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Mohamed IES, Oe H, Kamal NM, Mustafa HM, Gorafi YSA, Tahir ISA, Tsujimoto H, Tanaka H. Enhancing Wheat Flour Quality Through Introgression of High-Molecular-Weight Glutenin Subunits From Aegilops tauschii Accessions. FRONTIERS IN SUSTAINABLE FOOD SYSTEMS 2022. [DOI: 10.3389/fsufs.2022.887795] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/21/2023] Open
Abstract
Narrow genetic diversity in the wheat gene pool restricts the improvement of wheat quality traits. Aegilops tauschii possesses valuable genetic diversity that can be used to improve not only biotic and abiotic stresses in arid regions but also wheat yield and quality. Our study, which used 392 multiple synthetic derivatives (MSD) panel developed with Ae. tauschii Coss. introgressions, had three main aims: to explore the genetic diversity of high-molecular-weight glutenin subunits (HMW-GS), to investigate the dough strength and the relationship between protein content and grain yield, and to identify lines with a good flour quality. A wide range of allelic diversity was observed at the Glu-D1 locus, reflecting the impact of the different introgressed portions of Ae. tauschii, and a wide variation was found in dough strength even between lines having the same composition of HMW-GS. We report a negative impact on dough strength of subunit 5t+10t from Ae. tauschii and a relatively positive impact of subunit 2t+12.1t. We identified four MSD lines with significantly enhanced flour quality. Regressing the grain yield of the MSD lines against protein content showed no correlation between the two traits and identified lines with comparable grain yield to the recurrent parent and higher protein content. The identified MSD lines could provide a valuable genetic resource for enhancing the end-use quality of flour without any loss in productivity.
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Kwiatek MT, Belter J, Ulaszewski W, Skowrońska R, Noweiska A, Wiśniewska H. Molecular identification of triticale introgression lines carrying leaf rust resistance genes transferred from Aegilops kotschyi Boiss. and Ae. tauschii Coss. J Appl Genet 2021; 62:431-439. [PMID: 33990930 PMCID: PMC8357765 DOI: 10.1007/s13353-021-00635-2] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/03/2021] [Revised: 04/20/2021] [Accepted: 04/22/2021] [Indexed: 11/28/2022]
Abstract
Triticale (× Triticosecale Wittmack) is a commercial hybrid harboring wheat (Triticum sp.) and rye (Secale cereale L.) genomes. The limited genetic diversity of this crop resulted in the collapse of fungal disease resistance. Leaf rust disease, caused by Puccinia triticina Eriks., is reported to reduce the triticale yield significantly (more than 30%). There is a need to enlarge the genetic variability of this crop including leaf resistance genes. The main aim of this research was to evaluate the leaf rust resistance of the offspring of translocation lines of triticale carrying chromatin of Ae. tauschii and Ae. kotschyi. A reaction of seedlings of 200 plants of two triticale-Aegilops translocation lines (Bogo-2Dt.2R and Sekundo-2Sk.2R) was compared after inoculation with a natural mixture of P. triticina races, specific to triticale in controlled condition. Before inoculation, each plant was screened using molecular cytogenetics and molecular markers linked to leaf rust resistance genes. The presence of Aegilops chromosome segments was confirmed using genomic in situ hybridization (GISH). Lr39 and Lr54 leaf rust resistance genes were identified using Xgdm35 and S14 molecular markers, respectively. After inoculation, a significant improvement of resistance severity was observed in Sekundo-2Sk.2R in comparison with triticale cv. Sekundo plants. The resistance level of Bogo-2Dt.2R did not differ compared with triticale cv. Bogo plants. It was shown that Lr39 gene did not increase the leaf rust resistance level of triticale cv. Bogo.
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Affiliation(s)
- Michał T Kwiatek
- Department of Genomics, Institute of Plant Genetics of the Polish Academy of Sciences, Strzeszyńska 34, 60-479, Poznań, Poland. .,Department of Genetics and Plant Breeding, Poznan University of Life Sciences, Dojazd 11, 60-632, Poznań, Poland.
| | - Jolanta Belter
- Department of Genomics, Institute of Plant Genetics of the Polish Academy of Sciences, Strzeszyńska 34, 60-479, Poznań, Poland
| | - Waldemar Ulaszewski
- Department of Genomics, Institute of Plant Genetics of the Polish Academy of Sciences, Strzeszyńska 34, 60-479, Poznań, Poland
| | - Roksana Skowrońska
- Department of Genetics and Plant Breeding, Poznan University of Life Sciences, Dojazd 11, 60-632, Poznań, Poland
| | - Aleksandra Noweiska
- Department of Genetics and Plant Breeding, Poznan University of Life Sciences, Dojazd 11, 60-632, Poznań, Poland
| | - Halina Wiśniewska
- Department of Genomics, Institute of Plant Genetics of the Polish Academy of Sciences, Strzeszyńska 34, 60-479, Poznań, Poland
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Dinkar V, Jha SK, Mallick N, Niranjana M, Agarwal P, Sharma JB, Vinod. Molecular mapping of a new recessive wheat leaf rust resistance gene originating from Triticum spelta. Sci Rep 2020; 10:22113. [PMID: 33335131 PMCID: PMC7746701 DOI: 10.1038/s41598-020-78679-3] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2020] [Accepted: 11/27/2020] [Indexed: 11/09/2022] Open
Abstract
TSD276-2, a wheat genetic stock derived from the cross Agra Local/T. spelta 276 showed broad spectrum resistance against leaf rust pathogen. Genetic analysis was undertaken using F1, F2, F2:3 and BC1F1 generations derived from the cross TSD276-2/Agra Local. The results revealed a single recessive gene for leaf rust resistance, tentatively named as LrTs276-2, in TSD276-2. Molecular mapping of leaf rust resistance gene LrTs276-2 in TSD276-2 was done using SNP-based PCR and SSR markers. For Bulked Segregant Analysis (BSA), two bulks viz. resistant bulk and susceptible bulk, and the parents TSD276-2 and Agra Local were genotyped for SNPs using AFFYMETRIX 35K Wheat Breeders' AXIOM array. T. spelta 276 was also genotyped and used as a check. BSA indicated that the gene for leaf rust resistance in TSD276-2 is located on chromosome arm 1DS. Putatively linked SNPs on chromosome arm 1DS were converted into PCR-based markers. Polymorphic SSR markers on chromosome arm 1DS were also identified. Final linkage map was constructed using one SNP-based PCR and three SSR markers. The rust reaction and chromosomal location suggest that LrTs276-2 is a new leaf rust resistance gene which may be useful in broadening the genetic base of leaf rust resistance in wheat.
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Affiliation(s)
- Vishal Dinkar
- Division of Genetics, ICAR-Indian Agricultural Research Institute, New Delhi, 110012, India
| | - S K Jha
- Division of Genetics, ICAR-Indian Agricultural Research Institute, New Delhi, 110012, India
| | - Niharika Mallick
- Division of Genetics, ICAR-Indian Agricultural Research Institute, New Delhi, 110012, India
| | - M Niranjana
- Division of Genetics, ICAR-Indian Agricultural Research Institute, New Delhi, 110012, India
| | - Priyanka Agarwal
- Division of Genetics, ICAR-Indian Agricultural Research Institute, New Delhi, 110012, India
| | - J B Sharma
- Division of Genetics, ICAR-Indian Agricultural Research Institute, New Delhi, 110012, India
| | - Vinod
- Division of Genetics, ICAR-Indian Agricultural Research Institute, New Delhi, 110012, India.
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Mohler V, Schmolke M, Zeller FJ, Hsam SLK. Genetic analysis of Aegilops tauschii-derived seedling resistance to leaf rust in synthetic hexaploid wheat. J Appl Genet 2020; 61:163-168. [PMID: 31981185 PMCID: PMC7148280 DOI: 10.1007/s13353-020-00541-z] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/05/2019] [Revised: 12/19/2019] [Accepted: 01/08/2020] [Indexed: 10/29/2022]
Abstract
Seedling resistance to leaf rust available in the synthetic hexaploid wheat line Syn137 was characterised by means of cytogenetic and linkage mapping. Monosomic analysis located a single dominant gene for leaf rust resistance on chromosome 5D. Molecular mapping not only confirmed this location but also positioned the gene to the distal part of the long arm of chromosome 5D. A test of allelism showed that the gene, tentatively named LrSyn137, is independent but closely linked to Lr1. It appears that Syn137 is occasionally heterogeneous for Lr1 since the analysis of the Lr1-specific marker RGA567-5 in the genetic mapping population indicated the presence of Lr1. Syn137 represents another source of genetic variation that can be useful for the diversification of leaf rust resistance in wheat cultivars.
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Affiliation(s)
- Volker Mohler
- Bayerische Landesanstalt für Landwirtschaft (LfL), Institut für Pflanzenbau und Pflanzenzüchtung (IPZ), Am Gereuth 8, 85354, Freising, Germany.
| | - Michael Schmolke
- Plant Breeding, Centre for Food and Life Sciences Weihenstephan, Technical University of Munich, Liesel-Beckmann-Str. 2, 85354, Freising, Germany
| | - Friedrich J Zeller
- Plant Breeding, Centre for Food and Life Sciences Weihenstephan, Technical University of Munich, Liesel-Beckmann-Str. 2, 85354, Freising, Germany
| | - Sai L K Hsam
- Plant Breeding, Centre for Food and Life Sciences Weihenstephan, Technical University of Munich, Liesel-Beckmann-Str. 2, 85354, Freising, Germany
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Majka M, Kwiatek MT, Majka J, Wiśniewska H. Aegilops tauschii Accessions with Geographically Diverse Origin Show Differences in Chromosome Organization and Polymorphism of Molecular Markers Linked to Leaf Rust and Powdery Mildew Resistance Genes. FRONTIERS IN PLANT SCIENCE 2017; 8:1149. [PMID: 28702048 PMCID: PMC5487464 DOI: 10.3389/fpls.2017.01149] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/23/2017] [Accepted: 06/15/2017] [Indexed: 06/02/2023]
Abstract
Aegilops tauschii (2n = 2x = 14) is a diploid wild species which is reported as a donor of the D-genome of cultivated bread wheat. The main goal of this study was to examine the differences and similarities in chromosomes organization among accessions of Ae. tauschii with geographically diversed origin, which is believed as a potential source of genes, especially determining resistance to fungal diseases (i.e., leaf rust and powdery mildew) for breeding of cereals. We established and compared the fluorescence in situ hybridization patterns of 21 accessions of Ae. tauschii using various repetitive sequences mainly from the BAC library of wheat cultivar Chinese Spring. Results obtained for Ae. tauschii chromosomes revealed many similarities between analyzed accessions, however, some hybridization patterns were specific for accessions, which become from cognate regions of the World. The most noticeable differences were observed for accessions from China which were characterized by presence of distinct signals of pTa-535 in the interstitial region of chromosome 3D, less intensity of pTa-86 signals in chromosome 2D, as well as lack of additional signals of pTa-86 in chromosomes 1D, 5D, or 6D. Ae. tauschii of Chinese origin appeared homogeneous and separate from landraces that originated in western Asia. Ae. tauschii chromosomes showed similar hybridization patterns to wheat D-genome chromosomes, but some differences were also observed among both species. What is more, we identified reciprocal translocation between short arm of chromosome 1D and long arm of chromosome 7D in accession with Iranian origin. High polymorphism between analyzed accessions and extensive allelic variation were revealed using molecular markers associated with resistance genes. Majority of the markers localized in chromosomes 1D and 2D showed the diversity of banding patterns between accessions. Obtained results imply, that there is a moderate or high level of polymorphism in the genome of Ae. tauschii determined by a geographical origin, which we proved by cytogenetic and molecular markers analysis. Therefore, selected accessions might constitute an accessible source of variation for improvement of Triticeae species like wheat and triticale.
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Affiliation(s)
- Maciej Majka
- Cereal Genomics Team, Department of Genomics, Institute of Plant Genetics, Polish Academy of SciencesPoznań, Poland
| | - Michał T. Kwiatek
- Cereal Genomics Team, Department of Genomics, Institute of Plant Genetics, Polish Academy of SciencesPoznań, Poland
| | - Joanna Majka
- Plant Molecular Physiology and Cytogenetics Team, Department of Environmental Stress Biology, Institute of Plant Genetics, Polish Academy of SciencesPoznań, Poland
| | - Halina Wiśniewska
- Cereal Genomics Team, Department of Genomics, Institute of Plant Genetics, Polish Academy of SciencesPoznań, Poland
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Pasam RK, Bansal U, Daetwyler HD, Forrest KL, Wong D, Petkowski J, Willey N, Randhawa M, Chhetri M, Miah H, Tibbits J, Bariana H, Hayden MJ. Detection and validation of genomic regions associated with resistance to rust diseases in a worldwide hexaploid wheat landrace collection using BayesR and mixed linear model approaches. TAG. THEORETICAL AND APPLIED GENETICS. THEORETISCHE UND ANGEWANDTE GENETIK 2017; 130:777-793. [PMID: 28255670 DOI: 10.1007/s00122-016-2851-7] [Citation(s) in RCA: 17] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/04/2016] [Accepted: 12/28/2016] [Indexed: 05/26/2023]
Abstract
BayesR and MLM association mapping approaches in common wheat landraces were used to identify genomic regions conferring resistance to Yr, Lr, and Sr diseases. Deployment of rust resistant cultivars is the most economically effective and environmentally friendly strategy to control rust diseases in wheat. However, the highly evolving nature of wheat rust pathogens demands continued identification, characterization, and transfer of new resistance alleles into new varieties to achieve durable rust control. In this study, we undertook genome-wide association studies (GWAS) using a mixed linear model (MLM) and the Bayesian multilocus method (BayesR) to identify QTL contributing to leaf rust (Lr), stem rust (Sr), and stripe rust (Yr) resistance. Our study included 676 pre-Green Revolution common wheat landrace accessions collected in the 1920-1930s by A.E. Watkins. We show that both methods produce similar results, although BayesR had reduced background signals, enabling clearer definition of QTL positions. For the three rust diseases, we found 5 (Lr), 14 (Yr), and 11 (Sr) SNPs significant in both methods above stringent false-discovery rate thresholds. Validation of marker-trait associations with known rust QTL from the literature and additional genotypic and phenotypic characterisation of biparental populations showed that the landraces harbour both previously mapped and potentially new genes for resistance to rust diseases. Our results demonstrate that pre-Green Revolution landraces provide a rich source of genes to increase genetic diversity for rust resistance to facilitate the development of wheat varieties with more durable rust resistance.
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Affiliation(s)
- Raj K Pasam
- Department of Economic Development, Jobs, Transport and Recourses, AgriBio Centre for AgriBioscience, Bundoora, VIC, 3083, Australia
| | - Urmil Bansal
- Faculty of Agriculture and Environment, Plant Breeding Institute-Cobbitty, The University of Sydney, PMB4011, Narellan, NSW, 2567, Australia
| | - Hans D Daetwyler
- Department of Economic Development, Jobs, Transport and Recourses, AgriBio Centre for AgriBioscience, Bundoora, VIC, 3083, Australia
- School of Applied Systems Biology, La Trobe University, Bundoora, VIC, 3083, Australia
| | - Kerrie L Forrest
- Department of Economic Development, Jobs, Transport and Recourses, AgriBio Centre for AgriBioscience, Bundoora, VIC, 3083, Australia
| | - Debbie Wong
- Department of Economic Development, Jobs, Transport and Recourses, AgriBio Centre for AgriBioscience, Bundoora, VIC, 3083, Australia
| | - Joanna Petkowski
- Department of Economic Development, Jobs, Transport and Recourses, AgriBio Centre for AgriBioscience, Bundoora, VIC, 3083, Australia
| | - Nicholas Willey
- Faculty of Agriculture and Environment, Plant Breeding Institute-Cobbitty, The University of Sydney, PMB4011, Narellan, NSW, 2567, Australia
- Dow AgroSciences Australia Ltd, Unit 12A, 84 Barnes Street, Tamworth, NSW, 2340, Australia
| | - Mandeep Randhawa
- Faculty of Agriculture and Environment, Plant Breeding Institute-Cobbitty, The University of Sydney, PMB4011, Narellan, NSW, 2567, Australia
- International Maize and Wheat Improvement Center (CIMMYT), Carretera México-Veracruz Km. 45, El Batán, Texcoco, México, C.P. 56237, Mexico
| | - Mumta Chhetri
- Faculty of Agriculture and Environment, Plant Breeding Institute-Cobbitty, The University of Sydney, PMB4011, Narellan, NSW, 2567, Australia
| | - Hanif Miah
- Faculty of Agriculture and Environment, Plant Breeding Institute-Cobbitty, The University of Sydney, PMB4011, Narellan, NSW, 2567, Australia
| | - Josquin Tibbits
- Department of Economic Development, Jobs, Transport and Recourses, AgriBio Centre for AgriBioscience, Bundoora, VIC, 3083, Australia
| | - Harbans Bariana
- Faculty of Agriculture and Environment, Plant Breeding Institute-Cobbitty, The University of Sydney, PMB4011, Narellan, NSW, 2567, Australia.
| | - Matthew J Hayden
- Department of Economic Development, Jobs, Transport and Recourses, AgriBio Centre for AgriBioscience, Bundoora, VIC, 3083, Australia.
- School of Applied Systems Biology, La Trobe University, Bundoora, VIC, 3083, Australia.
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Gill BS. SNPing Aegilops tauschii genetic diversity and the birthplace of bread wheat. THE NEW PHYTOLOGIST 2013; 198:641-642. [PMID: 23577596 DOI: 10.1111/nph.12259] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Affiliation(s)
- Bikram S Gill
- Wheat Genetics Resource Center, Department of Plant Pathology, Throckmorton Plant Sciences Center, Kansas State University, Manhattan, KS, 66506-5502, USA
- Faculty of Science, Genomics and Biotechnology Section, Department of Biological Sciences, King Abdulaziz University, Jeddah, 21589, Saudi Arabia
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Laikova LI, Belan IA, Badaeva ED, Rosseeva LP, Shepelev SS, Shumny VK, Pershina LA. Development and study of spring bread wheat variety Pamyati Maystrenko with introgression of genetic material from synthetic hexaploid Triticum timopheevii Zhuk. × Aegilops tauschii Coss. RUSS J GENET+ 2013. [DOI: 10.1134/s1022795413010067] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
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10
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Pradhan GP, Prasad PVV, Fritz AK, Kirkham MB, Gill BS. Response of Aegilops species to drought stress during reproductive stages of development. FUNCTIONAL PLANT BIOLOGY : FPB 2012; 39:51-59. [PMID: 32480759 DOI: 10.1071/fp11171] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/03/2011] [Accepted: 10/28/2011] [Indexed: 06/11/2023]
Abstract
Drought stress is an important abiotic factor limiting wheat yield. Thirty-one accessions of Aegilops species belonging to five species were screened to identify species/accessions tolerant to drought stress and to measure traits associated with the tolerance. Plants were grown at full irrigation, 25/19°C day/night temperature and an 18h photoperiod. At anthesis (Feekes 10.5.1), drought stress was imposed by withholding water for 16 days. Controls were continuously irrigated. Drought stress decreased chlorophyll content, grain number, individual grain weight and grain yield by 31, 25, 68 and 76% compared with the control. Aegilops geniculata Roth had greater tolerance to drought stress for yield (48% decline from control) compared with other species (>73% decline from control). The tolerance was associated with higher grain number spike-1 and heavier grains. A. geniculata, GenBank accession number TA 10437, was highly tolerant to drought stress with <20% yield decline and a drought stress susceptibility index (DSI) <0.5, whereas TA 1802, TA 2061, TA 1814, TA 1819 were identified as moderately tolerant to drought stress (20-40% yield decline and DSI<1.0). Our results suggest a presence of genetic variability among Aegilops species that can be utilised in breeding wheat for tolerance to drought stress at reproductive stages.
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Affiliation(s)
- Gautam P Pradhan
- Department of Agronomy, Kansas State University, 2004 Throckmorton Plant Sciences Center, Manhattan, KS 66506, USA
| | - P V Vara Prasad
- Department of Agronomy, Kansas State University, 2004 Throckmorton Plant Sciences Center, Manhattan, KS 66506, USA
| | - Allan K Fritz
- Department of Agronomy, Kansas State University, 2004 Throckmorton Plant Sciences Center, Manhattan, KS 66506, USA
| | - Mary B Kirkham
- Department of Agronomy, Kansas State University, 2004 Throckmorton Plant Sciences Center, Manhattan, KS 66506, USA
| | - Bikram S Gill
- Department of Plant Pathology, Kansas State University, 4024 Throckmorton Plant Sciences Center, Manhattan, KS 66506, USA
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Mago R, Zhang P, Bariana HS, Verlin DC, Bansal UK, Ellis JG, Dundas IS. Development of wheat lines carrying stem rust resistance gene Sr39 with reduced Aegilops speltoides chromatin and simple PCR markers for marker-assisted selection. TAG. THEORETICAL AND APPLIED GENETICS. THEORETISCHE UND ANGEWANDTE GENETIK 2009; 119:1441-50. [PMID: 19756473 DOI: 10.1007/s00122-009-1146-7] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/09/2009] [Accepted: 08/21/2009] [Indexed: 05/20/2023]
Abstract
The use of major resistance genes is a cost-effective strategy for preventing stem rust epidemics in wheat crops. The stem rust resistance gene Sr39 provides resistance to all currently known pathotypes of Puccinia graminis f. sp. tritici (Pgt) including Ug99 (TTKSK) and was introgressed together with leaf rust resistance gene Lr35 conferring adult plant resistance to P. triticina (Pt), into wheat from Aegilops speltoides. It has not been used extensively in wheat breeding because of the presumed but as yet undocumented negative agronomic effects associated with Ae. speltoides chromatin. This investigation reports the production of a set of recombinants with shortened Ae. speltoides segments through induction of homoeologous recombination between the wheat and the Ae. speltoides chromosome. Simple PCR-based DNA markers were developed for resistant and susceptible genotypes (Sr39#22r and Sr39#50s) and validated across a set of recombinant lines and wheat cultivars. These markers will facilitate the pyramiding of ameliorated sources of Sr39 with other stem rust resistance genes that are effective against the Pgt pathotype TTKSK and its variants.
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Affiliation(s)
- Rohit Mago
- CSIRO Plant Industry, GPO Box 1600, Canberra, ACT, 2601, Australia.
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Evolution of new disease specificity at a simple resistance locus in a crop-weed complex: reconstitution of the Lr21 gene in wheat. Genetics 2009; 182:595-602. [PMID: 19364806 DOI: 10.1534/genetics.108.099614] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
The wheat leaf-rust resistance gene Lr21 was first identified in an Iranian accession of goatgrass, Aegilops tauschii Coss., the D-genome donor of hexaploid bread wheat, and was introgressed into modern wheat cultivars by breeding. To elucidate the origin of the gene, we analyzed sequences of Lr21 and lr21 alleles from 24 wheat cultivars and 25 accessions of Ae. tauschii collected along the Caspian Sea in Iran and Azerbaijan. Three basic nonfunctional lr21 haplotypes, H1, H2, and H3, were identified. Lr21 was found to be a chimera of H1 and H2, which were found only in wheat. We attempted to reconstitute a functional Lr21 allele by crossing the cultivars Fielder (H1) and Wichita (H2). Rust inoculation of 5876 F(2) progeny revealed a single resistant plant that proved to carry the H1H2 haplotype, a result attributed to intragenic recombination. These findings reflect how plants balance the penalty and the necessity of a resistance gene and suggest that plants can reuse "dead" alleles to generate new disease-resistance specificity, leading to a "death-recycle" model of plant-resistance gene evolution at simple loci. We suggest that selection pressure in crop-weed complexes contributes to this process.
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