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.

Mapping of a recessive gene for all-stage resistance to stripe rust in a wheat line derived from cultivated einkorn (Triticum monococcum L.).

Stripe rust, caused by Puccinia striiformis f. sp. tritici (Pst), is one of the most destructive fungal diseases of wheat. Cultivated einkorn (Triticum monococcum 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 Chinese prevalent 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 SNP array, flanked by markers KASP-1949-2 and KASP-1949-10 within a 3.3 cM genetic interval corresponding to 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 the marker-assisted selection.

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.

Molecular cytogenetic identification and nutritional composition evaluation of newly synthesized Triticum turgidum-Triticum boeoticum amphiploids (AABBAbAb).

Triticum boeoticum Boiss. (AbAb, 2n = 2x = 14) is a wheat-related species with the blue aleurone trait. In this study, 18 synthetic Triticum turgidum-Triticum boeoticum amphiploids were identified, which were derived from crosses between T. boeoticum and T. turgidum. Three probes (Oligo-pTa535, Oligo-pSc119.2, and Oligo-pTa713) for multicolor fluorescence in situ hybridization (mc-FISH) were combined with genomic in situ hybridization (GISH) to identify chromosomal composition. Seven nutritional indices (anthocyanins, protein, total essential amino acids TEAA, Fe, Zn, Mn and Cu) were measured, and the nutritional components of 18 synthetic amphiploids were comprehensively ranked by principal component analysis (PCA). The results showed that all three synthetic amphiploids used for cytological identification contained 42 chromosomes, including 14 A, 14 B, and 14 Ab chromosomes. The average anthocyanin content was 82.830 µg/g to 207.606 µg/g in the whole meal of the 17 blue-grained lines (Syn-ABAb-1 to Syn-ABAb-17), which was obviously higher than that in the yellow-grained line Syn-ABAb-18 (6.346 µg/g). The crude protein content was between 154.406 and 180.517 g/kg, and the EAA content was 40.193-63.558 mg/g. The Fe, Zn, Mn and Cu levels in the 17 blue-grained lines were 60.55 to 97.41 mg/kg, 60.55-97.41 mg/kg, 35.11 to 65.20 mg/kg and 5.74 to 7.22 mg/kg, respectively, which were higher than those in the yellow-grained line. The contribution of the first three principal components reached 84%. The first principal component was mainly anthocyanins, Fe, Zn and Mn. The second principal component contained protein and amino acids, and the third component contained only Cu. The top 5 Triticum turgidum-Triticum boeoticum amphiploids were Syn-ABAb-11, Syn-ABAb-17, Syn-ABAb-5, Syn-ABAb-8 and Syn-ABAb-4. These amphidiploids exhibited the potential to serve as candidates for hybridization with common wheat, as indicated by comprehensive score rankings, toward enhancing the nutritional quality of wheat.

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.

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.

Variation in the tonoplast cadmium transporter heavy metal ATPase 3 (HMA3) homolog gene in Aegilops tauschii.

The functionality of HMA3 is a key determinant controlling Cd accumulation in the shoots and grains of plants. Wild relatives of modern crop plants can serve as sources of valuable genetic variation for various traits. Here, resequencing of HMA3 homoeologous genes from Aegilops tauschii (the donor of the wheat D genome) was carried out to identify natural variation at both the nucleotide and polypeptide levels. HMA3 homoeologs are highly conserved, and 10 haplotypes were revealed based on 19 single nucleotide polymorphisms (eight induced single amino acid residue substitutions, including 2 altered amino acids in transmembrane domains) in 80 widely distributed Ae. tauschii accessions. The results provide genetic resources for low/no Cd concentration wheat improvement.

Haplotype Analysis Sheds Light on the Genetic Evolution of the Powdery Mildew Resistance Locus Pm60 in Triticum Species.

Wheat powdery mildew (Blumeria graminis f. sp. tritici, Bgt, recently clarified as B. graminis s. str.), is one of the most destructive diseases of wheat. Pm60 is a nucleotide-binding leucine-rich repeat (NLR) gene that confers race-specific resistance to Bgt. Allelic variants (Pm60, Pm60a, and Pm60b) were found in Triticum urartu and T. dicoccoides, the wild progenitors of wheat. In the present study, we studied the diversity of the Pm60 locus in a large set of wheat germplasm and found 20 tetraploid wheats harboring the Pm60 alleles, which correspond to three novel haplotypes (HapI-HapIII). HapI (Pm60 allele) and HapII (Pm60a allele) were present in domesticated tetraploid wheats, whereas HapIII (Pm60a allele) was identified in wild tetraploid T. araraticum. A sequence comparison of HapII and HapIII revealed that they differed by three SNPs and a GCC deletion. Results of the phylogenetic analysis revealed that HapII was more closely related to the functional haplotype MlIW172. Infection tests showed that HapII-carrying lines display a partial resistance response to Bgt#GH, while HapI was susceptible. Our results provide insights into the genetic evolution of the Pm60 locus and potential valuable alleles for powdery mildew resistance breeding.

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.

Development and identification of four new synthetic hexaploid wheat lines with solid stems.

Stem solidness is an important agronomic trait for increasing the ability of wheat to resist lodging. In this study, four new synthetic hexaploid wheat with solid stems were developed from natural chromosome doubling of F1 hybrids between a solid-stemmed durum wheat (Triticum turgidum ssp. durum, 2n = 4x = 28, AABB) and four Aegilops tauschii (2n = 2x = 14, DD) accessions. The solid expression of the second internode at the base of the stem was stable for two synthetic hexalpoid wheat Syn-SAU-117 and Syn-SAU-119 grown in both the greenhouse and field. The lodging resistance of four synthetic solid-stem wheats is stronger than that of CS, and Syn-SAU-116 has the strongest lodging resistance, followed by Syn-SAU-119. The paraffin sections of the second internode showed that four synthetic wheat lines had large outer diameters, well-developed mechanical tissues, large number of vascular bundles, and similar anatomical characteristics with solid-stemmed durum wheat. The chromosomal composition of four synthetic hexaploid wheat was identified by FISH (fluorescence in situ hybridization) using Oligo-pSc119.2-1 and Oligo-pTa535-1. At adult stage, all four synthetic hexaploid wheat showed high resistance to mixed physiological races of stripe rust pathogen (CYR31, CYR32, CYR33, CYR34). These synthetic hexaploid wheat lines provide new materials for the improvement of common wheat.

TbMYC4A Is a Candidate Gene Controlling the Blue Aleurone Trait in a Wheat-Triticum boeoticum Substitution Line.

Triticum boeoticum Boiss (AbAb, 2n = 2x = 14) is one of the sources of the blue grain trait controlled by blue aleurone layer 2 (Ba2). However, the underlying genes have not been cloned. In this study, a transcriptomic comparison between a blue-grained wheat-T. boeoticum substitution line and its wheat parent identified 41 unigenes related to anthocyanin biosynthesis and 29 unigenes related to transport. The bHLH transcription factor gene TbMYC4A showed a higher expression level in the blue-grained substitution line. TbMYC4A contained the three characteristic bHLH transcription factor domains (bHLH-MYC_N, HLH and ACT-like) and clustered with genes identified from other wheat lines with the blue grain trait derived from other Triticeae species. TbMYC4A overexpression confirmed that it was a functional bHLH transcription factor. The analysis of a TbMYC4A-specific marker showed that the gene was also present in T. boeoticum and T. monococcum with blue aleurone but absent in other Triticeae materials with white aleurone. These results indicate that TbMYC4A is a candidate gene of Ba2 controlling the blue aleurone trait. The isolation of TbMYC4A is helpful for further clarifying the genetic mechanism of the blue aleurone trait and is of great significance for breeding blue-grained wheat varieties.

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.

Chromosome Stability of Synthetic-Natural Wheat Hybrids.

Primary allopolyploids are not only ideal materials to study species evolution, but also important bridges in incorporating genetic diversity of wild species into crops. Primary allopolyploids typically exhibit chromosome instability that a disadvantage trait in crop breeding. Newly synthesized hexaploid wheat has been widely used in wheat genetics and breeding studies. To better understand the cytological and genetic basis of chromosome instability, this study investigated the chromosomes of a large number of seeds derived from the synthetic wheat SHW-L1 and its hybrids with natural wheat. SHW-L1 exhibited persistent chromosome instability since we observed a high frequent chromosome variation de novo generated from euploid SHW-L1 plants at the 14th generation of selfing (F14). High frequent chromosome variations were also observed in the F2 hybrids and most of the analyzed recombinant inbred lines (RILs) at F14, derived from the cross of SHW-L1 with common wheat variety Chuanmai 32. Chromosome instability was associated with frequent univalency during meiotic metaphase I. The experiment on reciprocal crosses between SHW-L1 and Chuanmai 32 indicated that cytoplasm has not obvious effects on chromosome instability. An analysis on 48 F14 RILs revealed chromosome variation frequency was not associated with the Ph1 alleles from either SHW-L1 or Chuanmai 32, rejecting the hypothesis that chromosome instability was due to the Ph1 role of synthetic wheat. In the analyzed RILs, chromosome instability influences the phenotype uniformity, showing as obvious trait differences among plants within a RIL. However, the analyzed commercial varieties only containing ∼12.5% genomic components of synthetic wheat were chromosomally stable, indicating that chromosome instability caused by synthetic wheat can be effectively overcome by increasing the genetic background of common wheat.

FISH karyotype comparison between Ab- and A-genome chromosomes using oligonucleotide probes.

Triticum boeoticum (2n = 2x = 14, AbAb) contains beneficial traits for common wheat improvement. The discrimination of Ab-genome chromosomes from A-genome chromosomes is an important step in gene transfer from T. boeoticum to common wheat. In this study, fluorescence in situ hybridization (FISH) analysis using nine oligonucleotide probes revealed high divergence between chromosomes of the common wheat germplasm Crocus and T. boeoticum accession G52. The combination of Oligo-pTa535-HM and Oligo-pSc119.2-HM can differentiate Ab and A chromosomes within homologous groups 2, 4, 5, and 6; chromosomes 2Ab and 6Ab can be identified by using (ACT)7, (CTT)7, and (GAA)7. The probes Oligo-pTa713 and (ACT)7 can be utilized for the identification of chromosomes 1Ab and 3Ab, respectively. Probes (CAG)7 and (CAC)7 can be applied in the identification of 7Ab. Moreover, probe combinations consisting of Oligo-pTa535-HM and (AAC)7 with (ACT)7 or (CTT)7 and of Oligo-pTa535-HM and Oligo-pTa713 with (CAC)7 or (CTT)7 will help discriminate the Ab-genome chromosomes of T. boeoticum. These probes are being used as potential markers to select common wheat Crocus-T. boeoticum G52 alien chromosome lines. Moreover, FISH patterns are highly divergent between Ab- and A-genome chromosomes, indicating that obvious chromosome structural variations arose during wheat evolution.

The Resurgence of Introgression Breeding, as Exemplified in Wheat Improvement.

Breeding progress in most crops has relied heavily on the exploitation of variation within the species' primary gene pool, a process which is destined to fail once the supply of novel variants has been exhausted. Accessing a crop's secondary gene pool, as represented by its wild relatives, has the potential to greatly expand the supply of usable genetic variation. The crop in which this approach has been most strongly championed is bread wheat (Triticum aestivum), a species which is particularly tolerant of the introduction of chromosomal segments of exotic origin thanks to the genetic buffering afforded by its polyploid status. While the process of introgression can be in itself cumbersome, a larger problem is that linkage drag and/or imperfect complementation frequently impose a yield and/or quality penalty, which explains the reluctance of breeders to introduce such materials into their breeding populations. Thanks to the development of novel strategies to induce introgression and of genomic tools to facilitate the selection of desirable genotypes, introgression breeding is returning as a mainstream activity, at least in wheat. Accessing variation present in progenitor species has even been able to drive genetic advance in grain yield. The current resurgence of interest in introgression breeding can be expected to result in an increased deployment of exotic genes in commercial wheat cultivars.

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.

Development, identification, and characterization of blue-grained wheat-Triticum boeoticum substitution lines.

Diploid wild einkorn wheat, Triticum boeoticum Boiss (AbAb, 2n = 2x = 14), is a wheat-related species with a blue aleurone layer. In this study, six blue-grained wheat lines were developed from F8 progeny of crosses between common wheat and T. boeoticum. The chromosome constitutions of these lines were characterized by fluorescence in situ hybridization (FISH) using the oligonucleotide probes Oligo-pTa535-1, Oligo-pSc119.2-1, Oligo-pTa71-2, and (AAC)7. Multicolor FISH using Oligo-pTa535-1, Oligo-pSc119.2-1, and Oligo-pTa71-2 identified all 42 common wheat chromosomes, while Oligo-pTa535-1 and (AAC)7 discriminated the 14 chromosomes of T. boeoticum. FISH revealed that all six blue-grained lines were wheat-T. boeoticum 4Ab (4B) disomic substitution lines. The substitution lines were validated by genotyping using the wheat 55 K single nucleotide polymorphism (SNP) array containing 53,063 markers. These 4Ab (4B) substitution lines represent novel germplasm for blue-grained wheat breeding. The FISH probes and SNP markers used here can be applied in the development of blue-grained wheat-Triticum boeoticum translocation lines.

A breeding strategy targeting the secondary gene pool of bread wheat: introgression from a synthetic hexaploid wheat.

KEY MESSAGE: Introgressing one-eighth of synthetic hexaploid wheat genome through a double top-cross plus a two-phase selection is an effective strategy to develop high-yielding wheat varieties. The continued expansion of the world population and the likely onset of climate change combine to form a major crop breeding challenge. Genetic advances in most crop species to date have largely relied on recombination and reassortment within a relatively narrow gene pool. Here, we demonstrate an efficient wheat breeding strategy for improving yield potentials by introgression of multiple genomic regions of de novo synthesized wheat. The method relies on an initial double top-cross (DTC), in which one parent is synthetic hexaploid wheat (SHW), followed by a two-phase selection procedure. A genotypic analysis of three varieties (Shumai 580, Shumai 969 and Shumai 830) released from this program showed that each harbors a unique set of genomic regions inherited from the SHW parent. The first two varieties were generated from very small populations, whereas the third used a more conventional scale of selection since one of bread wheat parents was a pre-breeding material. The three varieties had remarkably enhanced yield potential compared to those developed by conventional breeding. A widely accepted consensus among crop breeders holds that introducing unadapted germplasm, such as landraces, as parents into a breeding program is a risky proposition, since the size of the breeding population required to overcome linkage drag becomes too daunting. However, the success of the proposed DTC strategy has demonstrated that novel variation harbored by SHWs can be accessed in a straightforward, effective manner. The strategy is in principle generalizable to any allopolyploid crop species where the identity of the progenitor species is known.