Mapping of novel powdery mildew resistance gene(s) from <Emphasis Type="Italic">Agropyron cristatum</Emphasis> chromosome 2P

Key message

A physical map of Agropyron cristatum 2P chromosome was constructed for the first time and the novel powdery mildew resistance gene(s) from chromosome 2P was(were) also mapped.

Abstract

Agropyron cristatum (L.) Gaertn. (2n = 28, PPPP), a wild relative of common wheat, is highly resistant to powdery mildew. Previous studies showed that wheat-A. cristatum 2P disomic addition line II-9-3 displayed high resistance to powdery mildew, and the resistance was attributable to A. cristatum chromosome 2P. To utilize and physically map the powdery mildew resistance gene(s), 15 wheat-A. cristatum 2P translocation lines and three A. cristatum 2P deletion lines with different chromosomal segment sizes, obtained from II-9-3 using ⁶⁰Co-γ ray irradiation, were characterized using cytogenetic and molecular marker analysis. A. cristatum 2P chromosomal segments in the translocations were translocated to different wheat chromosomes, including 1A, 4A, 5A, 6A, 7A, 1B, 2B, 3B, 7B, 3D, 4D, and 6D. A physical map of the 2P chromosome was constructed with 82 STS markers, consisting of nine bins with 34 markers on 2PS and eight bins with 48 markers on 2PL. The BC₁F₂ populations of seven wheat-A. cristatum 2P translocation lines (2PT-3, 2PT-4, 2PT-5, 2PT-6, 2PT-8, 2PT-9, and 2PT-10) were developed by self-pollination, tested with powdery mildew and genotyped with 2P-specific STS markers. From these results, the gene(s) conferring powdery mildew resistance was(were) located on 2PL bin FL 0.66–0.86 and 19 2P-specific markers were identified in this bin. Moreover, two new powdery mildew-resistant translocation lines (2PT-4 and 2PT-5) with small 2PL chromosome segments were obtained. The newly developed wheat lines with powdery mildew resistance and the closely linked molecular markers will be valuable for wheat disease breeding in the future.

     

Identification and mapping of <Emphasis Type="Italic">MLIW30</Emphasis>, a novel powdery mildew resistance gene derived from wild emmer wheat

Powdery mildew, caused by Blumeria graminis f.sp. tritici (Bgt), is a destructive foliar disease of common wheat in areas with cool or maritime climates. Wild emmer wheat, Triticum turgidum ssp. dicoccoides, the progenitor of both domesticated tetraploid durum wheat and hexaploid bread wheat, harbors abundant genetic diversity related to resistance to powdery mildew that can be utilized for wheat improvement. An F₂ segregating population was obtained from a cross between resistant bread wheat line 2L6 and susceptible cultivar Liaochun 10, after which genetic analysis of F₂ and F₂-derived F₃ families was performed by inoculating plants with isolate Bgt E09. The results of this experiment demonstrated that powdery mildew resistance in 2L6, which was derived from wild emmer wheat accession IW30, was controlled by a single dominant gene, temporarily designated MLIW30. Nineteen SSR markers and two STS markers linked with MLIW30 were acquired by applying bulked segregant analysis. Finally, MLIW30 was located to the long arm of chromosome 4A and found to be flanked by simple sequence repeat markers XB1g2000.2 and XB1g2020.2 at 0.1 cM. Because no powdery mildew resistance gene in or derived from wild emmer wheat has been reported in wheat chromosome 4A, MLIW30 might be a novel Pm gene.     

Genetic analysis of a novel broad-spectrum powdery mildew resistance gene from the wheat-<Emphasis Type="Italic">Agropyron cristatum</Emphasis> introgression line Pubing 74

Main conclusion

A novel broad-spectrum powdery mildew resistance gene PmPB74 was identified in wheat- Agropyron cristatum introgression line Pubing 74. Development of wheat cultivars with broad-spectrum, durable resistance to powdery mildew has been restricted by lack of superior genetic resources. In this study, a wheat-A. cristatum introgression line Pubing 74, originally selected from a wide cross between the common wheat cultivar Fukuhokomugi (Fukuho) and Agropyron cristatum (L.) Gaertn (2n = 4x = 28; genome PPPP), displayed resistance to powdery mildew at both the seedling and adult stages. The putative alien chromosomal fragment in Pubing 74 was below the detection limit of genomic in situ hybridization (GISH), but evidence for other non-GISH-detectable introgressions was provided by the presence of three STS markers specific to A. cristatum. Genetic analysis indicated that Pubing 74 carried a single dominant gene for powdery mildew resistance, temporarily designated PmPB74. Molecular mapping showed that PmPB74 was located on wheat chromosome arm 5DS, and flanked by markers Xcfd81 and HRM02 at genetic distances of 2.5 and 1.7 cM, respectively. Compared with other lines with powdery mildew resistance gene(s) on wheat chromosome arm 5DS, Pubing 74 was resistant to all 28 Blumeria graminis f. sp tritici (Bgt) isolates from different wheat-producing regions of northern China. Allelism tests indicated that PmPB74 was not allelic to PmPB3558 or Pm2. Our work showed that PmPB74 is a novel gene with broad resistance to powdery mildew, and hence will be helpful in broadening the genetic basis of powdery mildew resistance in wheat.

     

Evolutionary divergence of the rye <Emphasis Type="Italic">Pm17</Emphasis> and <Emphasis Type="Italic">Pm8</Emphasis> resistance genes reveals ancient diversity

Key message

We have isolated a novel powdery mildew resistance gene in wheat that was originally introgressed from rye. Further analysis revealed evolutionary divergent history of wheat and rye orthologous resistance genes.

Abstract

Wheat production is under constant threat from a number of fungal pathogens, among them is wheat powdery mildew (Blumeria graminis f. sp. tritici). Deployment of resistance genes is the most economical and sustainable method for mildew control. However, domestication and selective breeding have narrowed genetic diversity of modern wheat germplasm, and breeders have relied on wheat relatives for enriching its gene pool through introgression. Translocations where the 1RS chromosome arm was introgressed from rye to wheat have improved yield and resistance against various pathogens. Here, we isolated the Pm17 mildew resistance gene located on the 1RS introgression in wheat cultivar ‘Amigo’ and found that it is an allele or a close paralog of the Pm8 gene isolated earlier from ‘Petkus’ rye. Functional validation using transient and stable transformation confirmed the identity of Pm17. Analysis of Pm17 and Pm8 coding regions revealed an overall identity of 82.9% at the protein level, with the LRR domains being most divergent. Our analysis also showed that the two rye genes are much more diverse compared to the variants encoded by the Pm3 gene in wheat, which is orthologous to Pm17/Pm8 as concluded from highly conserved upstream sequences in all these genes. Thus, the evolutionary history of these orthologous loci differs in the cereal species rye and wheat and demonstrates that orthologous resistance genes can take different routes towards functionally active genes. These findings suggest that the isolation of Pm3/Pm8/Pm17 orthologs from other grass species, additional alleles from the rye germplasm as well as possibly synthetic variants will result in novel resistance genes useful in wheat breeding.

     

Fine mapping of the chromosome 5B region carrying closely linked rust resistance genes <Emphasis Type="Italic">Yr47</Emphasis> and <Emphasis Type="Italic">Lr52</Emphasis> in wheat

Key message

Fine mapping of Yr47 and Lr52 in chromosome arm 5BS of wheat identified close linkage of the marker sun180 to both genes and its robustness for marker-assisted selection was demonstrated.

Abstract

The widely effective and genetically linked rust resistance genes Yr47 and Lr52 have previously been mapped in the short arm of chromosome 5B in two F₃ populations (Aus28183/Aus27229 and Aus28187/Aus27229). The Aus28183/Aus27229 F₃ population was advanced to generate an F₆ recombinant inbred line (RIL) population to identify markers closely linked with Yr47 and Lr52. Diverse genomic resources including flow-sorted chromosome survey sequence contigs representing the orthologous region in Brachypodium distachyon, the physical map of chromosome arm 5BS, expressed sequence tags (ESTs) located in the 5BS6-0.81-1.00 deletion bin and resistance gene analog contigs of chromosome arm 5BS were used to develop markers to saturate the target region. Selective genotyping was also performed using the iSelect 90 K Infinium wheat SNP assay. A set of SSR, STS, gene-based and SNP markers were developed and genotyped on the Aus28183/Aus27229 RIL population. Yr47 and Lr52 are genetically distinct genes that mapped 0.4 cM apart in the RIL population. The SSR marker sun180 co-segregated with Lr52 and mapped 0.4 cM distal to Yr47. In a high resolution mapping population of 600 F₂ genotypes Yr47 and Lr52 mapped 0.2 cM apart and marker sun180 was placed 0.4 cM distal to Lr52. The amplification of a different sun180 amplicon (195 bp) than that linked with Yr47 and Lr52 (200 bp) in 204 diverse wheat genotypes demonstrated its robustness for marker-assisted selection of these genes.

     

Characterization of <Emphasis Type="Italic">Pm59</Emphasis>, a novel powdery mildew resistance gene in Afghanistan wheat landrace PI 181356

Key message

A new powdery mildew resistance gene, designated Pm59, was identified in Afghanistan wheat landrace PI 181356, and mapped in the terminal region of the long arm of chromosome 7A.

Abstract

Powdery mildew, caused by Blumeria graminis f. sp. tritici (Bgt), is an important foliar disease of wheat worldwide. In the Great Plains of the USA, Bgt isolates virulent to widely used powdery mildew resistance genes, such as Pm3a, were previously identified. The objectives of this study were to characterize the powdery mildew resistance gene in Afghanistan landrace PI 181356, which exhibited high resistance to Bgt isolates collected in southern Great Plains, and identify molecular markers for marker-assisted selection. An F₂ population and F_2:3 lines derived from a cross between PI 181356 and OK1059060-126135-3 were used in this study. Genetic analysis indicated that PI 181356 carries a single dominant gene, designated Pm59, in the terminal region of the long arm of chromosome 7A. Pm59 was mapped to an interval between sequence tag site (STS) markers Xmag1759 and Xmag1714 with genetic distances of 0.4 cM distal to Xmag1759 and 5.7 cM proximal to Xmag1714. Physical mapping suggested that Pm59 is in the distal bin 7AL 0.99–1.00. Pm59 is a novel powdery mildew resistance gene, and confers resistance to Bgt isolates collected from the Great Plains and the state of Montana. Therefore, Pm59 can be used to breed powdery mildew-resistant cultivars in these regions. Xmag1759 is ideal for marker-assisted selection of Pm59 in wheat breeding.

     

Identification and marker-assisted transfer of a new powdery mildew resistance gene at the <Emphasis Type="Italic">Pm4</Emphasis> locus in common wheat

Powdery mildew, a wheat (Triticum aestivum L.) foliar disease caused by Blumeria graminis (DC.) E.O. Speer f. sp. tritici, imposes a constant challenge on wheat production in areas with cool or maritime climates. This study was conducted to identify and transfer the resistance gene in the newly identified common wheat accession ‘D29’. Genetic analysis of the F₂ population derived from a cross of D29 with the susceptible elite cultivar Y158 suggested a single dominant gene is responsible for the powdery mildew resistance in this germplasm. This gene was mapped to chromosome 2AL in a region flanked by microsatellite markers Xgdm93 and Xhbg327, and co-segregated with sequence-tagged site (STS) markers Xsts_bcd1231 and TaAetPR5. An allelic test indicated that the D29 gene was allelic to the Pm4 locus. To further evaluate the resistance conferred by this gene and develop new germplasms for breeding, this gene, as well as Pm4a and Pm4b, was transferred to Y158 through backcross and marker-assisted selection. In the resistance spectrum analysis, the D29 gene displayed a resistance spectrum distinguishable from the other Pm4 alleles, including Pm4a, Pm4b, and Pm4c, and thus was designated as Pm4e. The identification of new allelic variation at the Pm4 locus is important for understanding the resistance gene evolution and for breeding wheat cultivars with powdery mildew resistance.     

Development of V chromosome alterations and physical mapping of molecular markers specific to <Emphasis Type="Italic">Dasypyrum villosum</Emphasis>

Wheat-Dasypyrum villosum translocations were induced in the progeny of the amphiploid Triticum durum-D. villosum (AABBVV) by pollen irradiation. The rearranged V genome chromosomes were characterized by genomic/fluorescence in situ hybridization (GISH/FISH) and molecular markers. Twenty wheat-D. villosum translocation chromosomes were selected, including four centric, seven large segments, and nine small segments in a Chinese Spring (CS) background. The four centric translocations were subsequently identified by GISH/FISH and by molecular markers specific to chromosome arms of the Triticeae linkage groups. They were T5DL.4VL, T4BL.7VS, and T4BS.7VL as well as the compensating translocation T7AL.7VS. Using a combination of previously developed V chromosome alterations, 52 translocations or deletions that divided V chromosomes into 42 bins were employed for deletion mapping of molecular markers specific to D. villosum in a wheat background. Ninety-five expressed sequence tag (EST)-sequence-tagged site (STS) and seven SSR markers that were previously reported, as well as 72 STS markers screened in the present study, were physically allocated into 37 of 42 chromosome bins of D. villosum. Multiple loci of EST-STS markers were also mapped using CS nullisomic tetrasomic (NT) and ditelosomic (DT) genetic stocks. Most EST-STS homoeoloci were located on homoeologous chromosomes, suggesting a high degree of homology between the genomes of D. villosum and wheat. Four 4VL-specific markers detected homoeoloci on group 7 chromosomes of wheat, indicating that chromosome 4V of D. villosum shows some affinity to both wheat homoeologous groups 4 and 7. This is the first physical map of D. villosum, which will provide insight into the V genome for molecular breeding.     

<Emphasis Type="Italic">Pm61</Emphasis>: a recessive gene for resistance to powdery mildew in wheat landrace Xuxusanyuehuang identified by comparative genomics analysis

Key message

A single recessive powdery mildew resistance gene Pm61 from wheat landrace Xuxusanyuehuang was mapped within a 0.46-cM genetic interval spanning a 1.3-Mb interval of the genomic region of chromosome arm 4AL.

Abstract

Epidemics of powdery mildew incited by the biotrophic fungus Blumeria graminis f. sp. tritici (Bgt) have caused significant yield reductions in many wheat (Triticum aestivum)-producing regions. Identification of powdery mildew resistance genes is required for sustainable improvement of wheat for disease resistance. Chinese wheat landrace Xuxusanyuehuang was resistant to several Bgt isolates at the seedling stage. Genetic analysis based on the inoculation of Bgt isolate E09 on the F₁, F₂, and F_2:3 populations produced by crossing Xuxusanyuehuang to susceptible cultivar Mingxian 169 revealed that the resistance of Xuxusanyuehuang was controlled by a single recessive gene. Bulked segregant analysis and simple sequence repeat (SSR) mapping placed the gene on chromosome bin 4AL-4-0.80-1.00. Comparative genomics analysis was performed to detect the collinear genomic regions of Brachypodium distachyon, rice, sorghum, Aegilops tauschii, T. urartu, and T. turgidum ssp. dicoccoides. Based on the use of 454 contig sequences and the International Wheat Genome Sequence Consortium survey sequence of Chinese Spring wheat, four EST-SSR and seven SSR markers were linked to the gene. An F₅ recombinant inbred line population derived from Xuxusanyuehuang?×?Mingxian 169 cross was used to develop the genetic linkage map. The gene was localized in a 0.46-cM genetic interval between Xgwm160 and Xicsx79 corresponding to 1.3-Mb interval of the genomic region in wheat genome. This is a new locus for powdery mildew resistance on chromosome arm 4AL and is designated Pm61.

     

Production of wheat-rye substitution lines based on winter rye cultivars with karyotype identification by means of C-banding,GISH, and SSR markers

The study presents a continuation of the research aimed at producing of wheat-rye substitution lines (2n = 42) based on the cross (Triticum aestivum L. × Secale sereale L.) × Triticum aestivum L., and using winter rye cultivars Vyatka and Vietnamskaya Mestnaya. In BC 1 F 5 two lines were identified, having karyotypes in which a pair of homologous wheat chromosomes was substituted by a homeologous pair of rye chromosomes. The chromosome composition of these lines was analyzed using C-banding, GISH, and SSR markers. It was demonstrated that karyotype of each line included a single pair of rye chromosomes and lacked wheat-rye translocations. The rye chromosomes were identified, and the chromosomes of wheat, at which the substitutions occurred, were determined. The lines generated by crosses with rye of Vyatka and Vietnamskaya Mestnaya cultivars were designated 1Rv(1A) and 5Rviet(5A), respectively. Chromosome identification and classification of the lines makes it possible to use them in breeding programs and genetic studies.     

Identification of <Emphasis Type="Italic">Pm58</Emphasis> from <Emphasis Type="Italic">Aegilops tauschii</Emphasis>

Key message

A novel powdery mildew-resistance gene, designated Pm58, was introgressed directly from Aegilops tauschii to hexaploid wheat, mapped to chromosome 2DS, and confirmed to be effective under field conditions. Selectable KASP? markers were developed for MAS.

Abstract

Powdery mildew caused by Blumeria graminis (DC.) f. sp. tritici (Bgt) remains a significant threat to wheat (Triticum aestivum L.) production. The rapid breakdown of race-specific resistance to Bgt reinforces the need to identify novel sources of resistance. The d-genome species, Aegilops tauschii, is an excellent source of disease resistance that is transferrable to T. aestivum. The powdery mildew-resistant Ae. tauschii accession TA1662 (2n?=?2x?=?DD) was crossed directly with the susceptible hard white wheat line KS05HW14 (2n?=?6x?=?AABBDD) followed by backcrossing to develop a population of 96 BC₂F₄ introgression lines (ILs). Genotyping-by-sequencing was used to develop a genome-wide genetic map that was anchored to the Ae. tauschii reference genome. A detached-leaf Bgt assay was used to screen BC₂F_4:6 ILs, and resistance was found to segregate as a single locus (χ?=?2.0, P value?=?0.157). The resistance gene, referred to as Pm58, mapped to chromosome 2DS. Pm58 was evaluated under field conditions in replicated trials in 2015 and 2016. In both years, a single QTL spanning the Pm58 locus was identified that reduced powdery mildew severity and explained 21% of field variation (P value?<?0.01). KASP? assays were developed from closely linked GBS-SNP markers, a refined genetic map was developed, and four markers that cosegregate with Pm58 were identified. This novel source of powdery mildew-resistance and closely linked genetic markers will support efforts to develop wheat varieties with powdery mildew resistance.

     

Molecular mapping of powdery mildew resistance gene <Emphasis Type="Italic">PmSGD</Emphasis> in Chinese wheat landrace Shangeda using RNA-seq with bulk segregant analysis

Wheat powdery mildew, caused by the fungal pathogen Blumeria graminis f. sp. tritici (Bgt), is one of the most devastating diseases of wheat in China and causes serious yield losses. Resistance genes are urgently needed by wheat breeding programs to combat this disease. In the present study, genetic analysis of powdery mildew resistance was conducted on segregated F₂ and F_2:3 populations derived from the cross of Shangeda (providing good resistance to powdery mildew) and Chancellor (susceptible to powdery mildew). The results showed that the resistance of Shangeda to E09 was controlled by a single recessive gene, tentatively designated as PmSGD. In addition, RNA sequencing of the parental lines Shangeda and Chancellor and the corresponding bulked pools derived from homozygous resistant or susceptible F_2:3 lines was implemented to identify single-nucleotide polymorphisms (SNPs). The PmSGD gene was estimated to be located in the 240–250-Mb region of chromosome 7B based on the characteristics of putative SNP loci distributed on 21 wheat chromosomes. Among the developed SNP markers, 17 (57%) markers were linked to PmSGD flanked by SNP2-57 and SNP2-46, with genetic distances of 0.4 and 0.8 cM, respectively. The reaction patterns of Shangeda and cultivars (lines) carrying the Pm5e, Pmhym, mlxbd, and PmTm4 genes to 22 Bgt isolates indicated that PmSGD may be allelic or very closely linked to those genes. All of the SNP loci linked to PmSGD were used to test 38 cultivars with known Pm gene(s), and the results suggested that these SNP loci are useful for pyramiding PmSGD by marker-assisted selection.     

<Emphasis Type="Italic">Pm62</Emphasis>, an adult-plant powdery mildew resistance gene introgressed from <Emphasis Type="Italic">Dasypyrum villosum</Emphasis> chromosome arm 2VL into wheat

Key message

Pm62, a novel adult-plant resistance (APR) gene against powdery mildew, was transferred from D. villosum into common wheat in the form of Robertsonian translocation T2BS.2VL#5.

Abstract

Powdery mildew, which is caused by the fungus Blumeria graminis f. sp. tritici, is a major disease of wheat resulting in substantial yield and quality losses in many wheat production regions of the world. Introgression of resistance from wild species into common wheat has application for controlling this disease. A Triticum durum-Dasypyrum villosum chromosome 2V#5 disomic addition line, N59B-1 (2n?=?30), improved resistance to powdery mildew at the adult-plant stage, which was attributable to chromosome 2V#5. To transfer this resistance into bread wheat, a total of 298 BC₁F₁ plants derived from the crossing between N59B-1 and Chinese Spring were screened by combined genomic in situ hybridization and fluorescent in situ hybridization, 2V-specific marker analysis, and reaction to powdery mildew to confirm that a dominant adult-plant resistance gene, designated as Pm62, was located on chromosome 2VL#5. Subsequently, the 2VL#5 (2D) disomic substitution line (NAU1825) and the homozygous T2BS.2VL#5 Robertsonian translocation line (NAU1823), with normal plant vigor and full fertility, were identified by molecular and cytogenetic analyses of the BC₁F₂ generation. The effects of the T2BS.2VL#5 recombinant chromosome on agronomic traits were also evaluated in the F₂ segregation population. The results suggest that the translocated chromosome may have no distinct effect on plant height, 1000-kernel weight or flowering period, but a slight effect on spike length and seeds per spike. The translocation line NAU1823 has being utilized as a novel germplasm in breeding for powdery mildew resistance, and the effects of the T2BS.2VL#5 recombinant chromosome on yield-related and flour quality characters will be further assessed.

     

Specific features in using SNP markers developed for allopolyploid wheat

In this work, we analyzed 54 domestic cultivars of hexaploid (common) wheat Triticum aestivum L. (AABBDD genome) and accessions of tetraploid wheats of the Timopheevi group (AAGG) and rye Secale cereale (RR) using 21 SNP markers for common wheat. It was demonstrated that application of the SNP markers developed and verified for particular common wheat cultivars in allele-specific PCR analysis of other cultivars with different geographic origins could lead to an incorrect estimation of the similarity between the genotypes tested. The studied SNP markers of common wheat are inappropriate for analyzing genomes of other cereal species, in particular, T. timopheevii wheats and rye S. cereale.     

Molecular cytogenetic characterization of a new wheat-rye 1BL•1RS translocation line expressing superior stripe rust resistance and enhanced grain yield

Main conclusion

A new wheat-rye 1BL?1RS translocation line, with the characteristics of superior stripe rust resistance and high thousand-kernel weight and grain number per spike, was developed and identified from progenies of wheat-rye- Psathyrostachys huashanica trigeneric hybrids.

Abstract

The wheat-rye 1BL?1RS translocation line from Petkus rye has contributed substantially to the world wheat production. However, due to extensive growing of cultivars with disease resistance genes from short arm of rye chromosome 1R and coevolution of pathogen virulence and host resistance, these cultivars successively lost resistance to pathogens. In this study, a new wheat-rye line K13-868, derived from the progenies of wheat-rye-Psathyrostachys huashanica trigeneric hybrids, was identified and analyzed using fluorescence in situ hybridization (FISH), genomic in situ hybridization (GISH), acid polyacrylamide gel electrophoresis (A-PAGE), and molecular markers. Cytological studies indicated that the mean chromosome configuration of K13-868 at meiosis was 2n = 42 = 0.14 I + 18.78 II (ring) + 2.15 II (rod). Sequential FISH and GISH results demonstrated that K13-868 was a compensating wheat-rye 1BL?1RS Robertsonian translocation line. Acid PAGE analysis revealed that clear specific bands of rye 1RS were expressed in K13-868. Simple sequence repeat (SSR) and rye 1RS-specific markers ω-sec-p1/ω-sec-p2 and O-SEC5′-A/O-SEC3′-R suggested that the 1BS arm of wheat had been substituted by the 1RS arm of rye. At the seedling and adult growth stage, compared with its recurrent wheat parent SM51 and six other wheat cultivars containing the 1RS arm in southwestern China, K13-868 showed high levels of resistance to stripe rust (Puccinia striiformis f. sp. tritici, Pst) pathogens prevalent in China, which are virulent to Yr10 and Yr24/Yr26. In addition, K13-868 expresses higher thousand-kernel weight and more grain number per spike than these controls in two growing seasons, suggesting that this line may carry yield-related genes of rye. This translocation line, with significant characteristics of resistance to stripe rust and high thousand-kernel weight and grain number per spike, could be utilized as a valuable germplasm for wheat improvement.

     

Molecular mapping of a new temperature-sensitive gene <Emphasis Type="Italic">LrZH22</Emphasis> for leaf rust resistance in Chinese wheat cultivar Zhoumai 22

Leaf rust, caused by Puccinia triticina, is one of the most widespread diseases in common wheat globally. The Chinese wheat cultivar Zhoumai 22 is highly resistant to leaf rust at the seedling and adult stages. Seedlings of Zhoumai 22 and 36 lines with known leaf rust resistance genes were inoculated with 13 P. triticina races for gene postulation. The leaf rust response of Zhoumai 22 was different from those of the single gene lines. With the objective of identifying and mapping, the new gene(s) for resistance to leaf rust, F₁, F₂ plants and F_2:3 lines from the cross Zhoumai 22/Chinese Spring were inoculated with Chinese P. triticina race FHDQ at the seedling stage. A single dominant gene, tentatively designated LrZH22, conferred resistance. To identify other possible genes in Zhoumai 22, ten P. triticina races avirulent on Zhoumai 22 were used to inoculate 24 F_2:3 lines. The same gene conferred resistance to all ten avirulent races. A total of 1300 simple sequence repeat (SSR) markers and 36 EST markers on 2BS were used to test the parents, and resistant and susceptible bulks. Resistance gene LrZH22 was mapped in the chromosome bin 2BS1-0.53-0.75 and closely linked to six SSR markers (barc183, barc55, gwm148, gwm410, gwm374 and wmc474) and two EST markers (BF202681 and BE499478) on chromosome arm 2BS. The two closest flanking SSR loci were Xbarc55 and Xgwm374 with genetic distances of 2.4 and 4.8 cM from LrZH22, respectively. Six designated genes (Lr13, Lr16, Lr23, Lr35, Lr48 and Lr73) are located on chromosome arm 2BS. In seedling tests, LrZH22 was temperature sensitive, conferring resistance at high temperatures. The reaction pattern of Zhoumai 22 was different from that of RL 4031 (Lr13), RL 6005 (Lr16) and RL 6012 (Lr23), Lr35 and Lr48 are adult-plant resistance genes, and Lr73 is not sensitive to the temperature. Therefore, LrZH22 is likely to be a new leaf rust resistance gene or allele.     

Mapping of stripe rust resistance gene in an <Emphasis Type="Italic">Aegilops caudata</Emphasis> introgression line in wheat and its genetic association with leaf rust resistance

A pair of stripe rust and leaf rust resistance genes was introgressed from Aegilops caudata, a nonprogenitor diploid species with the CC genome, to cultivated wheat. Inheritance and genetic mapping of stripe rust resistance gene in backcross-recombinant inbred line (BC-RIL) population derived from the cross of a wheat–Ae. caudata introgression line (IL) T291-2(pau16060) with wheat cv. PBW343 is reported here. Segregation of BC-RILs for stripe rust resistance depicted a single major gene conditioning adult plant resistance (APR) with stripe rust reaction varying from TR-20MS in resistant RILs signifying the presence of some minor genes as well. Genetic association with leaf rust resistance revealed that two genes are located at a recombination distance of 13%. IL T291-2 had earlier been reported to carry introgressions on wheat chromosomes 2D, 3D, 4D, 5D, 6D and 7D. Genetic mapping indicated the introgression of stripe rust resistance gene on wheat chromosome 5DS in the region carrying leaf rust resistance gene LrAc, but as an independent introgression. Simple sequence repeat (SSR) and sequence-tagged site (STS) markers designed from the survey sequence data of 5DS enriched the target region harbouring stripe and leaf rust resistance genes. Stripe rust resistance locus, temporarily designated as YrAc, mapped at the distal most end of 5DS linked with a group of four colocated SSRs and two resistance gene analogue (RGA)-STS markers at a distance of 5.3 cM. LrAc mapped at a distance of 9.0 cM from the YrAc and at 2.8 cM from RGA-STS marker Ta5DS_2737450, YrAc and LrAc appear to be the candidate genes for marker-assisted enrichment of the wheat gene pool for rust resistance.     

Mapping and validation of a new QTL for adult-plant resistance to powdery mildew in Chinese elite bread wheat line Zhou8425B

Key message

Four QTLs for adult-plant resistance to powdery mildew were mapped in the Zhou8425B/Chinese Spring population, and a new QTL on chromosome 3B was validated in 103 wheat cultivars derived from Zhou8425B.

Abstract

Zhou8425B is an elite wheat (Triticum aestivum L.) line widely used as a parent in Chinese wheat breeding programs. Identification of genes for adult-plant resistance (APR) to powdery mildew in Zhou8425B is of high importance for continued controlling the disease. In the current study, the high-density Illumina iSelect 90K single-nucleotide polymorphism (SNP) array was used to map quantitative trait loci (QTL) for APR to powdery mildew in 244 recombinant inbred lines derived from the cross Zhou8425B/Chinese Spring. Inclusive composite interval mapping identified QTL on chromosomes 1B, 3B, 4B, and 7D, designated as QPm.caas-1BL.1, QPm.caas-3BS, QPm.caas-4BL.2, and QPm.caas-7DS, respectively. Resistance alleles at the QPm.caas-1BL.1, QPm.caas-3BS, and QPm.caas-4BL.2 loci were contributed by Zhou8425B, whereas that at QPm.caas-7DS was from Chinese Spring. QPm.caas-3BS, likely to be a new APR gene for powdery mildew resistance, was detected in all four environments. One SNP marker closely linked to QPm.caas-3BS was transferred into a semi-thermal asymmetric reverse PCR (STARP) marker and tested on 103 commercial wheat cultivars derived from Zhou8425B. Cultivars with the resistance allele at the QPm.caas-3BS locus had averaged maximum disease severity reduced by 5.3%. This STARP marker can be used for marker-assisted selection in improvement of the level of powdery mildew resistance in wheat breeding.

     

Fine mapping of the restorer gene <Emphasis Type="Italic">Rfp3</Emphasis> from an Iranian primitive rye (<Emphasis Type="Italic">Secale cereale</Emphasis> L.)

Key message

A comparative genetics approach allowed to precisely determine the map position of the restorer gene Rfp3 in rye and revealed that Rfp3 and the restorer gene Rfm1 in barley reside at different positions in a syntenic 4RL/6HS segment.

Abstract

Cytoplasmic male sterility (CMS) is a reliable and striking genetic mechanism for hybrid seed production. Breeding of CMS-based hybrids in cereals requires the use of effective restorer genes as an indispensable pre-requisite. We report on the fine mapping of a restorer gene for the Pampa cytoplasm in winter rye that has been tapped from the Iranian primitive rye population Altevogt 14160. For this purpose, we have mapped 41 gene-derived markers to a 38.8 cM segment in the distal part of the long arm of chromosome 4R, which carries the restorer gene. Male fertility restoration was comprehensively analyzed in progenies of crosses between a male-sterile tester genotype and 21 recombinant as well as six non-recombinant BC₄S₂ lines. This approach allowed us to validate the position of this restorer gene, which we have designated Rfp3, on chromosome 4RL. Rfp3 was mapped within a 2.5 cM interval and cosegregated with the EST-derived marker c28385. The gene-derived conserved ortholog set (COS) markers enabled us to investigate the orthology of restorer genes originating from different genetic resources of rye as well as barley. The observed localization of Rfp3 and Rfm1 in a syntenic 4RL/6HS segment asks for further efforts towards cloning of both restorer genes as an option to study the mechanisms of male sterility and fertility restoration in cereals.