Quantitative Trait Loci For Partial Resistance Research Articles

High-resolution mapping of genes involved in plant stage-specific partial resistance of barley to leaf rust

Partial resistance quantitative trait loci (QTLs) Rphq11 and rphq16 against Puccinia hordei isolate 1.2.1 were previously mapped in seedlings of the mapping populations Steptoe/Morex and Oregon Wolfe Barleys, respectively. In this study, QTL mapping was performed at adult plant stage for the two mapping populations challenged with the same rust isolate. The results suggest that Rphq11 and rphq16 are effective only at seedling stage, and not at adult plant stage. The cloning of several genes responsible for partial resistance of barley to P. hordei will allow elucidation of the molecular basis of this type of plant defence. A map-based cloning approach requires to fine-map the QTL in a narrow genetic window. In this study, Rphq11 and rphq16 were fine-mapped using an approach aiming at speeding up the development of plant material and simplifying its evaluation. The plant materials for fine-mapping were identified from early plant materials developed to produce QTL-NILs. The material was first selected to carry the targeted QTL in heterozygous condition and susceptibility alleles at other resistance QTLs in homozygous condition. This strategy took four to five generations to obtain fixed QTL recombinants (i.e., homozygous resistant at the Rphq11 or rphq16 QTL alleles, homozygous susceptible at the non-targeted QTL alleles). In less than 2 years, Rphq11 was fine-mapped into a 0.2-cM genetic interval and a 1.4-cM genetic interval for rphq16. The strongest candidate gene for Rphq11 is a phospholipid hydroperoxide glutathione peroxidase. Thus far, no candidate gene was identified for rphq16.

English

Many quantitative trait loci (QTLs) in different barley populations were discovered for resistance to Puccinia hordei and heterologous rust species. Partial resistance (host basal resistance) and nonhost resistance (nonhost basal resistance) in barley to leaf rusts are based on prehaustorial mechanism of resistance which is associated with papillae formation. They are mainly governed by genes with relatively small, quantitative effects, located on QTL. The genes for host basal resistance seem to play similar roles in basal resistance as those governing nonhost basal resistances. From different studies it was observed that these two resistance types are based on shared principles. Four and two quantitative trait loci-near isogenic lines (QTL-NILs), respectively, were developed for basal resistance and nonhost resistance QTLs using SusPtrit as recurrent parent. SusPtrit is a research line which is exceptionally susceptible to leaf rusts for which normally barley is a nonhost. They were infected with one homologous ( P. hordei isolate 1.2.1) and three heterologous ( Puccinia triticina isolate ‘Flamingo’, P. hordei -murini and P. hordei -secalini) leaf rusts in three replications at seedling stage to evaluate whether relatively large-effect QTLs show specificity in their reaction to homologous and heterologous rust isolates. The result showed that, the QTLs for host basal resistance and the QTL for nonhost basal resistance have a significant (P<0.05) effect on both homologous and heterologous rusts. Also, higher positive (P<0.05, r = 0.98) correlation was observed between different macroscopic and microscopic parameters measured indicating that there is a possible association between partial resistance quantitative trait loci and nonhost resistance quantitative trait loci. These give an indication that indeed, host and nonhost basal resistance are associated. Keywords: Partial resistance, nonhost resistance, near isogenic lines, barley

Characterization of resistance genes to rice blast fungus Magnaporthe oryzae in a “Green Revolution” rice variety

The indica rice variety Dee Geo Woo Gen (DGWG) was the source of the semidwarf gene (SD1) which played an important role in the Green Revolution. In the present study, resistance (R) genes to the US race (isolate) IB54 of Magnaporthe oryzae, causal agent of rice blast disease, was investigated. Two recombinant inbred line mapping populations, consisting of 175 and 224 individuals derived from crosses of DGWG with the straw hull weedy rice type PI653435 (AR-2001-1135; S population) and the black hull type PI653419 (MS-1996-9; B population), respectively, were used for mapping blast R genes and quantitative trait loci (QTLs). Two high-resolution linkage maps with 6,513 (S population) and 14,382 (B population) single nucleotide polymorphic markers derived from genotyping-by-sequencing data were used to map R genes. Two partial resistance QTLs, qBR1.1 and qBR6.1, and one major resistance QTL, qBR11.1, were identified in the B population. One partial resistance QTL, qBR6.1, and one major resistance QTL, qBR11.1, were confirmed with the S population. The total phenotypic variation of three resistance QTLs was 51 %, ranging from 1.12 to 47.62 %, in the B population. All three resistance QTLs were localized to relatively small genomic regions. The major resistance QTL, qBR11.1, was mapped to a 129-kb region on chromosome 11 near nine known blast R genes. Within this 129-kb region, three genes encoding putative nucleotide-binding site and leucine-rich repeat (LRR) disease resistance proteins and three genes encoding WRKY transcription factors WRKY61, WRKY63, and WRKY41 were identified as candidate genes of qBR11.1 and tentatively designated as Pi66(t). Identification of blast R genes in DGWG should help continued deployment of useful genes for improving crop productivity and resistance to rice blast disease.

Joint linkage QTL analyses for partial resistance to Phytophthora sojae in soybean using six nested inbred populations with heterogeneous conditions

Partial resistance to Phytophthora sojae in soybean is controlled by multiple quantitative trait loci (QTL). With traditional QTL mapping approaches, power to detect such QTL, frequently of small effect, can be limited by population size. Joint linkage QTL analysis of nested recombinant inbred line (RIL) populations provides improved power to detect QTL through increased population size, recombination, and allelic diversity. However, uniform development and phenotyping of multiple RIL populations can prove difficult. In this study, the effectiveness of joint linkage QTL analysis was evaluated on combinations of two to six nested RIL populations differing in inbreeding generation, phenotypic assay method, and/or marker set used in genotyping. In comparison to linkage analysis in a single population, identification of QTL by joint linkage analysis was only minimally affected by different phenotypic methods used among populations once phenotypic data were standardized. In contrast, genotyping of populations with only partially overlapping sets of markers had a marked negative effect on QTL detection by joint linkage analysis. In total, 16 genetic regions with QTL for partial resistance against P. sojae were identified, including four novel QTL on chromosomes 4, 9, 12, and 16, as well as significant genotype-by-isolate interactions. Resistance alleles from PI 427106 or PI 427105B contributed to a major QTL on chromosome 18, explaining 10-45% of the phenotypic variance. This case study provides guidance on the application of joint linkage QTL analysis of data collected from populations with heterogeneous assay conditions and a genetic framework for partial resistance to P. sojae.

Detection of partial resistance quantitative trait loci against Didymella pinodes in Medicago truncatula

Ascochyta blight caused by Didymella pinodes (formerly Mycosphaerella pinodes) is one of the most important fungal diseases of pea (Pisum sativum) worldwide that can also infect the model legume Medicago truncatula. The objective of this study was to identify quantitative trait loci (QTLs) controlling resistance to D. pinodes in M. truncatula. Response to D. pinodes was studied under controlled conditions in seedlings of a population derived from the cross J6 × F83005.5, two M. truncatula lines that are, respectively, resistant and susceptible to D. pinodes. A combined map using two different recombinant inbred line populations was then used to identify the genomic regions bearing putative QTLs and to improve the position of the QTLs. A single QTL associated with resistance to D. pinodes was detected on linkage group 2, explaining up to 13 % of the total phenotypic variation for relative disease severity against the pathogen. Two simple sequence repeat markers, MTE80 and mtic890 (3 cM apart) were the ones most significantly associated with the QTL. These markers are located in bacterial artifical chromosomes AC119409 and AC125474, respectively, both of them overlapping on M. truncatula chromosome 2. The integration of QTL analysis and genomics in M. truncatula will contribute to the development of new markers and facilitate the identification of candidate genes for Ascochyta blight resistance.

Durability of plant major resistance genes to pathogens depends on the genetic background, experimental evidence and consequences for breeding strategies

* The breakdown of plant resistance by pathogen populations is a limit to the genetic control of crop disease. Polygenic resistance is postulated as a durable alternative to defeated major resistance genes. Here, we tested this postulate in the pepper-Potato virus Y interaction. * The virus was selected for virulence towards monogenic and polygenic host resistance, using serial inoculations in laboratory and in natural epidemic conditions. The frequency of resistance breakdown and the genetic changes in the virus avirulence gene were analysed. * The monogenic resistance provided by the pvr2(3) gene was defeated at high frequency when introgressed in a susceptible genetic background whereas it was not when combined to partial resistance quantitative trait loci. The suppression of emergence of virulent mutants because of the genetic background resulted both from a differential selection effect and the necessity for the virus to generate multiple mutations. The virus adaptation to the polygenic resistance required a step-by-step selection with a primary selection for virulence towards the major gene, followed by selection for adaptation to the genetic background. * Polygenic resistance proved more durable than monogenic resistance, but breeding strategies giving priority to major resistance factors may jeopardize the progress in durability expected from polygenic resistance.

A Genome-Wide Meta-Analysis of Rice Blast Resistance Genes and Quantitative Trait Loci Provides New Insights into Partial and Complete Resistance

The completion of the genome sequences of both rice and Magnaporthe oryzae has strengthened the position of rice blast disease as a model to study plant-pathogen interactions in monocotyledons. Genetic studies of blast resistance in rice were established in Japan as early as 1917. Despite such long-term study, examples of cultivars with durable resistance are rare, partly due to our limited knowledge of resistance mechanisms. A rising number of blast resistance genes and quantitative trait loci (QTL) have been genetically described, and some have been characterized during the last 20 years. Using the rice genome sequence, can we now go a step further toward a better understanding of the genetics of blast resistance by combining all these results? Is such knowledge appropriate and sufficient to improve breeding for durable resistance? A review of bibliographic references identified 85 blast resistance genes and approximately 350 QTL, which we mapped on the rice genome. These data provide a useful update on blast resistance genes as well as new insights to help formulate hypotheses about the molecular function of blast QTL, with special emphasis on QTL for partial resistance. All these data are available from the OrygenesDB database.

Isolate specificity of quantitative trait loci for partial resistance of barley to Puccinia hordei confirmed in mapping populations and near‐isogenic lines

Partial resistance is considered race-nonspecific and durable, consistent with the concept of 'horizontal' resistance. However, detailed observations of partial resistance to leaf rust (Puccinia hordei) in barley (Hordeum vulgare) revealed small cultivar x isolate interactions, suggesting a minor-gene-for-minor-gene interaction model, similar to so-called 'vertical' resistance. Three consistent quantitative trait loci (QTLs), labelled Rphq2, Rphq3 and Rphq4, that were detected in the cross susceptible L94 x partially resistant Vada have been incorporated into the L94 background to obtain near-isogenic lines (NILs). Three isolates were used to map QTLs on seedlings of the L94 x Vada population and to evaluate the effect of each QTL on adult plants of the respective NILs under field conditions. Rphq2 had a strong effect in seedlings but almost no effect in adult plants, while Rphq3 was effective in seedlings and in adult plants against all three isolates. However, Rphq4 was effective in seedlings and in adult plants against two isolates but ineffective in both development stages against the third, demonstrating a clear and reproducible isolate-specific effect. The resistance governed by the three QTLs was not associated with a hypersensitive reaction. Those results confirm the minor-gene-for-minor-gene model suggesting specific interactions between QTLs for partial resistance and P. hordei isolates.

RFLP mapping of genes conferring complete and partial resistance to blast in a durably resistant rice cultivar.

Moroberekan, a japonica rice cultivar with durable resistance to blast disease in Asia, was crossed to the highly susceptible indica cultivar, CO39, and 281 F7 recombinant inbred (RI) lines were produced by single seed descent. The population was evaluated for blast resistance in the greenhouse and the field, and was analyzed with 127 restriction fragment length polymorphism (RFLP) markers. Two dominant loci associated with qualitative resistance to five isolates of the fungus were tentatively named Pi-5(t) and Pi-7(t). They were mapped on chromosomes 4 and 11, respectively. To identify quantitative trait loci (QTLs) affecting partial resistance, RI lines were inoculated with isolate PO6-6 of Pyricularia oryzae in polycyclic tests. Ten chromosomal segments were found to be associated with effects on lesion number (P < 0.0001 and LOD > 6.0). Three of the markers associated with QTLs for partial resistance had been reported to be linked to complete blast resistance in previous studies. QTLs identified in greenhouse tests were good predictors of blast resistance at two field sites. This study illustrates the usefulness of RI lines for mapping a complex trait such as blast resistance and suggests that durable resistance in the traditional variety, Moroberekan, involves a complex of genes associated with both partial and complete resistance.