Adult Plant Resistance Gene Research Articles

Lr34/Yr18/Sr57/Pm38 confers broad-spectrum resistance to fungal diseases via sinapyl alcohol transport for cell wall lignification in wheat

The widely recognized pleiotropic adult plant resistance gene Lr34 encodes an ATP-binding cassette transporter and plays an important role in breeding wheat for enhanced resistance to multiple fungal diseases. Despite its significance, the mechanisms underlying Lr34-mediated pathogen defense remain largely unknown. Our study demonstrates that wheat lines carrying the Lr34res allele exhibit thicker cell walls and enhanced resistance to fungal penetration compared to those without Lr34res. Transcriptome and metabolite profiling revealed that the lignin biosynthetic pathway is suppressed in lr34 mutants, indicating a disruption in cell wall lignification. Additionally, we discovered that lr34 mutant lines are hypersensitive to sinapyl alcohol, a major monolignol crucial for cell wall lignification. Yeast accumulation and efflux assays confirmed that the LR34 protein functions as a sinapyl alcohol transporter. Both genetic and virus-induced gene silencing experiments demonstrated that the disease resistance conferred by Lr34 can be enhanced by incorporating the TaCOMT-3B gene, which is responsible for the biosynthesis of sinapyl alcohol. Collectively, our findings provide novel insights into the role of Lr34 in disease resistance through mediating sinapyl alcohol transport and cell wall deposition, and highlight the synergistic effect of TaCOMT-3B and Lr34 against multiple fungal pathogens by mediating cell wall lignification in adult wheat plants.

Fine-mapping of LrN3B on wheat chromosome arm 3BS, one of the two complementary genes for adult-plant leaf rust resistance.

The common wheat line 4N0461 showed adult-plant resistance to leaf rust. 4N0461 was crossed with susceptible cultivars Nongda4503 and Shi4185 to map the causal resistance gene(s). Segregation of leaf rust response in F2 populations from both crosses was 9 resistant:7 susceptible, indicative of two complementary dominant resistance genes. The genes were located on chromosome arms 3BS and 4BL and temporarily named LrN3B and LrN4B, respectively. Subpopulations from 4N0461 × Nongda4503 with LrN3B segregating as a single allele were used to fine-map LrN3B locus. LrN3B was delineated in a genetic interval of 0.07cM, corresponding to 106kb based on the Chinese Spring reference genome (IWGSC RefSeq v1.1). Four genes were annotated in this region, among which TraesCS3B02G014800 and TraesCS3B02G014900 differed between resistant and susceptible genotypes, and both were required for LrN3B resistance in virus-induced gene silencing experiments. Diagnostic markers developed for checking the polymorphism of each candidate gene, can be used for marker-assisted selection in wheat breeding programs.

Transferring an Adult-Plant Stripe-Rust Resistance Gene Yr7VS from Chromosome 7V of Dasypyrum villosum (L.) to Bread Wheat.

Stripe rust (Puccinia striiformis West. f.sp. tritici, Pst) is a destructive disease that seriously threatens wheat production globally. Exploring novel resistance genes for use in wheat breeding is an urgent need, as continuous Pst evolution frequently leads to a breakdown of host resistance. Here, we identified a set of wheat-Dasypyrum villosum 01I139 (V#6) disomic introgression lines for the purpose of determining their responses to a mixture of Pst isolates CYR32, CYR33 and CYR34 at both seedling and adult-plant stages. The results showed that all introgression lines exhibited high susceptibility at the seedling stage, with infection-type (IT) scores in the range of 6-8, whereas, for chromosomes 5V#6 and 7V#6, disomic addition lines NAU5V#6-1 and NAU7V#6-1 displayed high resistance at the adult-plant stage, indicating that adult-plant resistance (APR) genes were located on them. Further, in order to transfer the stripe-rust resistance on chromosome 7V#6, four new wheat-D. villosum introgression lines were identified, by the use of molecular cytogenetic approaches, from the self-pollinated seeds of 7D and 7V#6, in double monosomic line NAU7V#6-2. Among them, NAU7V#6-3 and NAU7V#6-4 were t7V#6L and t7V#6S monosomic addition lines, and NAU7V#6-5 and NAU7V#6-6 were homozygous T7DS·7V#6L and T7DL·7V#6S whole-arm translocation lines. Stripe-rust tests and genetic analyses of chromosome 7V#6 introgression lines revealed a dominant APR gene designated as Yr7VS on the chromosome arm 7V#6S. Comparison with the homozygous T7DL·7V#6S translocation line and the recurrent parent NAU0686 showed no significant differences in yield-related traits. Thus, T7DL·7V#6S whole-arm translocation with the APR gene Yr7VS provided a valuable germplasm for breeding for resistance.

Assessment of genetic variability for wheat yellow rust resistance and Puccinia striiformis f.sp. tritici pathotypes from Pakistan

After their release, commercial wheat cultivars often experience a notable decline in effectiveness against yellow rust, highlighting the crucial importance of genetic resistance studies for developing durable solutions for sustainable wheat production. This study characterized 205 wheat genotypes for seedling and adult plant (Yr18) resistance genes, using 14 diverse races of Puccinia striiformis f.sp. tritici (Pst). Additionally, 75 Pst isolates sampled during 2008 were also analyzed. Seedling resistance in 16 genotypes remained undetermined. Twelve resistance patterns emerged: 82 genotypes showed susceptibility across all pathotypes, while 10 displayed consistent resistance. Fifty-seven genotypes were inferred to carry Yr2, Yr3, Yr6, Yr7, Yr9, and an unknown gene. Yr18 prevalence was observed in 29 cultivars but was absent in landraces. Eight resistance spectra were shared between landraces and cultivars, and four were cultivar-specific (Yr6+Yr7, Yr9+Yr17, and Yr18). Many landraces carried defeated resistance genes, as evident from prevalent pathotypes in the 2006 and 2008 surveys in Khyber Pakhtunkhwa, Pakistan. The low frequency of resistance genes in Pakistani cultivars underscores the need for increased use of undefeated seedling resistance genes and Yr18-associated cultivars for effective disease control.

The adult plant resistance (APR) genes Yr18, Yr29 and Yr46 in spring wheat showed significant effect against important yellow rust races under North-West European field conditions

Yellow rust caused by Puccinia striiformis f. sp. tritici (Pst) is one of the most important wheat diseases. Adult plant resistance (APR) genes have gained the attention of breeders and scientists because they show higher durability compared to major race-specific genes. Here, we determined the effect of the APR genes Yr18, Yr29 and Yr46 in North-West European field conditions against three currently important Pst races. We used three pairs of sibling wheat lines developed at CIMMYT, which consisted of a line with the functional resistance gene and a sibling with its non-functional allele. All APR genes showed significant effects against the Pst races Warrior and Warrior (–), and a race of the highly aggressive strain PstS2. The effects of Yr18 and Yr46 were especially substantial in slowing down disease progress. This effect was apparent in both Denmark, where susceptible controls reached 100 percent disease severity, and in United Kingdom where disease pressure was lower. We further validated field results by quantifying fungal biomass in leaf samples and by micro-phenotyping of samples collected during early disease development. Microscopic image analyses using deep learning allowed us to quantify separately the APR effects on leaf colonization and pustule formation. Our results show that the three APR genes can be used in breeding yellow rust resistant varieties of spring wheat to be grown in North-West European conditions, and that deep learning image analysis can be an effective method to quantify effects of APR on colonisation and pustule formation.

Fine mapping of QYrsv.swust-1BL for resistance to stripe rust in durum wheat Svevo.

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is a serious disease that affects wheat worldwide. There is a great need to develop cultivars with combinations of all-stage resistance (ASR) and adult-plant resistance (APR) genes for sustainable control of the disease. QYrsv.swust-1BL in the Italian durum wheat (Triticum turgidum ssp. durum) cultivar Svevo is effective against Pst races in China and Israel, and the gene has been previously mapped to the long arm of chromosome 1B. The gene is flanked by SNP (single nucleotide polymorphism) markers IWB5732 and IWB4839 (0.75 cM). In the present study, we used high-density 660K SNP array genotyping and the phenotypes of 137 recombinant inbred lines (RILs) to fine map the QYrsv.swust-1BL locus within a 1.066 Mb region in durum wheat Svevo (RefSeq Rel. 1.0) on chromosome arm 1BL. The identified 1.066 Mb region overlaps with a previously described map of Yr29/QYr.ucw-1BL, a stripe rust APR gene. Twenty-five candidate genes for QYrsv.swut-1BL were identified through comparing polymorphic genes within the 1.066 Mb region in the resistant cultivar. SNP markers were selected and converted to Kompetitive allele-specific polymerase chain reaction (KASP) markers. Five KASP markers based on SNP were validated in a F2 and F2:3 breeding population, providing further compelling evidence for the significant effects of QYrsv.swut-1BL. These markers should be useful in marker-assisted selection for incorporating Yr29/QYrsv.swust-1BL into new durum and common wheat cultivars for resistance to stripe rust.

High-density mapping of durable and broad-spectrum stripe rust resistance gene Yr30 in wheat.

The durable stripe rust resistance gene Yr30 was fine-mapped to a 610-kb region in which five candidate genes were identified by expression analysis and sequence polymorphisms. The emergence of genetically diverse and more aggressive races of Puccinia striiformis f. sp. tritici (Pst) in the past twenty years has resulted in global stripe rust outbreaks and the rapid breakdown of resistance genes. Yr30 is an adult plant resistance (APR) gene with broad-spectrum effectiveness and its durability. Here, we fine-mapped the YR30 locus to a 0.52-cM interval using 1629 individuals derived from residual heterozygous F5:6 plants in a Yaco"S"/Mingxian169 recombinant inbred line population. This interval corresponded to a 610-kb region in the International Wheat Genome Sequencing Consortium (IWGSC) RefSeq version 2.1 on chromosome arm 3BS harboring 30 high-confidence genes. Five genes were identified as candidate genes based on functional annotation, expression analysis by RNA-seq and sequence polymorphisms between cultivars with and without Yr30 based on resequencing. Haplotype analysis of the target region identified six haplotypes (YR30_h1-YR30_h6) in a panel of 1215 wheat accessions based on the 660K feature genotyping array. Lines with YR30_h6 displayed more resistance to stripe rust than the other five haplotypes. Near-isogenic lines (NILs) with Yr30 showed a 32.94% higher grain yield than susceptible counterparts when grown in a stripe rust nursery, whereas there was no difference in grain yield under rust-free conditions. These results lay a foundation for map-based cloning Yr30.

Identification and molecular mapping of resistance genes for adult-plant resistance to stripe rust in spring wheat germplasm line PI660076

Wheat is one of the major food crops worldwide. Stripe rust can cause a great loss of wheat yield, especially when the disease is prevalent. Chemical control not only causes the loss of resistance to stripe rust but also has a serious impact on the human body and environment. Therefore, the most economical measure to control wheat stripe rust is to cultivate resistant varieties. Rapid variation of stripe rust races often leads to rapid "loss" of resistance to stripe rust disease; therefore, breeders and researchers have to continuously explore new stripe rust resistance genes to provide new resistance sources for the rapid variation of stripe rust races. Previous studies have confirmed that PI660076, a spring wheat line, showed stripe rust resistance under natural conditions at the adult stage, which has great value in breeding programs. In this study, a recombinant inbred lines (RIL) population was constructed by crossing the wheat line PI660076 with the stripe rust-susceptible line AvS. Genotyping of the population was performed using a wheat 15 K SNP array. Three QTLs were identified using phenotypic data over four years across three environments. The resistance type of each QTL was determined by inoculating the RIL lines with single and homozygous QTL during the seedling and adult stages under controlled conditions. The all-stage resistance (ASR) QTL, QYr076.jaas-2A (flanked by SNP marker AX-11048464 and Kompetitive Allele-Specific PCR (KASP) marker KASP_4940) explained 7.13–16.58% and 6.95–7.25% of infection type (IT) and disease severity (DS), respectively. The adult-plant resistance (APR) resistance QTL, QYr076.jaas-4D.1 (flanked by KASP marker KASP_0795 and SNP marker AX-111567243,) explained 6.85–12.70% and 7.94–17.26% of IT and DS, respectively. The APR resistance QTL, QYr076.jaas-4D.2 flanked by KASP markers KASP_9130 and KASP_6535, explained 7.97–39.19% and 8.77–20.55% of the phenotypic variation in IT and DS, respectively. All the three QTLs are likely to be new. The obtained results lay a foundation for further utilization of the stripe rust-resistant line PI660076, as well as for fine mapping and molecular marker-assisted selection breeding.Graphical

The effects of Lr34 and Lr67 on Fusarium head blight resistance and deoxynivalenol accumulation in wheat

AbstractThe resistance gene Lr34 conditions durable disease resistance to many biotrophic wheat pathogens and has been incorporated into many wheat cultivars throughout the world. Lr67 is a similar adult plant resistance gene that also conditions resistance to multiple wheat diseases. Both genes significantly reduce disease severity and work in an additive manner with other rust resistance genes. To determine the effect of Lr34 and Lr67 on Fusarium head blight (FHB), two doubled haploid populations, segregating for each of these genes, were developed in the Thatcher wheat background by crossing near‐isogenic lines. Progeny from these populations were tested in five Fusarium graminearum‐inoculated disease nurseries and assessed for FHB symptoms, using a visual rating index (VRI), and deoxynivalenol (DON) accumulation. Both Lr34 and Lr67 significantly reduced visual symptoms of FHB and DON overall, though those effects were not significant in all environments. The overall reduction in the group of progeny with the resistant allele compared to the group with the susceptible allele for VRI was 9.4% for Lr34 and 6.8% for Lr67, while for DON it was 16.4% for Lr34 and 14.9% for Lr67. These genes represent important tools for improving resistance to FHB and many other diseases in wheat.

Identification of candidate genes for adult plant stripe rust resistance transferred from Aegilops ventricosa 2NvS into wheat via fine mapping and transcriptome analysis.

An adult plant gene for resistance to stripe rust was narrowed down to the proximal one-third of the 2NvS segment translocated from Aegilops ventricosa to wheat chromosome arm 2AS, and based on the gene expression analysis, two candidate genes were identified showing a stronger response at the adult plant stage compared to the seedling stage. The 2NvS translocation from Aegilops ventricosa, known for its resistance to various diseases, has been pivotal in global wheat breeding for more than three decades. Here, we identified an adult plant resistance (APR) gene in the 2NvS segment in wheat line K13-868. Through fine mapping in a segregating near-isogenic line (NIL) derived population of 6389 plants, the candidate region for the APR gene was narrowed down to between 19.36Mb and 33Mb in the Jagger reference genome. Transcriptome analysis in NILs strongly suggested that this APR gene conferred resistance to stripe rust by triggering plant innate immune responses. Based on the gene expression analysis, two disease resistance-associated genes within the candidate region, TraesJAG2A03G00588940 and TraesJAG2A03G00590140, exhibited a stronger response to Puccinia striiformis f. sp. tritici (Pst) infection at the adult plant stage than at the seedling stage, indicating that they could be potential candidates for the resistance gene. Additionally, we developed a co-dominant InDel marker, InDel_31.05, for detecting this APR gene. Applying this marker showed that over one-half of the wheat varieties approved in 2021 and 2022 in Sichuan province, China, carry this gene. Agronomic trait evaluation of NILs indicated that the 2NvS segment effectively mitigated the negative effects of stripe rust on yield without affecting other important agronomic traits. This study provided valuable insights for cloning and breeding through the utilization of the APR gene present in the 2NvS segment.

Comparisons of stripe rust response, grain yield and quality between fungicide sprayed and non-sprayed treatments for newly developed wheat lines carrying different genes for adult-plant resistance to stripe rust

Wheat genes Yr39, Yr48, Yr52, Yr59, and Yr62 conferring different levels of adult-plant resistance (APR) to stripe rust have been introgressed into Chinese wheat cultivars. To determine the effects of five newly developed wheat lines carrying these APR genes on reduction of stripe rust severity, grain yield, and quality, field experiments including non-sprayed and fungicide sprayed plots were conducted in the 2021 and 2022 growth seasons. Compared to the 100% relative area under the disease progress curve (rAUDPC) of a susceptible check cultivar, the resistant lines had relatively low rAUDPC values, ranging from 0.38% in SWUST-445 (Yr62) to 42.04% in SWUST-006 (Yr39 + Yr26) on average of the two years in the non-sprayed plots. The two-time fungicide applications almost completely controlled stripe rust on the susceptible check and the resistant lines in both years. Stripe rust significantly reduced yield by 29.96% in 2021 and by 84.40% in 2022, whereas the yield differences of the resistant lines were not significant in both years, except SWUST-006 and SWUST-445 in 2022. Stripe rust significantly reduced the thousand kernel weight (TKW) of the susceptible check by 30.36% on the average of both years but did not cause significant TKW reduction in the resistant lines. Except few cases, the various measurements for grain quality did not show significant differences for the susceptible check and resistant lines. The results showed that the resistant lines are useful for further development of wheat cultivars with adequate and durable resistance to stripe rust.

Identification of synthetic wheat lines with broadly effective stripe rust resistance

Wheat stripe/yellow rust (WYR), caused by Puccinia striiformis f. sp. tritici (Pst), is a major constraint in global wheat production. A set of 766 hexaploid synthetic wheat lines, including primary crosses of Triticum turgidum x Aegilops tauschii and their derivatives, were screened in artificially rust inoculated field nurseries for three seasons. From this set, a core set of 94 non-lodging lines with unique pedigrees and resistance to Pst that was consistent across years was established. The core set was tested for adult plant field response under field conditions for three seasons in Australia and at least one crop season in Ethiopia, India, Kenya, Nepal and Pakistan. It was also challenged with an array of well-defined Pst pathotypes at seedling growth stages in the greenhouse, and genotyped with molecular markers linked to the adult plant resistance (APR) genes Yr18, Yr36 and Yr46. Combined analysis of field rust responses, multi-pathotype seedling phenotyping and marker genotyping resolved seven classes of Pst resistance: uncatalogued (new) APR (UAPR, 11%), uncatalogued seedling resistance (USR, 46%), known seedling resistance (KSR, 5%), KSR + USR (2%), Yr18 + UAPR (4%), Yr18 + USR (29%) and Yr18 + KSR (3%). A majority of the lines carrying UAPR and USR either singly or in combination showed high levels of field resistance across all field sites and years of testing, demonstrating that these lines represent a valuable resource for breeding wheat for resistance to Pst.

Breeding for durable resistance against biotrophic fungal pathogens using transgenes from wheat

Breeding for resistant crops is a sustainable way to control disease and relies on the introduction of novel resistance genes. Here, we tested three strategies on how to use transgenes from wheat to achieve durable resistance against fungal pathogens in the field. First, we tested the highly effective, overexpressed single transgene Pm3e in the background of spring wheat cultivar Bobwhite in a long-term field trial over many years. Together with previous results, this revealed that transgenic wheat line Pm3e#2 conferred complete powdery mildew resistance during a total of nine field seasons without a negative impact on yield. Furthermore, overexpressed Pm3e provided resistance to powdery mildew isolates from our worldwide collection when crossed into the elite wheat cultivar Fiorina. Second, we pyramided the four overexpressed transgenes Pm3a, Pm3b, Pm3d, and Pm3f in the background of cultivar Bobwhite and showed that the pyramided line Pm3a,b,d,f was completely resistant to powdery mildew in five field seasons. Third, we performed field trials with three barley lines expressing adult plant resistance gene Lr34 from wheat during three field seasons. Line GLP8 expressed Lr34 under control of the pathogen-inducible Hv-Ger4c promoter and provided partial barley powdery mildew and leaf rust resistance in the field with small, negative effects on yield components which might need compensatory breeding. Overall, our study demonstrates and discusses three successful strategies for achieving fungal disease resistance of wheat and barley in the field using transgenes from wheat. These strategies might confer long-term resistance if applied in a sustainable way.

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.

Studying a collection of common-wheat varieties for leaf rust resistance, crop yield and grain quality in the environmental conditions of Novosibirsk region.

The relationship between a variety's genotype, environmental conditions and phytopathogenic load are the key factors contributing to high yields that should be taken into account in selecting donors for resistance and high manifestation of valuable traits. The study of leaf rust resistance in 49 common wheat varieties was carried out in the field against the natural pathogen background and under laboratory conditions using single-pustule isolates with virulence to Lr9 and Lr24. It has been shown that the varieties carrying alien genes Lr6Agi2 (Tulaikovskaya 10) and Lr6Agi1 (Voevoda) were resistant to leaf rust infection both in the field and in the laboratory. Varieties KWS Buran, KWS Akvilon, KW 240-3-13, and Etyud producing crop yields from 417 to 514 g/m2 comparable to the best standard variety Sibirskaya 17 can be reasonably used as Lr24 resistance gene donors under West Siberian conditions. Omskaya 44 variety showing crop yield of 440g/m2 can be used as a donor for Lr19 and partially effective Lr26. Varieties Tuleevskaya and Altayskaya 110 with Lr9 in their genomes are recommended for the development of resistance gene-pyramided genotypes. The highest protein and gluten contents were observed in the CS2A/2M sample, while KWS Buran, Altayskaya 110, Volgouralskaya, and KWS Akvilon showed the lowest values. Varieties CS2A/2M, Tulaikovskaya 10, Pavon, and Tuleevskaya were ranked the highest in micro- (Cu, Mn, Zn, Fe) and macronutrient (Ca, Mg, K) contents among the common wheat samples from the collection, while the lowest values for most elements were observed in KWS Buran, Novosibirskaya 15, and Volgouralskaya. Winter varieties demonstrating leaf rust resistance against the infectious background typically carry adult plant resistance genes (Lr34, Lr12, and Lr13), particularly combined with the juvenile Lr26 gene. The presence of Lr41 in a winter type line (KS 93 U 62) allowed it to maintain resistance against a leaf rust pathogen clone kLr24, despite the presence of Lr24 in the genotype. Varieties Doka and Cheshskaya 17 may act as donors of resistance genes Lr26 + Lr34 and Lr9 + Lr12 + Lr13 + Lr34, as well as sources of dwarfing without losses in winter hardiness and yield under West Siberian conditions.

Virulence Dynamics of the Barley Leaf Rust Pathogen (Puccinia hordei) in the United States from 1989 to 2020.

Barley leaf rust, caused by Puccinia hordei, is an important disease of barley worldwide. The pathogen can develop new races that overcome resistance genes, emphasizing the need for monitoring its virulence. This study characterized 519 P. hordei isolates collected in the United States from the 1989 to 2000 and 2010 to 2020 survey periods on 15 Rph (Reaction to Puccinia hordei) genes. We analyzed linearized infection type data to detect virulence patterns across the United States and in five geographical regions: Pacific/West (PW), Southwest (SW), Midwest (MW), Northeast (NE), and Southeast (SE). Over 32 years, we observed high mean infection scores for Rph1.a, Rph4.d, and Rph8.h; intermediate scores for Rph2.b, Rph9.i, Rph10.o, Rph11.p, and Rph13.x; and low scores for Rph3.c, Rph5.e, Rph5.f, Rph7.g, Rph9.z, Rph14.ab, and Rph15.ad. Virulence for Rph2.b, Rph3.c, Rph5.e, Rph9.z, Rph10.o, Rph11.p, and Rph13.x significantly differed between the two survey periods. From 1989 to 2020, regional patterns of virulence were found for Rph5.e, Rph5.f, Rph7.g, and Rph14.ab, while regionalities of virulence for Rph3.c, Rph9.i, Rph9.z were only observed in the 2010 to 2020 survey period. Virulence associations were also detected in the P. hordei population. Notably, isolates that were virulent to Rph5.e and Rph6.f were more likely to be avirulent to Rph7.g and Rph13.x, and vice versa. In decreasing order of effectiveness, Rph15.ad, Rph5.e, Rph3.c, Rph9.z, Rph7.g, Rph5.f, and Rph14.ab were the most effective Rph genes in the United States from 1989 to 2020. Pyramiding Rph15.ad with other widely effective Rph and adult plant resistance genes may provide long-lasting resistance against P. hordei.

Introgression of the bread wheat D genome encoded Lr34/Yr18/Sr57/Pm38/Ltn1 adult plant resistance gene into Triticum turgidum (durum wheat)

Key MessageLack of function of a D-genome adult plant resistance gene upon introgression into durum wheat.The wheat Lr34/Yr18/Sr57/Pm38/Ltn1 adult plant resistance gene (Lr34), located on chromosome arm 7DS, provides broad spectrum, partial, adult plant resistance to leaf rust, stripe rust, stem rust and powdery mildew. It has been used extensively in hexaploid bread wheat (AABBDD) and conferred durable resistance for many decades. These same diseases also occur on cultivated tetraploid durum wheat and emmer wheat but transfer of D genome sequences to those subspecies is restricted due to very limited intergenomic recombination. Herein we have introgressed the Lr34 gene into chromosome 7A of durum wheat. Durum chromosome substitution line Langdon 7D(7A) was crossed to Cappelli ph1c, a mutant derivative of durum cultivar Cappelli homozygous for a deletion of the chromosome pairing locus Ph1. Screening of BC1F2 plants and their progeny by KASP and PCR markers, 90 K SNP genotyping and cytology identified 7A chromosomes containing small chromosome 7D fragments encoding Lr34. However, in contrast to previous transgenesis experiments in durum wheat, resistance to wheat stripe rust was not observed in either Cappelli/Langdon 7D(7A) or Bansi durum plants carrying this Lr34 encoding segment due to low levels of Lr34 gene expression. Key message

Expression Profiling of the Slow Rusting Resistance Genes Lr34/Yr18 and Lr67/Yr46 in Common Wheat (Triticum aestivum L.) and Associated miRNAs Patterns.

The main efforts in common wheat (Triticum aestivum L.) breeding focus on yield, grain quality, and resistance to biotic and abiotic stresses. One of the major threats affecting global wheat cultivation and causing significant crop production losses are rust diseases, including leaf rust caused by a biotrophic fungus Puccinia triticina Eriks. Genetically determined resistance to leaf rust has been characterized in young plants (seedling resistance) as well as in plants at the adult plant stage. At the seedling stage, resistance is controlled vertically by major R genes, conferring a race-specific response that is highly effective but usually short-lived due to the rapid evolution of potentially virulent fungi. In mature plants, horizontal adult plant resistance (APR) was described, which provides long-term protection against multiple races of pathogens. A better understanding of molecular mechanisms underlying the function of APR genes would enable the development of new strategies for resistance breeding in wheat. Therefore, in the present study we focused on early transcriptomic responses of two major wheat APR genes, Lr34 and Lr67, and three complementary miRNAs, tae-miR9653b, tae-miR9773 and tae-miR9677b, to inoculation with P. triticina. Plant material consisted of five wheat reference varieties, Artigas, NP846, Glenlea, Lerma Rojo and TX89D6435, containing the Lr34/Yr18 and Lr67/Yr46 resistance genes. Biotic stress was induced by inoculation with fungal spores under controlled conditions in a phytotron. Plant material consisted of leaf tissue sampled before inoculation as well as 6, 12, 24 and 48 h postinoculation (hpi). The APR gene expression was quantified using real-time PCR with two reference genes, whereas miRNA was quantified using droplet digital PCR. This paper describes the resistance response of APR genes to inoculation with races of leaf rust-causing fungi that occur in central Europe. The study revealed high variability of expression profiles between varieties and time-points, with the prevalence of downregulation for APR genes and upregulation for miRNAs during the development of an early defense response. Nevertheless, despite the downregulation initially observed, the expression of Lr34 and Lr67 genes in studied cultivars was significantly higher than in a control line carrying wild (susceptible) alleles.

Investigating the Mechanisms Underlying the Durability and Sustainable Use of Pb1 Gene-Mediated High Field Resistance to Rice Panicle Blast

Rice blast, caused by Magnaporthe oryzae, poses a significant threat to rice production. Rice blast susceptibility has been observed in Japanese rice varieties with excellent eating quality. Enhancing blast resistance is essential to ensure minimal use of agricultural chemicals. Two types of blast resistance are observed: True resistance, which is a type of qualitative resistance expressed by a major gene, and field resistance, which is a type of quantitative resistance expressed by multiple micro-acting genes. ‘Resistance collapse’, in which a variety with a true resistance gene becomes diseased by blast fungus races compatible with the resistance gene, has been observed. Varieties carrying blast-resistance genes, such as Pb1 (panicle blast resistance 1), have been developed through DNA marker-assisted selection. In this review, we focus on the Pb1, which expresses strong quantitative resistance to panicle blast and has been widely used in Japan without showing ‘resistance collapse’ for 40 years. Pb1 is an ‘adult plant resistance gene’ that does not exert strong selection pressure on the blast population during the leaf blast stage, thus preventing the selective multiplication of Pb1-compatible blast strains. This epidemiological mechanism prevents ‘resistance collapse’. Interdisciplinary research and breeding are required to sustainably use genes that induce high field resistance.

A unique resistance mechanism is associated with RBgh2 barley powdery mildew adult plant resistance

Key messageGene expression at the RBgh2 locus indicates involvement in cAMP/G-protein-coupled signalling and innate immunity in barley powdery mildew adult plant resistance.Barley powdery mildew is a globally significant disease, responsible for reduced grain yield and quality. A major effect adult plant resistance gene, RBgh2, was previously found in a landrace from Azerbaijan. The atypical phenotype suggested different underlying genetic factors compared to conventional resistance genes and to investigate this, genome-wide gene expression was compared between sets of heterogeneous doubled haploids. RBgh2 resistance is recessive and induces both temporary genome-wide gene expression changes during powdery mildew infection together with constitutive changes, principally at the RBgh2 locus. Defence-related genes significantly induced included homologues of genes associated with innate immunity and pathogen recognition. Intriguingly, RBgh2 resistance does not appear to be dependent on salicylic acid signalling, a key pathway in plant resistance to biotrophs. Constitutive co-expression of resistance gene homologues was evident at the 7HS RBgh2 locus, while no expression was evident for a 6-transmembrane gene, predicted in silico to contain both G-protein- and calmodulin-binding domains. The gene was disrupted at the 5′ end, and G-protein-binding activity was suppressed. RBgh2 appears to operate through a unique mechanism that co-opts elements of innate immunity.