Identification and Validation of Quantitative Trait Loci for Wheat Dwarf Virus Resistance in Wheat (Triticum spp.)

The Past, Present, and Future of Wheat Dwarf Virus Management-A Review

Wheat dwarf disease (WDD) is an important disease of monocotyledonous species, including economically important cereals. The causative pathogen, wheat dwarf virus (WDV), is persistently transmitted mainly by the leafhopper Psammotettix alienus and can lead to high yield losses. Due to climate change, the periods of vector activity increased, and the vectors have spread to new habitats, leading to an increased importance of WDV in large parts of Europe. In the light of integrated pest management, cultivation practices and the use of resistant/tolerant host plants are currently the only effective methods to control WDV. However, knowledge of the pathosystem and epidemiology of WDD is limited, and the few known sources of genetic tolerance indicate that further research is needed. Considering the economic importance of WDD and its likely increasing relevance in the coming decades, this study provides a comprehensive compilation of knowledge on the most important aspects with information on the causal virus, its vector, symptoms, host range, and control strategies. In addition, the current status of genetic and breeding efforts to control and manage this disease in wheat will be discussed, as this is crucial to effectively manage the disease under changing environmental conditions and minimize impending yield losses.

Identification of Markers Associated with Wheat Dwarf Virus (WDV) Tolerance/Resistance in Barley (Hordeum vulgare ssp. vulgare) Using Genome-Wide Association Studies

Wheat dwarf virus (WDV) causes an important vector transmitted virus disease, which leads to significant yield losses in barley production. Due to the fact that, at the moment, no plant protection products are approved to combat the vector Psammotettix alienus, and this disease cannot be controlled by chemical means, the use of WDV-resistant or -tolerant genotypes is the most efficient method to control and reduce the negative effects of WDV on barley growth and production. In this study, a set of 480 barley genotypes were screened to identify genotypic differences in response to WDV, and five traits were assessed under infected and noninfected conditions. In total, 32 genotypes showed resistance or tolerance to WDV. Subsequently, phenotypic data of 191 out of 480 genotypes combined with 34,408 single-nucleotide polymorphisms (SNPs) were used for a genome-wide association study to identify quantitative trait loci (QTLs) and markers linked to resistance/tolerance to WDV. Genomic regions significantly associated with WDV resistance/tolerance in barley were identified on chromosomes 3H, 4H, 5H, and 7H for traits such as relative virus titer, relative performance of total grain weight, plant height, number of ears per plant, and thousand grain weight.

Evaluation of Resistance of Oilseed Rape Genotypes to Turnip Yellows Virus

Turnip yellows virus (TuYV), is one of the most important pathogens of oilseed rape, which has caused enormous yield losses in all growing regions of the world in recent years. Therefore, there is a need for resistant varieties for sustainable crop protection. We have investigated the resistance of known varieties and newly developed advanced-breeding lines of oilseed rape to TuYV in greenhouse and field trials. We have analysed the TuYV titre of individual genotypes inoculated with the virus using viruliferous aphids Myzus persicae. The genotypes 'DK Temptation' and 'Rescator' had the lowest and highest virus titres, respectively, and were used as resistant and susceptible models for comparative analyses with other genotypes. In the greenhouse, the best results were obtained with the genotypes 'OP-8143 DH' (2.94 × 10⁵ copies), OP-BN-72 (3.29 × 10⁵ copies), 'Navajo' (3.58 × 10⁵ copies) and 'SG-C 21215' (4.09 × 10⁵ copies), which reached virus titres about 2 times higher than the minimum virus concentration measured in 'DK Temptation' (1.80 × 10⁵ copies). In the field trials, the genotypes 'Navajo' (3.39 × 10⁵ copies), 'OP-8148 DH' (4.44 × 10⁵ copies), 'SG-C 21215' (6.80 × 10⁵ copies) and OP-8480 (7.19 × 10⁵ copies) had the lowest virus titres and reached about 3 times the virus titre of DK Temptation (2.54 × 10⁵ copies). Both trials showed that at least two commercial varieties (e.g., DK Temptation, Navajo) and three advanced breeding lines (e.g., OP-8143 DH, OP-BN-72, SG-C 21215) had low titres of the virus after TuYV infection. This indicates a high level of resistance to TuYV in 'Navajo' or the newly developed breeding lines and the basis of resistance is probably different from R54 (as in 'DK Temptation'). Furthermore, the greenhouse trials together with RT -qPCR-based virus titre analysis could be a cost-effective and efficient method to assess the level of resistance of a given genotype to TuYV infection compared to the field trials. However, further research is needed to identify the underlying mechanisms causing this difference in susceptibility.

Two major quantitative trait loci control wheat dwarf virus resistance in four related winter wheat populations

KEY MESSAGE

Qwdv.ifa-6A on chromosomes 6AL and Qwdv.ifa-1B on chromosome 1B are highly effective against wheat dwarf virus and act additively when combined. Wheat dwarf virus (WDV) is among the most damaging viral pathogens. Its prevalence has increased substantially in recent years, and it is predicted to increase even further due to global warming. There are limited options to control the virus. Growing resistant cultivars would safeguard crops, but most current wheat cultivars are highly susceptible. Thus, the aim of this study was to dissect the genetic architecture of WDV resistance in resistant germplasm and to identify quantitative trait loci (QTL) to support resistance breeding. QTL mapping was conducted using four related populations comprising 168, 105, 99 and 130 recombinant inbred lines. Populations were evaluated under field conditions for three years. Natural infestation was provoked by early autumn sowing. WDV symptom severity was visually assessed at two time points in spring. QTL analysis revealed two highly significant QTL with the major QTL Qwdv.ifa-6A mapping to the long arm of chromosome 6A between markers Tdurum_contig75700_411 (601,412,152 bp) and AX-95197581 (605,868,853 bp). Qwdv.ifa-6A descends from the Dutch experimental line SVP-72017 and was of high effect in all populations, explaining up to 73.9% of the phenotypic variance. The second QTL, Qwdv.ifa-1B, mapped to chromosome 1B and is putatively associated with the 1RS.1BL translocation, which was contributed by the CIMMYT line CM-82036. Qwdv.ifa-1B explained up to 15.8% of the phenotypic variance. Qwdv.ifa-6A and Qwdv.ifa-1B are among the first identified highly effective resistance QTL and represent valuable resources for improving WDV resistance in wheat.

Transcriptome Dynamics in Triticum aestivum Genotypes Associated with Resistance against the Wheat Dwarf Virus

Wheat dwarf virus (WDV) is one of the most important pathogens of cereal crops worldwide. To understand the molecular mechanism of resistance, here we investigated the comparative transcriptome of wheat genotypes with different levels of resistance (Svitava and Fengyou 3) and susceptibility (Akteur) to WDV. We found a significantly higher number of differentially expressed transcripts (DETs) in the susceptible genotype than in the resistant one (e.g., Svitava). The number of downregulated transcripts was also higher in the susceptible genotype than in the resistant one (Svitava) and the opposite was true for the upregulated transcripts. Further functional analysis of gene ontology (GO) enrichment identified a total of 114 GO terms for the DETs. Of these, 64 biological processes, 28 cellular components and 22 molecular function GO terms were significantly enriched. A few of these genes appear to have a specific expression pattern related to resistance or susceptibility to WDV infection. Validation of the expression pattern by RT-qPCR showed that glycosyltransferase was significantly downregulated in the susceptible genotype compared to the resistant genotypes after WDV infection, while CYCLIN-T1-3, a regulator of CDK kinases (cyclin-dependent kinase), was upregulated. On the other hand, the expression pattern of the transcription factor (TF) MYB (TraesCS4B02G174600.2; myeloblastosis domain of transcription factor) was downregulated by WDV infection in the resistant genotypes compared to the susceptible genotype, while a large number of TFs belonging to 54 TF families were differentially expressed due to WDV infection. In addition, two transcripts (TraesCS7A02G341400.1 and TraesCS3B02G239900.1) were upregulated with uncharacterised proteins involved in transport and regulation of cell growth, respectively. Altogether, our findings showed a clear gene expression profile associated with resistance or susceptibility of wheat to WDV. In future studies, we will explore the regulatory network within the same experiment context. This knowledge will broaden not only the future for the development of virus-resistant wheat genotypes but also the future of genetic improvement of cereals for resilience and WDV-resistance breeding.