Mapping of a Recessive Gene for All-Stage Resistance to Stripe Rust in a Wheat Line Derived from Cultivated Einkorn (Triticum monococcum).

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most destructive fungal diseases of wheat. Cultivated einkorn (Triticum monococcum L. ssp. monococcum, 2n = 2x = 14, AmAm), one of the founder crops of agriculture, harbors unexploited genetic sources for wheat improvement. An advanced wheat line, Z15-1949, with 42 chromosomes, selected from the hybrids of Pst-susceptible common wheat cultivar Crocus and resistant T. monococcum accession 10-1, exhibits high resistance to a mixture of the prevalent Chinese Pst races. Genetic analysis on F1, F2, and F2:3 generations of the cross between Z15-1949 and Pst-susceptible common wheat SY95-71 indicated that the resistance of Z15-1949 was conferred by a recessive gene, tentatively designated as YrZ15-1949. This gene was mapped to the short arm of chromosome 7D using the Wheat 55K single nucleotide polymorphism array, flanked by markers KASP-1949-2 and KASP-1949-10 within a 3.3-cM genetic interval corresponding to a 1.12-Mb physical region in the Chinese Spring reference genome V2.0. The gene differs from previously reported Yr genes on 7D based on their physical positions and is probably a novel gene. YrZ15-1949 would be a valuable resource for developing Pst-resistant wheat cultivars, and the linked markers could be used for marker-assisted selection.

Characterization and Mapping of a Rolling Leaf Mutant Allele rlT73 on Chromosome 1BL of Wheat.

Leaf rolling is regarded as an important morphological trait in wheat breeding. Moderate leaf rolling is helpful to keep leaves upright and improve the photosynthesis of plants, leading to increased yield. However, studies on the identification of genomic regions/genes associated with rolling leaf have been reported less frequently in wheat. In this study, a rolling leaf mutant, T73, which has paired spikelets, dwarfism, and delayed heading traits, was obtained from a common wheat landrace through ethyl methanesulfonate mutagenesis. The rlT73 mutation caused an increase in the number of epidermal cells on the abaxial side and the shrinkage of bulliform cells on the adaxial side, leading to an adaxially rolling leaf phenotype. Genetic analysis showed that the rolling leaf phenotype was controlled by a single recessive gene. Further Wheat55K single nucleotide polymorphism array-based bulked segregant analysis and molecular marker mapping delimited rlT73 to a physical interval of 300.29-318.33 Mb on the chromosome arm 1BL in the Chinese Spring genome. We show that a point mutation at the miRNA165/166 binding site of the HD zipper class III transcription factor on 1BL altered its transcriptional level, which may be responsible for the rolling leaf phenotype. Our results suggest the important role of rlT73 in regulating wheat leaf development and the potential of miRNA-based gene regulation for crop trait improvement.

Comparative analysis of Fusarium crown rot resistance in synthetic hexaploid wheats and their parental genotypes.

BACKGROUND: Fusarium crown rot (FCR) is a chronic disease of cereals worldwide. Compared with tetraploid wheat, hexaploid wheat is more resistant to FCR infection. The underlying reasons for the differences are still not clear. In this study, we compared FCR responses of 10 synthetic hexaploid wheats (SHWs) and their tetraploid and diploid parents. We then performed transcriptome analysis to uncover the molecular mechanism of FCR on these SHWs and their parents. RESULTS: We observed higher levels of FCR resistance in the SHWs compared with their tetraploid parents. The transcriptome analysis suggested that multiple defense pathways responsive to FCR infection were upregulated in the SHWs. Notably, phenylalanine ammonia lyase (PAL) genes, involved in lignin and salicylic acid (SA) biosynthesis, exhibited a higher level of expression to FCR infection in the SHWs. Physiological and biochemical analysis validated that PAL activity and SA and lignin contents of the stem bases were higher in SHWs than in their tetraploid parents. CONCLUSION: Overall, these findings imply that improved FCR resistance in SHWs compared with their tetraploid parents is probably related to higher levels of response on PAL-mediated lignin and SA biosynthesis pathways.

Transcriptomic analysis provides insight into the genetic regulation of shade avoidance in Aegilops tauschii.

BACKGROUND: Weeds are not only economically important but also fascinating models for studying the adaptation of species in human-mediated environments. Aegilops tauschii is the D-genome donor species of common wheat but is also a weed that influences wheat production. How shading stress caused by adjacent wheat plants affects Ae. tauschii growth is a fundamental scientific question but is also important in agriculture, such as for weed control and wheat breeding. RESULT: The present study indicated that shade avoidance is a strategy of Ae. tauschii in response to shading stress. Ae. tauschii plants exhibited growth increases in specific organs, such as stem and leaf elongation, to avoid shading. However, these changes were accompanied by sacrificing the growth of other parts of the plants, such as a reduction in tiller number. The two reverse phenotype responses seem to be formed by systemically regulating the expression of different genes. Fifty-six genes involved in the regulation of cell division and cell expansion were found to be downregulated, and one key upstream negative regulator (RPK2) of cell division was upregulated under shading stress. On the other hand, the upregulated genes under shading stress were mainly enriched in protein serine/threonine kinase activity and carbon metabolism, which are associated with cell enlargement, signal transduction and energy supply. The transcription factor WRKY72 may be important in regulating genes in response to shading stress, which can be used as a prior candidate gene for further study on the genetic regulation of shade avoidance. CONCLUSIONS: This study sheds new light on the gene expression changes and molecular processes involved in the response and avoidance of Ae. tauschii to shading stress, which may aid more effective development of shading stress avoidance or cultivars in wheat and other crops in the future.

Introgression of the bread wheat D genome encoded Lr34/Yr18/Sr57/Pm38/Ltn1 adult plant resistance gene into Triticum turgidum (durum wheat).

KEY MESSAGE: Lack of function of a D-genome adult plant resistance gene upon introgression into durum wheat. The wheat Lr34/Yr18/Sr57/Pm38/Ltn1 adult plant resistance gene (Lr34), located on chromosome arm 7DS, provides broad spectrum, partial, adult plant resistance to leaf rust, stripe rust, stem rust and powdery mildew. It has been used extensively in hexaploid bread wheat (AABBDD) and conferred durable resistance for many decades. These same diseases also occur on cultivated tetraploid durum wheat and emmer wheat but transfer of D genome sequences to those subspecies is restricted due to very limited intergenomic recombination. Herein we have introgressed the Lr34 gene into chromosome 7A of durum wheat. Durum chromosome substitution line Langdon 7D(7A) was crossed to Cappelli ph1c, a mutant derivative of durum cultivar Cappelli homozygous for a deletion of the chromosome pairing locus Ph1. Screening of BC1F2 plants and their progeny by KASP and PCR markers, 90 K SNP genotyping and cytology identified 7A chromosomes containing small chromosome 7D fragments encoding Lr34. However, in contrast to previous transgenesis experiments in durum wheat, resistance to wheat stripe rust was not observed in either Cappelli/Langdon 7D(7A) or Bansi durum plants carrying this Lr34 encoding segment due to low levels of Lr34 gene expression.

Identification and fine-mapping of QYrAS286-2BL conferring adult-plant resistance to stripe rust in cultivated emmer wheat.

KEY MESSAGE: A novel major adult-plant stripe rust resistance QTL derived from cultivated emmer wheat was mapped to a 123.6-kb region on wheat chromosome 2BL. Stripe rust, caused by the fungal pathogen Puccinia striiformis f. sp. tritici (Pst), is one of the most devastating diseases of wheat. Identification of new sources of resistance and their utilization in breeding programs is the effectively control strategy. The objective of this study was to identify and genetically characterize the stripe rust resistance derived from the cultivated emmer accession AS286. A recombinant inbred line population, developed from a cross between the susceptible durum wheat line langdon and AS286, was genotyped using the Wheat55K single nucleotide polymorphism array and evaluated in field conditions with a mixture of the prevalent Chinese Pst races (CYR32, CYR33, CYR34, Zhong4, and HY46) and in growth chamber with race CYR34. Three QTLs conferring resistance were mapped on chromosomes 1BS, 2BL, and 5BL, respectively. The QYrAS286-1BS and QYrAS286-2BL were stable with major effects, explaining 12.91% to 18.82% and 11.31% to 31.43% of phenotypic variation, respectively. QYrAS286-5BL was only detected based on growth chamber seedling data. RILs harboring both QYrAS286-1BS and QYrAS286-2BL showed high levels of stripe rust resistance equal to the parent AS286. The QYrAS286-2BL was only detected at the adult-plant stage, which is different from previously named Yr genes and inherited as a single gene. It was further mapped to a 123.6-kb region using KASP markers derived from SNPs identified by bulked segregant RNA sequencing (BSR-Seq). The identified loci enrich our stripe rust resistance gene pool, and the flanking markers developed here could be useful in marker-assisted selection for incorporating QYrAS286-2BL into wheat cultivars.

Cytogenetic Characterization and Molecular Marker Development for a Wheat-T. boeoticum 4Ab (4B) Disomic Substitution Line with Stripe Rust Resistance.

Triticum boeoticum (2n = 2x = 14, AbAb) is an important relative of wheat. This species tolerates many different types of environmental stresses, including drought, salt, and pathogenic infection, and is lower in dietary fiber and higher in antioxidants, protein (15 to 18%), lipids, and trace elements than common wheat. However, the gene transfer rate from this species to common wheat is low, and few species-specific molecular markers are available. In this study, the wheat-T. boeoticum substitution line Z1889, derived from a cross between the common wheat cultivar Crocus and T. boeoticum line G52, was identified using multicolor fluorescence in situ hybridization, multicolor genomic in situ hybridization, and a 55K single-nucleotide polymorphism array. Z1889 was revealed to be a 4Ab (4B) substitution line with a high degree of resistance to stripe rust pathogen strains prevalent in China. In addition, 22 4Ab chromosome-specific molecular markers and 11 T. boeoticum genome-specific molecular markers were developed from 1,145 4Ab chromosome-specific fragments by comparing the sequences generated by specific-length amplified fragment sequencing, with an efficiency of up to 55.0%. Furthermore, the specificity of these markers was verified in four species containing the Ab genome. These markers not only can be used for the detection of the 4Ab chromosome but also provide a basis for molecular marker-assisted, selection-based breeding in wheat.

Pyramiding of Adult-Plant Resistance Genes Enhances All-Stage Resistance to Wheat Stripe Rust.

Stripe rust, caused by Puccinia striiformis f. sp. tritici, is one of the most destructive diseases in wheat production. Pyramiding of adult-plant resistance (APR) genes is a promising strategy to increase durability of resistance. The stripe rust resistance (R) genes Yr18, Yr28, and Yr36 encode different protein families which confer partial resistance to a broad array of P. striiformis f. sp. tritici races. Here, we developed BC3F5 wheat lines representing all possible combinations of Yr18, Yr28, and Yr36 in a genetic background of the highly P. striiformis f. sp. tritici-susceptible wheat line SY95-71 that is widely used in stripe rust analysis. These lines enabled us to accurately evaluate these genes singly and in combination in a common genetic background. The adult plant resistance experiments were analyzed in the field, where stripe rust epidemics occurred frequently. The field results indicated that these partial R genes act additively in enhancing the levels of resistance, and a minimum of two-gene combinations can generate adequate stripe rust resistance. The Yr28 + Yr36 and Yr18 + Yr28 + Yr36 combinations also showed adequate resistance at the seedling stage, implying that APR gene pyramiding can achieve all-stage resistance. Meanwhile, the three genes were simultaneously introduced into elite wheat lines through gene-based marker selection. Elite lines exhibited strong all-stage resistance to stripe rust. This work provides valuable insights and resources for developing durable P. striiformis f. sp. tritici-resistant varieties and for elucidating the regulation mechanism of partial R gene pyramiding.

Identification of the Solid Stem Suppressor Gene Su-TdDof in Synthetic Hexaploid Wheat Syn-SAU-117.

Lodging is one of the most important factors affecting the high and stable yield of wheat worldwide. Solid-stemmed wheat has higher stem strength and lodging resistance than hollow-stemmed wheat does. There are many solid-stemmed varieties, landraces, and old varieties of durum wheat. However, the transfer of solid stem genes from durum wheat is suppressed by a suppressor gene located on chromosome 3D in common wheat, and only hollow-stemmed lines have been created. However, synthetic hexaploid wheat can serve as a bridge for transferring solid stem genes from tetraploid wheat to common wheat. In this study, the F1, F2, and F2:3 generations of a cross between solid-stemmed Syn-SAU-119 and semisolid-stemmed Syn-SAU-117 were developed. A single dominant gene, which was tentatively designated Su-TdDof and suppresses stem solidity, was identified in synthetic hexaploid wheat Syn-SAU-117 by using genetic analysis. By using bulked segregant RNA-seq (BSR-seq) analysis, Su-TdDof was mapped to chromosome 7DS and flanked by markers KASP-669 and KASP-1055 within a 4.53 cM genetic interval corresponding to 3.86 Mb and 2.29 Mb physical regions in the Chinese Spring (IWGSC RefSeq v1.1) and Ae. tauschii (AL8/78 v4.0) genomes, respectively, in which three genes related to solid stem development were annotated. Su-TdDof differed from a previously reported solid stem suppressor gene based on its origin and position. Su-TdDof would provide a valuable example for research on the suppression phenomenon. The flanking markers developed in this study might be useful for screening Ae. tauschii accessions with no suppressor gene (Su-TdDof) to develop more synthetic hexaploid wheat lines for the breeding of lodging resistance in wheat and further cloning the suppressor gene Su-TdDof.

Development and Characterization of Near-Isogenic Lines Derived from Synthetic Wheat Revealing the 2 kb Insertion in the PPD-D1 Gene Responsible for Heading Delay and Grain Number Improvement.

Spikelet number and grain number per spike are two crucial and correlated traits for grain yield in wheat. Photoperiod-1 (Ppd-1) is a key regulator of inflorescence architecture and spikelet formation in wheat. In this study, near-isogenic lines derived from the cross of a synthetic hexaploid wheat and commercial cultivars generated by double top-cross and two-phase selection were evaluated for the number of days to heading and other agronomic traits. The results showed that heading time segregation was conferred by a single incomplete dominant gene PPD-D1, and the 2 kb insertion in the promoter region was responsible for the delay in heading. Meanwhile, slightly delayed heading plants and later heading plants obviously have advantages in grain number and spikelet number of the main spike compared with early heading plants. Utilization of PPD-D1 photoperiod sensitivity phenotype as a potential means to increase wheat yield potential.

Genome-Wide Investigation and Functional Verification of the ZIP Family Transporters in Wild Emmer Wheat.

The zinc/iron-regulated transporter-like protein (ZIP) family has a crucial role in Zn homeostasis of plants. Although the ZIP genes have been systematically studied in many plant species, the significance of this family in wild emmer wheat (Triticum turgidum ssp. dicoccoides) is not yet well understood. In this study, a genome-wide investigation of ZIPs genes based on the wild emmer reference genome was conducted, and 33 TdZIP genes were identified. Protein structure analysis revealed that TdZIP proteins had 1 to 13 transmembrane (TM) domains and most of them were predicted to be located on the plasma membrane. These TdZIPs can be classified into three clades in a phylogenetic tree. They were annotated as being involved in inorganic ion transport and metabolism. Cis-acting analysis showed that several elements were involved in hormone, stresses, grain-filling, and plant development. Expression pattern analysis indicated that TdZIP genes were highly expressed in different tissues. TdZIP genes showed different expression patterns in response to Zn deficiency and that 11 genes were significantly induced in either roots or both roots and shoots of Zn-deficient plants. Yeast complementation analysis showed that TdZIP1A-3, TdZIP6B-1, TdZIP6B-2, TdZIP7A-3, and TdZIP7B-2 have the capacity to transport Zn. Overexpression of TdZIP6B-1 in rice showed increased Zn concentration in roots compared with wild-type plants. The expression levels of TdZIP6B-1 in transgenic rice were upregulated in normal Zn concentration compared to that of no Zn. This work provides a comprehensive understanding of the ZIP gene family in wild emmer wheat and paves the way for future functional analysis and genetic improvement of Zn deficiency tolerance in wheat.

Genome-Wide Survey and Functional Verification of the NAC Transcription Factor Family in Wild Emmer Wheat.

The NAC transcription factor (TF) family is one of the largest TF families in plants, which has been widely reported in rice, maize and common wheat. However, the significance of the NAC TF family in wild emmer wheat (Triticum turgidum ssp. dicoccoides) is not yet well understood. In this study, a genome-wide investigation of NAC genes was conducted in the wild emmer genome and 249 NAC family members (TdNACs) were identified. The results showed that all of these genes contained NAM/NAC-conserved domains and most of them were predicted to be located on the nucleus. Phylogenetic analysis showed that these 249 TdNACs can be classified into seven clades, which are likely to be involved in the regulation of grain protein content, starch synthesis and response to biotic and abiotic stresses. Expression pattern analysis revealed that TdNACs were highly expressed in different wheat tissues such as grain, root, leaves and shoots. We found that TdNAC8470 was phylogenetically close to NAC genes that regulate either grain protein or starch accumulation. Overexpression of TdNAC8470 in rice showed increased grain starch concentration but decreased grain Fe, Zn and Mn contents compared with wild-type plants. Protein interaction analysis indicated that TdNAC8470 might interact with granule-bound starch synthase 1 (TdGBSS1) to regulate grain starch accumulation. Our work provides a comprehensive understanding of the NAC TFs family in wild emmer wheat and establishes the way for future functional analysis and genetic improvement of increasing grain starch content in wheat.

Myb10-D confers PHS-3D resistance to pre-harvest sprouting by regulating NCED in ABA biosynthesis pathway of wheat.

Pre-harvest sprouting (PHS), the germination of grain before harvest, is a serious problem resulting in wheat yield and quality losses. Here, we mapped the PHS resistance gene PHS-3D from synthetic hexaploid wheat to a 2.4 Mb presence-absence variation (PAV) region and found that its resistance effect was attributed to the pleiotropic Myb10-D by integrated omics and functional analyses. Three haplotypes were detected in this PAV region among 262 worldwide wheat lines and 16 Aegilops tauschii, and the germination percentages of wheat lines containing Myb10-D was approximately 40% lower than that of the other lines. Transcriptome and metabolome profiling indicated that Myb10-D affected the transcription of genes in both the flavonoid and abscisic acid (ABA) biosynthesis pathways, which resulted in increases in flavonoids and ABA in transgenic wheat lines. Myb10-D activates 9-cis-epoxycarotenoid dioxygenase (NCED) by biding the secondary wall MYB-responsive element (SMRE) to promote ABA biosynthesis in early wheat seed development stages. We revealed that the newly discovered function of Myb10-D confers PHS resistance by enhancing ABA biosynthesis to delay germination in wheat. The PAV harboring Myb10-D associated with grain color and PHS will be useful for understanding and selecting white grained PHS resistant wheat cultivars.

Identification of a recessive gene YrZ15-1370 conferring adult plant resistance to stripe rust in wheat-Triticum boeoticum introgression line.

KEY MESSAGE: A novel recessive gene YrZ15-1370 derived from Triticum boeoticum confers adult-plant resistance to wheat stripe rust. Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most damaging diseases of wheat globally and resistance is the effectively control strategy. Triticum boeoticum Boiss (T. monococcum L. ssp. aegilopoides, 2n = 2x = 14, AbAb) accession G52 confers a high level of adult-plant resistance against a mixture of the Chinese prevalent Pst races. To transfer the resistance to common wheat, a cross was made between G52 and susceptible common wheat genotype Crocus. A highly resistant wheat-T. boeoticum introgression line Z15-1370 (F5 generation) with 42 chromosomes was selected cytologically and by testing with Pst races. F1, F2, and F2:3 generations of the cross between Z15-1370 and stripe rust susceptible common wheat Mingxian169 were developed. Genetic analysis revealed that the resistance in Z15-1370 was controlled by a single recessive gene, tentatively designated YrZ15-1370. Using the bulked segregant RNA-Seq (BSR-Seq) analysis, YrZ15-1370 was mapped to chromosome 6AL and flanked by markers KASP1370-3 and KASP-1370-5 within a 4.3 cM genetic interval corresponding to 1.8 Mb physical region in the Chinese Spring genome, in which a number of disease resistance-related genes were annotated. YrZ15-1370 differed from previously Yr genes identified on chromosome 6A based on its position and/or origin. The YrZ15-1370 would be a valuable resource for wheat resistance improvement and the flanking markers developed here could be useful tools for marker-assisted selection (MAS) in breeding and further cloning the gene.

A transcriptomic view of the ability of nascent hexaploid wheat to tolerate aneuploidy.

BACKGROUND: In contrast to most animal species, polyploid plant species are quite tolerant of aneuploidy. Here, the global transcriptome of four aneuploid derivatives of a synthetic hexaploid wheat line was acquired, with the goal of characterizing the relationship between gene copy number and transcript abundance. RESULTS: For most of the genes mapped to the chromosome involved in aneuploidy, the abundance of transcripts reflected the gene copy number. Aneuploidy had a greater effect on the strength of transcription of genes mapped to the chromosome present in a noneuploid dose than on that of genes mapped elsewhere in the genome. Overall, changing the copy number of one member of a homeologous set had little effect on the abundance of transcripts generated from the set of homeologs as a whole, consistent with the tolerance of aneuploidy exhibited by allopolyploids, whether in the form of a chromosomal deficit (monosomy) or chromosomal excess (trisomy). CONCLUSIONS: Our findings shed new light on the genetic regulation of homeoallele transcription and contribute to a deeper understanding of allopolyploid genome evolution, with implications for the breeding of polyploid crops.

A durum wheat adult plant stripe rust resistance QTL and its relationship with the bread wheat Yr80 locus.

KEY MESSAGE: A stripe rust resistance QTL in durum wheat maps near the bread wheat Yr80 locus with the latter reduced to 15 candidate genes. Some wheat adult plant resistance (APR) genes provide partial resistance in the later stages of plant development to rust diseases and are an important component in protecting wheat crops from these fungal pathogens. These genes provide protection in both bread wheat and durum wheat. Here, we have mapped APR to wheat stripe rust, caused by the fungal pathogen Puccinia striiformis f. sp. tritici, in a cross between durum cultivars Stewart and Bansi. Two resistance QTLs derived from the Stewart parent were identified in multi-generational field trials. One QTL is located on chromosome 1BL and maps to the previously identified Yr29/Lr46/Sr58/Pm39 multi-pathogen APR locus. The second locus, located on chromosome 3BL, maps near the recently described bread wheat APR gene, Yr80. Fine mapping in durum and bread wheat families shows that the durum 3BL locus and Yr80 are closely located, with the later APR gene reduced to 15 candidate genes present in the Chinese Spring genome sequence. Distorted segregation of the durum 3BL region was observed with the Stewart locus preferentially transmitted through pollen when compared with the equivalent Bansi region.

Karyotype mosaicism in early generation synthetic hexaploid wheats.

It is known that both the number and the structure of somatic chromosomes can vary in early generation hexaploid wheats. The phenomenon is generally assumed to arise as a result of the meiotic instability characteristic of freshly created allopolyploids. Here, an analysis of the somatic karyotype of a set of 33 early generation synthetic hexaploid wheats has revealed that variation, taking the form of sub-chromosomal fragments and inter-chromosomal translocations, can also arise in somatic tissue. A possible explanation for the observations was that karyotypic instability in early generation hexaploid wheat probably occurs not just during sporogenesis, but also in somatic tissue. However, other factors such as the use of nitrous oxide during the experiments could also cause the chromosome variations, and additional experimentation would be required to determine the most likely.

Development and characterization of Triticum turgidum - Aegilops comosa and T. turgidum - Ae. markgrafii amphidiploids.

Aegilops comosa and Ae. markgrafii are diploid progenitors of polyploidy species of Aegilops sharing M and C genomes, respectively. Transferring valuable genes/traits from Aegilops into wheat is an alternative strategy for wheat genetic improvement. The amphidiploids between diploid species of Aegilops and tetraploid wheat can act as bridges to overcome obstacles from direct hybridization and can be developed by the union of unreduced gametes. In this study, we developed seven Triticum turgidum - Ae. comosa and two T. turgidum - Ae. markgrafii amphidiploids. The unreduced gametes mechanisms, including first-division restitution (FDR) and single-division meiosis (SDM), were observed in triploid F1 hybrids of T. turgidum - Ae. comosa (STM) and T. turgidum - Ae. markgrafii (STC). Only FDR was observed in STC hybrids, whereas FDR or both FDR and SDM were detected in the STM hybrids. All seven pairs of M chromosomes of Ae. comosa and C chromosomes of Ae. markgrafii were distinguished by fluorescent in situ hybridization (FISH) probes pSc119.2 and pTa71 combinations with pTa-535 and (CTT)12/(ACT)7, respectively. Meanwhile, the chromosomes of tetraploid wheat and diploid Aegilops parents were distinguished by the same FISH probes. The amphidiploids possessed specific valuable traits such as multiple tillers, large seed size related traits, and stripe rust resistance that could be utilized in the genetic improvement of wheat.

Molecular Marker Based Design for Breeding Wheat Lines with Multiple Resistance and Superior Quality.

There has not been a major wheat stem rust epidemic worldwide since the 1970s, but the emergence of race TTKSK of Puccinia graminis f. sp. tritici in 1998 presented a great threat to the world wheat production. Single disease-resistance genes are usually effective for only several years before the pathogen changes genetically to overcome the resistance. Stripe rust caused by Puccinia striiformis f. sp. tritici (Pst) is one of the most common and persistent wheat diseases worldwide. The development of varieties with multiple resistance is the most economical and effective strategy for preventing stripe rust and stem rust, the two main rust diseases constraining wheat production. Plateau 448 has been widely used in the spring wheat growing region in northwest China, but it has become susceptible to stripe rust and is susceptible to TTKSK. To produce more durable resistance to race TTKSK as well as to stripe rust, four stem rust resistance genes (Sr33, Sr36, Sr-Cad, and Sr43) and three stripe rust resistance genes (Yr5, Yr18, and Yr26) were simultaneously introgressed into Plateau 448 to improve its stem rust (Ug99) and stripe rust resistance using a marker-assisted backcrossing strategy combined with phenotypic selection. We obtained 131 BC1F5 lines that pyramided two to four Ug99 resistance genes and one to two Pst resistance genes simultaneously. Thirteen of these lines were selected for their TTKSK resistance, and all of them exhibited near immunity or high resistance to TTKSK. Among the 131 pyramided lines, 95 showed high resistance to mixed Pst races. Nine lines exhibited not only high resistance to TTKSK and Pst but also better agronomic traits and high-molecular-weight glutenin subunit compositions than Plateau 448.

Genetic mapping of a major QTL promoting homoeologous chromosome pairing in a wheat landrace.

KEY MESSAGE: Common wheat landrace Kaixian-luohanmai carries a gene(s) that promotes homoeologous chromosome pairing. A major QTL responsible for this effect was mapped to chromosome arm 3AL. Polyhaploid hybrids of a Chinese common wheat landrace Kaixian-luohanmai (KL) and related species show increased levels of chromosome pairing. Over 90% of that pairing is between homoeologous arms of wheat chromosomes, with a very strong preference for pairing between homoeologs from genomes A and D. Wheat-rye pairing was also observed at low frequency. Two mapping populations were created from the hybrids of KL with two wheat genotypes top crossed to rye. Mean chiasmata numbers per plant were used as phenotypic data. Wheat 660 K and 15 K SNP arrays, DArT markers and SSR markers were used for genotyping of the top-cross ABDR hybrids. One major QTL, named QPh.sicau-3A, for increased homoeologous pairing was detected on chromosome arm 3AL, and it was responsible for ca. 16% of the total variation. This QTL was located in the interval 696-725 Mb in the Chinese Spring reference genome. SNP markers closely linked with QPh.sicau-3A were converted to KASP markers and validated for marker-assisted selection.