Wheat breeding programs have successfully harnessed the potential of elite germplasm pool and have contributed significantly to global food security. However, to obtain additional genetic gain, useful diversity for key traits from landraces, synthetics and wild relatives should be incorporated in breeding germplasm pool. Maladaptation and linkage drags are the bottlenecks in utilizing these exotic genepools for pre-breeding. A systematic, focused, large scale effort has been pursued at CIMMYT through a three-way cross (exotic x elite1 x elite2) population development strategy. Population was advanced through selected-bulk scheme in way to select relevant genetic diversity while maintaining large population sizes. A total of 984 advanced pre-breeding lines (PBLs) were evaluated in multiple environments for grain yield related traits, micronutrient content and diseases resistance (yellow rust, stem rust, powdery mildew, and karnal bunt). Potential useful lines for these traits have been identified. High-density genomic characterization of PBLs, parental elites and exotics was conducted through a "haplotype map" based approach, which revealed 16% (58/361) exotic specific haplotype block (HB) introgression in PBLs. Out of 58 exotic specific HBs, 12 (12/361 = 3%) were found associated with traits evaluated in the study. Three HBs, H1.28 (1A), H18.1 (6D) and H5.23 (2B) were significantly important as they showed consistent effects across environments for grain yield (1A and 6D) and yellow rust (2B). This significant contribution of exotics into PBLs opens avenues to mine and utilize their useful alleles in wheat improvement. This research describes systematic large-scale pre-breeding efforts, as proof of concept of exotic germplasm deployment to the breeding pipelines simultaneously enriching genetic knowledge through high-density genomics analysis. Genetic knowledge coupled with breeding efforts should provide substantial gain required for next generation wheat varietal improvement.
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Dr. Norman Borlaug stated that rust never sleeps and this enables rust pathogens to produce new strains capable of putting rust resistance genes to rest. These pathogens continue to pose threats to global wheat production. Wheat breeders have made significant progress to control rust outbreaks using conventional selection technologies; however, some critical shifts in pathogen populations have let them down. Rapid evolution in molecular marker technologies in the last 15 years and refinement of phenomic facilities have expedited the process of discovery and characterisation of rust resistance genes to underpin the development and validation of markers closely linked with genetically diverse sources of resistance. A high proportion of the formally named rust resistance genes were characterized in the 21st century and markers closely linked with these genes have been developed and validated. The marker tagged sources of resistance to three rust diseases have equipped the wheat breeding community with tools to deploy combinations of all stage and adult plant resistance genes in future wheat cultivars. The question that whether we have enough resistance genes discovered to compete against the ever-awake rust pathogens. In our opinion, we cannot be complacent and discovery needs to continue to ensure food security. This presentation will discuss the role of advances in phenomic and genomic technologies to achieve durable rust control in wheat.
Puccinia graminis f. sp. tritici (Pgt) is one of the most destructive pathogens of wheat. Fungal secreted proteins termed effectors play an important role in modulating the host cellular environment and suppressing the plant defense response to enable fungal growth. They also become targets of plant resistance (R) proteins. We have taken a genomics approach to initially identify candidate effectors. We have built a draft genome for a founder Australian Pgt isolate of pathotype (pt.) 21-0 (collected in 1954) by next generation DNA sequencing. A combination of reference-based assembly using the genome of the previously sequenced North American Pgt isolate CDL 75-36-700-3 (p7a) and de novo assembly resulted in a 92 Mbp reference genome for Pgt isolate 21-0. This draft genome was subsequently used to build a pan-genome based on five Australian Pgt isolates. Transcriptomes from germinated urediniospores and haustoria were separately assembled for pt. 21-0 and comparison of gene expression profiles showed differential expression in ~10% of the genes in germinated urediniospores as well as haustoria. A total of 1,924 secreted proteins were predicted from the 21-0 transcriptome, of which 586 were classified as haustorial secreted proteins (HSPs). We are currently exploring effector gene expression during infection of wheat to reduce this candidate list based on a common expression profile identified for Avr genes in the flax rust fungus. Comparison of 21-0 with two presumed clonal field derivatives (collected in 1982 and 1984) that had evolved virulence on four additional resistance genes (Sr5, Sr11, Sr27, SrSatu) identified mutations in 13 HSP effector candidates. These candidate effectors are being assessed for recognition in wheat accessions with the corresponding R genes using a bacterial type three secretion delivery system based on an engineered Pseudomonas fluorescence strain (Upadhyaya NM et al. Mol Plant Microbe Interact 27:255-264).
Plant breeders use naturally occurring resistance genes to fight plant diseases. However, new fungal strains rapidly emerge and defeat these genes. For almost a century, the wheat Lr34 gene has conferred a degree of stable resistance to the wheat rusts, making it one of the most important resistance genes. While sequence homology of the cloned Lr34 gene predicted that it encodes a putative ATP binding cassette (ABC) transporter protein belonging to the ABC G subfamily (also known as Pleiotropic Drug Resistance or PDR), its target transport substrate and mechanism of action remains enigmatic. In an effort to understand this transporter we designed several DNA constructs of the Lr34 gene and expressed them in yeast (Saccharomyces cerevisiae). Here we report the successful expression and purification of functional recombinant Lr34 protein. In vitro proteoliposome translocation assays identified the transport substrate of the Lr34Sus protein and demonstrated that the LR34Res protein has the same transport specificity. We also report the identification of related metabolites from flag leaves of Lr34-expressing wheat plants and discuss the functional relevance of these metabolites to the disease resistance and leaf tip necrosis (LTN) phenotypes caused by expression of Lr34Res.