Novel gene for adult plant resistance to Puccinia recondita in the wheat cultivar'Arjun'

Inheritance of resistance in the wheat cultivar‘Arjun’(HD 2009) against leaf rust pathotype 77–1 revealed that its durable resistance is attributable to a novel dominant adult plant resistance (APR) gene. Lr13, another gene reported in the cultivar played no role. This new gene is established as different from Lr34, the only effective APR gene from Triticum aestivum known for durability.     

Development and Validation of SCAR Markers Co-Segregating with an Agropyron Elongatum Derived Leaf Rust Resistance Gene Lr24 in Wheat

Summary An Agropyron elongatum-derived leaf rust resistance gene Lr24 located on chromosome 3DL of wheat was tagged with six random amplified polymorphic DNA (RAPD) markers which co-segregated with the gene. The markers were identified in homozygous resistant F₂ plants taken from a population segregating for leaf rust resistance generated from a cross between two near-isogenic lines (NILs) differing only for Lr24. Phenotyping was done by inoculating the plants with pathotype 77-5 of Puccinia triticina. To enable gene-specific selection, three RAPD markers (S1302₆₀₉, S1326₆₁₅ and OPAB-1₃₈₈) were successfully converted to polymorphic sequence characterized amplified region (SCAR) markers, amplifying only the critical DNA fragments co-segregating with Lr24. The SCAR markers were validated for specificity to the gene Lr24 in wheat NILs possessing Lr24 in 10 additional genetic backgrounds including the Thatcher NIL, but not to 43 Thatcher NILs possessing designated leaf rust resistance genes other than Lr24. This indicated the potential usefulness of these SCAR markers in marker assisted selection (MAS) and for pyramiding leaf rust resistance genes in wheat.     

Quantitative trait loci mapping of adult‐plant resistance to leaf rust in a Fundulea 900 × ‘Thatcher’ wheat cross

Wheat leaf rust (LR), caused by the obligate biotrophic fungus Puccinia triticina (Pt), is a destructive foliar disease of common wheat (Triticum aestivum L.) worldwide. The most effective, economic means to control the disease is resistant cultivars. The Romanian wheat line Fundulea 900 showed high resistance to LR in the field. To identify the basis of resistance to LR in Fundulea 900, a population of 188 F_2:3 lines from the cross Fundulea 900/‘Thatcher’ was phenotyped for LR severity during the 2010–2011, 2011–2012 and 2012–2013 cropping seasons in the field at Baoding, Hebei Province. Bulked segregant analysis and simple sequence repeat markers were used to identify the quantitative trait loci (QTLs) for LR adult‐plant resistance in the population. Three QTLs were detected and designated as QLr.hebau‐1BL, QLr.hebau‐2DS and QLr.hebau‐7DS. Based on the chromosome positions and molecular marker tests, QLr.hebau‐1BL is Lr46, and QLr.hebau‐7DS is Lr34. QLr.hebau‐2DS was derived from ‘Thatcher’ and was close to Lr22. This result suggests that Lr22b may confer residual resistance on field nurseries when challenged with isolates virulent on Lr22b, or another gene linked to Lr22b confers this resistance from ‘Thatcher’. This study confirms the value of Lr34 and Lr46 in breeding for LR resistance in China; the contribution of the QTL to chromosome 2D needs further validation.     

A leaf rust resistance gene, different from Lr34, associated with leaf tip necrosis in wheat

The gene Lr34 has contributed to durable resistance to leaf rust caused by Puccinia triticina in wheat worldwide. The closely associated leaf tip necrosis is generally used as the gene's marker. Lr34 has been postulated in many Indian bread wheat cultivars including ‘C 306’, based on the associated leaf tip necrosis and a few other field and glasshouse observations. The present study showed monogenic control of adult‐plant resistance in ‘C 306’ to leaf rust pathotype 77‐5 (121R63‐1). The F₂ segregation in the crosses between ‘C 306’ and the two known carriers of Lr34, ‘Line 897’ and ‘Jupateco 73’‘R’ fitted a digenic ratio. The F₃ families derived from the susceptible F₂ segregants were true breeding for susceptibility, proving the absence of Lr34 in ‘C 306’. The cross between ‘Line 897’ and ‘Jupateco 73’‘R’ did not segregate for susceptibility. Resistance in the cross ‘Agra Local’ (susceptible) × ‘C 306’ was associated with leaf tip necrosis, showing that the leaf rust resistance gene in ‘C 306’ was associated with leaf tip necrosis, but was different from Lr34. This gene is being temporarily designated as Lr‘C 306’. Hence, leaf tip necrosis cannot be considered as an exclusive marker for selecting Lr34 in wheat improvement.     

Genes Lr48 and Lr49 for hypersensitive adult plant leaf rust resistance in wheat (Triticum aestivum L.)

The genetic bases of leaf rust resistance in wheat (Triticum aestivum L.) line CSP44, selected from the Australian cultivar Condor, and Indian cultivar VL404, were studied. The reaction patterns of CSP44 and VL404 against Indian races 12, 77, 77-1, 77-2, 77-3, 77-4, 77-5 and 108 were different from reaction patterns shown by near-isogenic lines with known adult plant resistance (APR) genes, viz. Lr12, Lr13, Lr22b and Lr34. Although the reaction patterns of CSP44 and VL404 were similar to the near-isogenic line Tc+Lr22a, tests of allelism indicated absence of Lr22a in both CSP44 and VL404. On the basis of genetic studies, their resistances in field tests against race 77-5, the most virulent race from the Indian sub-continent, were each ascribed to two genes. One of the two genes in each wheat was identified to be the non-hypersensitive APR gene Lr34. The second APR genes in CSP44 and VL404 gave hypersensitive reaction types and were recessive and dominant, respectively. The gene in CSP44 was designated Lr48and the gene in VL404, Lr49. This revised version was published online in August 2006 with corrections to the Cover Date.     

Identification of a molecular marker linked to an Agropyron elongatum-derived gene Lr19 for leaf rust resistance in wheat

The leaf rust resistance gene Lr19, transferred from Agropyron elongatum into wheat (Triticum aestivum L.) imparts resistance to all pathotypes of leaf rust (Puccinia recondita f.sp. tritici) in South‐east Asia. A segregating F₂ population from a cross between the leaf rust resistant parent ‘HW 2046’ carrying Lr19 and a susceptible parent ‘Agra Local’ was screened in the phytotron against a virulent pathotype 77‐5 of leaf rust with the objective of identifying the molecular markers linked to Lr19. The gene was first tagged with a randomly amplified polymorphic DNA (RAPD) marker S73₇₂₈. The RAPD marker linked to the gene Lr19 which mapped at 6.4 ± 0.035 cM distance, was converted to a sequence characterized amplified region (SCAR) marker. The SCAR marker (SCS73₇₁₉) was specific to Lr19 and was not amplified in the near‐isogenic lines (NILs) carrying other equally effective alien genes Lr9, Lr28 and Lr32 enabling breeders to pyramid Lr19 with these genes.     

Evaluation of seedling and adult plant resistance to leaf rust in European wheat cultivars

Summary A set of 105 European wheat cultivars, comprising 68 cultivars with known seedling resistance genes and 37 cultivars that had not been tested previously, was tested for resistance to selected Australian pathotypes of P. triticina in seedling greenhouse tests and adult plant field tests. Only 4% of the cultivars were susceptible at all growth stages. Twelve cultivars lacked detectable seedling resistance to leaf rust, and among the remaining cultivars, 10 designated genes were present either singly or in combination. Lr13 was the most frequently detected gene, present in 67 cultivars, followed by the rye-derived gene Lr26, present in 19 cultivars. Other genes present were Lr1, Lr3a, Lr3ka, Lr10, Lr14a, Lr17b, Lr20 and Lr37. There was evidence for unidentified seedling resistance in addition to known resistance genes in 11 cultivars. Field tests with known pathotypes of P. triticina demonstrated that 57% of the cultivars carried adult plant resistance (APR) to P. triticina. The genetic identity of the APR is largely unknown. Genetic studies on selected cultivars with unidentified seedling resistances as well as all of those identified to carry APR are required to determine the number and inheritance of the genes involved, to determine their relationships with previously designated rust resistance genes, and to assess their potential value in breeding for resistance to leaf rust.     

Genetic Diversity for Adult Plant Resistance to Leaf Rust (Puccinia recondita) in Near-Isogenic Lines and in Indian Wheats

Variation for adult plant resistance in near-isogenic wheat lines carrying Lrl4b, Lrl4ab and Lr30 in a ‘Thatcher’ background indicated the possible presence of novel adult plant resistance genes effective against the Indian leaf rust population. Sixty-one wheats released for cultivation in India were grown in isolated nurseries. Each nursery was separately inoculated with one of four leaf rust pathotypes which had been selected to aid identification of resistance effective only in the adult plant stage. Seven distinct response groups were recognised and a minimum of six sources of adult plant resistance were postulated. In a group of 14 wheats, resistance was explained on the basis of the seedling response genes that were identified. Similar results for two years with pathotype 77-1 gave support to the reliability of field tests. Adult plant resistance (APR) sources were either race-specific or effective against all pathotypes used. Seedlings of cultivars with APR showed susceptible reactions. The possible presence of Lr34 in Indian wheats and its role in durable leaf rust resistance are discussed.     

Genetic basis of seedling-resistance to leaf rust in bread wheat 'Thatcher'

The bread wheat cultivar ‘Thatcher’ is documented to carry the gene Lr22b for adult‐plant resistance to leaf rust. Seedling‐resistance to leaf rust caused by Puccinia triticina in the bread wheat cultivar ‘Thatcher’, the background parent of the near‐isogenic lines for leaf rust resistance genes in wheat, is rare and no published information could be found on its genetic basis. The F₂ and F₃ analysis of the cross ‘Agra Local’ (susceptible) × ‘Thatcher’ showed that an apparently incompletely dominant gene conditioned seedling‐resistance in ‘Thatcher’ to the three ‘Thatcher’‐avirulent Indian leaf rust pathotypes – 0R8, 0R8‐1 and 0R9. Test of allelism revealed that this gene (temporarily designated LrKr1) was derived from ‘Kanred’, one of the parents of ‘Thatcher’. Absence of any susceptible F₂ segregants in a ‘Thatcher’ × ‘Marquis’ cross confirmed that an additional gene (temporarily designated LrMq1) derived from ‘Marquis’, another parent of ‘Thatcher’, was effective against pathotype 0R9 alone. These two genes as well as a second gene in ‘Kanred’ (temporarily designated LrKr2), which was effective against all the three pathotypes, but has not been inherited by ‘Thatcher’, seem to be novel, undocumented leaf rust resistance genes.     

Genetics of leaf rust resistance in three Americano landrace-derived wheat cultivars from Uruguay

A genetic analysis of the landrace‐derived wheat accessions Americano 25e, Americano 26n, and Americano 44d, from Uruguay was conducted to identify the leaf rust resistance genes present in these early wheat cultivars. The three cultivars were crossed with the leaf rust susceptible cultivar ‘Thatcher’ and approximately 80 backcross (BC1) F₂ families were derived for each cross. The BC1F₂ families and selected BC1F₄ lines were tested for seedling and adult plant leaf rust resistance with selected isolates of leaf rust, Puccinia triticina. The segregation and infection type data indicated that Americano 25e had seedling resistance genes Lr3, Lr16, an additional unidentified seedling gene, and one adult plant resistance gene that was neither Lr12 nor Lr13, and did not phenotypically resemble Lr34. Americano 26n was postulated to have genes Lr11, Lr12, Lr13, and Lr14a. Americano 44d appeared to have two possibly unique adult plant leaf rust resistance genes.     

Molecular genetics of race non-specific rust resistance in wheat

Over 150 resistance genes that confer resistance to either leaf rust, stripe rust or stem rust have been catalogued in wheat or introgressed into wheat from related species. A few of these genes from the ‘slow-rusting’ adult plant resistance (APR) class confer partial resistance in a race non-specific manner to one or multiple rust diseases. The recent cloning of two of these genes, Lr34/Yr18, a dual APR for leaf rust and stripe rust, and Yr36, a stripe rust APR gene, showed that they differ from other classes of plant resistance genes. Currently, seven Lr34/Yr18 haplotypes have been identified from sequencing the encoding ATP Binding Cassette transporter gene from diverse wheat germplasm of which one haplotype is commonly associated with the resistance phenotype. The paucity of well characterised APR genes, particularly for stem rust, calls for a focused effort in developing critical genetic stocks to delineate quantitative trait loci, construct specific BAC libraries for targeted APR genes to facilitate robust marker development for breeding applications, and the eventual cloning of the encoding genes.     

Alleles on the two dwarfing loci on 4B and 4D are main drivers of FHB‐related traits in the Canadian winter wheat population “Vienna” × “25R47”

Fusarium head blight (FHB), leaf rust and stem rust are among the most destructive wheat diseases. High‐yielding, native disease resistance sources are available in North America. The objective of this study was to map loci associated with FHB traits, leaf rust, stem rust and plant height in a “Vienna”/”25R47” population. DArT markers were used to generate a genetic map, and quantitative trait loci (QTL) analysis was performed by evaluating 113 doubled haploid lines across three environments in Ontario, Canada. FHB resistance QTL were identified on chromosomes 4D, 4B, 2D and 7A, while a QTL for leaf and stem rust resistance was identified on chromosome 1B. The dwarfing alleles of both Rht‐B1 and Rht‐D1 were associated with increased FHB index and DON content.     

Resistance to leaf rust in cultivars of bread wheat and durum wheat grown in Spain

A set of bread wheat and durum wheat cultivars adapted to Spanish conditions was tested for resistance against leaf rust caused by different pathotypes of Puccinia triticina in field trials and in growth chamber studies. Lower levels of resistance were found in durum wheat than in bread wheat. The most frequent Lr genes found in bread wheat were Lr1, Lr10, Lr13, Lr20, Lr26 and Lr28. In durum wheat, additional resistance genes that differed from the known Lr genes were identified. The level of partial resistance to leaf rust was in general low, although significant levels were identified in some bread wheat and durum wheat cultivars.     

Detection and inheritance of leaf rust resistance in common wheat lines Agra Local and IWP94

Genetic studies were undertaken to determine the number and identities of leaf rust resistance genes in common wheat lines Agra Local and IWP94. The infection type arrays of the two lines with eight pathotypes (pt.) of P. triticina were different from those of lines possessing known leaf rust resistance (Lr) genes. Agra Local possessed two recessive resistance genes, one conditioning resistance to pathotype 4R9-7, and the other, a temperature-sensitive factor, gave resistance to pt. 121R127 at high temperature (27°C). IWP94 was previously demonstrated to carry Lr23. From the present study IWP94 was determined to have at least four leaf rust resistance genes. The first of these was the same recessive gene conferring resistance to pathotype 4R9-7 which was found in Agra Local. A second partially dominant gene conferred resistance to pathotype 121R127 at high temperature and two additional recessive genes governed resistance to pathotype 93R15. When present together, these two recessive genes complemented each other and provided resistance to pathotype 69R13 as well. One of the two recessive genes conferring resistance to pathotypes 93R15 and 69R13 was Lr23.     

Rapid phenotyping of adult plant resistance in barley (Hordeum vulgare) to leaf rust under controlled conditions

Breeding for adult plant resistance (APR) is currently impeded by the low frequency of annual field‐based testing and variable environmental conditions. We developed and implemented a greenhouse‐based methodology for the rapid phenotyping of APR to leaf rust in barley to improve the efficacy of gene discovery and cloning. We assessed the effects of temperature (18 and 23°C) and growth stage (1–5 weeks) on the expression of APR in the greenhouse using 28 barley genotypes with both known and uncharacterized APR. All lines were susceptible in week 1, while lines carrying Rph20 and several with uncharacterized resistance expressed resistance as early as week 2. In contrast, lines lacking Rph20 and carrying either Rph23 and/or Rph24 expressed resistance from week 4. Resistant phenotypes were clearest at 18°C. A subset of 16 of the 28 lines were assessed for leaf rust across multiple national and international field sites. The greenhouse screening data reported in this study were highly correlated to most of the field sites, indicating that they provide comparable data on APR phenotypes for screening purposes.     

Rph23: A new designated additive adult plant resistance gene to leaf rust in barley on chromosome 7H

We report on a new adult plant resistance (APR) gene Rph23 conferring resistance to leaf rust in barley. The gene was identified and characterized from a doubled haploid population derived from an intercross between the Australian barley varieties Yerong (Y) and Franklin (F). Genetic analysis of adult plant field leaf rust scores of the Y/F population collected over three successive years indicated involvement of two highly additive genes controlling APR, one of which was named Rph23. The gene was mapped to chromosome 7HS positioned at a genetic distance 36.6 cM. Rph23 is closely linked to marker Ebmac0603, which is flanked by markers bPb‐8660 and bPb‐9601 with linkage distances of 0.8 and 5.1 cM, respectively. A PCR‐based marker was optimized for marker‐assisted selection of Rph23, and on the basis of this marker, the gene was postulated as being common in Australian and global barley germplasm. Pedigree and molecular marker analyses indicated that the six‐rowed black Russian landrace ‘LV‐Taganrog’ is the likely origin of Rph23.     

QTL mapping of adult‐plant resistance to stripe rust in a ‘Lumai 21 × Jingshuang 16’ wheat population

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is a devastating fungal disease in common wheat (Triticum aestivum L.) worldwide. Chinese wheat cultivars ‘Lumai 21’ and ‘Jingshuang 16’ show moderate levels of adult‐plant resistance (APR) to stripe rust in the field, and they showed a mean maximum disease severity (MDS) ranging from 24 to 56.7% and 26 to 59%, respectively, across different environments. The aim of this study was to identify quantitative trait loci (QTL) for resistance to stripe rust in an F₃ population of 199 lines derived from ‘Lumai 21’ × ‘Jingshuang 16’. The F₃ lines were evaluated for MDS in Qingshui, Gansu province, and Chengdu, Sichuan province, in the 2009–2010 and 2010–2011 cropping seasons. Five QTL for APR were detected on chromosomes 2B (2 QTL), 2DS, 4DL and 5DS based on mean MDS in each environment and averaged values from all three environments. These QTL were designated QYr.caas‐2BS.2, QYr.caas‐2BL.2, QYr.caas‐2DS.2, QYr.caas‐4DL.2 and QYr.caas‐5DS, respectively. QYr.caas‐2DS.2 and QYr.caas‐5DS were detected in all three environments, explaining 2.3–18.2% and 5.1–18.0% of the phenotypic variance, respectively. In addition, QYr.caas‐2BS.2 and QYr.caas‐2BL.2 colocated with QTL for powdery mildew resistance reported in a previous study. These APR genes and their linked molecular markers are potentially useful for improving stripe rust and powdery mildew resistances in wheat breeding.     

Effect of the leaf rust resistance gene Lr28 on grain yield and bread-making quality of wheat

Near‐isogenic lines carrying the Lr28 gene developed in five genetic backgrounds were tested for 2 years with and without fungicide treatment. The Lr28 gene increased grain yield, 1000‐grain weight and number of effective tillers per plant under heavy leaf rust infection with no negative effects on yield and bread‐making quality in rust‐free plots. Although a reduction in dough development time was found to be associated with Lr28, it can still be used extensively in wheat breeding programmes.     

Inheritance and QTL mapping of leaf rust resistance in the European winter wheat cultivar ‘Beaver’

Genetic studies were conducted on an European winter wheat cultivar, Beaver, to determine the mode of inheritance of leaf rust resistance at seedling and adult plant growth stages using a recombinant doubled haploid population, Beaver/Soissons. Greenhouse studies indicated the involvement of genes Lr13 and Lr26 in governing leaf rust resistance at seedling growth stages, whereas, adult plant resistance (APR) in the field with pathotypes carrying virulence individually for Lr13 and Lr26 showed trigenic inheritance for the population. Marker regression analysis of adult plant field data indicated the involvement of six significant QTLs (chromosomes 1B, 3B, 3D, 4B, 4D and 5A) in year 2005, four QTLs (1B, 3B, 4B and 5A) in 2006, and six QTLs (1A, 1B, 3B, 4A, 4B and 5A) in 2007 for reducing leaf rust severity. QTLs on chromosomes 1B, 4B and 5A were considered the most important because of their detection across years, whereas QTLs on chromosomes 1A, 3B, 3D and 4A were either inconsistent or non-significant and unexplained. Based on an association of closely linked markers with phenotypic data, putative single gene stocks were identified for each consistent QTL and crossing was initiated to develop populations segregating for each to permit fine mapping of the identified regions.     

Stripe rust resistance gene Yr18 and its suppressor gene in Chinese wheat landraces

The slow‐rusting and mildewing gene Yr18/Lr34/Pm38/Sr57 confers partial, durable resistance to multiple fungal pathogens and has its origins in China. A number of diagnostic markers were developed for this gene based on the gene sequence, but these markers do not always predict the presence of the resistant phenotype as some wheat varieties with the gene are susceptible to stripe rust in China. We hypothesized that these varieties have a suppressor of Yr18. This study was undertaken to determine the presence of Yr18, the suppressor and/or another resistance gene in 144 Chinese wheat landraces using molecular markers and stripe rust field data. Forty‐three landraces were predicted to have Yr18 based on the presence of the markers, but had final disease severities higher than 70%, indicating that this gene may be under the influence of a suppressor. Four of these landraces, ‘Sichuanyonggang 2’, ‘Baikemai’, ‘Youmai’ and ‘Zhangsihuang’, were chosen for genetic studies. Crosses were made between the lines and ‘Avocet S’, with further crosses of Sichuanyonggang 2 ×  ‘Huixianhong’ and Sichuanyonggang 2 ×  ‘Chinese Spring’. The F₁ plants of Sichuanyonggang 2/Chinese Spring was susceptible indicating the presence of a dominant suppressor gene. The results of genetic analyses of F_2:3 and BC₁F₂ families derived from these crosses indicated the presence of Yr18, a Yr18 suppressor and another additive resistance gene. The Yr18 region in Sichuanyonggang 2 was sequenced to ensure that it contained the functional allele. This is the first report of a suppressor of Yr18/Lr34/Pm38/Sr57 gene with respect to stripe rust response.