A candidate for Lr19, an exotic gene conditioning leaf rust resistance in wheat

Lr19, one of the few widely effective genes conferring resistance to leaf rust in wheat, was transferred from the wild relative Thinopyrum ponticum to durum wheat. Since Lr19 confers a hypersensitive response to the pathogen, it was considered likely that the gene would be a member of the major nucleotide-binding site (NBS)-leucine-rich repeat (LRR) plant R gene family. NBS profiling, based on PCR amplification of conserved NBS motifs, was applied to durum wheat–Th. ponticum recombinant lines involving different segments of the alien 7AgL chromosome arm, carrying or lacking Lr19. Differential PCR products were isolated and sequenced. From one such sequence (AG15), tightly linked to Lr19, a 4,121-bp full-length cDNA was obtained. Its deduced 1,258 amino acid sequence has the characteristic NBS-LRR domains of plant R gene products and includes a coiled-coil (CC) region typical of monocots. The genomic DNA sequence showed the presence of two exons and a short intron upstream of the predicted stop codon. Homology searches revealed considerable identity of AG15 with the cloned wheat resistance gene Pm3a and a lower similarity with wheat Lr1, Lr21, and Lr10. Quantitative PCR on leaf-rust-infected and non-infected Lr19 carriers proved AG15 to be constitutively expressed, as is common for R genes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The Lr34 adult plant rust resistance gene provides seedling resistance in durum wheat without senescence

The hexaploid wheat (Triticum aestivum) adult plant resistance gene, Lr34/Yr18/Sr57/Pm38/Ltn1, provides broad‐spectrum resistance to wheat leaf rust (Lr34), stripe rust (Yr18), stem rust (Sr57) and powdery mildew (Pm38) pathogens, and has remained effective in wheat crops for many decades. The partial resistance provided by this gene is only apparent in adult plants and not effective in field‐grown seedlings. Lr34 also causes leaf tip necrosis (Ltn1) in mature adult plant leaves when grown under field conditions. This D genome‐encoded bread wheat gene was transferred to tetraploid durum wheat (T. turgidum) cultivar Stewart by transformation. Transgenic durum lines were produced with elevated gene expression levels when compared with the endogenous hexaploid gene. Unlike nontransgenic hexaploid and durum control lines, these transgenic plants showed robust seedling resistance to pathogens causing wheat leaf rust, stripe rust and powdery mildew disease. The effectiveness of seedling resistance against each pathogen correlated with the level of transgene expression. No evidence of accelerated leaf necrosis or up‐regulation of senescence gene markers was apparent in these seedlings, suggesting senescence is not required for Lr34 resistance, although leaf tip necrosis occurred in mature plant flag leaves. Several abiotic stress‐response genes were up‐regulated in these seedlings in the absence of rust infection as previously observed in adult plant flag leaves of hexaploid wheat. Increasing day length significantly increased Lr34 seedling resistance. These data demonstrate that expression of a highly durable, broad‐spectrum adult plant resistance gene can be modified to provide seedling resistance in durum wheat.     

Leaf tip necrosis, molecular markers and β1-proteasome subunits associated with the slow rusting resistance genes Lr46/Yr29

Resistance based on slow-rusting genes has proven to be a useful strategy to develop wheat cultivars with durable resistance to rust diseases in wheat. However this type of resistance is often difficult to incorporate into a single genetic background due to the polygenic and additive nature of the genes involved. Therefore, markers, both molecular and phenotypic, are useful tools to facilitate the use of this type of resistance in wheat breeding programs. We have used field assays to score for both leaf and yellow rust in an Avocet-YrA × Attila population that segregates for several slow-rusting leaf and yellow rust resistance genes. This population was analyzed with the AFLP technique and the slow-rusting resistance locus Lr46/Yr29 was identified. A common set of AFLP and SSR markers linked to the Lr46/Yr29 locus was identified and validated in other recombinant inbred families developed from single chromosome recombinant populations that segregated for Lr46. These populations segregated for leaf tip necrosis (LTN) in the field, a trait that had previously been associated with Lr34/Yr18. We show that LTN is also pleiotropic or closely linked to the Lr46/Yr29 locus and suggest that a new Ltn gene designation should be given to this locus, in addition to the one that already exists for Lr34/Yr18. Coincidentally, members of a small gene family encoding β-1 proteasome subunits located on group 1L and 7S chromosomes implicated in plant defense were linked to the Lr34/Yr18 and Lr46/Yr29 loci.     

Powdery mildew resistance and Lr34/Yr18 genes for durable resistance to leaf and stripe rust cosegregate at a locus on the short arm of chromosome 7D of wheat

The incorporation of effective and durable disease resistance is an important breeding objective for wheat improvement. The leaf rust resistance gene Lr34 and stripe rust resistance gene Yr18 are effective at the adult plant stage and have provided moderate levels of durable resistance to leaf rust caused by Puccinia triticina Eriks. and to stripe rust caused by Puccinia striiformis Westend. f. sp. tritici. These genes have not been separated by recombination and map to chromosome 7DS in wheat. In a population of 110 F₇ lines derived from a Thatcher × Thatcher isogenic line with Lr34/Yr18, field resistance to leaf rust conferred by Lr34 and to stripe rust resistance conferred by Yr18 cosegregated with adult plant resistance to powdery mildew caused by Blumeria graminis (DC) EO Speer f. sp. tritici. Lr34 and Yr18 were previously shown to be associated with enhanced stem rust resistance and tolerance to barley yellow dwarf virus infection. This chromosomal region in wheat has now been linked with resistance to five different pathogens. The Lr34/Yr18 phenotypes and associated powdery mildew resistance were mapped to a single locus flanked by microsatellite loci Xgwm1220 and Xgwm295 on chromosome 7DS.     

Gamma irradiation induced deletions in an alien chromosome segment of the wheat 'Indis' and their use in gene mapping.

Deletion mutants were produced in a translocated chromosome segment derived from Thinopyrum distichum (Thunb.) L?ve. Spikes of the translocation line 'Indis' were irradiated with gamma rays at dosages of 15, 20, and 25 Gy. The irradiated spikes were pollinated with 'Inia 66' pollen and the F2 and F3 generations screened for translocation mutants, using the genes for leaf rust resistance and yellow endosperm pigmentation as markers. Finally, endopeptidase polymorphisms were utilized to select mutant translocation homozygotes within each of 29 families. An investigation of polymorphisms at the alpha-Amy-D2 and Wsp-D1 loci of chromosome arm 7DL revealed that 'Indis' did not produce an alpha-AMY-D2 product, but it did produce a novel WSP-D1 protein. The mutants were characterized for their leaf and stem rust resistances and the presence of WSP-D1 and yellow flour pigments. The stem rust resistance gene could not be accurately mapped. The linear order of the remaining loci on 7DL was centromere - leaf rust resistance - Wsp-D1 and yellow pigment. The data obtained suggested that the 'Indis' translocation has homo(eo)logy to the Lr19 translocation and homoeology to 7DL of common wheat.     

The Pontin series of recombinant alien translocations in bread wheat: single translocations integrating combinations of Bdv2, Lr19 and Sr25 disease-resistance genes from Thinopyrum intermedium and Th. ponticum

Two bread wheat lines each with a translocation on chromosome 7DL from either Thinopyrum intermedium (TC5 and TC14) or Thinopyrum ponticum (T4m), were hybridized in a ph1b mutant background to enhance recombination between the two translocated chromosomal segments. The frequency of recombinants was high in lines derived from the larger and similar-sized translocations (TC5/T4m), but much lower when derived from different-sized translocations (TC14/T4m). Recombinant translocations contained combinations of resistance genes Bdv2, Lr19 and Sr25 conferring resistance to Barley yellow dwarf virus (BYDV), leaf rust and stem rust, respectively. Their genetic composition was identified using bioassays and molecular markers specific for the two progenitor Thinopyrum species. This set of 7DL Th. ponticum/intermedium recombinant translocations was termed the Pontin series. In addition to Thinopyrum markers, the size of the translocation was estimated with the aid of wheat markers mapped on each of the 7DL deletion bins. Bioassays for BYDV, leaf rust and stem rust were performed under greenhouse and field conditions. Once separated from ph1b background, the Pontin recombinant translocations were stable and showed normal inheritance in successive backcrosses. The reported Pontin translocations integrate important resistance genes in a single linkage block which will allow simultaneous selection of disease resistance. Combinations of Bdv2 + Lr19 or Lr19 + Sr25 in both long and short translocations, are available to date. The smaller Pontins, comprising only 20 % of the distal portion of 7DL, will be most attractive to breeders.     

A genetic map of <Emphasis Type="Italic">Lophopyrum ponticum</Emphasis> chromosome 7E,harboring resistance genes to Fusarium head blight and leaf rust

The leaf rust resistance gene Lr19 and Fusarium head blight (FHB) resistance quantitative trait loci (QTL) derived from the wild wheatgrass Lophopyrum ponticum have been located on chromosome 7E. The main objectives of the present study were to develop a genetic map of chromosome 7E and map the two resistance loci using a population of 237 F_7:8 recombinant inbred lines (RILs) derived from a cross between two Thatcher-L. ponticum substitution lines, K11463 (7el₁(7D)) and K2620 (7el₂(7D)). 532 G-SSR, E-SSR and STS markers from wheat chromosome group 7 were screened in the parent lines. Of these, 118 markers were polymorphic, with a polymorphism frequency of 22.2%. A genetic map of L. ponticum chromosome 7E was constructed with 64 markers, covering 95.76 cM, with an average genetic distance of 1.47 cM between markers. The major FHB resistance locus, temporarily assigned as FhbLoP, was mapped to the very distal region of the long arm of chromosome 7E within a 3.71 cM interval flanked by Xcfa2240 and Xswes19, which accounts for 30.46% of the phenotypic variance. Lr19 was bracketed by Xwmc273 and XBE404744, with a map distance of 1.54 and 1.43 cM from either side, respectively. The closely linked markers identified in this study will be helpful for marker-assisted introgression of the L. ponticum-derived FhbLoP and Lr19 genes into elite cultivars of wheat, and the development of a genetic map will accelerate the map-based cloning of these two genes.     

Pathogenic Specialization of Puccinia triticina in Andalusia from 1998 to 2000

Wheat leaf rust (Puccinia triticina) is becoming a serious concern in Spanish wheat, especially on durum wheat where acreage has enormously increased. Host resistance is the preferred method of disease control, but the virulence spectrum of the leaf rust population in Spain is currently unknown. In order to deploy effective Lr genes, this study was conducted to characterize the virulence spectrum of leaf rust in Andalusia (Spain). Isolates were obtained from surveys of wheat fields across Andalusia from 1998 to 2000. From 56 isolates phenotyped, 35 pathotypes were identified. Virulence to Lr10, Lr11, Lr14a, Lr14b and Lr18 was high (>96%), while virulence to Lr9 and Lr24 were not found. None of the isolates collected from durum wheat were virulent to Lr1, Lr3, Lr3ka, Lr3bg, Lr15, Lr16 and Lr17, while many of the isolates collected on bread wheat showed virulence on these genes, indicating a certain specialization in the leaf rust infecting durum wheat. Population dynamics of current wheat leaf rust pathotypes in terms of mutation and migration are discussed.     

An introgression on wheat chromosome 4DL in RL6077 (Thatcher*6/PI 250413) confers adult plant resistance to stripe rust and leaf rust (Lr67)

Adult plant resistance (APR) to leaf rust and stripe rust derived from the wheat (Triticum aestivum L.) line PI250413 was previously identified in RL6077 (=Thatcher*6/PI250413). The leaf rust resistance gene in RL6077 is phenotypically similar to Lr34 which is located on chromosome 7D. It was previously hypothesized that the gene in RL6077 could be Lr34 translocated to another chromosome. Hybrids between RL6077 and Thatcher and between RL6077 and 7DS and 7DL ditelocentric stocks were examined for first meiotic metaphase pairing. RL6077 formed chain quadrivalents and trivalents relative to Thatcher and Chinese Spring; however both 7D telocentrics paired only as heteromorphic bivalents and never with the multivalents. Thus, chromosome 7D is not involved in any translocation carried by RL6077. A genome-wide scan of SSR markers detected an introgression from chromosome 4D of PI250413 transferred to RL6077 through five cycles of backcrossing to Thatcher. Haplotype analysis of lines from crosses of Thatcher × RL6077 and RL6058 (Thatcher*6/PI58548) × RL6077 showed highly significant associations between introgressed markers (including SSR marker cfd71) and leaf rust resistance. In a separate RL6077-derived population, APR to stripe rust was also tightly linked with cfd71 on chromosome 4DL. An allele survey of linked SSR markers cfd71 and cfd23 on a set of 247 wheat lines from diverse origins indicated that these markers can be used to select for the donor segment in most wheat backgrounds. Comparison of RL6077 with Thatcher in field trials showed no effect of the APR gene on important agronomic or quality traits. Since no other known Lr genes exist on chromosome 4DL, the APR gene in RL6077 has been assigned the name Lr67.     

Identification and validation of molecular markers linked to the leaf rust resistance gene Lr19 in wheat

A leaf rust resistance gene Lr19 on the chromosome 7DL of wheat derived from Agropyron elongatum was tagged with random amplified polymorphic DNA (RAPD) and microsatellite markers. The F₂ population of 340 plants derived from a cross between the leaf rust resistant near-isogenic line (NIL) of Thatcher (Tc + Lr19) and leaf rust susceptible line Agra Local that segregated for dominant monogenic leaf rust resistance was utilized for generating the mapping population. The molecular markers were mapped in the F₂ derived F₃ homozygous population of 140 seedlings. Sixteen RAPD markers were identified as linked to the alien gene Lr19 among which eight were in a coupling phase linkage. Twelve RAPD markers co-segregated with Lr19 locus. Nine microsatellite markers located on the long arm of chromosome 7D were also mapped as linked to the gene Lr19, including 7 markers which co-segregated with Lr19 locus, thus generating a saturated region carrying 25 molecular markers linked to the gene Lr19 within 10.2 ± 0.062 cM on either side of the locus. Two RAPD markers S265₅₁₂ and S253₇₃₇ which flanked the locus Lr19 were converted to sequence characterized amplified region markers SCS265₅₁₂ and SCS253₇₃₆, respectively. The marker SCS265₅₁₂ was linked with Lr19 in a coupling phase and the marker SCS253₇₃₆ was linked in a repulsion phase, which when used together mimicked one co-dominant marker capable of distinguishing the heterozygous resistant seedlings from the homozygous resistant. The molecular markers were validated on NILs mostly in Thatcher background isogenic for 44 different Lr genes belonging to both native and alien origin. The validation for polymorphism in common leaf rust susceptible cultivars also confirmed the utility of these tightly linked markers to the gene Lr19 in marker-assisted selection.     

The introgressed segment carrying rust resistance genes Yr17, Lr37 and Sr38 in wheat can be assayed by a cloned disease resistance gene-like sequence

A cloned gene sequence (Vrga1D), with features of the nucleotide-binding-site leucine-rich repeat class of disease resistance (R) gene sequence super family, was previously shown to belong to a family of five gene members derived from a Triticum ventricosum Ces. (syn. Aegilops ventricosa Tausch) segment in wheat (Triticum aestivum L.). This gene family was introgressed, together with the linked rust resistance genes Yr17, Lr37 and Sr38 from T. ventricosum, to wheat chromosome 2AS. An independently derived T. ventricosum segment carrying a leaf rust resistance gene in a French wheat cultivar, was shown to exhibit a rust resistance response equivalent to Lr37 as well as Yr17 and Sr38. DNA probes from different regions of the Vrga1D clone consistently detected the presence of RFLPs associated with the introgressed segment carrying the resistance genes Yr17, Lr37 and Sr38 present in diverse wheat genotypes from Australia, Canada, France and the UK. Our results showed that the transfer of the T. ventricosum- derived Vrga1 gene members and the rust resistance genes were always accompanied by the loss of a corresponding set of Vrga1-related gene members in recipient wheat cultivars presumed to be of homoeoallelic origin. A PCR assay, based on sequences from the 3"-untranslated region of a Vrga1 gene member isolated from the T. ventricosum donor line of the introgressed segment, was developed. The PCR assay detected the presence of the introgressed rust resistance genes across the diverse wheat backgrounds and should be useful in marker- assisted selection in wheat breeding. Received: 24 December 1999 / Accepted: 13 June 2000     

Development of a molecular marker for the adult plant leaf rust resistance gene Lr35 in wheat

The objective of this work was to develop a marker for the adult plant leaf rust resistance gene Lr35. The Lr35 gene was originally introgressed into chromosome 2B from Triticum speltoides, a diploid relative of wheat. A segregating population of 96 F ₂ plants derived from a cross between the resistant line ThatcherLr35 and the susceptible variety Frisal was analysed. Out of 80 RFLP probes previously mapped on wheat chromosome 2B, 51 detected a polymorphism between the parents of the cross. Three of them were completely linked with the resistance gene Lr35. The co-segregating probe BCD260 was converted into a PCR-based sequence-tagged-site (STS) marker. A set of 48 different breeding lines derived from several European breeding programs was tested with the STS marker. None of these lines has a donor for Lr35 in its pedigree and all of them reacted negatively with the STS marker. As no leaf rust races virulent on Lr35 have been found in different areas of the world, the STS marker for the Lr35 resistance gene is of great value to support the introgression of this gene in combination with other leaf rust (Lr) genes into breeding material by marker-assisted selection. Received: 14 December 1998 / Accepted: 30 January 1999     

The adult plant rust resistance loci Lr34/Yr18 and Lr46/Yr29 are important determinants of partial resistance to powdery mildew in bread wheat line Saar

Powdery mildew, caused by Blumeria graminis f. sp. tritici is a major disease of wheat (Triticum aestivum L.) that can be controlled by resistance breeding. The CIMMYT bread wheat line Saar is known for its good level of partial and race non-specific resistance, and the aim of this study was to map QTLs for resistance to powdery mildew in a population of 113 recombinant inbred lines from a cross between Saar and the susceptible line Avocet. The population was tested over 2 years in field trials at two locations in southeastern Norway and once in Beijing, China. SSR markers were screened for association with powdery mildew resistance in a bulked segregant analysis, and linkage maps were created based on selected SSR markers and supplemented with DArT genotyping. The most important QTLs for powdery mildew resistance derived from Saar were located on chromosomes 7DS and 1BL and corresponded to the adult plant rust resistance loci Lr34/Yr18 and Lr46/Yr29. A major QTL was also located on 4BL with resistance contributed by Avocet. Additional QTLs were detected at 3AS and 5AL in the Norwegian testing environments and at 5BS in Beijing. The population was also tested for leaf rust (caused by Puccinia triticina) and stripe rust (caused by P. striiformis f. sp. tritici) resistance and leaf tip necrosis in Mexico. QTLs for these traits were detected on 7DS and 1BL at the same positions as the QTLs for powdery mildew resistance, and confirmed the presence of Lr34/Yr18 and Lr46/Yr29 in Saar. The powdery mildew resistance gene at the Lr34/Yr18 locus has recently been named Pm38. The powdery mildew resistance gene at the Lr46/Yr29 locus is designated as Pm39.     

Lr68: a new gene conferring slow rusting resistance to leaf rust in wheat

The common wheat cultivar Parula possesses a high level of slow rusting, adult plant resistance (APR) to all three rust diseases of wheat. Previous mapping studies using an Avocet-YrA/Parula recombinant inbred line (RIL) population showed that APR to leaf rust (Puccinia triticina) in Parula is governed by at least three independent slow rusting resistance genes: Lr34 on 7DS, Lr46 on 1BL, and a previously unknown gene on 7BL. The use of field rust reaction and flanking markers identified two F₆ RILs, Arula1 and Arula2, from the above population that lacked Lr34 and Lr46 but carried the leaf rust resistance gene in 7BL, hereby designated Lr68. Arula1 and Arula2 were crossed with Apav, a highly susceptible line from the cross Avocet-YrA/Pavon 76, and 396 F₄-derived F₅ RILs were developed for mapping Lr68. The RILs were phenotyped for leaf rust resistance for over 2 years in Ciudad Obregon, Mexico, with a mixture of P. triticina races MBJ/SP and MCJ/SP. Close genetic linkages with several DNA markers on 7BL were established using 367 RILs; Psy1-1 and gwm146 flanked Lr68 and were estimated at 0.5 and 0.6 cM, respectively. The relationship between Lr68 and the race-specific seedling resistance gene Lr14b, located in the same region and present in Parula, Arula1 and Arula2, was investigated by evaluating the RILs with Lr14b-avirulent P. triticina race TCT/QB in the greenhouse. Although Lr14b and Lr68 homozygous recombinants in repulsion were not identified in RILs, γ-irradiation-induced deletion stocks that lacked Lr68 but possessed Lr14b showed that Lr68 and Lr14b are different loci. Flanking DNA markers that are tightly linked to Lr68 in a wide array of genotypes can be utilized for selection of APR to leaf rust.     

A study of modified forms of the Lr19 translocation of common wheat

 Following the induction of allosyndetic pairing between the Thinopyrum-derived Lr19 translocation in ‘Indis’ wheat and homoeologous wheat chromatin, eight suspected recombinants for the Lr19 region were recovered. These selections were characterised for marker loci that were previously used to construct a physical map of the Lr19 segment. At the same time near-isogenic lines were developed for some of the selected segments and tested for seedling leaf-rust resistance in order to confirm the presence of Lr19. It appeared that three of the four white-endosperm selections do not possess Lr19 and only one, 88M22-149, is a true Lr19 recombinant. The resistance gene in the three non-Lr19 selections resides on chromosome 6B, appears to derive from ‘Indis’, and was selected unintentionally during backcrossing. The pedigree of ‘Indis’ is suspect and it is believed that the Lr19 translocation in ‘Indis’ is in reality the Th. ponticum-derived (T4) segment rather than being of Th. distichum origin as was believed earlier. The white-endosperm recombinant, 88M22-149, retained the complete Lr19 resistance and was apparently re-located to chromosome arm 7BL in a double-crossover event. 88M22-149 has lost the Sd1 gene and often shows strong self-elimination in translocation heterozygotes. This effect may result from additional gametocidal loci or from an altered chromosome structure following re-location of the segment. 88M22-149 in fact contains a duplicated region involving the Wsp-B1 locus. Three selections had partially white endosperms and expressed Lr19 and other Thinopyrum marker alleles. Polymorphisms for the available markers confirmed that the translocated segment in at least one of them had been shortened through recombination with chromosome arm 7DL. Further markers need to be studied in order to determine whether the translocation in the remaining two partially white recombinants had also undergone recombination with wheat. The eighth selection has yellow endosperm and appears to self-eliminate in certain translocation heterozygotes. No evidence of recombination could be found with the markers used. If the latter selections are in fact recombinants they may prove useful in attempts to unravel the complex segregation distortion mechanism. Received: 8 August 1996 / Accepted: 10 January 1997     

Association mapping of leaf rust response in durum wheat

Resistance to leaf rust (Puccinia triticina Eriks.) is a main objective for durum wheat (Triticum durum Desf.) breeding. Association mapping on germplasm collections is now being used as an additional approach for the discovery and validation of major genes/QTLs. In this study, a collection of 164 elite durum wheat accessions suitable for association mapping has been tested for leaf rust response at the seedling stage and under field conditions (adult plant stage). Seedling tests were carried out with 25 selected isolates from durum wheat, bread wheat and triticale, while field experiments were carried out in artificially inoculated plots in Italy and in Mexico. The collection has been profiled with 225 simple sequence repeat (SSR) loci of known map position and a PCR assay targeting Ppd-A1. Associations showing highly consistent experiment-wise significances across leaf rust isolates and field trials were mainly detected for the 7BL distal chromosome (chr.) region (harbouring Lr14 from cultivar Llareta INIA and QLr.ubo-7B.2 from cultivar Creso) and for two chr. regions located in chrs. 2A and 2B. Additionally, isolate-specific associations and/or associations with smaller effects in the field trials were identified in most of the chromosomes. The chr. 7BL distal region was investigated in detail through haplotyping with 15 SSR markers, revealing that the Creso and Llareta INIA alleles are identical by descent at 6 adjacent SSR loci in the most distal 7BL region spanning 8 cM. Association mapping allowed us to further refine the map location of the Lr14/QLr.ubo-7B.2 resistance gene to the most distal region of the linkage group, tagged by Xcfa2257.2, Xgwm344.2 and Xwmc10. The resistant haplotype is present in a number of accessions (ca. 15% of the accessions included in the collection) from the Italian, CIMMYT and ICARDA breeding programmes. Therefore, this chr. 7BL region can be considered as the most important source of resistance to leaf rust currently exploited by durum breeders in the Mediterranean areas. Furthermore, the field trials at the adult plant stage allowed us to identify marker associations (e.g. chrs. 2BL and 3BS, proximal regions; chr. 7BS, distal region) which suggest the presence of minor QTLs for slow-rusting resistance.     

An RGA – like marker detects all known Lr21 leaf rust resistance gene family members in Aegilops tauschii and wheat

Leaf rust is one of the most important diseases of wheat worldwide, particularly in the Great Plains region of the USA. One long-term strategy for the control of this disease may be through durable genetic resistance by gene pyramiding. An important step in this strategy is identifying molecular markers linked to different leaf rust-resistance genes. Here we report the molecular tagging of a leaf rust-resistance gene that may have the potential for durable resistance through further genetic manipulation and gene pyramiding. Lr39 was previously designated for a leaf rust-resistance gene introgressed from Aegilops tauschii accession TA1675 into the common wheat germplasm WGRC2. Lr40 was designated for a gene derived from Ae. tauschii accession TA1649 and is present in germplasm WGRC7. These genes are now believed to be allelic to Lr21, which was transferred to wheat from a different accession of Ae. tauschii. Molecular mapping of Lr39 and Lr40 indicates that both genes come from TA1649. WGRC2 and WRGC7 also have a similar infection type against rust culture PRTUS6. We suggest the designation of the gene in WGRC2 should be changed to Lr40. RFLP marker KSUD14 (locus Xksud14) was found 0.2-cM proximal to Lr40 in a WGRC2/Wichita F₂ population (218 individuals), and co-segregated with the gene in a WGRC7/ Wichita F₂ population (165 individuals). A PCR-based molecular marker developed from the sequence-tagged-site (STS) of Xksud14 was mapped to the same locus as the RFLP marker KSUD14 in both populations. KSUD14 has the structure of a resistance gene analog (RGA) including kinase2a and kinase3 domains similar to the Cre3 gene of wheat and the rust resistance gene Rp1-D of maize. When the PCR products amplified from KSU14 STS were cleaved with restriction enzyme MspI, an 885-bp fragment was found in WGRC2, WGRC7, the Lr21 near-isogenic line, and eight accessions of Ae. tauschii shown to have resistance gene alleles at the Lr21 locus. The KSUD14 PCR-based assay provides an excellent marker for Lr40 and Lr21 in diverse wheat breeding and wild Ae. tauschii populations. Received: 22 December 2000 / Accepted: 12 February 2001     

QTL mapping of multiple foliar disease and root-lesion nematode resistances in wheat

A genetic linkage map, based on a cross between the synthetic hexaploid CPI133872 and the bread wheat cultivar Janz, was established using 111 F₁-derived doubled haploid lines. The population was phenotyped in multiple years and/or locations for seven disease resistance traits, namely, Septoria tritici blotch (Mycosphaeralla graminicola), yellow leaf spot also known as tan spot (Pyrenophora tritici-repentis), stripe rust (Puccinia striiformis f. sp. tritici), leaf rust (Puccinia triticina), stem rust (Puccinia graminis f. sp. tritici) and two species of root-lesion nematode (Pratylenchyus thornei and P. neglectus). The DH population was also scored for coleoptile colour and the presence of the seedling leaf rust resistance gene Lr24. Implementation of a multiple-QTL model identified a tightly linked cluster of foliar disease resistance QTL in chromosome 3DL. Major QTL each for resistance to Septoria tritici blotch and yellow leaf spot were contributed by the synthetic hexaploid parent CPI133872 and linked in repulsion with the coincident Lr24/Sr24 locus carried by parent Janz. This is the first report of linked QTL for Septoria tritici blotch and yellow leaf spot contributed by the same parent. Additional QTL for yellow leaf spot were detected in 5AS and 5BL. Consistent QTL for stripe rust resistance were identified in chromosomes 1BL, 4BL and 7DS, with the QTL in 7DS corresponding to the Yr18/Lr34 region. Three major QTL for P. thornei resistance (2BS, 6DS, 6DL) and two for P. neglectus resistance (2BS, 6DS) were detected. The recombinants combining resistance to Septoria tritici blotch, yellow leaf spot, rust diseases and root-lesion nematodes from parents CPI133872 and Janz constitute valuable germplasm for the transfer of multiple disease resistance into new wheat cultivars.     

Cytogenetic and molecular mapping of the leaf rust resistance gene Lr39 in wheat

Leaf rust, caused by the fungus Puccinia triticina Eriks,is one of the most serious diseases of wheat (Triticum aestivum AABBDD, 2n=6x=42) worldwide. Growing resistant cultivars is an efficient and economical method of reducing losses to leaf rust. Here we report a new leaf rust resistance gene, Lr39, transferred from Aegilops tauschii into common wheat. Lr39 conditions both seedling and adult plant resistance to the leaf rust pathogen. The inter- and intra-chromosomal mapping of the Lr39 gene showed that it is different from all previously described Lr genes. We used monosomic analysis for the inter-chromosomal mapping and wheat microsatellite markers for the intra-chromosomal mapping. The monosomic and ditelosomic analysis indicated that Lr39 is independent of the centromere on the short arm of chromosome 2D. Eight microsatellite markers for 2DS were used for linkage analysis on a population of 57 F₂ plants derived from a cross of an Ae. tauschii-derived wheat, cv. Wichita line TA4186 (possessing Lr39), with Wichita monosomics for the D-genome chromosomes. The microsatellite marker analysis confirmed the location of the gene on 2DS. Three markers were polymorphic and linked to the gene. The closest marker Xgwm210 mapped 10.7 cM from Lr39. The location of Lr39 near the telomere of 2DS distinguishes it from the Lr2 and Lr22 loci, which are located on 2DS proximal to Xgwm210. Received: 19 April 2000 / Accepted: 15 May 2000     

Leaf rust resistance genes of wheat: identification in cultivars and resistance sources

Thirty-seven wheat cultivars originating from seven European countries were examined by using sequence tagged site (STS) markers for seven Lr (leaf rust = brown rust) resistance genes against the fungal pathogen of wheat Puccinia recondita f. sp. tritici (Lr9, Lr10, Lr19, Lr24, Lr26 and Lr37). Additionally, 22 accessions with various Lr genes from two germplasm collections were tested. A Scar (sequence-characterized amplified region) marker for Lr24 and a CAPS (Cleaved Amplified Polymorphic Sequence) marker for Lr47 were also used to identify those genes in the wheat accessions. Each marker amplified one specific DNA fragment. Three Lr gene markers were identified in wheat cultivars (Lr10, Lr26 and Lr37). Another four markers (Lr9, Lr19, Lr24 and Lr47) were found in breeding lines carrying leaf rust resistance genes. The results were compared with leaf rust resistance gene postulations made in previous studies, based on multipathotype testing. Markers for Lr10, Lr26 and Lr37 may be useful in marker-assisted breeding.