Development and characterization of expressed sequence tag-derived microsatellite markers for the wheat stem rust fungus Puccinia graminis f. sp. tritici

A highly multiplexed assay to monitor pathogenicity, fungicide resistance and gene flow in the fungal wheat pathogen Zymoseptoria tritici

Crop pathogens pose severe risks to global food production due to the rapid rise of resistance to pesticides and host resistance breakdowns. Predicting future risks requires monitoring tools to identify changes in the genetic composition of pathogen populations. Here we report the design of a microfluidics-based amplicon sequencing assay to multiplex 798 loci targeting virulence and fungicide resistance genes, and randomly selected genome-wide markers for the fungal pathogen Zymoseptoria tritici. The fungus causes one of the most devastating diseases on wheat showing rapid adaptation to fungicides and host resistance. We optimized the primer design by integrating polymorphism data from 632 genomes of the same species. To test the performance of the assay, we genotyped 192 samples in two replicates. Analysis of the short-read sequence data generated by the assay showed a fairly stable success rate across samples to amplify a large number of loci. The performance was consistent between samples originating from pure genomic DNA as well as material extracted directly from infected wheat leaves. In samples with mixed genotypes, we found that the assay recovers variations in allele frequencies. We also explored the potential of the amplicon assay to recover transposable element insertion polymorphism relevant for fungicide resistance. As a proof-of-concept, we show that the assay recovers the pathogen population structure across French wheat fields. Genomic monitoring of crop pathogens contributes to more sustainable crop protection and yields.

SSR Variability of Stem Rust Pathogen on Spring Bread Wheat in Russia

Wheat stem rust, caused by Puccinia graminis f. sp. tritici, which used to be a harmful disease of winter wheat in the southern part of Russia, has been largely affecting the yield of spring bread wheat in the territories of the temperate climate zone since 2009. In total, 222 P. graminis f. sp. tritici isolates were obtained from samples of susceptible cultivars of spring bread wheat in Central and Volga regions and Omsk and Novosibirsk provinces in 2019. Genotyping of the isolates was carried out at 16 simple-sequence repeat (SSR) loci. Number of alleles, proportion of heterozygotes, and deviation from Hardy-Weinberg equilibrium were determined at each SSR locus. Based on genetic variability of SSR genotypes, it was shown that the P. graminis f. sp. tritici population is subdivided into two large clusters in the territory of the Russian temperate climate zone: the "European" population (the Central region) and the "Asian" one (the Volga region and two main wheat provinces of Western Siberia). Both of the P. graminis f. sp. tritici populations are characterized by a mixed mode of reproduction (sexual and clonal) but different sources of inoculum seem to shape a genotype structure within them. A group of P. graminis f. sp. tritici genotypes with high variability, the inbreeding coefficient closed to zero, and low observed heterozygosity was revealed among samples from Omsk. Moreover, two singular SSR genotypes identified among the Asian samples of P. graminis f. sp. tritici isolates should attract special attention in the monitoring of stem rust in order to disclose unexpected rapid changes of the pathogen in the corresponding regions and to prevent disease outbreak.

Wheat Stem Rust Detection and Race Characterization in Tunisia

Climate changes over the past 25 years have led to conducive conditions for invasive and transboundary fungal disease occurrence, including the re-emergence of wheat stem rust disease, caused by Puccinia graminis f.sp. tritici (Pgt) in East Africa, Europe, and the Mediterranean basin. Since 2018, sporadic infections have been observed in Tunisia. In this study, we investigated Pgt occurrence at major Tunisian wheat growing areas. Pgt monitoring, assessment, and sampling from planted trap nurseries at five different locations over two years (2021 and 2022) revealed the predominance of three races, namely TTRTF (Clade III-B), TKKTF (Clade IV-F), and TKTTF (Clade IV-B). Clade III-B was the most prevalent in 2021 as it was detected at all locations, while in 2022 Pgt was only reported at Beja and Jendouba, with the prevalence of Clade IV-B. The low levels of disease incidence during these two years and Pgt population diversity suggest that this fungus most likely originated from exotic incursions and that climate factors could have caused disease establishment in Tunisia. Further evaluation under the artificial disease pressure of Tunisian wheat varieties and weather-based modeling for early disease detection in the Mediterranean area could be helpful in monitoring and predicting wheat stem rust emergence and epidemics.

Stem rust on barberry species in Europe: Host specificities and genetic diversity

The increased emergence of cereal stem rust in southern and western Europe, caused by the pathogen Puccinia graminis, and the prevalence of alternate (sexual) host, Berberis species, have regained attention as the sexual host may serve as source of novel pathogen variability that may pose a threat to cereal supply. The main objective of the present study was to investigate the functional role of Berberis species in the current epidemiological situation of cereal stem rust in Europe. Surveys in 11 European countries were carried out from 2018 to 2020, where aecial infections from five barberry species were collected. Phylogenetic analysis of 121 single aecial clusters of diverse origin using the elongation factor 1-α gene indicated the presence of different special forms (aka formae speciales) of P. graminis adapted to different cereal and grass species. Inoculation studies using aecial clusters from Spain, United Kingdom, and Switzerland resulted in 533 stem rust isolates sampled from wheat, barley, rye, and oat, which confirmed the presence of multiple special forms of P. graminis. Microsatellite marker analysis of a subset of 192 sexually-derived isolates recovered on wheat, barley and rye from the three populations confirmed the generation of novel genetic diversity revealed by the detection of 135 multilocus genotypes. Discriminant analysis of principal components resulted in four genetic clusters, which grouped at both local and country level. Here, we demonstrated that a variety of Berberis species may serve as functional alternate hosts for cereal stem rust fungi and highlights the increased risks that the sexual cycle may pose to cereal production in Europe, which calls for new initiatives within rust surveillance, epidemiological research and resistance breeding.

Elucidating the Population Structure and Genetic Diversity of Indian Puccinia striiformis f. sp. tritici Pathotypes Based on Microsatellite Markers

In India, systematic wheat yellow rust survey and pathotype (race) analysis work began in 1930. However, information on population structure and genetic diversity of yellow rust pathogen has not been available. To address this, we conducted studies on population structure and genetic diversity of Puccinia striiformis f. sp. tritici (Pst) pathotypes using 38 simple sequence repeat primer-pairs. Bayesian assignment and discriminant analysis of principal components indicated the presence of two distinct Pst subpopulations (Pop1 and Pop2) along with 37.9% admixed pathotypes. The unweighted pair-group method with arithmetic mean also categorized these pathotypes into two major clusters. Principal coordinates analysis explained 20.06 and 12.50% variance in horizontal and vertical coordinates, respectively. Index of association (I_A) and the standardized index of association ([Formula: see text]) values showed that Pst subpopulations reproduced asexually (clonally). In total, 102 alleles were detected, with the expected heterozygosity (H_exp) per locus ranging from 0.13 to 0.73, with a mean of 0.47. The average polymorphic information content value of 0.40 indicated high genetic diversity among pathotypes. Analysis of molecular variance revealed 12% of the total variance between subpopulations, 11% among the pathotypes of each subpopulation, and 77% within pathotypes. A significant moderate level of genetic differentiation (F_ST = 0.122, P < 0.001) and gene flow (N_m = 1.80) were observed between subpopulations. The Pst virulence phenotypes showed a weak positive correlation (R² = 0.027, P < 0.02) with molecular genotypes.

Wheat Stem Rust Back in Europe: Diversity, Prevalence and Impact on Host Resistance

The objective of this study was to investigate the re-emergence of a previously important crop pathogen in Europe, Puccinia graminis f.sp. tritici, causing wheat stem rust. The pathogen has been insignificant in Europe for more than 60 years, but since 2016 it has caused epidemics on both durum wheat and bread wheat in local areas in southern Europe, and additional outbreaks in Central- and West Europe. The prevalence of three distinct genotypes/races in many areas, Clade III-B (TTRTF), Clade IV-B (TKTTF) and Clade IV-F (TKKTF), suggested clonal reproduction and evolution by mutation within these. None of these genetic groups and races, which likely originated from exotic incursions, were detected in Europe prior to 2016. A fourth genetic group, Clade VIII, detected in Germany (2013), was observed in several years in Central- and East Europe. Tests of representative European wheat varieties with prevalent races revealed high level of susceptibility. In contrast, high diversity with respect to virulence and Simple Sequence Repeat (SSR) markers were detected in local populations on cereals and grasses in proximity to Berberis species in Spain and Sweden, indicating that the alternate host may return as functional component of the epidemiology of wheat stem rust in Europe. A geographically distant population from Omsk and Novosibirsk in western Siberia (Russia) also revealed high genetic diversity, but clearly different from current European populations. The presence of Sr31-virulence in multiple and highly diverse races in local populations in Spain and Siberia stress that virulence may emerge independently when large geographical areas and time spans are considered and that Sr31-virulence is not unique to Ug99. All isolates of the Spanish populations, collected from wheat, rye and grass species, were succesfully recovered on wheat, which underline the plasticity of host barriers within P. graminis. The study demonstrated successful alignment of two genotyping approaches and race phenotyping methodologies employed by different laboratories, which also allowed us to line up with previous European and international studies of wheat stem rust. Our results suggest new initiatives within disease surveillance, epidemiological research and resistance breeding to meet current and future challenges by wheat stem rust in Europe and beyond.

Genetic Diversity and Population Structure Reveal Cryptic Genetic Variation and Long Distance Migration of Puccinia graminis f. sp. tritici in the Indian Subcontinent

Stem rust caused by Puccinia graminis f. sp. tritici (Pgt) is a devastating disease of wheat worldwide since time immemorial. Several wheat stem rust outbreaks have been reported worldwide including India. Approximately 7 mha wheat area in central and peninsular India is highly vulnerable to stem rust epidemics. In this study, a repository of 29 single genotype uredospore pathotypes, representing five geographical regions, was characterized by investigating their virulence phenotype and simple sequence repeat (SSR) genotypes using 37 reproducible polymorphic SSR markers, 32 of which had ≥ 0.50 polymorphic information content (PIC) value. Virulence phenotypes were used to evaluate the virulence frequency (VF) and construct a hypothetical evolutionary hierarchy of these pathotypes. We projected seven lineages to explain the evolutionary pattern of the Pgt population. The VF of these pathotypes ranged between 0% and 100%. The virulence-based neighbor-joining (NJ) cluster analysis grouped Pgt pathotypes into five virulence groups. Likewise, five molecular groups were categorized using molecular genotypes. The molecular grouping was supported by principal coordinate analysis (PCoA), which revealed 25% of the cumulative variance contributed by the first two axes. Analysis of molecular variance (AMOVA) revealed 8 and 92% of the variation among and within the populations, respectively. The Mantel test confirmed a positive but weak correlation (R 2 = 0.15) between virulence phenotypes and SSR genotypes. The highest and lowest values of different genetic diversity parameters (Na, Ne, I, He, uHe, and %P) revealed maximum and minimum variability in the Pgt population from Maharashtra and Uttar Pradesh, respectively. The population structure analysis clustered 29 Pgt pathotypes into two subpopulations and an admixture. Our results demonstrated that there was significant genetic diversity among Pgt pathotypes resulting from their long-distance dispersal ability complemented by gene flow. These findings provide insights into the virulence patterns, genetic variations, and possible evolution of Pgt pathotypes, which would support strategic stem rust resistance breeding.

Phenotyping and Genotyping Analyses Reveal the Spread of Puccinia striiformis f. sp. tritici Aeciospores From Susceptible Barberry to Wheat in Qinghai of China

Puccinia striiformis f. sp. tritici Eriks., the cause of wheat yellow or stripe rust on wheat, undergoes sexual reproduction on barberry, but it is unclear if barberry plays any role in stripe rust epidemics under natural conditions. P. striiformis f. sp. tritici was isolated from its alternate host barberry (Berberis spp.) and primary host wheat in the vicinity of barberry by inoculation of aeciospores and urediniospores on Mingxian 169 cultivar in Qinghai province of China in 2018. The P. striiformis f. sp. tritici isolates from barberry and wheat were characterized to virulence patterns by inoculation on 24 differentials bearing Yr gene under control conditions and analyzed using 12 polymorphic simple sequence repeat (SSR) markers. The occurrence frequency of P. striiformis f. sp. tritici on barberry was 1.87% by inoculation aecia, collected from barberry on Mingxian 169 of wheat. A close virulence relationship was presented between P. striiformis f. sp. tritici isolates from both barberry and wheat based on virulence simple matching coefficient and principal coordinates analysis (PCoA). Additionally, the same genetic ancestry, based on structure analysis by STRUCTURE program and genetic relationship analyses using discriminant analysis of principal components and PCoA, was shared between P. striiformis f. sp. tritici isolates from barberry and those from wheat. Together, all the results indicated that the role of barberry in providing aeciospores as an inoculum source causing wheat stripe rust epidemic in Qinghai in spring is of considerable importance.

A Homolog of the Arabidopsis TIME FOR COFFEE Gene Is Involved in Nonhost Resistance to Wheat Stem Rust in Brachypodium distachyon

Plants resist infection by pathogens using both preexisting barriers and inducible defense responses. Inducible responses are governed in a complex manner by various hormone signaling pathways. The relative contribution of hormone signaling pathways to nonhost resistance to pathogens is not well understood. In this study, we examined the molecular basis of disrupted nonhost resistance to the fungal species Puccinia graminis, which causes stem rust of wheat, in an induced mutant of the model grass Brachypodium distachyon. Through bioinformatic analysis, a 1-bp deletion in the mutant genotype was identified that introduces a premature stop codon in the gene Bradi1g24100, which is a homolog of the Arabidopsis thaliana gene TIME FOR COFFEE (TIC). In Arabidopsis, TIC is central to the regulation of the circadian clock and plays a crucial role in jasmonate signaling by attenuating levels of the transcription factor protein MYC2, and its mutational disruption results in enhanced susceptibility to the hemibiotroph Pseudomonas syringae. Our similar finding for an obligate biotroph suggests that the biochemical role of TIC in mediating disease resistance to biotrophs is conserved in grasses, and that the correct modulation of jasmonate signaling during infection by Puccinia graminis may be essential for nonhost resistance to wheat stem rust in B. distachyon.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY 4.0 International license.

A genome-wide genetic linkage map and reference quality genome sequence for a new race in the wheat pathogen Pyrenophora tritici-repentis

Pyrenophora tritici-repentis is an ascomycete fungus that causes tan spot of wheat. The disease has a worldwide distribution and can cause significant yield and quality losses in wheat production. The fungal pathogen is homothallic in nature, which means it can undergo sexual reproduction by selfing to produce pseudothecia on wheat stubble for seasonal survival. Since homothallism precludes the development of bi-parental fungal populations, no genetic linkage map has been developed for P. tritici-repentis for mapping and map-based cloning of fungal virulence genes. In this work, we created two heterothallic strains by deleting one of the mating type genes in each of two parental isolates 86-124 (race 2) and AR CrossB10 (a new race) and developed a bi-parental fungal population between them. The draft genome sequences of the two parental isolates were aligned to the Pt-1C-BFP reference sequence to mine single nucleotide polymorphisms (SNPs). A total of 225 SNP markers were developed for genotyping the entire population. Additionally, 75 simple sequence repeat, and two gene markers were also developed and used in the genotyping. The resulting linkage map consisted of 13 linkage groups spanning 5,075.83 cM in genetic distance. Because the parental isolate AR CrossB10 is a new race and produces Ptr ToxC, it was sequenced using long-read sequencing platforms and de novo assembled into contigs. The majority of the contigs were further anchored into chromosomes with the aid of the linkage maps. The whole genome comparison of AR CrossB10 to the reference genome of M4 revealed a few chromosomal rearrangements. The genetic linkage map and the new AR CrossB10 genome sequence are valuable tools for gene cloning in P. tritici-repentis.

Chromosomal composition analysis and molecular marker development for the novel Ug99-resistant wheat-Thinopyrum ponticum translocation line WTT34

A novel Ug99-resistant wheat-Thinopyrum ponticum translocation line was produced, its chromosomal composition was analyzed and specific markers were developed. Stem rust caused by Puccinia graminis f. sp. tritici Eriks. & E. Henn (Pgt) has seriously threatened global wheat production since Ug99 race TTKSK was first detected in Uganda in 1998. Thinopyrum ponticum is near immune to Ug99 races and may be useful for enhancing wheat disease resistance. Therefore, developing new wheat-Th. ponticum translocation lines that are resistant to Ug99 is crucial. In this study, a novel wheat-Th. ponticum translocation line, WTT34, was produced. Seedling and field evaluation revealed that WTT34 is resistant to Ug99 race PTKST. The resistance was derived from the alien parent Th. ponticum. Screening WTT34 with markers linked to Sr24, Sr25, Sr26, Sr43, and SrB resulted in the amplification of different DNA fragments from Th. ponticum, implying WTT34 carries at least one novel stem rust resistance gene. Genomic in situ hybridization (GISH), multicolor fluorescence in situ hybridization (mc-FISH), and multi-color GISH (mc-GISH) analyses indicated that WTT34 carries a T5DS·5DL-Th translocation, which was consistent with wheat660K single-nucleotide polymorphism (SNP) array results. The SNP array also uncovered a deletion event in the terminal region of chromosome 1D. Additionally, the homeology between alien segments and the wheat chromosomes 2A and 5D was confirmed. Furthermore, 51 PCR-based markers derived from the alien segments of WTT34 were developed based on specific-locus amplified fragment sequencing (SLAF-seq). These markers may enable wheat breeders to rapidly trace Th. ponticum chromosomal segments carrying Ug99 resistance gene(s).

Principal approaches and achievements in studying race composition of wheat stem rust

 Wheat stem rust caused by the biotrophic fungus Puccinia graminis f. sp. tritici is a dangerous disease that seriously damages the economics in many countries of the world. The review contains information about epidemics of wheat stem rust and causes of their emergence worldwide. Recently wheat stem rust epidemics have been recorded in the northern regions of Kazakhstan and on the territories adjacent to Omsk Region of Russia. It has been shown that severe wheat stem rust epidemics occur mainly due to the emergence of new virulent races of the disease agent and to growing susceptible wheat cultivars. New methods of studying the race composition of the fungus are described as well as the use of the previous and current differential sets for race determination of P. graminis f. sp. tritici. The results of developing molecular markers and assessing their effectiveness in studying stem rust races are presented. Wheat stem rust races dominant in major grain-growing countries of the globe and their typical peculiarities are described. The paper contains information on identifcation of race Ug99 and of its variations including data on areas of their dissemination and on their virulence to Sr-resistance genes. The existence and emergence of other races of the agent potentially dangerous for commercially important genes for stem rust resistance is also described. Currently in nature strongly virulent races of P. graminis f. sp. tritici are circulating with wide geographical coverage and their virulence is absolutely different from the virulence of race Ug99. Historical and modern data on studying the race composition of the pathogen in Kazakhstan are summarized. It is stated that the use of the old standard differential set and an incomplete North American system of race nomenclature in experiments prevents measuring similarity between Kazakhstani races and the worldwide known races of the pathogen. It has been shown that there is a need to continue studies on the intraspecies structure of the disease agent’s population in Kazakhstan with the use of the modern differential set, on determination of race composition and ways of emergence of new races potentially dangerous for commercial wheat varieties.

Development and Characterization of Novel Genic-SSR Markers in Apple-Juniper Rust Pathogen Gymnosporangium yamadae (Pucciniales: Pucciniaceae) Using Next-Generation Sequencing

The Apple-Juniper rust, Gymnosporangium yamadae, is an economically important pathogen of apples and junipers in Asia. The absence of markers has hampered the study of the genetic diversity of this widespread pathogen. In our study, we developed twenty-two novel microsatellite markers for G. yamadae from randomly sequenced regions of the transcriptome, using next-generation sequencing methods. These polymorphic markers were also tested on 96 G. yamadae individuals from two geographical populations. The allele numbers ranged from 2 to 9 with an average value of 6 per locus. The polymorphism information content (PIC) values ranged from 0.099 to 0.782 with an average value of 0.48. Furthermore, the observed (H_O) and expected (H_E) heterozygosity ranged from 0.000 to 0.683 and 0.04 to 0.820, respectively. These novel developed microsatellites provide abundant molecular markers for investigating the genetic structure and genetic diversity of G. yamadae, which will help us to better understand disease epidemics and the origin and migration routes of the Apple-Juniper rust pathogen. Further studies will also be completed to dissect how human activities influence the formation of current population structures. Furthermore, these SSR (simple sequence repeat) markers can also be used as tools to identify virulence by mapping the whole genomes of different virulent populations. These markers will, thus, assist the development of effective risk-assessment models and management systems for the Apple-Juniper rust pathogen.

Genetic mapping of a major gene in triticale conferring resistance to bacterial leaf streak

A major gene conferring resistance to bacterial leaf streak was mapped to chromosome 5R in triticale. Bacterial leaf streak (BLS), caused by Xanthomonas translucens pv. undulosa (Xtu), is an important disease of wheat and triticale around the world. Although resistance to BLS is limited in wheat, several triticale accessions have high levels of resistance. To characterize the genetic basis of this resistance, we developed triticale mapping populations using a resistant accession (Siskiyou) and two susceptible accessions (UC38 and Villax St. Jose). Bulked segregant analysis in an F₂ population derived from the cross of Siskiyou × UC38 led to the identification of a simple sequence repeat (SSR) marker (XSCM138) on chromosome 5R that co-segregated with the resistance gene. The cross of Siskiyou × Villax St. Jose was advanced into an F_2:5 recombinant inbred line population and evaluated for BLS reaction. Genetic linkage maps on this population were assembled with markers generated using genotyping-by-sequencing as well as several SSR markers previously identified on 5R. Quantitative trait locus (QTL) mapping revealed a single major QTL on chromosome 5R, underlined by the same SSR marker as in the Siskiyou × UC38 population. The F₁ hybrids of the two crosses were highly resistant to BLS, indicating that resistance is largely dominant. This work will facilitate introgression of this rye-derived BLS resistance gene into the wheat genome by molecular marker-mediated chromosome engineering.

Multiple genotypes within aecial clusters in Puccinia graminis and Puccinia coronata: improved understanding of the biology of cereal rust fungi

BACKGROUND

Cereal rust fungi (Puccinia spp.) are among the most economically important plant pathogens. These fungi have a complex life cycle, including five spore stages and two hosts. They infect one grass host on which they reproduce clonally and cause the cereal rust diseases, while the alternate host is required for sexual reproduction. Although previous studies clearly demonstrate the importance of the alternate host in creating genetic diversity in cereal rust fungi, little is known about the amount of novel genotypes created in each successful completion of a sexual reproduction event.

RESULTS

In this study, single sequence repeat markers were used to study the genotypic diversity within aecial clusters by genotyping individual aecial cups. Two common cereal rusts, Puccinia graminis causing stem rust and Puccinia coronata the causal agent of crown rust were investigated. We showed that under natural conditions, a single aecial cluster usually include several genotypes, either because a single pycnial cluster is fertilized by several different pycniospores, or because aecia within the cluster are derived from more than one fertilized adjoining pycnial cluster, or a combination of both.

CONCLUSION

Our results imply that although sexual events in cereal rust fungi in most regions of the world are relatively rare, the events that occur may still significantly contribute to the genetic variation within the pathogen populations.

Identification and characterization of simple sequence repeats (SSRs) for population studies of Puccinia novopanici

Switchgrass (Panicum virgatum L.) can be severely affected by rust disease. Recently switchgrass rust caused by P. emaculata (now confirmed to be Puccinia novopanici) has received most of the attention by the research community because this pathogen is responsible for reducing the biomass production and biofuel feedstock quality of switchgrass. Microsatellite markers found in the literature were either not informative (no allele frequency) or showed few polymorphisms in the target populations, therefore additional markers are needed for future studies of the genetic variation and population structure of P. novopanici. This study reports the development and characterization of novel simple sequence repeat (SSR) markers from a Puccinia emaculata s.l. microsatellite-enriched library and expressed sequence tags (ESTs). Microsatellites were evaluated for polymorphisms on P. emaculata s.l. urediniospores collected in Iowa (IA), Mississippi (MS), Oklahoma (OK), South Dakota (SD) and Virginia (VA). Puccinia novopanici single spore whole genome amplifications were used as templates to validate the SSR reactions protocol and to assess a preliminary population genetics statistics of the pathogen. Eighteen microsatellite markers were polymorphic (average PIC=0.72) on individual urediniospores, with an average of 8.3 alleles per locus (range 3 to 17). Of the 49 SSRs loci initially identified in P. emaculata s.l., 18 were transferable to P. striiformis f. sp. tritici, 23 to P. triticina, 20 to P. sorghi and 31 to P. andropogonis. Thus, these markers could be useful for DNA fingerprinting and population structure analysis for population genetics, epidemiology and ecological studies of P. novopanici and potentially other related Puccinia species.

Emergence and Spread of New Races of Wheat Stem Rust Fungus: Continued Threat to Food Security and Prospects of Genetic Control

Race Ug99 (TTKSK) of Puccinia graminis f. sp. tritici, detected in Uganda in 1998, has been recognized as a serious threat to food security because it possesses combined virulence to a large number of resistance genes found in current widely grown wheat (Triticum aestivum) varieties and germplasm, leading to its potential for rapid spread and evolution. Since its initial detection, variants of the Ug99 lineage of stem rust have been discovered in Eastern and Southern African countries, Yemen, Iran, and Egypt. To date, eight races belonging to the Ug99 lineage are known. Increased pathogen monitoring activities have led to the identification of other races in Africa and Asia with additional virulence to commercially important resistance genes. This has led to localized but severe stem rust epidemics becoming common once again in East Africa due to the breakdown of race-specific resistance gene SrTmp, which was deployed recently in the 'Digalu' and 'Robin' varieties in Ethiopia and Kenya, respectively. Enhanced research in the last decade under the umbrella of the Borlaug Global Rust Initiative has identified various race-specific resistance genes that can be utilized, preferably in combinations, to develop resistant varieties. Research and development of improved wheat germplasm with complex adult plant resistance (APR) based on multiple slow-rusting genes has also progressed. Once only the Sr2 gene was known to confer slow rusting APR; now, four more genes-Sr55, Sr56, Sr57, and Sr58-have been characterized and additional quantitative trait loci identified. Cloning of some rust resistance genes opens new perspectives on rust control in the future through the development of multiple resistance gene cassettes. However, at present, disease-surveillance-based chemical control, large-scale deployment of new varieties with multiple race-specific genes or adequate levels of APR, and reducing the cultivation of susceptible varieties in rust hot-spot areas remains the best stem rust management strategy.

Identification and mapping of Sr46 from Aegilops tauschii accession CIae 25 conferring resistance to race TTKSK (Ug99) of wheat stem rust pathogen

Mapping studies confirm that resistance to Ug99 race of stem rust pathogen in Aegilops tauschii accession Clae 25 is conditioned by Sr46 and markers linked to the gene were developed for marker-assisted selection. The race TTKSK (Ug99) of Puccinia graminis f. sp. tritici, the causal pathogen for wheat stem rust, is considered as a major threat to global wheat production. To address this threat, researchers across the world have been devoted to identifying TTKSK-resistant genes. Here, we report the identification and mapping of a stem rust resistance gene in Aegilops tauschii accession CIae 25 that confers resistance to TTKSK and the development of molecular markers for the gene. An F2 population of 710 plants from an Ae. tauschii cross CIae 25 × AL8/78 were first evaluated against race TPMKC. A set of 14 resistant and 116 susceptible F2:3 families from the F2 plants were then evaluated for their reactions to TTKSK. Based on the tests, 179 homozygous susceptible F2 plants were selected as the mapping population to identify the simple sequence repeat (SSR) and sequence tagged site (STS) markers linked to the gene by bulk segregant analysis. A dominant stem rust resistance gene was identified and mapped with 16 SSR and five new STS markers to the deletion bin 2DS5-0.47-1.00 of chromosome arm 2DS in which Sr46 was located. Molecular marker and stem rust tests on CIae 25 and two Ae. tauschii accessions carrying Sr46 confirmed that the gene in CIae 25 is Sr46. This study also demonstrated that Sr46 is temperature-sensitive being less effective at low temperatures. The marker validation indicated that two closely linked markers Xgwm210 and Xwmc111 can be used for marker-assisted selection of Sr46 in wheat breeding programs.

Development and characterization of wheat lines carrying stem rust resistance gene Sr43 derived from Thinopyrum ponticum

Wheat lines carrying Ug99-effective stem rust resistance gene Sr43 on shortened alien chromosome segments were produced using chromosome engineering, and molecular markers linked to Sr43 were identified for marker-assisted selection. Stem rust resistance gene Sr43, transferred into common wheat (Triticum aestivum) from Thinopyrum ponticum, is an effective gene against stem rust Ug99 races. However, this gene has not been used in wheat breeding because it is located on a large Th. ponticum 7el(2) chromosome segment, which also harbors genes for undesirable traits. The objective of this study was to eliminate excessive Th. ponticum chromatin surrounding Sr43 to make it usable in wheat breeding. The two original translocation lines KS10-2 and KS24-1 carrying Sr43 were first analyzed using simple sequence repeat (SSR) markers and florescent genomic in situ hybridization. Six SSR markers located on wheat chromosome arm 7DL were identified to be associated with the Th. ponticum chromatin in KS10-2 and KS24-1. The results confirmed that KS24-1 is a 7DS·7el(2)L Robertsonian translocation as previously reported. However, KS10-2, which was previously designated as a 7el(2)S·7el(2)L-7DL translocation, was identified as a 7DS-7el(2)S·7el(2)L translocation. To reduce the Th. ponticum chromatin carrying Sr43, a BC(2)F(1) population (Chinese Spring//Chinese Spring ph1bph1b*2/KS10-2) containing ph1b-induced homoeologous recombinants was developed, tested with stem rust, and genotyped with the six SSR markers identified above. Two new wheat lines (RWG33 and RWG34) carrying Sr43 on shortened alien chromosome segments (about 17.5 and 13.7 % of the translocation chromosomes, respectively) were obtained, and two molecular markers linked to Sr43 in these lines were identified. The new wheat lines with Sr43 and the closely linked markers provide new resources for improving resistance to Ug99 and other races of stem rust in wheat.

Genetic variation in Puccinia graminis collected from oats, rye, and barberry

Puccinia graminis, the causal agent of stem rust, was collected from its alternate host barberry (Berberis spp.) and two different uredinial hosts, oats (Avena sativa) and rye (Secale cereale). The samples were analyzed using 11 polymorphic simple sequence repeat (SSR) markers. There were large differences between fungal populations on oats (P. graminis f. sp. avenae) and rye (P. graminis f. sp. secalis), and the genetic variation within the different formae speciales was also high. It was possible to distinguish between the two formae speciales on barberry. Additional genotypic groups not present in the field samples from oats and rye were also identified on barberry. Our results confirm the importance of barberry in maintaining the populations of P. graminis in Sweden and the importance of the sexual stage for the survival of the pathogen.

Virulence profile and genetic structure of a North Dakota population of Pyrenophora teres f. teres, the causal agent of net form net blotch of barley

A Pyrenophora teres f. teres population in North Dakota was analyzed for virulence variation and genetic diversity using 75 monospore isolates that were collected across a 4-year period (2004 to 2007) from two North Dakota State University agricultural experiment stations at Fargo and Langdon. Pathogenicity tests by inoculation onto 22 barley differential lines at seedling stage revealed 49 pathotypes, indicating a wide range of pathogenic diversity. Two-way analysis of variance of disease ratings revealed a significant difference in the virulence among isolates and in the resistance among barley lines, as well as in the interactions between the two. 'CI5791', 'Algerian', and 'Heartland' were three barley lines showing a high level of seedling resistance to all North Dakota isolates tested; however, many previously reported resistance genes have been overcome. Forty multilocus genotypes were identified from this set of isolates by genotyping at 13 simple-sequence repeat loci. High percentages of clonal cultures were detected in the samplings from 2005 and 2007 in Fargo and 2005 in Langdon. Using a clone-corrected sample set, the mean gene diversity (h) was estimated to be 0.58, approximately the same for both locations. The calculated Wright's F(ST) value is small (0.11) but was significantly >0, indicating a significant differentiation between the Fargo and Langdon populations. In the gametic disequilibrium test, only 3 of 78 possible pairwise comparisons over all isolates showed significant (P < 0.05) nonrandom association, suggesting a random mating mode. Our results suggest that the populations from the two locations are derived from a common source and undergo frequent recombination. This research provides important information for barley breeders regarding development and deployment of cultivars with resistance to net form net blotch in this region.

Mining and survey of simple sequence repeats in wheat rust Puccinia sp

The abundance and inherent potential for extensive allelic variations in simple sequence repeats (SSRs) or microsatellites resulted in valuable source for genetic markers in eukaryotes. In this study, we analyzed and compared the abundance and organisation of SSR in the genome of two important fungal pathogens of wheat, brown or leaf rust (Puccinia triticina) and black or stem rust (Puccinia graminis f. sp. tritici). P. triticina genome with two fold genome size as compared to P. graminis tritici has lower relative abundance and SSR density. The distribution pattern of different SSR motifs provides the evidence of greater accumulation of dinucleotide followed by trinucleotide repeats. More than two-hundred different types of repeat motifs were observed in the genomes. The longest SSR motifs varied in both genomes and some of the repeat motifs are found in higher frequency. The information about survey of relative abundance, relative density, length and frequency of different repeat motifs in Puccinia sp. will be useful for developing SSR markers that could find several applications in analysis of fungal genome such as genetic diversity, population genetics, race identification and acquisition of new virulence.

Targeted introgression of a wheat stem rust resistance gene by DNA marker-assisted chromosome engineering

Chromosome engineering is a useful strategy for transfer of alien genes from wild relatives into modern crops. However, this strategy has not been extensively used for alien gene introgression in most crops due to low efficiency of conventional cytogenetic techniques. Here, we report an improved scheme of chromosome engineering for efficient elimination of a large amount of goatgrass (Aegilops speltoides) chromatin surrounding Sr39, a gene that provides resistance to multiple stem rust races, including Ug99 (TTKSK) in wheat. The wheat ph1b mutation, which promotes meiotic pairing between homoeologous chromosomes, was employed to induce recombination between wheat chromosome 2B and goatgrass 2S chromatin using a backcross scheme favorable for inducing and detecting the homoeologous recombinants with small goatgrass chromosome segments. Forty recombinants with Sr39 with reduced surrounding goatgrass chromatin were quickly identified from 1048 backcross progenies through disease screening and molecular marker analysis. Four of the recombinants carrying Sr39 with a minimal amount of goatgrass chromatin (2.87-9.15% of the translocated chromosomes) were verified using genomic in situ hybridization. Approximately 97% of the goatgrass chromatin was eliminated in one of the recombinants, in which a tiny goatgrass chromosome segment containing Sr39 was retained in the wheat genome. Localization of the goatgrass chromatin in the recombinants led to rapid development of three molecular markers tightly linked to Sr39. The new wheat lines and markers provide useful resources for the ongoing global effort to combat Ug99. This study has demonstrated great potential of chromosome engineering in genome manipulation for plant improvement.

Gene discovery in EST sequences from the wheat leaf rust fungus Puccinia triticina sexual spores, asexual spores and haustoria, compared to other rust and corn smut fungi

Background

Rust fungi are biotrophic basidiomycete plant pathogens that cause major diseases on plants and trees world-wide, affecting agriculture and forestry. Their biotrophic nature precludes many established molecular genetic manipulations and lines of research. The generation of genomic resources for these microbes is leading to novel insights into biology such as interactions with the hosts and guiding directions for breakthrough research in plant pathology.

Results

To support gene discovery and gene model verification in the genome of the wheat leaf rust fungus, Puccinia triticina (Pt), we have generated Expressed Sequence Tags (ESTs) by sampling several life cycle stages. We focused on several spore stages and isolated haustorial structures from infected wheat, generating 17,684 ESTs. We produced sequences from both the sexual (pycniospores, aeciospores and teliospores) and asexual (germinated urediniospores) stages of the life cycle. From pycniospores and aeciospores, produced by infecting the alternate host, meadow rue (Thalictrum speciosissimum), 4,869 and 1,292 reads were generated, respectively. We generated 3,703 ESTs from teliospores produced on the senescent primary wheat host. Finally, we generated 6,817 reads from haustoria isolated from infected wheat as well as 1,003 sequences from germinated urediniospores. Along with 25,558 previously generated ESTs, we compiled a database of 13,328 non-redundant sequences (4,506 singlets and 8,822 contigs). Fungal genes were predicted using the EST version of the self-training GeneMarkS algorithm. To refine the EST database, we compared EST sequences by BLASTN to a set of 454 pyrosequencing-generated contigs and Sanger BAC-end sequences derived both from the Pt genome, and to ESTs and genome reads from wheat. A collection of 6,308 fungal genes was identified and compared to sequences of the cereal rusts, Puccinia graminis f. sp. tritici (Pgt) and stripe rust, P. striiformis f. sp. tritici (Pst), and poplar leaf rust Melampsora species, and the corn smut fungus, Ustilago maydis (Um). While extensive homologies were found, many genes appeared novel and species-specific; over 40% of genes did not match any known sequence in existing databases. Focusing on spore stages, direct comparison to Um identified potential functional homologs, possibly allowing heterologous functional analysis in that model fungus. Many potentially secreted protein genes were identified by similarity searches against genes and proteins of Pgt and Melampsora spp., revealing apparent orthologs.

Conclusions

The current set of Pt unigenes contributes to gene discovery in this major cereal pathogen and will be invaluable for gene model verification in the genome sequence.

A first genome assembly of the barley fungal pathogen Pyrenophora teres f. teres

Background

Pyrenophora teres f. teres is a necrotrophic fungal pathogen and the cause of one of barley's most important diseases, net form of net blotch. Here we report the first genome assembly for this species based solely on short Solexa sequencing reads of isolate 0-1. The assembly was validated by comparison to BAC sequences, ESTs, orthologous genes and by PCR, and complemented by cytogenetic karyotyping and the first genome-wide genetic map for P. teres f. teres.

Results

The total assembly was 41.95 Mbp and contains 11,799 gene models of 50 amino acids or more. Comparison against two sequenced BACs showed that complex regions with a high GC content assembled effectively. Electrophoretic karyotyping showed distinct chromosomal polymorphisms between isolates 0-1 and 15A, and cytological karyotyping confirmed the presence of at least nine chromosomes. The genetic map spans 2477.7 cM and is composed of 243 markers in 25 linkage groups, and incorporates simple sequence repeat markers developed from the assembly. Among predicted genes, non-ribosomal peptide synthetases and efflux pumps in particular appear to have undergone a P. teres f. teres-specific expansion of non-orthologous gene families.

Conclusions

This study demonstrates that paired-end Solexa sequencing can successfully capture coding regions of a filamentous fungal genome. The assembly contains a plethora of predicted genes that have been implicated in a necrotrophic lifestyle and pathogenicity and presents a significant resource for examining the bases for P. teres f. teres pathogenicity.