Genetic Dissection of Adult Plant Resistance to Sharp Eyespot Using an Updated Genetic Map of Niavt14 × Xuzhou25 Winter Wheat Recombinant Inbred Line Population

Wheat sharp eyespot, a disease mainly caused by soil-borne fungus Rhizoctonia cerealis, is a threat to world wheat production. Wheat’s genetic resistance to sharp eyespot is a potential approach to reducing the application of fungicides and farming practice inputs. To identify the genetic basis of sharp eyespot resistance in Niavt14, a recombinant inbred line population comprising 215 F8 lines from Niavt14/Xuzhou25 was developed. An earlier linkage map (148 SSR markers) was updated with 5,792 polymorphic Affymetrix AxiomTM 55K SNPs to a new map of 5684.2 cM with 1,406 non-redundant markers. The new linkage map covered all 21 chromosomes of common wheat and showed a good collinearity with the IWGSC RefSeq v1.0 genome. We conducted quantitative trait locus (QTL) mapping for sharp eyespot resistance using the adult plant response data from the field of five consecutive growing seasons and one greenhouse test. Two stable QTL on chromosomes 2B and 7D that were identified in the previous study were confirmed, and three novel stable QTL, explaining 4.0 to 17.5% phenotypic variation, were mapped on 1D, 6D, and 7A, which were independent of QTL for phenology and plant height. The QTL on 1D, 2B, 6D, and 7A showed low frequencies in 384 landraces (0-10%) and 269 elite cultivars (5-23%) from the southern winter wheat region and the Yellow and Huai River Valley facultative wheat region in China, respectively. These identified QTL could be used in wheat breeding programs for improving sharp eyespot resistance through marker-assisted selection.

Development of a Rapid Approach for Detecting Sharp Eyespot Resistance in Seedling-Stage Wheat and Its Application in Chinese Wheat Cultivars

Sharp eyespot, caused by Rhizoctonia cerealis, has become one of the most severe diseases affecting global wheat production in recent decades. Quick and efficient screening methods are required to accelerate the development of cultivars for sharp eyespot resistance in wheat breeding. Here, a two-step colonized wheat kernels (TSCWK) method for the inoculation and classification of sharp eyespot resistance in seedlings was established in a greenhouse. After preliminary verification of the reliability of the method in two replicates, 196 wheat cultivars were assessed for sharp eyespot resistance, and significant correlations were identified among the four replicates (r = 0.78 to 0.84; P < 0.01). Furthermore, the 196 cultivars were scored for sharp eyespot resistance at the milk-ripe stage using traditional toothpick inoculation in the field. Correlation and linear regression analysis showed that the application of this approach at the seedling stage showed good consistency with the traditional field method. Moreover, the scoring of 442 cultivars using the TSCWK method indicated that most cultivars from the Huanghuai valley were susceptible to R. cerealis, suggesting an urgent need to improve sharp eyespot resistance in this region. Additionally, the relative resistance index of sharp eyespot decreased in the surveyed cultivars of the region with time. This study offers a rapid and effective approach for the identification of wheat sharp eyespot resistance and provides valuable germplasm for improving sharp eyespot resistance in wheat breeding.

Yield reduction historically associated with the Aegilops ventricosa 7DV introgression is genetically and physically distinct from the eyespot resistance gene Pch1

Key message Yield penalty and increased grain protein content traits associated with Aegilops ventricosa 7D introgression have been mapped for the first time, and they are physically distinct from the eyespot resistance locus Pch1. Abstract Wheat wild relatives represent an important source of genetic variation, but introgression of agronomically relevant genes, such as for disease resistance, may lead to the simultaneous introduction of genetically linked deleterious traits. Pch1 is a dominant gene, conferring resistance to eyespot and was introgressed to wheat from Aegilops ventricosa as part of a large segment of the 7DV chromosome. This introgression has been associated with a significant yield reduction and a concomitant increase in grain protein content. In this study, we evaluated both traits and their relationship to the location of the Pch1 gene. We found that both QTLs were clearly distinct from the Pch1 gene, being located on a different linkage group to Pch1. In addition, we found that the QTL for increased grain protein content was strong and consistent across field trials, whereas the yield penalty QTL was unstable and environmentally dependent. The yield and grain protein content QTLs were genetically linked and located in the same linkage group. This finding is due in part to the small size of the population, and to the restricted recombination between wheat 7D and Ae. ventricosa 7Dv chromosomes. Although recombination in this interval is rare, it does occur. A recombinant line containing Pch1 and 7D_KASP6, the marker associated with increase in grain protein content, but not Xwmc221, the marker associated with the yield penalty effect, was identified.

How do eyespot resistance genes transferred into winter wheat breeding lines affect their yield?

Eyespot can reduce yields, even up to 50%. There are four genetically characterized resistances in wheat varieties, controlled by: (1) the Pch1 gene, transferred from Aegilops ventricosa; (2) the Pch2 gene, originating from wheat variety Capelle Desprez; (3) the Pch3 gene, originating from Dasypyrum villosum; and (4) the Q.Pch.jic-5A gene, a quantitative trait locus (QTL) located on chromosome 5A of Capelle Desprez. However, those loci have drawbacks, such as linkage of Pch1 with deleterious traits and limited effectiveness of Pch2 against the disease. Here we present an initial study which aims to characterize wheat pre-registration breeding lines carrying 12 eyespot resistance genes, consider their resistance expression in inoculation tests and the influence of resistance genotypes on the yield. We selected four groups of breeding lines, carrying: (1) the Pch1 gene alone: one line; (2) the Pch2 gene alone: four lines; (3) the Q.Pch.jic-5A gene alone: one line; and (4) Pch1 + Q.Pch.jic-5A: three lines. For the first time, the effect of the combination of Pch1 and Q.Pch.jic-5A genes was compared with resistance conferred by Pch1 or Q.Pch.jic-5A alone. We found significant differences between infection scores evaluated in resistant lines carrying Pch1 and Q.Pch.jic-5A alone, while no differences in terms of the level of resistance expression were detected between Pch1 alone and Pch1 + Q.Pch.jic-5A, and between wheat lines carrying Pch1 and Pch2 alone. Moreover, we demonstrated that the Pch1 gene, together with an Ae. ventricosa segment, caused statistically significant yield losses, both as a single eyespot resistance source or in a combination with Q.Pch.jic-5A. Yield scores showed that wheat lines with Q.Pch.jic-5A had the highest yields, similar to the yielding potential of Pch2-bearing lines and control varieties.