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.     

Genetic analysis and mapping of adult plant resistance loci to leaf rust in durum wheat cultivar Bairds

Key message

New leaf rust adult plant resistance (APR) QTL QLr.cim - 6BL was mapped and confirmed the known pleotropic APR gene Lr46 effect on leaf rust in durum wheat line Bairds.

Abstract

CIMMYT-derived durum wheat line Bairds displays an adequate level of adult plant resistance (APR) to leaf rust in Mexican field environments. A recombinant inbred line (RIL) population developed from a cross of Bairds with susceptible parent Atred#1 was phenotyped for leaf rust response at Ciudad Obregon, Mexico, during 2013, 2014, 2015 and 2016 under artificially created epidemics of Puccinia triticina (Pt) race BBG/BP. The RIL population and its parents were genotyped with the 50 K diversity arrays technology (DArT) sequence system and simple sequence repeat (SSR) markers. A genetic map comprising 1150 markers was used to map the resistance loci. Four significant quantitative trait loci (QTLs) were detected on chromosomes 1BL, 2BC (centromere region), 5BL and 6BL. These QTLs, named Lr46, QLr.cim-2BC, QLr.cim-5BL and QLr.cim-6BL, respectively, explained 13.5–60.8%, 9.0–14.3%, 2.8–13.9%, and 11.6–29.4%, respectively, of leaf rust severity variation by the inclusive composite interval mapping method. All of these resistance loci were contributed by the resistant parent Bairds, except for QLr.cim-2BC, which came from susceptible parent Atred#1. Among these, the QTL on chromosome 1BL was the known pleiotropic APR gene Lr46, whereas QLr.cim-6BL, a consistently detected locus, should be a new leaf rust resistance locus in durum wheat. The mean leaf rust severity of RILs carrying all four QTLs ranged from 8.0 to 17.5%, whereas it ranged from 10.9 to 38.5% for three QTLs (Lr46 + 5BL + 6BL) derived from the resistant parent Bairds. Two RILs with four QTLs combinations can be used as sources of complex APR in durum wheat breeding.

     

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.     

Genetic analysis of leaf rust resistance genes and associated markers in the durable resistant wheat cultivar Sinvalocho MA

In the cross of the durable leaf rust resistant wheat Sinvalocho MA and the susceptible line Gama6, four specific genes were identified: the seedling resistance gene Lr3, the adult plant resistance (APR) genes LrSV1 and LrSV2 coming from Sinvalocho MA, and the seedling resistance gene LrG6 coming from Gama6. Lr3 was previously mapped on 6BL in the same cross. LrSV1 was mapped on chromosome 2DS where resistance genes Lr22a and Lr22b have been reported. Results from rust reaction have shown that LrSV1 from Sinvalocho is not the same allele as Lr22b and an allelism test with Lr22a showed that they could be alleles or closely linked genes. LrSV1 was mapped in an 8.5-cM interval delimited by markers gwm296 distal and gwm261 proximal. Adult gene LrSV2 was mapped on chromosome 3BS, cosegregating with gwm533 in a 7.2-cM interval encompassed by markers gwm389 and gwm493, where other disease resistance genes are located, such as seedling gene Lr27 for leaf rust, Sr2 for stem rust, QTL Qfhs.ndsu-3BS for resistance to Fusarium gramineum and wheat powdery mildew resistance. The gene LrG6 was mapped on chromosome 2BL, with the closest marker gwm382 at 0.6 cM. Lines carrying LrSV1, LrSV2 and LrG6 tested under field natural infection conditions, showed low disease infection type and severity, suggesting that this kind of resistance can be explained by additive effects of APR and seedling resistance genes. The identification of new sources of resistance from South American land races and old varieties, supported by modern DNA technology, contributes to sustainability of agriculture through plant breeding.     

Mapping and characterization of the new adult plant leaf rust resistance gene <Emphasis Type="Italic">Lr77</Emphasis> derived from Santa Fe winter wheat

Key message

A new gene for adult plant leaf rust resistance in wheat was mapped to chromosome 3BL. This gene was designated as Lr77.

Abstract

‘Santa Fe’ is a hard red winter cultivar that has had long-lasting resistance to the leaf rust fungus, Puccinia triticina. The objective of this study was to determine the chromosome location of the adult plant leaf rust resistance in Santa Fe wheat. A partial backcross line of ‘Thatcher’ (Tc) wheat with adult plant leaf rust resistance derived from Santa Fe was crossed with Thatcher to develop a Thatcher//Tc*2/Santa Fe F₆ recombinant inbred line (RIL) population. The RIL population and parental lines were evaluated for segregation of leaf rust resistance in three field plot tests and in an adult plant greenhouse test. A genetic map of the RIL population was constructed using 90,000 single-nucleotide polymorphism (SNP) markers with the Illumina Infinium iSelect 90K wheat bead array. A significant quantitative trait locus for reduction of leaf rust severity in all four tests was found on chromosome 3BL that segregated as a single adult plant resistance gene. The RILs with the allele from the resistant parent for SNP marker IWB10344 had lower leaf rust severity and a moderately resistant to moderately susceptible response compared to the susceptible RILs and Thatcher. The gene derived from Santa Fe on chromosome 3BL was designated as Lr77. Kompetitive allele-specific polymerase chain reaction assay markers linked to Lr77 on 3BL should be useful for selection of wheat germplasm with this gene.

     

New slow-rusting leaf rust and stripe rust resistance genes <Emphasis Type="Italic">Lr67</Emphasis> and <Emphasis Type="Italic">Yr46</Emphasis> in wheat are pleiotropic or closely linked

The common wheat genotype ‘RL6077’ was believed to carry the gene Lr34/Yr18 that confers slow-rusting adult plant resistance (APR) to leaf rust and stripe rust but located to a different chromosome through inter-chromosomal reciprocal translocation. However, haplotyping using the cloned Lr34/Yr18 diagnostic marker and the complete sequencing of the gene indicated Lr34/Yr18 is absent in RL6077. We crossed RL6077 with the susceptible parent ‘Avocet’ and developed F₃, F₄ and F₆ populations from photoperiod-insensitive F₃ lines that were segregating for resistance to leaf rust and stripe rust. The populations were characterized for leaf rust resistance at two Mexican sites, Cd. Obregon during the 2008–2009 and 2009–2010 crop seasons, and El Batan during 2009, and for stripe rust resistance at Toluca, a third Mexican site, during 2009. The F₃ population was also evaluated for stripe rust resistance at Cobbitty, Australia, during 2009. Most lines had correlated responses to leaf rust and stripe rust, indicating that either the same gene, or closely linked genes, confers resistance to both diseases. Molecular mapping using microsatellites led to the identification of five markers (Xgwm165, Xgwm192, Xcfd71, Xbarc98 and Xcfd23) on chromosome 4DL that are associated with this gene(s), with the closest markers being located at 0.4 cM. In a parallel study in Canada using a Thatcher × RL6077 F₃ population, the same leaf rust resistance gene was designated as Lr67 and mapped to the same chromosomal region. The pleiotropic, or closely linked, gene derived from RL6077 that conferred stripe rust resistance in this study was designated as Yr46. The slow-rusting gene(s) Lr67/Yr46 can be utilized in combination with other slow-rusting genes to develop high levels of durable APR to leaf rust and stripe rust in wheat.     

Genetic research as the valid base of strategies for breeding rust resistant wheats

It is known that few wheat cultivars maintain their resistance to rust diseases for a long period of time, particularly when crop populations become genetically more uniform. A number of genetically diverse, so far unexploited, sources of rust resistance in the natural as well as mutagenized population of wheat cultivars were identified. Several of these genes were placed in agronomically superior well-adapted backgrounds so that they could be used as pre-breeding stocks for introducing genetic diversity for resistance in a crop population. Some of these stocks when employed as parents in several cross combinations in a breeding programme have generated a number of promising cultivars with diversity for resistance.Many presently grown wheats in India, near-isogenic lines each with Lr14b, Lr14ab, Lr30 and certain international cultivars were identified as possessing diverse sources of adult plant resistance (APR) to leaf rust. Prolonged leaf rust resistance in some of the Indian cultivars was attributed to the likely presence of Lr34 either alone or in combination with other APR components. Tests of allelism carried out in certain cultivars that continue to show adequate levels of field resistance confirm the presence of Lr34, which explains the role that this gene has played in imparting durability for resistance to leaf rust. Also, Lr34 in combination with other APR components increases the levels of resistance, which suggests that combination of certain APR components should be another important strategy for breeding cultivars conferring durable and adequate levels of resistance. A new adult plant leaf rust resistance source that seems to be associated with durability in Arjun has been postulated. Likewise, cultivars possessing Sr2 in combination with certain other specific genes have maintained resistance to stem rust.Further, non-specific resistances that were transferred across widely different genotypes into two of the popular Indian wheats provided easily usable materials to the national breeding programmes for imparting durable resistance to stripe rust.     

QTL mapping of adult-plant resistances to stripe rust and leaf rust in Chinese wheat cultivar Bainong 64

Stripe rust and leaf rust, caused by Puccinia striiformis Westend. f. sp. tritici Erikss. and P. triticina, respectively, are devastating fungal diseases of common wheat (Triticum aestivum L.). Chinese wheat cultivar Bainong 64 has maintained acceptable adult-plant resistance (APR) to stripe rust, leaf rust and powdery mildew for more than 10?years. The aim of this study was to identify quantitative trait loci/locus (QTL) for resistance to the two rusts in a population of 179 doubled haploid (DH) lines derived from Bainong 64?×?Jingshuang 16. The DH lines were planted in randomized complete blocks with three replicates at four locations. Stripe rust tests were conducted using a mixture of currently prevalent P. striiformis races, and leaf rust tests were performed with P. triticina race THTT. Leaf rust severities were scored two or three times, whereas maximum disease severities (MDS) were recorded for stripe rust. Using bulked segregant analysis (BSA) and simple sequence repeat (SSR) markers, five independent loci for APR to two rusts were detected. The QTL on chromosomes 1BL and 6BS contributed by Bainong 64 conferred resistance to both diseases. The loci identified on chromosomes 7AS and 4DL had minor effects on stripe rust response, whereas another locus, close to the centromere on chromosome 6BS, had a significant effect only on leaf rust response. The loci located on chromosomes 1BL and 4DL also had significant effects on powdery mildew response. These were located at the same positions as the Yr29/Lr46 and Yr46/Lr67 genes, respectively. The multiple disease resistance locus for APR on chromosome 6BS appears to be new. All three genes and their closely linked molecular markers could be used in breeding wheat cultivars with durable resistance to multiple diseases.     

Molecular and phenotypic characterization of seedling and adult plant leaf rust resistance in a world wheat collection

Genetic resistance is the most effective approach to managing wheat leaf rust. The aim of this study was to characterize seedling and adult plant leaf rust resistance of a world wheat collection. Using controlled inoculation with ten races of Puccinia triticina, 14 seedling resistance genes were determined or postulated to be present in the collection. Lr1, Lr3, Lr10 and Lr20 were the most prevalent genes around the world while Lr9, Lr14b, Lr3ka and/or Lr30 and Lr26 were rare. To confirm some gene postulations, the collection was screened with gene-specific molecular markers for Lr1, Lr10, Lr21 and Lr34. Although possessing the Lr1 and/or Lr10 gene-specific marker, 51 accessions showed unexpected high infection types to P. triticina race BBBD. The collection was tested in the field, where rust resistance ranged from nearly immune or highly resistant with severity of 1 % and resistant host response to highly susceptible with severity of 84 % and susceptible host response. The majority of the accessions possessing the adult plant resistance (APR) gene Lr34 had a maximum rust severity of 0–35 %, similar to or better than accession RL6058, a Thatcher-Lr34 near-isogenic line. Many accessions displayed an immune response or a high level of resistance under field conditions, likely as a result of synergy between APR genes or between APR and seedling resistance genes. However, accessions with three or more seedling resistance genes had an overall lower field severity than those with two or fewer. Immune or highly resistant accessions are potential sources for improvement of leaf rust resistance. In addition, some lines were postulated to have known but unidentified genes/alleles or novel genes, also constituting potentially important sources of novel resistance.     

Analysis of leaf and stripe rust severities reveals pathotype changes and multiple minor QTLs associated with resistance in an Avocet?×?Pastor wheat population

Leaf rust and stripe rust are important diseases of wheat world-wide and deployment of cultivars with genetic resistance is an effective and environmentally sound control method. The use of minor, additive genes conferring adult plant resistance (APR) has been shown to provide resistance that is durable. The wheat cultivar ‘Pastor’ originated from the CIMMYT breeding program that focuses on minor gene-based APR to both diseases by selecting and advancing generations alternately under leaf rust and stripe rust pressures. As a consequence, Pastor has good resistance to both rusts and was used as the resistant parent to develop a mapping population by crossing with the susceptible ‘Avocet’. All 148 F₅ recombinant inbred lines were evaluated under artificially inoculated epidemic environments for leaf rust (3 environments) and stripe rust (4 environments, 2 of which represent two evaluation dates in final year due to the late build-up of a new race virulent to Yr31) in Mexico. Map construction and QTL analysis were completed with 223 polymorphic markers on 84 randomly selected lines in the population. Pastor contributed Yr31, a moderately effective race-specific gene for stripe rust resistance, which was overcome during this study, and this was clearly shown in the statistical analysis. Linked or pleiotropic chromosomal regions contributing to resistance against both pathogens included Lr46/Yr29 on 1BL, the Yr31 region on 2BS, and additional minor genes on 5A, 6B and 7BL. Other minor genes for leaf rust resistance were located on 1B, 2A and 2D and for stripe rust on 1AL, 1B, 3A, 3B, 4D, 6A, 7AS and 7AL. The 1AL, 1BS and 7AL QTLs are in regions that were not identified previously as having QTLs for stripe rust resistance. The development of uniform and severe epidemics facilitated excellent phenotyping, and when combined with multi-environment analysis, resulted in the relatively large number of QTLs identified in this study.     

Physical and genetic mapping of amplified fragment length polymorphisms and the leaf rust resistance Lr3 gene on chromosome 6BL of wheat

The Argentinian wheat cultivar Sinvalocho MA carries the Lr3 gene for leaf rust resistance on distal chromosome 6BL. In this cultivar, 33 spontaneous susceptible lines were isolated and cytogenetically characterized by C-banding. The analysis revealed deletions on chromosome 6BL in most lines. One line was nulli-6B, two lines were ditelo 6BS, two, three, and ten lines had long terminal deletions of 40, 30, and 20%, respectively, three lines showed very small terminal deletions, and one line had an intercalary deletion of 11%. Physical mapping of 55 amplified fragment length polymorphism (AFLP) markers detected differences between deletions and led to the division of 6BL into seven bins delimited by deletion breakpoints. The most distal bin, with a length smaller than 5% of 6BL, contained 22 AFLP markers and the Lr3 gene. Polymorphism for nine AFLPs between Sinvalocho MA and the rust leaf susceptible cultivar Gamma 6 was used to construct a linkage map of Lr3. This gene is at a genetic distance of 0.9 cM from a group of seven closely linked AFLPs. The location of the gene in a high recombinogenic region indicated a physical distance of approximately 1 Mb to the markers.     

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.     

Molecular mapping of leaf rust resistance gene LrNJ97 in Chinese wheat line Neijiang 977671

Neijiang 977671 and 19 near-isogenic lines with known leaf rust resistance genes were inoculated with 12 pathotypes of Puccinia triticina for postulation of leaf rust resistance genes effective at the seedling stage. The reaction pattern of Neijiang 977671 differed from those of the lines with known leaf rust resistance genes used in the test, indicating that Neijiang 977671 may carry a new leaf rust resistance gene(s). With the objective of identifying and mapping the new gene for resistance to leaf rust, F₁ and F₂ plants, and F_2:3 families, from Neijiang 977671 × Zhengzhou 5389 (susceptible) were inoculated with Chinese P. triticina pathotype FHNQ in the greenhouse. Results from the F₂ and F_2:3 populations indicated that a single dominant gene, temporarily designated LrNJ97, conferred resistance. In order to identify other possible genes in Neijiang 977671 other eight P. triticina pathotypes avirulent on Neijiang 977671 were used to inoculate 25 F_2:3 families. The results showed that at least three leaf rust resistance genes were deduced in Neijiang 977671. Bulked segregant analysis was performed on equal amounts of genomic DNA from 20 resistant and 20 susceptible F₂ plants. SSR markers polymorphic between the resistant and susceptible bulks were used to analyze the F_2:3 families. LrNJ97 was linked to five SSR loci on chromosome 2BL. The two closest flanking SSR loci were Xwmc317 and Xbarc159 at genetic distances of 4.2 and 2.2 cM, respectively. At present two designated genes (Lr50 and Lr58) are located on chromosome 2BL. In the seedling tests, the reaction pattern of LrNJ97 was different from that of Lr50. Lr50 and Lr58 were derived from T. armeniacum and Ae. triuncialis, respectively, whereas according to the pedigree of Neijiang 977671 LrNJ97 is from common wheat. Although seeds of lines with Lr58 were not available, it was concluded that LrNJ97 is likely to be a new leaf rust resistance gene.     

QTL mapping of adult-plant resistance to leaf rust in a RIL population derived from a cross of wheat cultivars Shanghai 3/Catbird and Naxos

Key message

Six QTL for adult plant resistance to leaf rust, including two QTL effective against additional diseases, were identified in a RIL population derived from a cross between Shanghai 3/Catbird and Naxos.

Abstract

Leaf rust is an important wheat disease and utilization of adult-plant resistance (APR) may be the best approach to achieve long-term protection from the disease. The CIMMYT spring wheat line Shanghai 3/Catbird (SHA3/CBRD) showed a high level of APR to Chinese Puccinia triticina pathotypes in the field. To identify APR genes in this line, a mapping population of 164 recombinant inbred lines (RILs) was developed from a cross of this line and Naxos, a moderately susceptible German cultivar. The RILs were evaluated for final disease severity (FDS) at Baoding, Hebei province, and Zhoukou, Henan province, in the 2010–2011 and 2011–2012 cropping seasons. QTL analysis detected one major QTL derived from SHA3/CBRD on chromosome 2BS explaining from 15 to 37 % of the phenotypic variance across environments. In addition one minor resistance QTL on chromosome 1AL from SHA3/CBRD and four minor QTL from Naxos on chromosomes 2DL, 5B, 7BS, and 7DS were also detected. SHA3/CBRD also possessed seedling resistance gene Lr26, and Naxos contained Lr1 based on gene postulation following tests with an array of P. triticina pathotypes and molecular marker assays. These seedling resistance and APR genes and their closely linked molecular markers are potentially useful for improving leaf rust resistance in wheat breeding programs.     

Molecular mapping of leaf rust resistance gene <Emphasis Type="Italic">LrZH84</Emphasis> in Chinese wheat line Zhou 8425B

Leaf rust, caused by Puccinia triticina, is one of the most widespread diseases in common wheat (Triticum aestivum L.) worldwide. With the objective of identifying and mapping new genes for resistance to leaf rust, F₁, F₂ plants and F₃ lines from a cross between resistant line Zhou 8425B and susceptible line Chinese Spring were inoculated with Chinese P. triticina races THTT and MBHP in the greenhouse. A total of 793 pairs of SSR primers were used to test the parents and resistant and susceptible bulks. Seven polymorphic chromosome 1B markers were used for genotyping the F₂ and F₃ populations. Zhou 8425B carried a single dominant resistance gene, temporarily designated LrZH84, linked to SSR markers gwm582 and barc8 with genetic distances of 3.9 and 5.2 cM, respectively. The Xbarc8 allele co-segregated with Lr26 in the F₃ population. The Xgwm582 allele associated with LrZH84 was identified as a leaf rust resistance gene and shown to be present in the Predgornaia 2 parent of Zhou 8425B. The seedling reaction pattern of LrZH84 was different from those of lines with Lr26, Lr33, Lr44 and Lr46, all of which are located in chromosome 1B. It was concluded that LrZH84 is likely to be a new leaf rust resistance gene.     

Identification of microsatellite markers linked to leaf rust resistance gene <Emphasis Type="Italic">Lr25</Emphasis> in wheat

The leaf rust resistance gene Lr25, transferred from Secale cereale L. into wheat and located on chromosome 4B, imparts resistance to all pathotypes of leaf rust in South-East Asia. In an F₂-derived F₃ population, created by crossing TcLr25 that carries the gene Lr25 for leaf rust resistance with leaf rust-susceptible parent Agra Local, three microsatellite markers located on the long arm of chromosome 4B were found to be linked to the Lr25 locus. The donor parent TcLr25 is a near-isogenic line derived from the variety Thatcher. The most virulent pathotype of leaf rust in the South-East Asian region, designated 77–5 (121R63-1), was used for challenging the population under artificially controlled conditions. The marker Xgwm251 behaved as a co-dominant marker placed 3.8 cM away from the Lr25 locus on 4BL. Two null allele markers, Xgwm538 and Xgwm6, in the same linkage group were located at a distance of 3.8 cM and 16.2 cM from the Lr25 locus, respectively. The genetic sequence of Xgwm251, Lr25, Xgwm538, and Xgwm6 covered a total length of 20 cM on 4BL. The markers were validated for their specificity to Lr25 resistance in a set of 43 wheat genetic stocks representing 43 other Lr genes.     

Effect of gene Lr34 in the enhancement of resistance to leaf rust of wheat

Summary Leaf rust resistance gene Lr34 is present in many wheat cultivars throughout the world that have shown durable resistance to leaf rust. Fourteen pair-wise combinations of Lr34 and seedling leaf rust resistance genes were developed by intercrossing near isogenic Thatcher lines. In both seedling and adult plant tests homozygous paired combinations of specific resistance genes with Lr34 had enhanced resistance relative to either parent to different numbers of isolates that were avirulent to the additional resistance genes. The TcLr34, 18 line also expressed enhanced resistance to specific isolates virulent to Lr18 in seedling and adult plant stages. In rust nursery tests, homozygous lines were more resistant than either parent, if the additional leaf rust gene conditioned an effective of resistance when present singly. The ability of Lr34 to interact with other genes conditioning effective resistance may contribute to the durability of leaf rust resistance in cultivars with Lr34. Contribution 1453 Agriculture Canada     

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.     

Molecular characterization of durum and common wheat recombinant lines carrying leaf rust resistance (Lr19) and yellow pigment (Y) genes from Lophopyrum ponticum

Chromosome 7E from Lophopyrum ponticum carries a valuable leaf rust resistant gene designated Lr19. This gene has not been widely used in common wheat breeding because of linkage with the yellow pigment gene Y. This gene tints flour yellow, reducing its appeal in bread making. However, a high level of yellow pigment is desirable in durum wheat breeding. We produced 97 recombinant chromosomes between L. ponticum transfer 7D.7E#1 and its wheat homoeologues, using the ph1b mutation that promotes homoeologous pairing. We characterized a subset of 37 of these lines with 11 molecular markers and evaluated their resistance to leaf rust and the abundance of yellow pigment. The Lr19 gene was mapped between loci Xwg420 and Xmwg2062, whereas Y was mapped distal to Xpsr687, the most distal marker on the long arm of chromosome 7. A short terminal 7EL segment translocated to 7A, including Lr19 and Y (line 1-23), has been transferred to durum wheat by backcrossing. The presence of this alien segment significantly increased the abundance of yellow pigment. The Lr19 also conferred resistance to a new durum leaf rust race from California and Mexico that is virulent on most durum wheat cultivars. The new durum lines with the recombinant 7E segment will be useful parents to increase yellow pigment and leaf rust resistance in durum wheat breeding programs. For the common wheat breeding programs, we selected the recombinant line 1-96, which has an interstitial 7E segment carrying Lr19 but not Y. This recombinant line can be used to improve leaf rust resistance without affecting flour color. The 7EL/7DL 1-96 recombinant chromosome did not show the meiotic self-elimination previously reported for a 7EL/7BL translocation.     

NBS-LRR sequence family is associated with leaf and stripe rust resistance on the end of homoeologous chromosome group 1S of wheat

A detailed RFLP map was constructed of the distal end of the short arm of chromosome 1D of Aegilops tauschii and wheat. At least two unrelated resistance-gene analogs (RGAs) mapped close to known leaf rust resistance genes (Lr21 and Lr40) located distal to seed storage protein genes on chromosome 1DS. One of the two RGA clones, which was previously shown to be part of a candidate gene for stripe rust resistance (Yr10) located within the homoeologous region on 1BS, identified at least three gene family members on chromosome 1DS of Ae. tauschii. One of the gene members co-segregated with the leaf rust resistance genes, Lr21 and Lr40, in Ae. tauschii and wheat segregating families. Hence, a RGA clone derived from a candidate gene for stripe rust resistance located on chromosome 1BS detected candidate genes for leaf rust resistance located in the corresponding region on 1DS of wheat. Received: 10 January 2000 / Accepted: 25 March 2000