QTL mapping of resistance to tan spot induced by race 2 of Pyrenophora tritici-repentis in tetraploid wheat.

KEY MESSAGE: A total of 12 QTL conferring resistance to tan spot induced by a race 2 isolate, 86-124, were identified in three tetraploid wheat mapping populations. Durum is a tetraploid species of wheat and an important food crop. Tan spot, caused by the necrotrophic fungal pathogen Pyrenophora tritici-repentis (Ptr), is a major foliar disease of both tetraploid durum wheat and hexaploid bread wheat. Understanding the Ptr-wheat interaction and identifying major QTL can facilitate the development of resistant cultivars and effectively mitigate the negative effect of this disease. Over 100 QTL have already been discovered in hexaploid bread wheat, whereas few mapping studies have been conducted in durum wheat. Utilizing resistant resources and identifying novel resistant loci in tetraploid wheat will be beneficial for the development of tan spot-resistant durum varieties. In this study, we evaluated four interconnected tetraploid wheat populations for their reactions to the race 2 isolate 86-124, which produces Ptr ToxA. Tsn1, the wheat gene that confers sensitivity to Ptr ToxA, was not associated with tan spot severity in any of the four populations. We found a total of 12 tan spot-resistant QTL among the three mapping populations. The QTL located on chromosomes 3A and 5A were detected in multiple populations and co-localized with race-nonspecific QTL identified in other mapping studies. Together, these QTL can confer high levels of resistance and can be used for the improvement in tan spot resistance in both hexaploid bread and durum wheat breeding. Two QTL on chromosomes 1B and 7A, respectively, were found in one population when inoculated with a ToxA knockout strain 86-124ΔToxA only, indicating that their association with tan spot was induced by other unidentified necrotrophic effectors, but under the absence of Ptr ToxA. In addition to removal of the known dominant susceptibility genes, integrating major race-nonspecific resistance loci like the QTL identified on chromosome 3A and 5A in this study could confer high and stable tan spot resistance in durum wheat.

Genetic Mapping and Prediction Analysis of FHB Resistance in a Hard Red Spring Wheat Breeding Population.

Fusarium head blight (FHB) is one of the most destructive diseases in wheat worldwide. Breeding for FHB resistance is hampered by its complex genetic architecture, large genotype by environment interaction, and high cost of phenotype screening. Genomic selection (GS) is a powerful tool to enhance improvement of complex traits such as FHB resistance. The objectives of this study were to (1) investigate the genetic architecture of FHB resistance in a North Dakota State University (NDSU) hard red spring wheat breeding population, (2) test if the major QTL Fhb1 and Fhb5 play an important role in this breeding population; and (3) assess the potential of GS to enhance breeding efficiency of FHB resistance. A total of 439 elite spring wheat breeding lines from six breeding cycles were genotyped using genotyping-by-sequencing (GBS) and 102,147 SNP markers were obtained. Evaluation of FHB severity was conducted in 10 unbalanced field trials across multiple years and locations. One QTL for FHB resistance was identified and located on chromosome arm 1AL, explaining 5.3% of total phenotypic variation. The major type II resistance QTL Fhb1 only explained 3.1% of total phenotypic variation and the QTL Fhb5 was not significantly associated with FHB resistance in this breeding population. Our results suggest that integration of many genes with medium/minor effects in this breeding population should provide stable FHB resistance. Genomic prediction accuracies of 0.22-0.44 were obtained when predicting over breeding cycles in this study, indicating the potential of GS to enhance the improvement of FHB resistance.

Genome-Wide Association and Prediction of Grain and Semolina Quality Traits in Durum Wheat Breeding Populations.

Grain yield and semolina quality traits are essential selection criteria in durum wheat breeding. However, high phenotypic screening costs limit selection to relatively few breeding lines in late generations. This selection paradigm confers relatively low selection efficiency due to the advancement of undesirable lines into expensive yield trials for grain yield and quality trait testing. Marker-aided selection can enhance selection efficiency, especially for traits that are difficult or costly to phenotype. The aim of this study was to identify major quality trait quantitative trait loci (QTL) for marker-assisted selection (MAS) and to explore potential application of genomic selection (GS) in a durum wheat breeding program. In this study, genome-wide association mapping was conducted for five quality traits using 1184 lines from the North Dakota State University (NDSU) durum wheat breeding program. Several QTL associated with test weight, semolina color, and gluten strength were identified. Genomic selection models were developed and forward prediction accuracies of 0.27 to 0.66 were obtained for the five quality traits. Our results show the potential for grain and semolina quality traits to be selected more efficiently through MAS and GS with further refinement. Considerable opportunity exists to extend these techniques to other traits such as grain yield and agronomic characteristics, further improving breeding efficiency in durum cultivar development.

Dissecting Plant Chromosomes by the Use of Ionizing Radiation.

Radiation treatment of genomes is used to generate chromosome breaks for numerous applications. This protocol describes the preparation of seeds and the determination of the optimal level of irradiation dosage for the creation of a radiation hybrid (RH) population. These RH lines can be used to generate high-resolution physical maps for the assembly of sequenced genomes as well as the fine mapping of genes. This procedure can also be used for mutation breeding and forward/reverse genetics.

Novel nuclear-cytoplasmic interaction in wheat (Triticum aestivum) induces vigorous plants.

Interspecific hybridization can be considered an accelerator of evolution, otherwise a slow process, solely dependent on mutation and recombination. Upon interspecific hybridization, several novel interactions between nuclear and cytoplasmic genomes emerge which provide additional sources of diversity. The magnitude and essence of intergenomic interactions between nuclear and cytoplasmic genomes remain unknown due to the direction of many crosses. This study was conducted to address the role of nuclear-cytoplasmic interactions as a source of variation upon hybridization. Wheat (Triticum aestivum) alloplasmic lines carrying the cytoplasm of Aegilops mutica along with an integrated approach utilizing comparative quantitative trait locus (QTL) and epigenome analysis were used to dissect this interaction. The results indicate that cytoplasmic genomes can modify the magnitude of QTL controlling certain physiological traits such as dry matter weight. Furthermore, methylation profiling analysis detected eight polymorphic regions affected by the cytoplasm type. In general, these results indicate that novel nuclear-cytoplasmic interactions can potentially trigger an epigenetic modification cascade in nuclear genes which eventually change the genetic network controlling physiological traits. These modified genetic networks can serve as new sources of variation to accelerate the evolutionary process. Furthermore, this variation can synthetically be produced by breeders in their programs to develop epigenomic-segregating lines.