Wheat Chromosome 5A Research Articles

Development and Molecular Cytogenetic Identification of a New Wheat-Psathyrostachys huashanica Keng Translocation Line Resistant to Powdery Mildew.

Psathyrostachys huashanica Keng, a wild relative of common wheat with many desirable traits, is an invaluable source of genetic material for wheat improvement. Few wheat–P. huashanica translocation lines resistant to powdery mildew have been reported. In this study, a wheat–P. huashanica line, E24-3-1-6-2-1, was generated via distant hybridization, ethyl methanesulfonate (EMS) mutagenesis, and backcross breeding. A chromosome karyotype of 2n = 44 was observed at the mitotic stage in E24-3-1-6-2-1. Genomic in situ hybridization (GISH) analysis revealed four translocated chromosomes in E24-3-1-6-2-1, and P. huashanica chromosome-specific marker analysis showed that the alien chromosome fragment was from the P. huashanica 4Ns chromosome. Moreover, fluorescence in situ hybridization (FISH) analysis demonstrated that reciprocal translocation had occurred between the P. huashanica 4Ns chromosome and the wheat 3D chromosome; thus, E24-3-1-6-2-1 carried two translocations: T3DS·3DL-4NsL and T3DL-4NsS. Translocation also occurred between wheat chromosomes 2A and 4A. At the adult stage, E24-3-1-6-2-1 was highly resistant to powdery mildew, caused by prevalent pathotypes in China. Further, the spike length, numbers of fertile spikelets, kernels per spike, thousand-kernel weight, and grain yield of E24-3-1-6-2-1 were significantly higher than those of its wheat parent 7182 and addition line 24-6-3-1. Thus, this translocation line that is highly resistant to powdery mildew and has excellent agronomic traits can be used as a novel promising germplasm for breeding resistant and high-yielding cultivars.

Evaluation of a Novel Molecular Marker Associated with the Tan Spot Disease Response in Wheat

After nearly 40 years of DNA molecular marker development in plant breeding, the wheat research community has amassed an extensive collection of molecular markers which have been widely and successfully used for selection of agronomic, physiological and disease resistance traits in wheat breeding programs. Tan spot is a major fungal disease of wheat and a significant global economic challenge and is caused by the necrotrophic fungal pathogen Pyrenophora tritici-repentis (Ptr). Here, the potential for using a PCR-based marker (Ta1AS3422) present on the short arm of wheat chromosome 1A, was evaluated for effectiveness in distinguishing tan spot disease susceptibility. The marker was initially screened against 40 commercial Australian hexaploid wheat varieties, and those that amplified the marker had an overall lower disease score (2.8 ± 0.7 for seedlings and 2.4 ± 0.4 for plants at the tillering stage), compared to those lacking the marker which exhibited a higher disease score (3.6 ± 0.8 for both growth stages). The potential of Ta1AS3422 as a marker for the tan spot disease response was further assessed against a panel of 100 commercial Australian hexaploid wheat varieties. A significant association was observed between marker absence/presence and tan spot disease rating (Pearson’s chi-squared test, χ2 (6) = 20.53, p = 0.002), with absence of Ta1AS3422 associated with susceptibility. This simple and cost-effective PCR-based marker may be useful for varietal improvement against tan spot, although further work is required to validate its effectiveness.

Transfer of fusarium head blight resistance from Thinopyrum elongatum to bread wheat cultivar Chinese Spring

The diploid form of tall wheatgrass, Thinopyrum elongatum (Host) D.R. Dewey (2n = 2x = 14, EE genome), has a high level of resistance to fusarium head blight. The symptoms did not spread beyond the inoculated florets following point inoculation. Using a series of E-genome chromosome additions in a bread wheat cultivar Chinese Spring (CS) background, the resistance was found to be localized to the long arm of chromosome 7E. The CS mutant ph1b was used to induce recombination between chromosome 7E, present in the 7E(7D) substitution and homoeologous wheat chromosomes. Multivalent chromosome associations were detected in the BC1 hybrids, confirming the effectiveness of the ph1b mutant. Genetic markers specific for chromosome 7E were used to estimate the size of the 7E introgression in the wheat genome. Using single sequence repeat (SSR) markers specific for homoeologous wheat chromosome 7, introgressions were detected on wheat chromosomes 7A, 7B, and 7D. Some of the introgression lines were resistant to fusarium head blight.

Discovery of a susceptibility factor for Fusarium head blight on chromosome 7A of wheat.

Discovery and mapping of a susceptibility factor located on the short arm of wheat chromosome 7A whose deletion makes plants resistant to Fusarium head blight. Fusarium head blight (FHB) disease of wheat caused by Fusarium spp. deteriorates both quantity and quality of the crop. Manipulation of susceptibility factors, the plant genes facilitating disease development, offers a novel and alternative strategy for enhancing FHB resistance in plants. In this study, a major effect susceptibility gene for FHB was identified on the short arm of chromosome 7A (7AS). Nullisomic-tetrasomic lines for homoeologous group-7 of wheat revealed dosage effect of the gene, with tetrasomic 7A being more susceptible than control Chinese Spring wheat, qualifying it as a genuine susceptibility factor. Five chromosome 7A inter-varietal substitution lines and a tetraploid Triticum dicoccoides 7A substitution line showed similar susceptibility as that of Chinese Spring, indicating toward the commonality of the susceptibility factor among these diverse genotypes. The susceptibility factor was named as Sf-Fhb-7AS and mapped on chromosome 7AS to a 48.5-50.5Mb peri-centromeric region between del7AS-3 and del7AS-8. Our results showed that deletion of Sf-Fhb-7AS imparts 50-60% type 2 FHB resistance and its manipulation can be used to enhance resistance against FHB in wheat.

Chromosomal composition analysis and molecular marker development for the novel Ug99-resistant wheat-Thinopyrum ponticum translocation line WTT34.

A novel Ug99-resistant wheat-Thinopyrum ponticum translocation line was produced, its chromosomal composition was analyzed and specific markers were developed. Stem rust caused by Puccinia graminis f. sp. tritici Eriks. & E. Henn (Pgt) has seriously threatened global wheat production since Ug99 race TTKSK was first detected in Uganda in 1998. Thinopyrum ponticum is near immune to Ug99 races and may be useful for enhancing wheat disease resistance. Therefore, developing new wheat-Th. ponticum translocation lines that are resistant to Ug99 is crucial. In this study, a novel wheat-Th. ponticum translocation line, WTT34, was produced. Seedling and field evaluation revealed that WTT34 is resistant to Ug99 race PTKST. The resistance was derived from the alien parent Th. ponticum. Screening WTT34 with markers linked to Sr24, Sr25, Sr26, Sr43, and SrB resulted in the amplification of different DNA fragments from Th. ponticum, implying WTT34 carries at least one novel stem rust resistance gene. Genomic in situ hybridization (GISH), multicolor fluorescence in situ hybridization (mc-FISH), and multi-color GISH (mc-GISH) analyses indicated that WTT34 carries a T5DS·5DL-Th translocation, which was consistent with wheat660K single-nucleotide polymorphism (SNP) array results. The SNP array also uncovered a deletion event in the terminal region of chromosome 1D. Additionally, the homeology between alien segments and the wheat chromosomes 2A and 5D was confirmed. Furthermore, 51 PCR-based markers derived from the alien segments of WTT34 were developed based on specific-locus amplified fragment sequencing (SLAF-seq). These markers may enable wheat breeders to rapidly trace Th. ponticum chromosomal segments carrying Ug99 resistance gene(s).

Development and use of miRNA-derived SSR markers for the study of genetic diversity, population structure, and characterization of genotypes for breeding heat tolerant wheat varieties.

Heat stress is an important abiotic factor that limits wheat production globally, including south-east Asia. The importance of micro (mi) RNAs in gene expression under various biotic and abiotic stresses is well documented. Molecular markers, specifically simple sequence repeats (SSRs), play an important role in the wheat improvement breeding programs. Given the role of miRNAs in heat stress-induced transcriptional regulation and acclimatization, the development of miRNA-derived SSRs would prove useful in studying the allelic diversity at the heat-responsive miRNA-genes in wheat. In the present study, efforts have been made to identify SSRs from 96 wheat heat-responsive miRNA-genes and their characterization using a panel of wheat genotypes with contrasting reactions (tolerance/susceptible) to heat stress. A set of 13 miRNA-derived SSR markers were successfully developed as an outcome. These miRNA-SSRs are located on 11 different common wheat chromosomes (2A, 3A, 3B, 3D, 4D, 5A, 5B, 5D, 6A, 6D, and 7A). Among 13 miRNA-SSRs, seven were polymorphic on a set of 37 selected wheat genotypes. Within these polymorphic SSRs, three makers, namely HT-169j, HT-160a, and HT-160b, were found promising as they could discriminate heat-tolerant and heat-susceptible genotypes. This is the first report of miRNA-SSR development in wheat and their deployment in genetic diversity and population structure studies and characterization of trait-specific germplasm. The study suggests that this new class of molecular makers has great potential in the marker-assisted breeding (MAB) programs targeted at improving heat tolerance and other adaptability or developmental traits in wheat and other crops.

Development of KASP markers for wheat greenbug resistance gene Gb5

AbstractGreenbug [Schizaphis graminum (Rondani)] is a destructive pest of wheat (Triticum aestivum L.) worldwide. Incorporation of wheat greenbug resistance genes in wheat‐breeding pipelines is critical to improve wheat production. The greenbug resistance gene Gb5, originally identified in Triticum speltoides chromosome 7S and transferred to wheat chromosome 7A, confers resistance to several economically important greenbug biotypes. The T. speltoides chromosome segment carrying Gb5 was previously shortened by means of homoeologous recombination induced by the ph1b gene, which minimized linkage drag and made Gb5 a useful gene for improvement of greenbug resistance in wheat. This study characterized the shortened T. speltoides chromosome segment in PI 603919 and developed kompetitive allele‐specific polymerase chain reaction (KASP) markers to facilitate its introgression in wheat breeding. A set of 21 simple sequence repeat (SSR) markers specific to wheat 7AL chromosome were developed to determine the length and location of the T. speltoides chromosome segment. The T. speltoides segment in PI 603919 was estimated to be about 79.5–87.8 Mb in the Chinese Spring reference sequence IWGSC RefSeq v.1.0. A recombinant inbred line (RIL) population derived from a cross between CI 17884, which carries the original wheat‐T. speltoides 7A‐7S translocation chromosome segment, and Bainong 418 was genotyped using the genotyping‐by‐sequencing (GBS) approach and evaluated for responses to greenbug biotype E. Three GBS‐derived single nucleotide polymorphisms (SNPs) on the shortened translocation segment were converted to KASP markers. These KASP markers were evaluated for efficacy to select Gb5 in wheat breeding.

Genetic analysis revealed a quantitative trait loci (QTL2A.K) on short arm of chromosome 2A associated with yellow rust resistance in wheat (Triticum aestivum L.)

Yellow rust, caused by Puccinia striiformis, is one of the most devastating diseases in wheat. A synthetic by elite recombinant inbred line (RIL) population derived from a cross, Botnol/Aegilops squarrosa (666)//Kachu was evaluated for yellow rust resistance in two different environments in Mexico. The population was subjected to DArT-seq analysis for an in-depth genetic characterization. A major effect rust resistance QTL (QTL2A.K) explaining up to 45% phenotypic variance was found to be contributed by Kachee, an elite line of International Maize and Wheat Improvement Center (CIMMYT) Mexico. The QTL2A.K was found to be contributed by a segment of 2NS Chromosome of Triticum ventricosum translocated into the short arm of bread wheat chromosome 2A (QTL2A.K). The position of QTL2A.K was confirmed using T. ventricosum specific primer VENTRIUP-LN2. Identified genomic regions are being introgressed in to the popular but susceptible wheat varieties through marker-assisted breeding for enhancing yellow rust resistance.

High Resolution Melting and Insertion Site-Based Polymorphism Markers for Wheat Variability Analysis and Candidate Genes Selection at Drought and Heat MQTL Loci

The practical use of molecular markers is facilitated by cost-effective detection techniques. In this work, wheat insertion site-based polymorphisms (ISBP) markers were set up for genotyping using high-resolution melting analysis (HRM). Polymorphic HRM-ISBP assays were developed for wheat chromosomes 4A and 3B and used for wheat variability assessment. The marker sequences were mapped against the wheat genome reference sequence, targeting interesting genes. Those genes were located within or in proximity to previously described quantitative trait loci (QTL) or meta-quantitative trait loci (MQTL) for drought and heat stress tolerance, and also yield and yield related traits. Eighteen of the markers used tagged drought related genes and, interestingly, eight of the genes were differentially expressed under different abiotic stress conditions. These results confirmed HRM as a cost-effective and efficient tool for wheat breeding programs.

The MYB transcription factor TaPHR3-A1 is involved in phosphate signaling and governs yield-related traits in bread wheat.

Improved inorganic phosphate (Pi) use efficiency in crops will be important for sustainable agriculture. Exploring molecular mechanisms that regulate Pi uptake could provide useful information for breeding wheat with improved Pi use efficiency. Here, a TaPHR3-A1 (Gene ID: TraesCS7A02G415800) ortholog of rice OsPHR3 that functions in transcriptional regulation of Pi signaling was cloned from wheat chromosome 7A. Ectopic expression of TaPHR3-A1 in Arabidopsis and rice produced enhanced vegetative growth and more seeds. Overexpression in transgenic rice led to increased biomass, grain number, and primary panicle branching by 61.23, 42.12, and 36.34% compared with the wild type. Transgenic wheat lines with down-regulation of TaPHR3-A1 exhibited retarded growth and root hair development at the seedling stage, and showed yield-related effects at the adult stage when grown in both low- and sufficient Pi conditions, indicating that TaPHR3-A1 positively regulated tolerance to low Pi. Introgression lines further confirmed the effect of TaPHR3-A1 in improving grain number. The Chinese wheat mini core collection and a recombinant inbred line analysis demonstrated that the favorable allele TaPHR3-A1-A associated with higher grain number was positively selected in breeding. A TaPHR3-A1-derived cleaved amplified polymorphic sequence marker effectively identified haplotype TaPHR3-A1-A. Our results suggested that TaPHR3-A1 was a functional regulatory factor for Pi uptake and provided useful information for marker-assisted selection for high yield in wheat.

Introgression of powdery mildew resistance from Aegilops triuncialis into wheat through induced homeologous pairing

Powdery mildew is a serious fungal disease of wheat caused by Blumeria graminis f. sp. tritici. Chromosome 5U of Aegilops triuncialis carrying powdery mildew resistance was induced to recombine with chromosome 5A of wheat using ph1b deletion line. The resulting recombinant progeny lines were tested for resistance and screened with molecular markers. Among 367 progeny lines 8 were immune to powdery mildew, 53 were moderately resistant, 231 moderately susceptible and 75 highly susceptible. Screening of 5U/5A recombinant lines with 5A specific SSR markers indicated that a major powdery mildew resistance gene from A. triuncialis was introgressed on the long arm of wheat chromosome 5A, along with two other loci, one located in the terminal end of the same arm and the other on the short arm of 5A. Meiotic metaphase-I pairing indicated that chromosome pairing in the resistant recombinant lines was regular with 21 bivalents and high seed set. The DNA markers associated with the introgressions can be used in marker assisted selection for improved powdery mildew resistance in wheat.

Transfer of grain softness from 5U-5A wheat-Aegilops triuncialis substitution line to bread wheat through induced homeologous pairing

Bread wheat sustains genes for grain softness on “Ha” locus of short arm of 5D chromosome. Pina-D1 and Pinb-D1 alleles of “Ha” locus together are responsible for soft grain texture of wheat. The wheat cultivar WL711 had wild type Pinb-D1a and mutated Pina allele, imparting hard kernel texture. A 5U-5A substitution line of Aegilops triuncialis had extra soft grains. Efforts were made to transfer puroindolines from Aegilops triuncialis 5U as 5U-5A substitution line BTC17 to its homeologous chromosome 5A of bread wheat. A total of 367 5U-5A substitution recombinants developed through induced homeologous pairing by ph1b deletion of Ph1 locus were screened for the presence of Pina and Pinb. Only 23 lines were found to have both Pina and Pinb. Grain hardness index of recombinants with functional puroindolines was reduced from 6 to 67%. Deletion mapping of wheat 5A chromosome specific SSR markers revealed the maintenance of synteny in 5U and 5A homeologous chromosomes and puroindolines were found to be translocated at the telomeric end of 5A short arm. Translocation of puroindolines from 5U was confirmed by sequencing of Pina and Pinb genes. The Pina nucleotide sequence was found 100% similar to Aegilops markgrafii (CC). Puroindoline proteins extracted from the selected softer lines exhibited significant antimicrobial activity against gram positive and gram negative bacteria. Chromosome 5U puroindolines transferred to 5A may be pyramided with 5D of soft wheat to develop extra soft varieties for superior biscuit making quality.

Comparative analysis of chromosome 2A molecular organization in diploid and hexaploid wheat

Diploid A genome wheat species harbor immense genetic variability which has been targeted and proven useful in wheat improvement. Development and deployment of sequence-based markers hasopened avenues for comparative analysis, gene transfer and marker assisted selection (MAS) using high throughput cost effective genotyping techniques. Chromosome 2A of wheat is known to harbor several economically important genes. The present study aimed at identification of genic sequences corresponding to full length cDNAs and mining of SSRs and ISBPs from 2A draft sequence assembly of hexaploid wheat cv. Chinese Spring for marker development. In total, 1029 primer pairs including 478 gene derived, 501 SSRs and 50 ISBPs were amplified in diploid A genome species Triticum monococcum and T. boeoticum identifying 221 polymorphic loci. Out of these, 119 markers were mapped onto a pre-existing chromosome 2A genetic map consisting of 42 mapped markers. The enriched genetic map constituted 161 mapped markers with final map length of 549.6 cM. Further, 2A genetic map of T. monococcum was anchored to the physical map of 2A of cv. Chinese Spring which revealed several rearrangements between the two species. The present study generated a highly saturated genetic map of 2A and physical anchoring of genetically mapped markers revealed a complex genetic architecture of chromosome 2A that needs to be investigated further.

Genetic mapping using a wheat multi-founder population reveals a locus on chromosome 2A controlling resistance to both leaf and glume blotch caused by the necrotrophic fungal pathogen Parastagonospora nodorum

Key messageA locus on wheat chromosome 2A was found to control field resistance to both leaf and glume blotch caused by the necrotrophic fungal pathogen Parastagonospora nodorum.The necrotrophic fungal pathogen Parastagonospora nodorum is the causal agent of Septoria nodorum leaf blotch and glume blotch, which are common wheat (Triticum aestivum L.) diseases in humid and temperate areas. Susceptibility to Septoria nodorum leaf blotch can partly be explained by sensitivity to corresponding P. nodorum necrotrophic effectors (NEs). Susceptibility to glume blotch is also quantitative; however, the underlying genetics have not been studied in detail. Here, we genetically map resistance/susceptibility loci to leaf and glume blotch using an eight-founder wheat multiparent advanced generation intercross population. The population was assessed in six field trials across two sites and 4 years. Seedling infiltration and inoculation assays using three P. nodorum isolates were also carried out, in order to compare quantitative trait loci (QTL) identified under controlled conditions with those identified in the field. Three significant field resistance QTL were identified on chromosomes 2A and 6A, while four significant seedling resistance QTL were detected on chromosomes 2D, 5B and 7D. Among these, QSnb.niab-2A.3 for field resistance to both leaf blotch and glume blotch was detected in Norway and the UK. Colocation with a QTL for seedling reactions against culture filtrate from a Norwegian P. nodorum isolate indicated the QTL could be caused by a novel NE sensitivity. The consistency of this QTL for leaf blotch at the seedling and adult plant stages and culture filtrate infiltration was confirmed by haplotype analysis. However, opposite effects for the leaf blotch and glume blotch reactions suggest that different genetic mechanisms may be involved.

Genome-wide identification, characterization, and expression profiling of SPX gene family in wheat

The SPX gene family, ubiquitous in all vascular plants, plays a critical role in plant development and growth as well as in response to phosphorus stress. Based on genomic census, 46 TaSPX genes were identified in the wheat genome. All of them are evenly distributed on 13 of the 21 wheat chromosomes and chromosome 7A contains the largest members. As many as 57 gene specific SSRs were discovered among genomic sequences of identified TaSPXs. MicroRNA target analysis revealed that TaSPX genes were targeted by 9 different miRNAs including tae-miR1120a, tae-miR1120b-3p, tae-miR1120c-5p, tae-miR1122b-3p, tae-miR1122c-3p, tae-miR1130a, tae-miR1130b-3p, tae-miR1137a, and tae-miR1137b-5p. Expression profiles derived from transcriptome data and real-time quantitative PCR revealed that TaSPX genes were significantly induced by Pi starvation. The modeled 3D structure of wheat SPX proteins shared high level of homology with template structures, providing information to understand their functions at proteomic level. We have also refined the modeled 3D structures on 10 ns using molecular dynamics simulations for conformational stability. The discovered members of SPX gene family and their targeting miRNAs may provide resource for genetic improvement and promote P use efficiency in cereals.

Genome-wide identification and expression profiling of trihelix gene family under abiotic stresses in wheat

BackgroundThe trihelix gene family is a plant-specific transcription factor family that plays important roles in plant growth, development, and responses to abiotic stresses. However, to date, no systemic characterization of the trihelix genes has yet been conducted in wheat and its close relatives.ResultsWe identified a total of 94 trihelix genes in wheat, as well as 22 trihelix genes in Triticum urartu, 29 in Aegilops tauschii, and 31 in Brachypodium distachyon. We analyzed the chromosomal locations and orthology relations of the identified trihelix genes, and no trihelix gene was found to be located on chromosome 7A, 7B, or 7D of wheat, thereby reflecting the uneven distributions of wheat trihelix genes. Phylogenetic analysis indicated that the 186 identified trihelix proteins in wheat, rice, B. distachyon, and Arabidopsis were clustered into five major clades. The trihelix genes belonging to the same clades usually shared similar motif compositions and exon/intron structural patterns. Five pairs of tandem duplication genes and three pairs of segmental duplication genes were identified in the wheat trihelix gene family, thereby validating the supposition that more intrachromosomal gene duplication events occur in the genome of wheat than in that of other grass species. The tissue-specific expression and differential expression profiling of the identified genes under cold and drought stresses were analyzed by using RNA-seq data. qRT-PCR was also used to confirm the expression profiles of ten selected wheat trihelix genes under multiple abiotic stresses, and we found that these genes mainly responded to salt and cold stresses.ConclusionsIn this study, we identified trihelix genes in wheat and its close relatives and found that gene duplication events are the main driving force for trihelix gene evolution in wheat. Our expression profiling analysis demonstrated that wheat trihelix genes responded to multiple abiotic stresses, especially salt and cold stresses. The results of our study built a basis for further investigation of the functions of wheat trihelix genes and provided candidate genes for stress-resistant wheat breeding programs.

Aegilops tauschii Genome Sequence: A Framework for Meta-analysis of Wheat QTLs.

Numerous quantitative trait loci (QTL) have been mapped in tetraploid and hexaploid wheat and wheat relatives, mostly with simple sequence repeat (SSR) or single nucleotide polymorphism (SNP) markers. To conduct meta-analysis of QTL requires projecting them onto a common genomic framework, either a consensus genetic map or genomic sequence. The latter strategy is pursued here. Of 774 QTL mapped in wheat and wheat relatives found in the literature, 585 (75.6%) were successfully projected onto the Aegilops tauschii pseudomolecules. QTL mapped with SNP markers were more successfully projected (92.2%) than those mapped with SSR markers (66.2%). The QTL were not distributed homogeneously along chromosome arms. Their frequencies increased in the proximal-to-distal direction but declined in the most distal regions and were weakly correlated with recombination rates along the chromosome arms. Databases for projected SSR markers and QTL were constructed and incorporated into the Ae. tauschii JBrowse. To facilitate meta-QTL analysis, eight clusters of QTL were used to estimate standard deviations () of independently mapped QTL projected onto the Ae. tauschii genome sequence. The standard deviations were modeled as an exponential decay function of recombination rates along the Ae. tauschii chromosomes. We implemented four hypothesis tests for determining the membership of query QTL. The hypothesis tests and estimation procedure for were implemented in a web portal for meta-analysis of projected QTL. Twenty-one QTL for Fusarium head blight resistance mapped on wheat chromosomes 3A, 3B, and 3D were analyzed to illustrate the use of the portal for meta-QTL analyses.

A genetic system on chromosome arm 1BL of wild emmer causes distorted segregation in common wheat

Nonrandom segregation ratios of alleles 'segregation distortion' can have a striking impact on transmission genetics, and with widespread availability of genetic markers has been shown to be a frequent phenomenon. To investigate the possible effect of genetic interaction on segregation distortion and genetic map construction, the segregation and mapping of genetic markers locatedon wheat chromosomes 1A and 1B were followed in four recombinant substitution line (RSL) populations, produced using four chromosome-arm substitution lines (CASLs 1AS, 1AL, 1BS and 1BL) of wild emmer (Triticum turgidum var. dicoccoides, accession TTD140) in the background of the common wheat (T. aestivum) cultivar Bethlehem (BLH), each crossed to BLH itself. Using these four RSL populations, four genetic maps of chromosome 1 arms were constructed. A total of 22 genetic markers representing 19 loci were assigned to chromosome 1A, and 32 markers representing 30 loci were assigned to 1B. For chromosome 1B, two linkage maps were also constructed using RFLP data of an F2 population derived from the same cross combination as the RSLs. The RSL and F2 maps varied in genetic distances, but showed the same linear order of DNA markers. Segregation analysis revealed strong selection against BLH alleles on chromosome 1B, skewing the allelic frequency distribution in favour of TTD in both F2 and RSL populations at all marker loci. On the contrary, strong selection against TTD alleles on chromosome 1A was detected for some loci in the BLH × CASL1AL RSLs, and their distribution was significantly skewed to BLH. F2 populations always showed more segregation distortion than the corresponding RSLs. More markers near the region of chromosome 1B shared by both CASL1BS and 1BL (∼55 cM on chromosome 1B across the centromere) showed significantly distorted segregation in the BLH × CASL1BL population than in thecorresponding BLH × CASL1BS populations. Six markers located on chromosome 1A region shared by CASL1AS and 1AL showed significantly distorted segregation in 1AL-RSL, while no marker showed distorted segregation in 1AS-RSL. These results indicated that genetic factor(s) in the centromere region cause the distorted segregation of genetic markers on wheat chromosome 1B.

Silencing of the Wheat Protein Phosphatase 2A Catalytic Subunit TaPP2Ac Enhances Host Resistance to the Necrotrophic Pathogen Rhizoctonia cerealis.

Eukaryotic type 2A protein phosphatases (protein phosphatase 2A, PP2A) consist of a scaffold subunit A, a regulatory subunit B, and a catalytic subunit C. Little is known about the roles of PP2Ac proteins that are involved in plant responses to necrotrophic fungal pathogens. Sharp eyespot, caused by the necrotrophic fungus Rhizoctonia cerealis, is a destructive disease of wheat (Triticum aestivum), an important staple food crop. Here, we isolated TaPP2Ac-4D from wheat, which encodes a catalytic subunit of the heterotrimeric PP2A, and characterized its properties and role in plant defense response to R. cerealis. Based on the sequence alignment of TaPP2Ac-4D with the draft sequences of wheat chromosomes from the International Wheat Genome Sequencing Consortium (IWGSC), it was found that TaPP2Ac-4D gene is located on the long arm of the wheat chromosome 4D and has two homologs assigned on wheat chromosomes 4A and 4B. Sequence and phylogenetic tree analyses revealed that the TaPP2Ac protein is a typical member of the PP2Ac family and belongs to the subfamily II. TaPP2Ac-4B and TaPP2Ac-4D displayed higher transcriptional levels in the R. cerealis-susceptible wheat cultivar Wenmai 6 than those seen in the resistant wheat line CI12633. The transcriptional levels of TaPP2Ac-4B and TaPP2Ac-4D were significantly elevated in wheat R. cerealis after infection and upon H2O2 treatment. Virus-induced gene silencing results revealed that the transcriptional knockdown of TaPP2Ac-4D and TaPP2Ac-4B significantly increased wheat resistance to R. cerealis infection. Meanwhile, the transcriptional levels of certain pathogenesis-related (PR) and reactive oxygen species (ROS)-scavenging enzyme encoding genes were increased in TaPP2Ac-silenced wheat plants. These results suggest that TaPP2Ac-4B and TaPP2Ac-4D negatively regulate defense response to R. cerealis infection possibly through modulation of the expression of certain PR and ROS-scavenging enzyme genes in wheat. This study reveals a novel function of the plant PP2Ac genes in plant immune responses.

Reassessment of the evolution of wheat chromosomes 4A, 5A, and 7B

Key messageComparison of genome sequences of wild emmer wheat and Aegilops tauschii suggests a novel scenario of the evolution of rearranged wheat chromosomes 4A, 5A, and 7B.Past research suggested that wheat chromosome 4A was subjected to a reciprocal translocation T(4AL;5AL)1 that occurred in the diploid progenitor of the wheat A subgenome and to three major rearrangements that occurred in polyploid wheat: pericentric inversion Inv(4AS;4AL)1, paracentric inversion Inv(4AL;4AL)1, and reciprocal translocation T(4AL;7BS)1. Gene collinearity along the pseudomolecules of tetraploid wild emmer wheat (Triticum turgidum ssp. dicoccoides, subgenomes AABB) and diploid Aegilops tauschii (genomes DD) was employed to confirm these rearrangements and to analyze the breakpoints. The exchange of distal regions of chromosome arms 4AS and 4AL due to pericentric inversion Inv(4AS;4AL)1 was detected, and breakpoints were validated with an optical Bionano genome map. Both breakpoints contained satellite DNA. The breakpoints of reciprocal translocation T(4AL;7BS)1 were also found. However, the breakpoints that generated paracentric inversion Inv(4AL;4AL)1 appeared to be collocated with the 4AL breakpoints that had produced Inv(4AS;4AL)1 and T(4AL;7BS)1. Inv(4AS;4AL)1, Inv(4AL;4AL)1, and T(4AL;7BS)1 either originated sequentially, and Inv(4AL;4AL)1 was produced by recurrent chromosome breaks at the same breakpoints that generated Inv(4AS;4AL)1 and T(4AL;7BS)1, or Inv(4AS;4AL)1, Inv(4AL;4AL)1, and T(4AL;7BS)1 originated simultaneously. We prefer the latter hypothesis since it makes fewer assumptions about the sequence of events that produced these chromosome rearrangements.