Lr46, and Lr67. Interestingly, lines carrying Lr67 were mostly landraces from India and Pakistan, reconfirming the likely origin of the gene. Rapid phenotypic screening using a method that integrates assessment at both seedling and adult growth stages under accelerated growth conditions (i.e., constant light and controlled temperature) identified 50 lines carrying APR. Levels of APR corresponded well with phenotypes obtained in a field nursery inoculated using the same pathotype (R2 = 0.82). The second year of field testing, using a mixture of pathotypes with additional virulence for race-specific APR genes (Lr13 and Lr37), identified a subset of 13 lines that consistently displayed high levels of APR across years and pathotypes. These lines provide useful sources of resistance for future research. A strategy combining rapid generation advance coupled with phenotyping under controlled conditions could accelerate introgression of these potentially novel alleles into adapted genetic backgrounds.
To respond faster to the changing climate, evolving pathogens and to feed a global population of 9–10 billion by 2050, plant breeders are exploring more efficient crop improvement strategies. In this study, we applied novel methodology for rapid trait introgression to the European two-rowed barley cultivar Scarlett. Scarlett is widely-grown in Argentina and is preferred for malting and brewing, yet lacks adequate disease resistance. We used four donor lines combining multiple disease resistance (i.e. leaf rust, net and spot forms of net blotch and spot blotch) in a modified backcross strategy, which incorporated both multi-trait phenotypic screens and the rapid generation advance technology ‘speed breeding’, to develop 87 BC1F3:4 Scarlett introgression lines (ILs) within two years. Phenotyping this set of lines in disease nurseries located in Australia and Uruguay revealed the ILs had high levels of multiple disease resistance. Preliminary yield testing of the 12 most promising ILs in Argentina identified three ILs that were significantly higher yielding than Scarlett at Balcarce, whereas all 12 ILs displayed yield equivalent to Scarlett at Tres Arroyos. We propose that this approach is useful to rapidly transfer genes for multiple target traits into adapted cereal cultivars or pyramiding desirable traits in elite breeding material.
Wheat stem rust (Puccinia graminis f. sp. tritici Eriks. and E. Henn.) is one of the most destructive diseases world-wide. Races belonging to Ug99 (or TTKSK) continue to cause crop losses in East Africa and threaten global wheat production. Developing and deploying wheat varieties with multiple race-specific genes or complex adult plant resistance is necessary to achieve durability. In the present study, we applied genome-wide association studies (GWAS) for identifying loci associated with the Ug99 stem rust resistance (SR) in a panel of wheat lines developed at the International Maize and Wheat Improvement Center (CIMMYT). Genotyping was carried out using the wheat 9K iSelect single nucleotide polymorphism (SNP) chip. Phenotyping was done in the field in Kenya by infection of Puccinia graminis f. sp. tritici race TTKST, the Sr24-virulent variant of Ug99. Marker-trait association identified 12 SNP markers significantly associated with resistance. Among them, 7 were mapped on five chromosomes. Markers located on chromosomes 4A and 4B overlapped with the location of the Ug99 resistance genes SrND643 and Sr37, respectively. Markers identified on 7DL were collocated with Sr25. Additional significant markers were located in the regions where no Sr gene has been reported. The chromosome location for five of the SNP markers was unknown. A BLASTN search of the NCBI database using the flanking sequences of the SNPs associated with Ug99 resistance revealed that several markers were linked to plant disease resistance analogues, while others were linked to regulatory factors or metabolic enzymes. A KASP (Kompetitive Allele Specific PCR) assay was used for validating six marker loci linked to genes with resistance to Ug99. Of those, four co-segregated with the Sr25-pathotypes while the rest identified unknown resistance genes. With further investigation, these markers can be used for marker-assisted selection in breeding for Ug99 stem rust resistance in wheat.