Race Non-specific Resistance Research Articles

Phytopathological screening of common winter wheat lines for resistance to Puccinia triticina

Abstract. Leaf rust on wheat caused by P. triticina is the most widespread fungal disease worldwide and in Bulgaria. The development of wheat genotypes resistant to leaf rust is related to the different types of resistance and ways of gene inheritance. The different varieties have different susceptibility during the different stages of their ontogenetic development. The study of the initial material is an important step in the breeding for resistance and it is an ongoing process. Two hundred and forty-six breeding lines were tested against the local population of P. triticina during 2015-2017, under conditions of infection field and in a greenhouse at Dobrudzha Agricultural Institute – General Toshevo, Bulgaria. Twenty lines were selected, which demonstrated different degrees of resistance under infection field conditions and which showed resistance to some of the seven pathotypes used in the study. The identification of the pathotypes was based on the type of reaction of 20 monogenic lines. The reaction of isogenic lines was compared with the reaction of the selection lines and the results showed that the reaction of line 165/05-46 corresponded to isogenic lines carrying genes Lr 9, Lr 19 and Lr 25. The response of the other 3 lines (245/03-72, 13/04-97, 14/06-110) partially coincided with the response of the above mentioned genes. The reaction of the other 3 lines (169/05-69, 14/06-81, 8/08-25) partially coincided with the reaction of isogenic lines carrying genes Lr 2a, Lr 28 and Lr 29. It was determined that seven of the lines were carriers of a race-specific gene and that it was necessary to subject these lines to further breeding and genetic studies to determine presence of race-specific resistance. Five of the lines (72/05-53, 61/03-75, 106/04-189, 268/03-151, 79/04-131) combined incomplete resistance at young stage with resistance at adult stage. Four of the tested lines (68/06-78, 29/02-133-38, 79/04-125-28, 197/05-33) exhibited typical race non-specific resistance or APR. Four lines (13/04-165, 14/05-119, 19/06-79 and 61/03-81) demonstrated “slow rusting” type of resistance. Twenty of the studied lines were carriers of effective resistance and can be included in the breeding programs as donors of resistance to P. triticina.

Race specific and non-specific resistance to Magnaporthe oryzae and QTL mapping in wild introgression lines using the standard differential system

Race specific and non-specific resistance to Magnaporthe oryzae and QTL mapping in wild introgression lines using the standard differential system

QTL Mapping for Adult-Plant Resistance to Leaf Rust in Italian Wheat Cultivar Libellula.

Wheat leaf rust (Lr), which is caused by Puccinia triticina Eriks. (Pt), is one of the most important wheat diseases affecting wheat production globally. Using resistant wheat cultivars is the most economical and environmentally friendly way to control leaf rust. The Italian wheat cultivar Libellula has demonstrated good resistance to Lr in field studies. To identify the genetic basis of Lr resistance in 'Libellula', 248 F6 recombinant inbred lines from the cross 'Libellula'/'Huixianhong' was phenotyped for Lr severity in seven environments: the 2014/2015, 2016/2017, 2017/2018, and 2018/2019 cropping seasons at Baoding, Hebei Province, and the 2016/2017, 2017/2018, and 2018/2019 crop seasons at Zhoukou, Henan Province. Bulked segregant analysis and simple sequence repeat markers were then used to identify the quantitative trait loci (QTLs) for Lr adult-plant resistance in the population. Six QTLs were consequently detected and designated as QLr.hebau-1AL and QLr.hebau-1AS that were presumed to be new and QLr.hebau-1BL, QLr.hebau-3AL, QLr.hebau-4BL, and QLr.hebau-7DS that were identified at similar physical positions as previously reported QTLs. Based on chromosome positions and molecular marker tests, QLr.hebau-1BL and QLr.hebau-7DS share similar flanking markers with Lr46 and Lr34, respectively. Lr46 and Lr34 are race nonspecific adult plant resistance (APR) genes for leaf rust and stripe rust and powdery mildew. QLr.hebau-4BL showed multiple disease resistance to leaf rust, stripe rust, Fusarium head blight, and powdery mildew. The QTL identified in this study, as well as their closely linked markers, may potentially be used in marker-assisted selection in wheat breeding.

Seedling and adult plant resistance to leaf rust in some Bulgarian common wheat lines.

The response of 250 common winter wheat breeding lines was investigated for resistance to the causative agent of Puccinia triticina under conditions of an infected field on the territory of Dobrudzha Agricultural Institute - General Toshevo, Bulgaria, during three successive seasons. Twenty lines with different degrees of resistance under field conditions were selected. Multi-pathotype testing was used to study the response of these lines at seedling stage under greenhouse conditions to individual pathotypes of P. triticina. Based on the response of the lines at seedling and adult stages, we found out that 20 % of them carried race-specific resistance. One of the lines (99/08-52) reacted with full resistance to the pathotypes used under greenhouse conditions. The reaction demonstrated by this line coincided with the response of isogenic lines carrying the genes Lr9, Lr19, Lr22a, Lr22b and Lr25. The other three lines (19/06- 108, 82/08-43 and 82/08-35) showed a resistant reaction to 6 or 5 of the pathotypes used in the study. Their response partially coincided with the reaction of 5 isogenic lines, and the presence of some of these genes in the above lines is quite possible. Lines carrying this type of resistance are to be subjected to further genetic and breeding investigations to prove the presence of a race-specific gene. Twenty-five percent of the lines combined partial race-specific resistance at seedling stage with the resistance of race non-specific nature at adult stage. Forty percent of all studied lines carried race non-specific resistance, and 15 % of the lines possessed resistance of the "slow rusting" type. As a result of the study we carried out, the lines that demonstrated stable resistance to leaf rust can provide sufficient protection of the host and can be included in the breeding programs for developing varieties resistant to P. triticina.

Toward combining qualitative race-specific and quantitative race-nonspecific disease resistance by genomic selection

Key messageA novel genomic selection strategy offers the unique opportunity to develop qualitative race-specific resistant varieties that possess high levels of the more durable quantitative race-nonspecific resistance in their genetic background.Race-specific qualitative resistance genes (R-genes) are conferring complete resistance in many pathosystems, but are frequently overcome by new virulent pathogen races. Once the deployed R-genes are overcome, a wide variation of quantitative disease resistance (QDR) can be observed in a set of previously race-specific, i.e., completely resistant genotypes—a phenomenon known as “vertifolia effect.” This race-nonspecific QDR is considered to be more durable in the long term, but provides merely a partial protection against pathogens. This simulation study aimed to detangle race-specific R-gene-mediated resistance of pending selection candidates and the QDR in their genetic background by employing different genomic selection strategies. True breeding values that reflected performance data for rust resistance in wheat were simulated, and used in a recurrent genomic selection based on several prediction models and training population designs. Using training populations that were devoid of race-specific R-genes was thereby pivotal for an efficient improvement of QDR in the long term. Marker-assisted preselection for the presence of R-genes followed by a genomic prediction for accumulating the many small to medium effect loci underlying QDR in the genetic background of race-specific resistant genotypes appeared furthermore to be a promising approach to select simultaneously for both types of resistance. The practical application of such a knowledge-driven genomic breeding strategy offers the opportunity to develop varieties with multiple layers of resistance, which have the potential to prevent intolerable crop losses under epidemic situations by displaying a high level of QDR even when race-specific R-genes have been overcome by evolving pathogen populations.

RESISTANCE IN COMMON BEAN(PHASEOLUS VULGARIS L.) TO UROMYCES APPENDICULATUS IN BULGARIA

Common bean rust, caused by Uromycesappendiculatus, is one of the most devastating diseases in common bean. Breeding for resistant cultivars is the most effective and eco-friendly method for disease control. Although there is no targeted breeding program in Bulgaria to create resistant cultivars to the pathogen, periodic screening of the virulent diversity of U. appendiculatus and sources of resistance to the identified races/pathotypes is carried out. During the last twenty years, four investigations were made to estimate virulence diversity in pathogen population. Eleven U. appendiculatus races (20-0, 20-1, 20-2, 20-3, 20-19, 29-0, 29-1, 28-1, 52-3, 20-16 and 20-18) are identified, distributed in Rhodope mountains and North Bulgaria. Chronologically each of the studies was accompanied or followed by a screening for sources of resistance to the pathogen under field conditions. Еach study found different numbers of resistant accessions depending on the virulent potential of the population used. In three of the studies, cultivars that showed resistant phenotype under field conditions were screened against a set of races, distributed in the country in the same time, under greenhouse. Bulgarian cultivar ‘Beslet’ showed resistant reaction under field and greenhouse in all investigation. The resistance of the cultivar to races 20-1 and 20-19 is governed by a single dominant gene. This cultivar can be successfully used in a breeding program for race-specific resistance. Fifty accessions with race-nonspecific resistance, expressed by low disease intensity and resistant/susceptible infection type, are identified in two investigations under field conditions.

Multiplex PCR assay for the simultaneous identification of race specific and non-specific leaf resistance genes in wheat (Triticum aestivum L.)

Race-nonspecific resistance is a key to sustainable management of pathogens in bread wheat (Triticum aestivum L.) breeding. It is more durable compared to race-specific immunity, conferred by the major genes (R), which are often overcome by pathogens. The accumulation of the genes, which provide the resistance to a specific race of a pathogen, together with the introduction of race-non-specific resistance genes is the most effective strategy aimed at preventing the breakdown of genetically conditioned immunity. PCR markers improved the productivity and accuracy of classical plant breeding by means of marker-assisted selection (MAS). Multiplexing assays provide increased throughput, reduced reaction cost, and conservation of limited sample material, which are beneficial for breeding purposes. Here, we described the process of customizing multiplex PCR assay for the simultaneous identification of the major leaf rust resistance genes Lr19, Lr24, Lr26, and Lr38, as well as the slow rusting, race-nonspecific resistance genes: Lr34 and Lr68, in thirteen combinations. The adaptation of PCR markers for multiplex assays relied on: (1) selection of primers with an appropriate length; (2) selection of common annealing/extension temperature for given primers; and (3) PCR mixture modifications consisting of increased concentration of primers for the scanty band signals or decreased concentration of primers for the strong bands. These multiplex PCR protocols can be integrated into a marker-assisted selection of the leaf rust-resistant wheat genotypes.

Simultaneous editing of three homoeologues of TaCIPK14 confers broad-spectrum resistance to stripe rust in wheat.

Wheat stripe rust caused by the fungus Puccinia striiformis f. sp. tritici (Pst) is one of the most destructive wheat diseases resulting in significant losses to wheat production worldwide. The development of disease-resistant varieties is the most economical and effective measure to control diseases. Altering the susceptibility genes that promote pathogen compatibility via CRISPR/Cas9-mediated gene editing technology has become a new strategy for developing disease-resistant wheat varieties. Calcineurin B-like protein (CBL)-interacting protein kinases (CIPKs) has been demonstrated to be involved in defence responses during plant-pathogen interactions. However, whether wheat CIPK functions as susceptibility factor is still unclear. Here, we isolated a CIPK homoeologue gene TaCIPK14 from wheat. Knockdown of TaCIPK14 significantly increased wheat resistance to Pst, whereas overexpression of TaCIPK14 resulted in enhanced wheat susceptibility to Pst by decreasing different aspects of the defence response, including accumulation of ROS and expression of pathogenesis-relative genes. We generated wheat Tacipk14 mutant plants by simultaneous modification of the three homoeologues of wheat TaCIPK14 via CRISPR/Cas9 technology. The Tacipk14 mutant lines expressed race-nonspecific (RNS) broad-spectrum resistance (BSR) to Pst. Moreover, no significant difference was found in agronomic yield traits between Tacipk14 mutant plants and Fielder control plants under greenhouse and field conditions. These results demonstrate that TaCIPK14 acts as an important susceptibility factor in wheat response to Pst, and knockout of TaCIPK14 represents a powerful strategy for generating new disease-resistant wheat varieties with BSR to Pst.

Harnessing genetic resistance to rusts in wheat and integrated rust management methods to develop more durable resistant cultivars.

Wheat is one of the most important staple foods on earth. Leaf rust, stem rust and stripe rust, caused by Puccini triticina, Puccinia f. sp. graminis and Puccinia f. sp. striiformis, respectively, continue to threaten wheat production worldwide. Utilization of resistant cultivars is the most effective and chemical-free strategy to control rust diseases. Convectional and molecular biology techniques identified more than 200 resistance genes and their associated markers from common wheat and wheat wild relatives, which can be used by breeders in resistance breeding programmes. However, there is continuous emergence of new races of rust pathogens with novel degrees of virulence, thus rendering wheat resistance genes ineffective. An integration of genomic selection, genome editing, molecular breeding and marker-assisted selection, and phenotypic evaluations is required in developing high quality wheat varieties with resistance to multiple pathogens. Although host genotype resistance and application of fungicides are the most generally utilized approaches for controlling wheat rusts, effective agronomic methods are required to reduce disease management costs and increase wheat production sustainability. This review gives a critical overview of the current knowledge of rust resistance, particularly race-specific and non-race specific resistance, the role of pathogenesis-related proteins, non-coding RNAs, and transcription factors in rust resistance, and the molecular basis of interactions between wheat and rust pathogens. It will also discuss the new advances on how integrated rust management methods can assist in developing more durable resistant cultivars in these pathosystems.

Pojava prouzrokovača žutomrke pegavosti lista pšenice (Pyrenophora tritici-repentis) u proizvodnoj 2021/2022. godini

Pyrenophora tritici-repentis (Died.) Drechs, the agent of tan spot, is an economically important pathogen of wheat. It causes grain quality distortion and yield losses up to 53%. Ptr is a necrotrophic pathogen and produces three types of necrotrophic effectors (NEs) (Ptr ToxA, Ptr ToxB and Ptr ToxC) that induce necrosis and chlorosis. The genetic basis of the Ptr-host interaction also includes genes that provide race-nonspecific resistance. In 2022, the percentage of genotypes infected with Ptr in the collections of "disease nurseries" ranged from 11.4% to 94.3%. The lowest number of infected genotypes (11.4%) was recorded in breeding pro-grams for resistance to prevalent pathogens. A low number of genotypes infected with Ptr was also found in the collection of wild relatives (18.4%), while the highest percentage of genotypes susceptible to Ptr was found in varieties originating from Kazakhstan (94.4%). Disease index of Ptr ranged from a trace level up to 50% or more. The length of the Ptr conidia ranged from 73.4 mm to 107.2 mm. Conidia width ranged from 15.5 to 22.8 mm. The results of this study indicated on genotypes that could be used as sources of resistance to Ptr, and also could be introduce in further studies of the mechanisms that provide resistance for Ptr.

Identification of Winter Habit Bread Wheat Landraces in the National Small Grains Collection with Resistance to Emerging Stem Rust Pathogen Variants.

Wheat stem rust caused by Puccinia graminis f. sp. tritici is a widespread and recurring threat to wheat production. Emerging P. graminis f. sp. tritici variants are rapidly overcoming major gene resistance deployed in wheat cultivars and new sources of race-nonspecific resistance are urgently needed. The National Small Grains Collection (NSGC) contains thousands of wheat landrace accessions that may harbor unique and broadly effective sources of resistance to emerging P. graminis f. sp. tritici variants. All NSGC available facultative and winter-habit bread wheat landraces were tested in a field nursery in St. Paul, Minnesota, against a bulk collection of six common U.S. P. graminis f. sp. tritici races. Infection response and severity data were collected on 9,192 landrace accessions at the soft-dough stage and resistant accessions were derived from single spikes. Derived accessions were tested in St. Paul a second time to confirm resistance and in a field nursery in Njoro, Kenya against emerging races of P. graminis f. sp. tritici with virulence to many known resistance genes including Sr24, Sr31, Sr38, and SrTmp. Accessions resistant in the St. Paul field were also tested at the seedling stage with up to 13 P. graminis f. sp. tritici races, including TTKSK and TKTTF, and with 19 molecular markers linked with known stem rust resistance genes or genes associated with modern breeding practices. Forty-five accessions were resistant in both U.S. and Kenya field nurseries and lacked alleles linked with known stem rust resistance genes. Accessions with either moderate or strong resistance in the U.S. and Kenya field nurseries and with novel seedling resistance will be prioritized for further study.

Accumulating candidate genes for broad-spectrum resistance to rice blast in a drought-tolerant rice cultivar

Biotic stresses, including diseases, severely affect rice production, compromising producers’ ability to meet increasing global consumption. Understanding quantitative responses for resistance to diverse pathogens can guide development of reliable molecular markers, which, combined with advanced backcross populations, can accelerate the production of more resistant varieties. A candidate gene (CG) approach was used to accumulate different disease QTL from Moroberekan, a blast-resistant rice variety, into Vandana, a drought-tolerant variety. The advanced backcross progeny were evaluated for resistance to blast and tolerance to drought at five sites in India and the Philippines. Gene-based markers were designed to determine introgression of Moroberekan alleles for 11 CGs into the progeny. Six CGs, coding for chitinase, HSP90, oxalate oxidase, germin-like proteins, peroxidase and thaumatin-like protein, and 21 SSR markers were significantly associated with resistance to blast across screening sites. Multiple lines with different combinations, classes and numbers of CGs were associated with significant levels of race non-specific resistance to rice blast and sheath blight. Overall, the level of resistance effective in multiple locations was proportional to the number of CG alleles accumulated in advanced breeding lines. These disease resistant lines maintained tolerance to drought stress at the reproductive stage under blast disease pressure.

Evaluation of horizontal resistance of sunflower (Helianthus annuus L.) to downy mildew (Plasmopara halstedii)

Downy mildew is one of the most important diseases of sunflower crop around the world caused by Plasmopara halstedii (Farl.) Berl.et de Toni. The aim of our study was to evaluate the horizontal resistance of elite and prospective lines to downy mildew in field conditions. Experiments were conducted at All-Russia Research Institute of Oil Crops (VNIIMK) in 2016–2020. Released and prospective sunflower lines bred at VNIIMK were studied, divided into four groups on the base of their race-specific resistance in laboratory tests. Experimental design was randomized blocks with two replications. Evaluation was made by recording all the recognizable symptoms of downy mildew from emergence till flowering. The results obtained allowed us to identify the lines with the highest race non-specific resistance to downy mildew: VK 678 (5.3%), VK 653 (7.3%), VA 760 (3.3%), VA 93 (2.0%) and VK 732 (6.3%). The final aim is the development of sunflower hybrids with the most durable resistance to downy mildew, combining one parental line with the major gene (Pl15) most effective for the specific location and the second parental line with a high level of horizontal resistance to downy mildew.

RESISTANCE IN COMMON BEAN TO UROMYCES APPENDICULATUS UNDER FIELD AND GREENHOUSE CONDITIONS

Bean rust, caused by Uromyces appendiculatus, is a major disease in common bean which occurs annually in The Rhodope Mountains and sporadic in the plains of Bulgaria. The present study aims to find sources of resistance in common bean to the pathogen for using in a breeding program. The reaction of fifty-five Phaseolus vulgaris accessions to the pathogen was monitored under field condition. Infection type, disease intensity and area under the disease progress curve were calculated. Twelve common bean cultivars were inoculated with eight pathotypes of races 20-2, 20-16, and 20-18 in the greenhouse, and infection type was estimated. Twelve accessions had an immune reaction, eight accessions had resistant a reaction, two accessions had a middle resistant reaction, and seven accessions had a susceptible reaction to U. appendiculatus population in both field estimations. Five cultivars showed resistant phenotype to the eight pathotypes in the greenhouse, four of which were resistant in the field (Abritus, Beslet, Trakiya, and Prelom). Five cultivars had a susceptible or resistant reaction to the pathotypes of the same race, resulting from different interaction between resistant genes in the host and virulent genes in the pathogen. Nine accessions showed race-nonspecific resistance in the field expressed in low disease intensity and susceptible/resistant phenotype.

Differential Gene Expression Analysis of Wheat Breeding Lines Reveal Molecular Insights in Yellow Rust Resistance under Field Conditions

The evolution of pathogens in the changing climate raises new challenges for wheat production. Yellow rust is one of the major wheat diseases worldwide, leading to an increased use of fungicides to prevent significant yield losses. The enhancement of the resistance potential of wheat cultivars is a necessary and environmentally friendly solution for sustainable wheat production. In this study, we aimed to identify the differentially expressed genes induced upon yellow rust infection in the field. Reference and de novo based transcriptome analysis was performed among the resistant and susceptible lines of a bi-parental population to study the global transcriptome changes in contrasting wheat genotypes. Based on the analysis, the de novo transcriptome analysis approach was found to be more supportive for field studies. Expression profiles, gene ontology, KEGG pathway analysis and enrichment studies indicated the relation between differentially expressed genes of wheat and yellow rust infection. The h0igh expression of genes related to non-race specific resistance along with pathogen-specific resistance might be a reason for the better resistance ability of a resistant wheat genotype in the field. The targeted metagenomic analysis of wheat samples revealed that Puccinia striiformis tritici was the most dominant pathogen along with other pathogens on the collected leaf material and validating the disease scoring carried out in the field and transcriptomics analyses.

Влияние сортов озимой пшеницы с разными типами устойчивости к возбудителю бурой ржавчины на изменение структуры популяции по признаку вирулентности

Abstract. A change in the varietal composition of wheat causes response changes in the population of the fungus, which evolves conjugately. Population studies of the causative agent of leaf rust of wheat are carried out in many countries. Comparison of populations of P. triticina is mainly carried out geographically. The purpose of the study was to analyze the dynamics of the virulence of North Caucasus Puccinia triticina populations under the influence of winter wheat varieties with different types of leaf rust resistance. Methods. The study was carried out in 2018 in the experimental field and greenhouses All-Russian Research Institute of Biological Plant Protection. Two varieties of winter wheat: Kralya and Vershina with different types of resistance to leaf rust, were used for the experiment. Cultivar Kralya had race-specific resistance; cultivar a Vershina had race-nonspecific resistance to leaf rust. Virulence analysis of the impact of each cultivar on the structure of the pathogen population based on the results of differentiation of P. triticina populations, collected from each cultivar and compared to the mixed pathogens population. As a result, it was found that the genotype of a winter wheat variety with race-specific resistance significantly affects the virulence of the P. triticina population. The frequency of virulent pathogen isolates to most lines with Lr genes has changed under the influence of driving selection. Therefore, the wide regionalization of varieties with race-specific resistance will contribute to selective pressure on the P. triticina population and the accumulation of virulent phenotypes in it. The genotype of a wheat variety with nonspecific resistance did not cause significant changes in the gene pool of the virulence of the leaf rust pathogen population. In accordance with the obtained values of the Nei index, the pathogen population collected from a variety with race-specific resistance has significant differences with the combined population (N = 0.68), in contrast to the population of P. triticina collected from the Vershina variety (N = 0.41). The results obtained confirm the importance of screening varieties by resistance types for further decision making on their placement. Varieties with race-specific resistance are best used for mosaic placement with subsequent mandatory rotation in time and space. And varieties with nonspecific resistance, which are weakly affected by all races of the pathogen and cannot be carriers of infection, can be used over large areas for a longer time, combined with varieties from various groups, including their own. Scientific novelty. There have been practically no studies on the effect of wheat varieties with different types of resistance on the virulence of rust fungi populations, although this direction is of great practical interest for both selection and varietal placement.

Exploring Mechanisms of Quantitative Resistance to Leptosphaeria maculans (Blackleg) in the Cotyledons of Canola (Brassica napus) Based on Transcriptomic and Microscopic Analyses.

Using resistant cultivars is a common approach to managing blackleg of canola/rapeseed caused by Leptosphaeria maculans (Lm). Quantitative resistance (QR), as opposed to major-gene resistance, is of interest because it is generally more durable, due to its multi-genetic basis. However, the mechanisms and genes underlying QR are mostly unknown. In this study, potential QR modes of action in “74-44 BL” was explored. This Canadian canola cultivar showed moderate but consistent race-nonspecific resistance at the cotyledon and adult-plant stages. A susceptible cultivar, “Westar”, was used as a control. After inoculation, the lesions developed more slowly on the cotyledons of 74-44 BL than those of Westar. We used RNA sequencing (-RNA-seq) to identify genes and their functions, putatively related to this resistance, and found that genes involved in programmed cell death (PCD), reactive oxygen species (ROS), signal transduction or intracellular endomembrane transport were most differentially expressed. ROS production was assessed in relation to Lm hyphal growth and lesion size; it occurred beyond the tissue colonized by Lm in 74-44 BL and appeared to trigger rapid cell death, limiting cotyledon colonization by Lm. In contrast, Lm grew more rapidly in Westar, often catching up with the ring of ROS and surpassing lesion boundaries. It appears that QR in 74-44 BL cotyledons is associated with limited colonization by Lm possibly mediated via ROS. The RNA-seq data also showed a link between ROS, signal transduction, and endomembrane vesicle trafficking, as well as PCD in the resistance. These results provide a starting point for a better understanding of the mechanisms behind QR against Lm in canola.

Identification of novel genetic factors underlying the host-pathogen interaction between barley (Hordeum vulgare L.) and powdery mildew (Blumeria graminis f. sp. hordei)

Powdery mildew is an important foliar disease of barley (Hordeum vulgare L.) caused by the biotrophic fungus Blumeria graminis f. sp. hordei (Bgh). The understanding of the resistance mechanism is essential for future resistance breeding. In particular, the identification of race-nonspecific resistance genes is important because of their regarded durability and broad-spectrum activity. We assessed the severity of powdery mildew infection on detached seedling leaves of 267 barley accessions using two poly-virulent isolates and performed a genome-wide association study exploiting 201 of these accessions. Two-hundred and fourteen markers, located on six barley chromosomes are associated with potential race-nonspecific Bgh resistance or susceptibility. Initial steps for the functional validation of four promising candidates were performed based on phenotype and transcription data. Specific candidate alleles were analyzed via transient gene silencing as well as transient overexpression. Microarray data of the four selected candidates indicate differential regulation of the transcription in response to Bgh infection. Based on our results, all four candidate genes seem to be involved in the responses to powdery mildew attack. In particular, the transient overexpression of specific alleles of two candidate genes, a potential arabinogalactan protein and the barley homolog of Arabidopsis thaliana’s Light-Response Bric-a-Brac/-Tramtrack/-Broad Complex/-POxvirus and Zinc finger (AtLRB1) or AtLRB2, were top candidates of novel powdery mildew susceptibility genes.

Genome-wide association mapping of tan spot resistance in a worldwide collection of durum wheat.

Resistance to tan spot in durum wheat involves race-nonspecific QTL and necrotrophic insensitivity gene. Tan spot, caused bythe necrotrophic fungus Pyrenophoratritici-repentis, is a major foliar disease on all cultivated wheat crops worldwide. Compared to common wheat, much less work has been done to investigate the genetic basis of tan spot resistance in durum. Here, we conducted disease evaluations, necrotrophic effector (NE) sensitivity assays and a genome-wide association study using a collection of durum accessions. The durum panel segregated for the reaction to disease inoculations and NE infiltrations with eighteen accessions being highly resistant to all races and most of them insensitive to both PtrToxA and PtrToxB. Over 65,000SNP markers were developed from genotyping-by-sequencing for the association mapping. As expected, sensitivity to PtrToxA and PtrToxB was mapped to the chromosome arms 5BL and 2BS, respectively. For the fungal inoculations, a quantitative trait locus (QTL) on chromosome 3B was associated with resistance to all races and likely corresponds to the race-nonspecific resistance QTL previously identified in common wheat. The Tsn1locus was not significantly associated with tan spot caused by the PtrToxA-producing isolates Pti2 and 86-124, but the Tsc2 locus was significantly associated with tan spot caused by the PtrToxB-producing isolate DW5. Another QTL on chromosome arm 1AS was associated with tan spot caused by the PtrToxC-producing isolate Pti2 and likely corresponds to the Tsc1 locus. Additional QTL for specific races was identified on chromosome 1B and 3B. Our work highlights the complexity of genetic resistance to tan spot and further confirms that the Ptr ToxA-Tsn1 interaction plays no significant role in disease development in tetraploid wheat.

Identification of a major dominant gene for race-nonspecific tan spot resistance in wild emmer wheat.

A single dominant gene found in tetraploid and hexaploid wheat controls broad-spectrum race-nonspecific resistance to the foliar disease tan spot caused by Pyrenophora tritici-repentis. Tan spot is an important foliar disease of durum and common wheat caused by the necrotrophic fungal pathogen Pyrenophora tritici-repentis. Genetic studies in common wheat have shown that pathogen-produced necrotrophic effectors interact with host genes in an inverse gene-for-gene manner to cause disease, but quantitative trait loci (QTLs) with broad race-nonspecific resistance also exist. Less work has been done to understand the genetics of tan spot interactions in durum wheat. Here, we evaluated a set of Langdon durum-wild emmer (Triticum turgidum ssp. dicoccoides) disomic chromosome substitution lines for reaction to four P. tritici-repentis isolates representing races 1, 2, 3, and 5 to identify wild emmer chromosomes potentially containing tan spot resistance genes. Chromosome 3B from the wild emmer accession IsraelA rendered the tan spot-susceptible durum cultivar Langdon resistant to all four fungal isolates. Genetic analysis indicated that a single dominant gene, designated Tsr7, governed resistance. Detailed mapping experiments showed that the Tsr7 locus is likely the same as the race-nonspecific QTL previously identified in the hexaploid wheat cultivars BR34 and Penawawa. Four user-friendly SNP-based semi-thermal asymmetric reverse PCR (STARP) markers cosegregated with Tsr7 and should be useful for marker-assisted selection of resistance. In addition to 3B, other wild emmer chromosomes contributed moderate levels of tan spot resistance, and, as has been shown previously for tetraploid wheat, the Tsn1-Ptr ToxA interaction was not associated with susceptibility. This is the first report of a major dominant gene governing resistance to tan spot in tetraploid wheat.