Major Resistance Genes Research Articles

Molecular mechanisms of resistance to Myzus persicae conferred by the peach Rm2 gene: A multi-omics view.

The transcriptomic and metabolomic responses of peach to Myzus persicae infestation were studied in Rubira, an accession carrying the major resistance gene Rm2 causing antixenosis, and GF305, a susceptible accession. Transcriptome and metabolome showed both a massive reconfiguration in Rubira 48 hours after infestation while GF305 displayed very limited changes. The Rubira immune system was massively stimulated, with simultaneous activation of genes encoding cell surface receptors involved in pattern-triggered immunity and cytoplasmic NLRs (nucleotide-binding domain, leucine-rich repeat containing proteins) involved in effector-triggered immunity. Hypersensitive reaction featured by necrotic lesions surrounding stylet punctures was supported by the induction of cell death stimulating NLRs/helpers couples, as well as the activation of H2O2-generating metabolic pathways: photorespiratory glyoxylate synthesis and activation of the futile P5C/proline cycle. The triggering of systemic acquired resistance was suggested by the activation of pipecolate pathway and accumulation of this defense hormone together with salicylate. Important reduction in carbon, nitrogen and sulphur metabolic pools and the repression of many genes related to cell division and growth, consistent with reduced apices elongation, suggested a decline in the nutritional value of apices. Finally, the accumulation of caffeic acid conjugates pointed toward their contribution as deterrent and/or toxic compounds in the mechanisms of resistance.

Geographical Correlation and Genetic Diversity of Newly Emerged Races within the Ug99 Lineage of Stem Rust Pathogen, Puccinia graminis f. sp. tritici, in Different Wheat-Producing Areas.

Wheat stem rust caused by Puccinia graminis f. sp. tritici is one of the most destructive wheat diseases worldwide. Identifying stem rust races in general, Ug99 lineage particularly, and determining resistance genes are critical goals for disease assessment. Thirty wheat varieties and monogenic lines with major stem rust resistance genes (Sr) were examined here over the course of three succeeding seasons from 2020 to 2022. Fourteen stem rust races have been identified in ten African countries, as well as Central and West Asia and North Africa (CWANA) and ten European countries. The Ug99 group (Clade I) included four races (TTKSK, TTKST, TTKTK, and TTKTT) and was reported in five African countries (Egypt, Kenya, Rwanda, Tanzania, and Uganda) and Iran, but none of the European countries. On the other hand, none of the races in Clade III-B (TTRTF) and Clade IV-B (TKTTF and TTTTF) were found in Egypt. Furthermore, Egyptian races were clustered separately from races identified from other countries, and six races were found only in Egypt, including PKSTC, RKTTH, TKTTC, TTTSK, TCKTC, and TKTTH. Races from Kenya, Tanzania, Uganda, Rwanda, and Iran were all closely associated with one another, according to correlation analysis. However, most races identified from other investigated regions, including Eritrea, Spain, Ethiopia, Morocco, Italy, Poland, Kenya, Tanzania, and Uganda, were adversely linked with Egyptian races. The diagnostic 350 bp long PCR fragment linked with virulence to Sr31, Clement (Sr31), and Brigardier (Sr31) was used to identify the TTKSK (Ug99) race. The identification of the regional associations and genetic diversity of newly emerged races within the Ug99 lineage of P. graminis tritici in Africa, Asia, and Europe is one of the key goals of this study. It will help plant breeders to develop new resistant lines against the virulent races, especially TTKSK (Ug99) and TTTSK. This helps in ensuring global food security in the context of climate change.

Characterization of Foliar Fungal Endophyte Communities from White Pine Blister Rust Resistant and Susceptible Pinus flexilis in Natural Stands in the Southern Rocky Mountains

Fungal endophytic communities in needles of field-grown Pinus flexilis previously inferred to carry major gene resistance (R) to white pine blister rust (WPBR) or to lack it (S) were surveyed to identify unique microbes that may be recruited by WPBR-resistant genotypes. Resistant and susceptible trees were sampled in each of 11 P. flexilis populations for a total of 50 trees sampled. Through next-generation sequencing, this study showed a diverse needle mycobiota in P. flexilis, of which many remain unknown, regardless of the presence or absence of the WPBR resistance gene, Cr4. Ascomycota dominated the mycobiota (88.9%) followed by Basidiomycota (4.4%) and Chytridiomycota (0.03%), and the remaining 6.7% were unclassified. Shared ( n = 105) and unique ( n = 48 in R and n = 49 in S) fungal taxa, including differentially abundant operational taxonomic units, were identified that could provide insights into core mycobiota and host genotype-specific fungal groups. Marginal variation of the fungal diversity and structure was observed between host genotypes, which indicates that neither Cr4 nor the physiological differences associated with the presence or absence of the gene affects mycobiota recruitment. Instead, other parameters, including host size (diameter at breast height) and site elevation, significantly influenced the variability of the composition and structure of the fungal endophytic community. Further investigations are needed to understand the relationship of unique or differentially abundant taxa with one genotype or the other, and to determine the role of the needle mycobiota in WPBR disease development in natural stands of P. flexilis.

Role of tomato wild relatives in breeding disease-free varieties

Cultivated tomato (Solanum lycopersicum) is one of the most economically important and widely grown vegetable crops worldwide. However, tomato plants are often affected by biotic and abiotic stresses that reduce yield and affect fruit quality. Phenotypic diversity is evident in cultivated tomatoes, particularly for horticultural traits, but genetic diversity is rather narrow. Major disease resistance genes for different pathogens such as viruses, fungi, bacteria and nematodes are mainly derived from wild tomato species and introgressed into cultivated tomatoes. Here, we list the major disease and insect-pest resistance genes identified in S. pimpinellifolium, S. habrochaites, S. peruvianum, S. chilense, S. pennellii, S. galapagense, S. arcanum and S. neorickii with perspective on the gap between current knowledge on tomato wild relatives and the knowledge that is needed.

The Rm1 and Rm2 Resistance Genes to Green Peach Aphid (Myzus persicae) Encode the Same TNL Proteins in Peach (Prunus persica L.).

The green peach aphid (GPA), Myzus persicae, is an important pest of the peach crop. Three major dominant resistance genes have already been detected, Rm1 in the Weeping Flower Peach (WFP) clone, Rm2 in the Rubira clone, and Rm3 in the Fen Shouxing clone. In this study, after NGS resequencing of WFP and Rubira, we found that their genomic sequences in the Rm1 and Rm2 region were similar but very different from that of the susceptible reference peach Lovell. We constructed a BAC library for the GPA-resistant WFP and screened four BAC clones to sequence the target region. The new sequence was 61.7 Kb longer than Lovell and was annotated with four different TIR_NBS_LRR genes. Among them, the TNL1 gene was very overexpressed in WFP leaves 24 h after GPA infestation. This gene was also present and expressed in the Rubira clone and had the same sequence as the candidate Rm3 gene, supporting the hypothesis that the three genes share the same origin. In addition, we identified a second TNL, TNL2, located at 35.4 Kb from TNL1 and slightly overexpressed after GPA infestation. Kasp and size molecular markers were designed for use in marker-assisted selection and were validated in a peach segregating population.

Scarcity of major resistance genes against Verticillium wilt caused by Verticillium dahliae

AbstractVerticillium dahliae is a soil‐borne fungal pathogen that causes vascular wilt disease in numerous plant species. The only described qualitative resistances against V. dahliae are the Ve1 gene and the V2 locus in tomato. These resistances have been overcome by virulent strains. We tried to identify additional resistances. Out of the methods we tested, comparing the canopy area of V. dahliae‐inoculated plants with mock‐inoculated plants yielded the best discriminative power in resistance tests. Out of six wild tomato accessions that were previously reported to possess some resistance, Solanum pimpinellifolium G1.1596 and Solanum cheesmanii G1.1615 displayed the lowest stunting and the least colonization by V. dahliae. Recombinant inbred line (RIL) populations were developed of both populations. No QTLs were identified in the G1.1596 RIL population. In the G1.1615 population, four small‐effect QTLs were associated with reduced stunting. Many studies in other hosts also failed to discover major resistance genes against V. dahliae. We hypothesize that the scarcity of major resistance genes against V. dahliae is caused by its endophytic behaviour in nature. The limited damage in nature would not lead to evolutionary pressure to evolve major resistances. However, in agriculture, V. dahliae can behave more pathogenic, leading to serious damage.

Stress responsive OsHyPRP16 promoter driven early expression of resistance gene Pi54 potentiate the resistance against Magnaporthe oryzae in transgenic rice

The rice Hybrid Proline Rich Protein (HyPRP) encoding gene, OsHyPRP16 expression exhibit early upregulation in response to Magnaporthe oryzae inoculation. Here, we functionally characterized the OsHyPRP16 promoter through deletion analysis in transgenic Arabidopsis using GUS (β-glucuronidase) reporter assay. The promoter fragments, sequentially deleted from the 5′ end could induce differential GUS activity in response to stresses induced by different hormones and abiotic stress conditions. In addition, a strong GUS induction was observed in M. oryzae inoculated transgenic Arabidopsis. Based on the insilico and stress-inducibility of D1 promoter fragment against various phytohormones and rice blast fungus, and with no basal activity under control conditions, we rationally selected D1 promoter fragment to drive the expression of a major rice blast resistance gene; Pi54 in the genetic background of blast susceptible TP309 rice line. The D1 promoter fragment was able to induce the expression of Pi54 at immediate-early stages of M. oryzae infection in transgenic rice. The transgenic plants with Pi54 under the control of D1 promoter fragment displayed complete resistance against M. oryzae infection as compared to control plants. The present study suggests that the D1 fragment of OsHyPRP16 promoter is a valuable tool for breeding and development of rice lines with early-inducible and pathogen-responsive enhanced disease resistance.

Conversion of sheath blight susceptible indica and japonica rice cultivars into moderately resistant through expression of antifungal β-1,3-glucanase transgene from Trichoderma spp.

Rice is an important food crop for three billion people worldwide. The crop is vulnerable to several diseases. Sheath blight caused by fungal pathogen Rhizoctonia solani is a significant threat to rice cultivation accounting for up to 50% yield losses. The pathogen penetrates leaf blades and sheaths, leading to plant necrosis; and major disease resistance gene against the pathogen is not available. This study describes development of sheath blight resistant transgenic indica and japonica rice cultivars through introduction of antifungal β-1,3-glucanase transgene cloned from Trichoderma. The transgene integration and expression in transformed T0 rice plants was examined by PCR, RT-PCR, qRT-PCR demonstrating up to 5-fold higher expression as compared to non-transgenic plants. The bioassay of T0, T1 and homozygous T2 progeny plants with virulent R. solani isolate revealed that plants carrying high level of β-1,3-glucanase expression displayed moderately resistant reaction to the pathogen. The optical micrographs of leaf sheath cells from moderately resistant plant after pathogen inoculation displayed presence of a few hyphae with sparse branching; on the contrary, pathogen hyphae in susceptible non-transgenic plant cells were present in abundance with profuse hyphal branching and forming prominent infection cushions. The disease severity in T2 progeny plants was significantly less as compared to non-transgenic plants confirming role of β-1,3-glucanase in imparting resistance.

Genetic detection and identification of the candidate resistance gene for a major QTL resistant to Puccinia polysora in maize

Southern corn rust (SCR) is a major foliar disease in maize. Identification the major QTL and candidate resistance gene is prerequisite in maize resistance breeding programs. To identify major resistance loci/gene(s) resistant to SCR, a recombinant inbred line (RIL) population derived from a cross between 975 and 12 (resistant) and Lx9801 (susceptible) was analyzed. In total four QTLs were identified, which were located on chromosomes 5, 9 and 10, respectively. Among them, a major QTL on chromosome 10 (qSCR3), were detected in all the environments, which can explain 70.3–78.4% of the total phenotypic variation. Fine-mapping of qSCR3 delimited the QTL to an interval of ∼225 Kb. Using transcripts assembly of the 975–12 and a susceptible mutant (975-12S), an NLR gene, Zm00025ab420970, was identified as the candidate gene for qSCR3, which is a new allele/gene resistant to SCR in maize. Since qSCR3 is a major QTL resistant to SCR, it can be used as a desirable resistance source in maize resistance breeding programs.

Blocked at the Stomatal Gate, a Key Step of Wheat Stb16q-Mediated Resistance to Zymoseptoria tritici.

Septoria tritici blotch (STB), caused by the fungus Zymoseptoria tritici, is among the most threatening wheat diseases in Europe. Genetic resistance remains one of the main environmentally sustainable strategies to efficiently control STB. However, the molecular and physiological mechanisms underlying resistance are still unknown, limiting the implementation of knowledge-driven management strategies. Among the 22 known major resistance genes (Stb), the recently cloned Stb16q gene encodes a cysteine-rich receptor-like kinase conferring a full broad-spectrum resistance against Z. tritici. Here, we showed that an avirulent Z. tritici inoculated on Stb16q quasi near isogenic lines (NILs) either by infiltration into leaf tissues or by brush inoculation of wounded tissues partially bypasses Stb16q-mediated resistance. To understand this bypass, we monitored the infection of GFP-labeled avirulent and virulent isolates on Stb16q NILs, from germination to pycnidia formation. This quantitative cytological analysis revealed that 95% of the penetration attempts were unsuccessful in the Stb16q incompatible interaction, while almost all succeeded in compatible interactions. Infectious hyphae resulting from the few successful penetration events in the Stb16q incompatible interaction were arrested in the sub-stomatal cavity of the primary-infected stomata. These results indicate that Stb16q-mediated resistance mainly blocks the avirulent isolate during its stomatal penetration into wheat tissue. Analyses of stomatal aperture of the Stb16q NILs during infection revealed that Stb16q triggers a temporary stomatal closure in response to an avirulent isolate. Finally, we showed that infiltrating avirulent isolates into leaves of the Stb6 and Stb9 NILs also partially bypasses resistances, suggesting that arrest during stomatal penetration might be a common major mechanism for Stb-mediated resistances.

Assessment of Rice Sheath Blight Resistance Including Associations with Plant Architecture, as Revealed by Genome-Wide Association Studies

BackgroundSheath blight (ShB) disease caused by Rhizoctonia solani Kühn, is one of the most economically damaging rice (Oryza sativa L.) diseases worldwide. There are no known major resistance genes, leaving only partial resistance from small-effect QTL to deploy for cultivar improvement. Many ShB-QTL are associated with plant architectural traits detrimental to yield, including tall plants, late maturity, or open canopy from few or procumbent tillers, which confound detection of physiological resistance.ResultsTo identify QTL for ShB resistance, 417 accessions from the Rice Diversity Panel 1 (RDP1), developed for association mapping studies, were evaluated for ShB resistance, plant height and days to heading in inoculated field plots in Arkansas, USA (AR) and Nanning, China (NC). Inoculated greenhouse-grown plants were used to evaluate ShB using a seedling-stage method to eliminate effects from height or maturity, and tiller (TN) and panicle number (PN) per plant. Potted plants were used to evaluate the RDP1 for TN and PN. Genome-wide association (GWA) mapping with over 3.4 million SNPs identified 21 targeted SNP markers associated with ShB which tagged 18 ShB-QTL not associated with undesirable plant architecture traits. Ten SNPs were associated with ShB among accessions of the Indica subspecies, ten among Japonica subspecies accessions, and one among all RDP1 accessions. Across the 18 ShB QTL, only qShB4-1 was not previously reported in biparental mapping studies and qShB9 was not reported in the GWA ShB studies. All 14 PN QTL overlapped with TN QTL, with 15 total TN QTL identified. Allele effects at the five TN QTL co-located with ShB QTL indicated that increased TN does not inevitably increase disease development; in fact, for four ShB QTL that overlapped TN QTL, the alleles increasing resistance were associated with increased TN and PN, suggesting a desirable coupling of alleles at linked genes.ConclusionsNineteen accessions identified as containing the most SNP alleles associated with ShB resistance for each subpopulation were resistant in both AR and NC field trials. Rice breeders can utilize these accessions and SNPs to develop cultivars with enhanced ShB resistance along with increased TN and PN for improved yield potential.

Genomic Traits Associated with Virulence and Antimicrobial Resistance of Invasive Group B Streptococcus Isolates with Reduced Penicillin Susceptibility from Elderly Adults.

ABSTRACTThis study aimed to investigate genomic traits underlying the antimicrobial resistance and virulence of multidrug-resistant (MDR) group B streptococci with reduced penicillin susceptibility (PRGBS) recovered from elderly patients with bloodstream infections, which remain poorly characterized. The pangenome was found to be open, with the predicted pan- and core genome sizes being 3,531 and 1,694 genes, respectively. Accessory and unique genes were enriched for the Clusters of Orthologous Groups (COG) categories L, Replication, recombination, and repair, and K, Transcription. All MDR PRGBS isolates retained a core virulence gene repertoire (bibA, fbsA/-B/-C, cspA, cfb, hylB, scpB, lmb, and the cyl operon), supporting an invasive ability similar to that of the other invasive GBS, penicillin-susceptible GBS (PSGBS), and noninvasive PRGBS isolates. The putative sequence type 1 (ST1)-specific AlpST-1 virulence gene was also retained among the serotype Ia/ST1 PRGBS isolates. In addition to tet(M) and erm(B), mef(A)-msr(D) elements or the high-level gentamicin resistance gene aac(6′)-aph(2″), which are both rare in PSGBS, were detected among those MDR PRGBS isolates. In the core single-nucleotide polymorphism (SNP) phylogenetic tree, all invasive ST1 PRGBS isolates with serotypes Ia and III were placed together in a clade with a recombination rate of 3.97, which was 36 times higher than the value found for a clade formed by serotype V/ST1 PSGBS isolates derived mostly from human blood. ST1 has been the predominant sequence type among the PRGBS isolates in Japan, and serotypes Ia and III have been very rare among the ST1 PSGBS isolates. Thus, these lineages that mostly consisted of serotypes Ia/ST1 and III/ST1 PRGBS could possibly emerge through recombination within the ST1 populations.IMPORTANCE Streptococcus agalactiae, or group B Streptococcus (GBS), is recognized as the leading cause of neonatal invasive infections. However, an increasing incidence of invasive GBS infections among nonpregnant adults, particularly the elderly and those with underlying diseases, has been observed. There is a trend toward the increasing occurrence of penicillin nonsusceptibility among GBS clinical isolates, from 4.8% in 2008 to 5.8% in 2020 in Japan. Also, in the United States, the frequency of adult invasive GBS isolates suggestive of β-lactam nonsusceptibility increased from 0.7% in 2015 to 1.0% in 2016. In adults, mortality has been significantly higher among patients with bacteremia than among those without bacteremia. Our study revealed that invasive GBS with reduced penicillin susceptibility (PRGBS) isolates harbor major virulence and resistance genes known among GBS, highlighting the need for large population-based genomic surveillance studies to better understand the clinical relevance of invasive PRGBS isolates.

Genetic Resources of Cereal Crops for Aphid Resistance.

The genetic resources of cereal crops in terms of resistance to aphids are reviewed. Phytosanitary destabilization led to a significant increase in the harmfulness of this group of insects. The breeding of resistant plant genotypes is a radical, the cheapest, and environmentally safe way of pest control. The genetic homogeneity of crops hastens the adaptive microevolution of harmful organisms. Both major and minor aphid resistance genes of cereal plants interact with insects differentially. Therefore, rational breeding envisages the expansion of the genetic diversity of cultivated varieties. The possibilities of replenishing the stock of effective resistance genes by studying the collection of cultivated cereals, introgression, and creating mutant forms are considered. The interaction of insects with plants is subject to the gene-for-gene relationship. Plant resistance genes are characterized by close linkage and multiple allelism. The realizing plant genotype depends on the phytophage biotype. Information about the mechanisms of constitutional and induced plant resistance is discussed. Resistance genes differ in terms of stability of expression. The duration of the period when varieties remain resistant is not related either to its phenotypic manifestation or to the number of resistance genes. One explanation for the phenomenon of durable resistance is the association of the virulence mutation with pest viability.

Registration of three Gossypium barbadense L. American pima‐like germplasm lines (PSSJ‐FRP01, PSSJ‐FRP02, and PSSJ‐FRP03) with improved resistance to Fusarium wilt race 4 and good fiber quality

AbstractOver the past 18 yr, Fusarium wilt race 4 (FOV4) has affected cotton (Gossypium spp.) production in the San Joaquin Valley of California, and more recently this pathogen was formally identified in 2017 in the El Paso, TX, region and in 2019 in New Mexico. The primary purpose for the germplasm release is to provide cotton breeders with needed alternative sources for continuing to improve FOV4 resistance in pima cotton. PSSJ‐FRP01 (Reg. no. GP‐1107, PI 699963) (PS‐592) originated from Pima S‐6 × G. barbadense pool accessions GB1368/k‐7616. Pima S‐6 possesses a major gene(s) for FOV4 resistance. GB1368/k‐7616 is Uzbekistan accession introduction with different plant structure, large leaves, columnar growth, and a more clustered fruiting habit when compared to most American pima. PSSJ‐FRP02 (Reg. no. GP‐1108, PI 699964) and PSSJ‐FRP03 (Reg. no. GP‐1109, PI 699965) originated from PS‐592 × SA‐3208 (LIAO MIAN 7 HAO). SA‐3208 is moderately resistant to FOV4. Multiple selections and recurrent cycles of selections were applied to these new germplasm lines based on an asymptomatic single plant after greenhouse FOV4 inoculations and/or infested field evaluations from F1 to F4. In subsequent field and greenhouse tests in 2019–2021, the germplasm lines showed improved resistance to FOV4 based on low percentage mortality and vascular staining and fiber quality when compared to commercial cultivars. PSSJ‐FRP01, PSSJ‐FRP02, and PSSJ‐FRP03 lines will help to continue reducing the vulnerability of the pima cotton industry to this fungal pathogen and to advance efforts to broaden the genetic base of FOV4 resistance, which is critical to the pima cotton industry in the United States and Uzbekistan.

Deciphering Host Plant Resistance Mechanisms against Tungro Virus in Rice: A Comprehensive Exploration

Rice tungro virus (RTV) remains a significant threat to global rice production, causing substantial yield losses and economic hardships for farmers. Host plant resistance stands out as a sustainable and environmentally friendly approach to mitigate the impact of RTV. This review provides a comprehensive exploration of the host plant resistance mechanisms against RTV in rice and delve into the genetic basis of resistance, highlighting major resistance genes and quantitative trait loci (QTLs) associated with RTV resistance. Furthermore, discuss the molecular mechanisms underlying resistance, including recognition, signaling pathways, and defense responses triggered upon RTV infection. The role of genetic diversity in shaping resistance patterns and the potential for marker-assisted selection (MAS) in breeding RTV-resistant rice cultivars are also examined. Additionally, explore emerging strategies such as genome editing and RNA interference (RNAi) for enhancing RTV resistance in rice. Insights gained from this comprehensive exploration of host plant resistance mechanisms provide valuable guidance for breeding programs and crop management strategies aimed at enhancing RTV resistance and ensuring sustainable rice production.

SNPs in a Large Genomic Scaffold Are Strongly Associated withCr1R, Major Gene for Resistance to White Pine Blister Rust in Range-Wide Samples of Sugar Pine (Pinus lambertiana)

Sugar pine, Pinus lambertiana Douglas, is a keystone species of montane forests from Baja California to southern Oregon. Like other North American white pines, populations of sugar pine have been greatly reduced by the disease white pine blister rust (WPBR) caused by a fungal pathogen, Cronartium ribicola, that was introduced into North America early in the twentieth century. Major gene resistance to WPBR segregating in natural populations has been documented in sugar pine. Indeed, the dominant resistance gene in this species, Cr1, was genetically mapped, although not precisely. Genomic single nucleotide polymorphisms (SNPs) placed in a large scaffold were reported to be associated with the allele for this major gene resistance (Cr1R). Forest restoration efforts often include sugar pine seed derived from the rare resistant individuals (typically Cr1R/Cr1r) identified through an expensive 2-year phenotypic testing program. To validate and geographically characterize the variation in this association and investigate its potential to expedite genetic improvement in forest restoration, we developed a simple PCR-based, diploid genotyping of DNA from needle tissue. By applying this to range-wide samples of susceptible and resistant (Cr1R) trees, we show that the SNPs exhibit a strong, though not complete, association with Cr1R. Paralleling earlier studies of the geographic distribution of Cr1R and the inferred demographic history of sugar pine, the resistance-associated SNPs are marginally more common in southern populations, as is the frequency of Cr1R. Although the strength of the association of the SNPs with Cr1R and thus, their predictive value, also varies with geography, the potential value of this new tool in quickly and efficiently identifying candidate WPBR-resistant seed trees is clear.

The barley leaf rust resistance gene Rph3 encodes a predicted membrane protein and is induced upon infection by avirulent pathotypes of Puccinia hordei

Leaf rust, caused by Puccinia hordei, is an economically significant disease of barley, but only a few major resistance genes to P. hordei (Rph) have been cloned. In this study, gene Rph3 was isolated by positional cloning and confirmed by mutational analysis and transgenic complementation. The Rph3 gene, which originated from wild barley and was first introgressed into cultivated Egyptian germplasm, encodes a unique predicted transmembrane resistance protein that differs from all known plant disease resistance proteins at the amino acid sequence level. Genetic profiles of diverse accessions indicated limited genetic diversity in Rph3 in domesticated germplasm, and higher diversity in wild barley from the Eastern Mediterranean region. The Rph3 gene was expressed only in interactions with Rph3-avirulent P. hordei isolates, a phenomenon also observed for transcription activator-like effector-dependent genes known as executors conferring resistance to Xanthomonas spp. Like known transmembrane executors such as Bs3 and Xa7, heterologous expression of Rph3 in N. benthamiana induced a cell death response. The isolation of Rph3 highlights convergent evolutionary processes in diverse plant-pathogen interaction systems, where similar defence mechanisms evolved independently in monocots and dicots.

Efficacy of Blackleg Major Resistance Genes in B. napus in Germany.

Leptosphaeria maculans is one of the major pathogens of oilseed rape (B. napus). It causes blackleg disease, which accounts for significant yield losses worldwide. Using cultivars that harbor major resistance (R) genes is one of the most effective control methods. However, the efficacy of major R genes is related to the frequency of the corresponding avirulence (Avr) genes in a L. maculans population. In this paper, we report the Avr profiles of L. maculans populations and the ratio of its mating types in Northern and Central regions of Germany. Eleven Avr genes in five-hundred and seventy-four isolates were characterized either by applying cotyledon tests on a B. napus differential set or by amplifying avirulence gene-specific PCR markers. Fifty-two races were determined, among which the most dominant race was Avrlm6, -7, -11, AvrlepR1, -R2. Results showed that the resistance gene Rlm2 is 100% ineffective, some other major R genes such as Rlm1, Rlm3, Rlm4 and LepR3 are partially effective (with corresponding Avr frequencies ≤ 42%), while LepR1, LepR2, Rlm6, Rlm11 and Rlm7 can still provide relatively effective resistance in the German fields investigated (with corresponding Avr frequencies of 63–100%). Sexual reproduction is a factor that enhances the potential of L. maculans to evolve under selection pressure. Mating types of the L. maculans populations did not deviate from the ratio of 1:1 in the examined regions, indicating that sexual reproduction and ascospores play central roles in the L. maculans lifecycle. Overall, this study provides an important dataset for the establishment of a strategic plan to preserve the efficacies of major R genes in Germany by applying cultivar rotations of oilseed rape.

Comparative Association Mapping Reveals Conservation of Major Gene Resistance to White Pine Blister Rust in Southwestern White Pine (Pinus strobiformis) and Limber Pine (P. flexilis).

All native North American white pines are highly susceptible to white pine blister rust (WPBR) caused by Cronartium ribicola. Understanding genomic diversity and molecular mechanisms underlying genetic resistance to WPBR remains one of the great challenges in improvement of white pines. To compare major gene resistance (MGR) present in two species, southwestern white pine (Pinus strobiformis) Cr3 and limber pine (P. flexilis) Cr4, we performed association analyses of Cr3-controlled resistant traits using single nucleotide polymorphism (SNP) assays designed with Cr4-linked polymorphic genes. We found that ∼70% of P. flexilis SNPs were transferable to P. strobiformis. Furthermore, several Cr4-linked SNPs were significantly associated with the Cr3-controlled traits in P. strobiformis families. The most significantly associated SNP (M326511_1126R) almost colocalized with Cr4 on the Pinus consensus linkage group 8, suggesting that Cr3 and Cr4 might be the same R locus, or have localizations very close to each other in the syntenic region of the P. strobiformis and P. flexilis genomes. M326511_1126R was identified as a nonsynonymous SNP, causing amino acid change (Val376Ile) in a putative pectin acetylesterase, with coding sequences identical between the two species. Moreover, top Cr3-associated SNPs were further developed as TaqMan genotyping assays, suggesting their usefulness as marker-assisted selection (MAS) tools to distinguish genotypes between quantitative resistance and MGR. This work demonstrates the successful transferability of SNP markers between two closely related white pine species in the hybrid zone, and the possibility for deployment of MAS tools to facilitate long-term WPBR management in P. strobiformis breeding and conservation.

Mapping Blast Resistance Genes in Rice Varieties 'Minghui 63' and 'M-202'.

Rice blast caused by the fungus Magnaporthe oryzae (syn. Magnaporthe grisea) is one of the most lethal diseases for sustainable rice production worldwide. Blast resistance mediated by major resistance genes is often broken down after a short period of deployment, while minor blast resistance genes, each providing a small effect on disease reactions, are more durable. In the present study, we first evaluated disease reactions of two rice breeding parents 'Minghui 63' and 'M-202' with 11 blast races, IA45, IB1, IB45, IB49, IB54, IC1, IC17, ID1, IE1, IG1, and IH1, commonly present in the United States, under greenhouse conditions using a category disease rating resembling infection types under field conditions. 'Minghui 63' exhibited differential resistance responses in comparison with those of 'M-202' to the tested blast races. A recombinant inbred line (RIL) population of 275 lines from a cross between 'Minghui 63' and 'M-202' was also evaluated with the above-mentioned blast races. The population was genotyped with 156 simple sequence repeat (SSR) and insertion and deletion (Indel) markers. A linkage map with a genetic distance of 1,022.84 cM was constructed using inclusive composite interval mapping (ICIM) software. A total of 10 resistance QTLs, eight from 'Minghui 63' and two from 'M-202', were identified. One major QTL, qBLAST2 on chromosome 2, was identified by seven races/isolates. The remaining nine minor resistance QTLs were mapped on chromosomes 1, 3, 6, 9, 10, 11, and 12. These findings provide useful genetic markers and resources to tag minor blast resistance genes for marker-assisted selection in rice breeding program and for further studies of underlying genes.