Phosphomannomutase (PMM) is an essential enzyme in eukaryotes. However, little is known about PMM gene and function in crop plants. Here, we report molecular evolutionary and biochemical analysis of PMM genes in bread wheat and related Triticeae species.
Two sets of homoeologous PMM genes (TaPMM-1 and 2) were found in bread wheat, and two corresponding PMM genes were identified in the diploid progenitors of bread wheat and many other diploid Triticeae species. The duplication event yielding PMM-1 and 2 occurred before the radiation of diploid Triticeae genomes. The PMM gene family in wheat and relatives may evolve largely under purifying selection. Among the six TaPMM genes, the transcript levels of PMM-1 members were comparatively high and their recombinant proteins were all enzymatically active. However, PMM-2 homoeologs exhibited lower transcript levels, two of which were also inactive. TaPMM-A1, B1 and D1 were probably the main active isozymes in bread wheat tissues. The three isozymes differed from their counterparts in barley and Brachypodium distachyon in being more tolerant to elevated test temperatures.
Our work identified the genes encoding PMM isozymes in bread wheat and relatives, uncovered a unique PMM duplication event in diverse Triticeae species, and revealed the main active PMM isozymes in bread wheat tissues. The knowledge obtained here improves the understanding of PMM evolution in eukaryotic organisms, and may facilitate further investigations of PMM function in the temperature adaptability of bread wheat.
The patterns of expression of homoeologous genes in hexaploid bread wheat have been intensively studied in recent years, but the interaction between structural genes and their homoeologous regulatory genes remained unclear. The question was as to whether, in an allopolyploid, this interaction is genome-specific, or whether regulation cuts across genomes. The aim of the present study was cloning, sequence analysis, mapping and expression analysis of F3H (flavanone 3-hydroxylase – one of the key enzymes in the plant flavonoid biosynthesis pathway) homoeologues in bread wheat and study of the interaction between F3H and their regulatory genes homoeologues – Rc (red coleoptiles).
PCR-based cloning of F3H sequences from hexaploid bread wheat (Triticum aestivum L.), a wild tetraploid wheat (T. timopheevii) and their putative diploid progenitors was employed to localize, physically map and analyse the expression of four distinct bread wheat F3H copies. Three of these form a homoeologous set, mapping to the chromosomes of homoeologous group 2; they are highly similar to one another at the structural and functional levels. However, the fourth copy is less homologous, and was not expressed in anthocyanin pigmented coleoptiles. The presence of dominant alleles at the Rc-1 homoeologous loci, which are responsible for anthocyanin pigmentation in the coleoptile, was correlated with F3H expression in pigmented coleoptiles. Each dominant Rc-1 allele affected the expression of the three F3H homoeologues equally, but the level of F3H expression was dependent on the identity of the dominant Rc-1 allele present. Thus, the homoeologous Rc-1 genes contribute more to functional divergence than do the structural F3H genes.
The lack of any genome-specific relationship between F3H-1 and Rc-1 implies an integrative evolutionary process among the three diploid genomes, following the formation of hexaploid wheat. Regulatory genes probably contribute more to the functional divergence between the wheat genomes than do the structural genes themselves. This is in line with the growing consensus which suggests that although heritable morphological traits are determined by the expression of structural genes, it is the regulatory genes which are the prime determinants of allelic identity.
Plant and animal methyltransferases are key enzymes involved in DNA methylation at cytosine residues, required for gene expression control and genome stability. Taking advantage of the new sequence surveys of the wheat genome recently released by the International Wheat Genome Sequencing Consortium, we identified and characterized MET1 genes in the hexaploid wheat Triticum aestivum (TaMET1).
Nine TaMET1 genes were identified and mapped on homoeologous chromosome groups 2A/2B/2D, 5A/5B/5D and 7A/7B/7D. Synteny analysis and evolution rates suggest that the genome organization of TaMET1 genes results from a whole genome duplication shared within the grass family, and a second gene duplication, which occurred specifically in the Triticeae tribe prior to the speciation of diploid wheat. Higher expression levels were observed for TaMET1 homoeologous group 2 genes compared to group 5 and 7, indicating that group 2 homoeologous genes are predominant at the transcriptional level, while group 5 evolved into pseudogenes. We show the connection between low expression levels, elevated evolution rates and unexpected enrichment in CG-dinucleotides (CG-rich isochores) at putative promoter regions of homoeologous group 5 and 7, but not of group 2 TaMET1 genes. Bisulfite sequencing reveals that these CG-rich isochores are highly methylated in a CG context, which is the expected target of TaMET1.
We retraced the evolutionary history of MET1 genes in wheat, explaining the predominance of group 2 homoeologous genes and suggest CG-DNA methylation as one of the mechanisms involved in wheat genome dynamics.
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DNA methylation; Evolution; Genome dynamics; CG-rich isochores
Polyploidization is considered one of the main mechanisms of plant genome evolution. The presence of multiple copies of the same gene reduces selection pressure and permits sub-functionalization and neo-functionalization leading to plant diversification, adaptation and speciation. In bread wheat, polyploidization and the prevalence of transposable elements resulted in massive gene duplication and movement. As a result, the number of genes which are non-collinear to genomes of related species seems markedly increased in wheat.
We used new-generation sequencing (NGS) to generate sequence of a Mb-sized region from wheat chromosome arm 3DS. Sequence assembly of 24 BAC clones resulted in two scaffolds of 1,264,820 and 333,768 bases. The sequence was annotated and compared to the homoeologous region on wheat chromosome 3B and orthologous loci of Brachypodium distachyon and rice. Among 39 coding sequences in the 3DS scaffolds, 32 have a homoeolog on chromosome 3B. In contrast, only fifteen and fourteen orthologs were identified in the corresponding regions in rice and Brachypodium, respectively. Interestingly, five pseudogenes were identified among the non-collinear coding sequences at the 3B locus, while none was found at the 3DS locus.
Direct comparison of two Mb-sized regions of the B and D genomes of bread wheat revealed similar rates of non-collinear gene insertion in both genomes with a majority of gene duplications occurring before their divergence. Relatively low proportion of pseudogenes was identified among non-collinear coding sequences. Our data suggest that the pseudogenes did not originate from insertion of non-functional copies, but were formed later during the evolution of hexaploid wheat. Some evidence was found for gene erosion along the B genome locus.
Wheat; BAC sequencing; Homoeologous genomes; Gene duplication; Non-collinear genes; Allopolyploidy
Physical maps employing libraries of bacterial artificial chromosome (BAC) clones are essential for comparative genomics and sequencing of large and repetitive genomes such as those of the hexaploid bread wheat. The diploid ancestor of the D-genome of hexaploid wheat (Triticum aestivum), Aegilops tauschii, is used as a resource for wheat genomics. The barley diploid genome also provides a good model for the Triticeae and T. aestivum since it is only slightly larger than the ancestor wheat D genome. Gene co-linearity between the grasses can be exploited by extrapolating from rice and Brachypodium distachyon to Ae. tauschii or barley, and then to wheat.
We report the use of Ae. tauschii for the construction of the physical map of a large distal region of chromosome arm 3DS. A physical map of 25.4 Mb was constructed by anchoring BAC clones of Ae. tauschii with 85 EST on the Ae. tauschii and barley genetic maps. The 24 contigs were aligned to the rice and B. distachyon genomic sequences and a high density SNP genetic map of barley. As expected, the mapped region is highly collinear to the orthologous chromosome 1 in rice, chromosome 2 in B. distachyon and chromosome 3H in barley. However, the chromosome scale of the comparative maps presented provides new insights into grass genome organization. The disruptions of the Ae. tauschii-rice and Ae. tauschii-Brachypodium syntenies were identical. We observed chromosomal rearrangements between Ae. tauschii and barley. The comparison of Ae. tauschii physical and genetic maps showed that the recombination rate across the region dropped from 2.19 cM/Mb in the distal region to 0.09 cM/Mb in the proximal region. The size of the gaps between contigs was evaluated by comparing the recombination rate along the map with the local recombination rates calculated on single contigs.
The physical map reported here is the first physical map using fingerprinting of a complete Triticeae genome. This study demonstrates that global fingerprinting of the large plant genomes is a viable strategy for generating physical maps. Physical maps allow the description of the co-linearity between wheat and grass genomes and provide a powerful tool for positional cloning of new genes.
Mutational inactivation of plant genes is an essential tool in gene function studies. Plants with inactivated or deleted genes may also be exploited for crop improvement if such mutations/deletions produce a desirable agronomical and/or quality phenotype. However, the use of mutational gene inactivation/deletion has been impeded in polyploid plant species by genetic redundancy, as polyploids contain multiple copies of the same genes (homoeologous genes) encoded by each of the ancestral genomes. Similar to many other crop plants, bread wheat (Triticum aestivum L.) is polyploid; specifically allohexaploid possessing three progenitor genomes designated as 'A', 'B', and 'D'. Recently modified TILLING protocols have been developed specifically for mutation detection in wheat. Whilst extremely powerful in detecting single nucleotide changes and small deletions, these methods are not suitable for detecting whole gene deletions. Therefore, high-throughput methods for screening of candidate homoeologous gene deletions are needed for application to wheat populations generated by the use of certain mutagenic agents (e.g. heavy ion irradiation) that frequently generate whole-gene deletions.
To facilitate the screening for specific homoeologous gene deletions in hexaploid wheat, we have developed a TaqMan qPCR-based method that allows high-throughput detection of deletions in homoeologous copies of any gene of interest, provided that sufficient polymorphism (as little as a single nucleotide difference) amongst homoeologues exists for specific probe design. We used this method to identify deletions of individual TaPFT1 homoeologues, a wheat orthologue of the disease susceptibility and flowering regulatory gene PFT1 in Arabidopsis. This method was applied to wheat nullisomic-tetrasomic lines as well as other chromosomal deletion lines to locate the TaPFT1 gene to the long arm of chromosome 5. By screening of individual DNA samples from 4500 M2 mutant wheat lines generated by heavy ion irradiation, we detected multiple mutants with deletions of each TaPFT1 homoeologue, and confirmed these deletions using a CAPS method. We have subsequently designed, optimized, and applied this method for the screening of homoeologous deletions of three additional wheat genes putatively involved in plant disease resistance.
We have developed a method for automated, high-throughput screening to identify deletions of individual homoeologues of a wheat gene. This method is also potentially applicable to other polyploidy plants.
A cytogenetic map of wheat was constructed using FISH with cDNA probes. FISH markers detected homoeology and chromosomal rearrangements of wild relatives, an important source of genes for wheat improvement.
To transfer agronomically important genes from wild relatives to bread wheat (Triticum aestivum L., 2n = 6x = 42, AABBDD) by induced homoeologous recombination, it is important to know the chromosomal relationships of the species involved. Fluorescence in situ hybridization (FISH) can be used to study chromosome structure. The genomes of allohexaploid bread wheat and other species from the Triticeae tribe are colinear to some extent, i.e., composed of homoeoloci at similar positions along the chromosomes, and with genic regions being highly conserved. To develop cytogenetic markers specific for genic regions of wheat homoeologs, we selected more than 60 full-length wheat cDNAs using BLAST against mapped expressed sequence tags and used them as FISH probes. Most probes produced signals on all three homoeologous chromosomes at the expected positions. We developed a wheat physical map with several cDNA markers located on each of the 14 homoeologous chromosome arms. The FISH markers confirmed chromosome rearrangements within wheat genomes and were successfully used to study chromosome structure and homoeology in wild Triticeae species. FISH analysis detected 1U-6U chromosome translocation in the genome of Aegilops umbellulata, showed colinearity between chromosome A of Ae. caudata and group-1 wheat chromosomes, and between chromosome arm 7S#3L of Thinopyrum intermedium and the long arm of the group-7 wheat chromosomes.
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Using Roche/454 technology, we sequenced the chloroplast genomes of 12 Triticeae species, including bread wheat, barley and rye, as well as the diploid progenitors and relatives of bread wheat Triticum urartu, Aegilops speltoides and Ae. tauschii. Two wild tetraploid taxa, Ae. cylindrica and Ae. geniculata, were also included. Additionally, we incorporated wild Einkorn wheat Triticum boeoticum and its domesticated form T. monococcum and two Hordeum spontaneum (wild barley) genotypes. Chloroplast genomes were used for overall sequence comparison, phylogenetic analysis and dating of divergence times. We estimate that barley diverged from rye and wheat approximately 8–9 million years ago (MYA). The genome donors of hexaploid wheat diverged between 2.1–2.9 MYA, while rye diverged from Triticum aestivum approximately 3–4 MYA, more recently than previously estimated. Interestingly, the A genome taxa T. boeoticum and T. urartu were estimated to have diverged approximately 570,000 years ago. As these two have a reproductive barrier, the divergence time estimate also provides an upper limit for the time required for the formation of a species boundary between the two. Furthermore, we conclusively show that the chloroplast genome of hexaploid wheat was contributed by the B genome donor and that this unknown species diverged from Ae. speltoides about 980,000 years ago. Additionally, sequence alignments identified a translocation of a chloroplast segment to the nuclear genome which is specific to the rye/wheat lineage. We propose the presented phylogeny and divergence time estimates as a reference framework for future studies on Triticeae.
The Protein Disulfide Isomerase (PDI) gene family encodes several PDI and PDI-like proteins containing thioredoxin domains and controlling diversified metabolic functions, including disulfide bond formation and isomerisation during protein folding. Genomic, cDNA and promoter sequences of the three homoeologous wheat genes encoding the "typical" PDI had been cloned and characterized in a previous work. The purpose of present research was the cloning and characterization of the complete set of genes encoding PDI and PDI like proteins in bread wheat (Triticum aestivum cv Chinese Spring) and the comparison of their sequence, structure and expression with homologous genes from other plant species.
Eight new non-homoeologous wheat genes were cloned and characterized. The nine PDI and PDI-like sequences of wheat were located in chromosome regions syntenic to those in rice and assigned to eight plant phylogenetic groups. The nine wheat genes differed in their sequences, genomic organization as well as in the domain composition and architecture of their deduced proteins; conversely each of them showed high structural conservation with genes from other plant species in the same phylogenetic group. The extensive quantitative RT-PCR analysis of the nine genes in a set of 23 wheat samples, including tissues and developmental stages, showed their constitutive, even though highly variable expression.
The nine wheat genes showed high diversity, while the members of each phylogenetic group were highly conserved even between taxonomically distant plant species like the moss Physcomitrella patens. Although constitutively expressed the nine wheat genes were characterized by different expression profiles reflecting their different genomic organization, protein domain architecture and probably promoter sequences; the high conservation among species indicated the ancient origin and diversification of the still evolving gene family. The comprehensive structural and expression characterization of the complete set of PDI and PDI-like wheat genes represents a basis for the functional characterization of this gene family in the hexaploid context of bread wheat.
Nitrogen uptake and the efficient absorption and metabolism of nitrogen are essential elements in attempts to breed improved cereal cultivars for grain or silage production. One of the enzymes related to nitrogen metabolism is glutamine-2-oxoglutarate amidotransferase (GOGAT). Together with glutamine synthetase (GS), GOGAT maintains the flow of nitrogen from NH4+ into glutamine and glutamate, which are then used for several aminotransferase reactions during amino acid synthesis.
The aim of the present work was to identify and analyse the structure of wheat NADH-GOGAT genomic sequences, and study the expression in two durum wheat cultivars characterized by low and high kernel protein content. The genomic sequences of the three homoeologous A, B and D NADH-GOGAT genes were obtained for hexaploid Triticum aestivum and the tetraploid A and B genes of Triticum turgidum ssp. durum. Analysis of the gene sequences indicates that all wheat NADH-GOGAT genes are composed of 22 exons and 21 introns. The three hexaploid wheat homoeologous genes have high conservation of sequence except intron 13 which shows differences in both length and sequence. A comparative analysis of sequences among di- and mono-cotyledonous plants shows both regions of high conservation and of divergence. qRT-PCR performed with the two durum wheat cvs Svevo and Ciccio (characterized by high and low protein content, respectively) indicates different expression levels of the two NADH-GOGAT-3A and NADH-GOGAT-3B genes.
The three hexaploid wheat homoeologous NADH-GOGAT gene sequences are highly conserved – consistent with the key metabolic role of this gene. However, the dicot and monocot amino acid sequences show distinctive patterns, particularly in the transit peptide, the exon 16–17 junction, and the C-terminus. The lack of conservation in the transit peptide may indicate subcellular differences between the two plant divisions - while the sequence conservation within enzyme functional domains remains high. Higher expression levels of NADH-GOGAT are associated with higher grain protein content in two durum wheats.
ERECTA encodes a receptor-like kinase and is proposed as a candidate for determining transpiration efficiency of plants. Two genes homologous to ERECTA in Arabidopsis were identified on chromosomes 6 (TaER2) and 7 (TaER1) of bread wheat (Triticum aestivum L.), with copies of each gene on the A, B and D genomes of wheat. Similar expression patterns were observed for TaER1 and TaER2 with relatively higher expression of TaER1 in flag leaves of wheat at heading (Z55) and grain-filling (Z73) stages. Significant variations were found in the expression levels of both TaER1 and TaER2 in the flag leaves at both growth stages among 48 diverse bread wheat varieties. Based on the expression of TaER1 and TaER2, the 48 wheat varieties could be classified into three groups having high (5 varieties), medium (27 varieties) and low (16 varieties) levels of TaER expression. Significant differences were also observed between the three groups varying for TaER expression for several transpiration efficiency (TE)- related traits, including stomatal density (SD), transpiration rate, photosynthetic rate (A), instant water use efficiency (WUEi) and carbon isotope discrimination (CID), and yield traits of biomass production plant-1 (BYPP) and grain yield plant-1 (GYPP). Correlation analysis revealed that the expression of TaER1 and TaER2 at the two growth stages was significantly and negatively associated with SD (P<0.01), transpiration rate (P<0.05) and CID (P<0.01), while significantly and positively correlated with flag leaf area (FLA, P<0.01), A (P<0.05), WUEi (P<0.05), BYPP (P<0.01) and GYPP (P<0.01), with stronger correlations for TaER1 than TaER2 and at grain-filling stage than at heading stage. These combined results suggested that TaER involved in development of transpiration efficiency -related traits and yield in bread wheat, implying a function for TaER in regulating leaf development of bread wheat and contributing to expression of these traits. Moreover, the results indicate that TaER could be exploitable for manipulating important agronomical traits in wheat improvement.
Plant non-specific lipid transfer proteins (nsLTPs) are encoded by multigene families and possess physiological functions that remain unclear. Our objective was to characterize the complete nsLtp gene family in rice and arabidopsis and to perform wheat EST database mining for nsLtp gene discovery.
In this study, we carried out a genome-wide analysis of nsLtp gene families in Oryza sativa and Arabidopsis thaliana and identified 52 rice nsLtp genes and 49 arabidopsis nsLtp genes. Here we present a complete overview of the genes and deduced protein features. Tandem duplication repeats, which represent 26 out of the 52 rice nsLtp genes and 18 out of the 49 arabidopsis nsLtp genes identified, support the complexity of the nsLtp gene families in these species. Phylogenetic analysis revealed that rice and arabidopsis nsLTPs are clustered in nine different clades. In addition, we performed comparative analysis of rice nsLtp genes and wheat (Triticum aestivum) EST sequences indexed in the UniGene database. We identified 156 putative wheat nsLtp genes, among which 91 were found in the 'Chinese Spring' cultivar. The 122 wheat non-redundant nsLTPs were organized in eight types and 33 subfamilies. Based on the observation that seven of these clades were present in arabidopsis, rice and wheat, we conclude that the major functional diversification within the nsLTP family predated the monocot/dicot divergence. In contrast, there is no type VII nsLTPs in arabidopsis and type IX nsLTPs were only identified in arabidopsis. The reason for the larger number of nsLtp genes in wheat may simply be due to the hexaploid state of wheat but may also reflect extensive duplication of gene clusters as observed on rice chromosomes 11 and 12 and arabidopsis chromosome 5.
Our current study provides fundamental information on the organization of the rice, arabidopsis and wheat nsLtp gene families. The multiplicity of nsLTP types provide new insights on arabidopsis, rice and wheat nsLtp gene families and will strongly support further transcript profiling or functional analyses of nsLtp genes. Until such time as specific physiological functions are defined, it seems relevant to categorize plant nsLTPs on the basis of sequence similarity and/or phylogenetic clustering.
The high level of identity among duplicated homoeologous genomes in tetraploid pasta wheat presents substantial challenges for de novo transcriptome assembly. To solve this problem, we develop a specialized bioinformatics workflow that optimizes transcriptome assembly and separation of merged homoeologs. To evaluate our strategy, we sequence and assemble the transcriptome of one of the diploid ancestors of pasta wheat, and compare both assemblies with a benchmark set of 13,472 full-length, non-redundant bread wheat cDNAs.
A total of 489 million 100 bp paired-end reads from tetraploid wheat assemble in 140,118 contigs, including 96% of the benchmark cDNAs. We used a comparative genomics approach to annotate 66,633 open reading frames. The multiple k-mer assembly strategy increases the proportion of cDNAs assembled full-length in a single contig by 22% relative to the best single k-mer size. Homoeologs are separated using a post-assembly pipeline that includes polymorphism identification, phasing of SNPs, read sorting, and re-assembly of phased reads. Using a reference set of genes, we determine that 98.7% of SNPs analyzed are correctly separated by phasing.
Our study shows that de novo transcriptome assembly of tetraploid wheat benefit from multiple k-mer assembly strategies more than diploid wheat. Our results also demonstrate that phasing approaches originally designed for heterozygous diploid organisms can be used to separate the close homoeologous genomes of tetraploid wheat. The predicted tetraploid wheat proteome and gene models provide a valuable tool for the wheat research community and for those interested in comparative genomic studies.
Transcriptome assembly; multiple k-mer assembly; wheat; polyploid; Triticum urartu; Triticum turgidum; pseudogenes; phasing; gene prediction
Bread wheat (Triticum aestivum) has a large, complex and hexaploid genome consisting of A, B and D homoeologous chromosome sets. Therefore each wheat gene potentially exists as a trio of A, B and D homoeoloci, each of which may contribute differentially to wheat phenotypes. We describe a novel approach combining wheat cytogenetic resources (chromosome substitution ‘nullisomic-tetrasomic’ lines) with next generation deep sequencing of gene transcripts (RNA-Seq), to directly and accurately identify homoeologue-specific single nucleotide variants and quantify the relative contribution of individual homoeoloci to gene expression.
We discover, based on a sample comprising ~5-10% of the total wheat gene content, that at least 45% of wheat genes are expressed from all three distinct homoeoloci. Most of these genes show strikingly biased expression patterns in which expression is dominated by a single homoeolocus. The remaining ~55% of wheat genes are expressed from either one or two homoeoloci only, through a combination of extensive transcriptional silencing and homoeolocus loss.
We conclude that wheat is tending towards functional diploidy, through a variety of mechanisms causing single homoeoloci to become the predominant source of gene transcripts. This discovery has profound consequences for wheat breeding and our understanding of wheat evolution.
Wheat; Wheat transcriptome; mRNA-Seq; Diploidization; Homoeologues; Polyploidy
The endoplasmic reticulum chaperone binding protein (BiP) is an important functional protein, which is involved in protein synthesis, folding assembly, and secretion. In order to study the role of BiP in the process of wheat seed development, we cloned three BiP homologous cDNA sequences in bread wheat (Triticum aestivum), completed by rapid amplification of cDNA ends (RACE), and examined the expression of wheat BiP in wheat tissues, particularly the relationship between BiP expression and the subunit types of HMW-GS using near-isogenic lines (NILs) of HMW-GS silencing, and under abiotic stress.
Sequence analysis demonstrated that all BiPs contained three highly conserved domains present in plants, animals, and microorganisms, indicating their evolutionary conservation among different biological species. Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) revealed that TaBiP (Triticum aestivum BiP) expression was not organ-specific, but was predominantly localized to seed endosperm. Furthermore, immunolocalization confirmed that TaBiP was primarily located within the protein bodies (PBs) in wheat endosperm. Three TaBiP genes exhibited significantly down-regulated expression following high molecular weight-glutenin subunit (HMW-GS) silencing. Drought stress induced significantly up-regulated expression of TaBiPs in wheat roots, leaves, and developing grains.
The high conservation of BiP sequences suggests that BiP plays the same role, or has common mechanisms, in the folding and assembly of nascent polypeptides and protein synthesis across species. The expression of TaBiPs in different wheat tissue and under abiotic stress indicated that TaBiP is most abundant in tissues with high secretory activity and with high proportions of cells undergoing division, and that the expression level of BiP is associated with the subunit types of HMW-GS and synthesis. The expression of TaBiPs is developmentally regulated during seed development and early seedling growth, and under various abiotic stresses.
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Wheat; BiP; Cloning; Expression; HMW-GS silencing; Drought stress
The bread wheat (Triticum aestivum L.) genotype “Chinese Spring” (“CS”) is the reference base in wheat genetics and genomics. Pericentric rearrangements in this genotype were systematically assessed by analyzing homoeoloci for a set of nonredundant genes from Brachypodium distachyon, Triticum urartu, and Aegilops tauschii in the CS chromosome shotgun sequence obtained from individual chromosome arms flow-sorted from CS aneuploid lines. Based on patterns of their homoeologous arm locations, 551 genes indicated the presence of pericentric inversions in at least 10 of the 21 chromosomes. Available data from deletion bin-mapped expressed sequence tags and genetic mapping in wheat indicated that all inversions had breakpoints in the low-recombinant gene-poor pericentromeric regions. The large number of putative intrachromosomal rearrangements suggests the presence of extensive structural differences among the three subgenomes, at least some of which likely occurred during the production of the aneuploid lines of this hexaploid wheat genotype. These differences could have significant implications in wheat genome research where comparative approaches are used such as in ordering and orientating sequence contigs and in gene cloning.
chromosomal rearrangement; comparative genomics; pericentric inversion; pericentromeric regions; translocation; Chinese Spring
Bread wheat is not only an important crop, but its large (17 Gb), highly repetitive, and hexaploid genome makes it a good model to study the organization and evolution of complex genomes. Recently, we produced a high quality reference sequence of wheat chromosome 3B (774 Mb), which provides an excellent opportunity to study the evolutionary dynamics of a large and polyploid genome, specifically the impact of single gene duplications.
We find that 27 % of the 3B predicted genes are non-syntenic with the orthologous chromosomes of Brachypodium distachyon, Oryza sativa, and Sorghum bicolor, whereas, by applying the same criteria, non-syntenic genes represent on average only 10 % of the predicted genes in these three model grasses. These non-syntenic genes on 3B have high sequence similarity to at least one other gene in the wheat genome, indicating that hexaploid wheat has undergone massive small-scale interchromosomal gene duplications compared to other grasses. Insertions of non-syntenic genes occurred at a similar rate along the chromosome, but these genes tend to be retained at a higher frequency in the distal, recombinogenic regions. The ratio of non-synonymous to synonymous substitution rates showed a more relaxed selection pressure for non-syntenic genes compared to syntenic genes, and gene ontology analysis indicated that non-syntenic genes may be enriched in functions involved in disease resistance.
Our results highlight the major impact of single gene duplications on the wheat gene complement and confirm the accelerated evolution of the Triticeae lineage among grasses.
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In the developing endosperm of bread wheat (Triticum aestivum), seed storage proteins are produced on the rough endoplasmic reticulum (ER) and transported to protein bodies, specialized vacuoles for the storage of protein. The functionally important gluten proteins of wheat are transported by two distinct routes to the protein bodies where they are stored: vesicles that bud directly off the ER and transport through the Golgi. However, little is known about the processing of glutenin and gliadin proteins during these steps or the possible impact on their properties. In plants, the RabD GTPases mediate ER-to-Golgi vesicle transport. Available sequence information for Rab GTPases in Arabidopsis, rice, Brachypodium and bread wheat was compiled and compared to identify wheat RabD orthologs. Partial genetic sequences were assembled using the first draft of the Chinese Spring wheat genome. A suitable candidate gene from the RabD clade (TaRabD2a) was chosen for down-regulation by RNA interference (RNAi), and an RNAi construct was used to transform wheat plants. All four available RabD genes were shown by qRT-PCR to be down-regulated in the transgenic developing endosperm. The transgenic grain was found to produce flour with significantly altered processing properties when measured by farinograph and extensograph. SE-HPLC found that a smaller proportion of HMW-GS and large proportion of LMW-GS are incorporated into the glutenin macropolymer in the transgenic dough. Lower protein content but a similar protein profile on SDS-PAGE was seen in the transgenic grain.
Triticum aestivum; endosperm; Rab GTPase; trafficking; gluten; breadmaking
In higher plants, inorganic nitrogen is assimilated via the glutamate synthase cycle or GS-GOGAT pathway. GOGAT enzyme occurs in two distinct forms that use NADH (NADH-GOGAT) or Fd (Fd-GOGAT) as electron carriers. The goal of the present study was to characterize wheat Fd-GOGAT genes and to assess the linkage with grain protein content (GPC), an important quantitative trait controlled by multiple genes.
We report the complete genomic sequences of the three homoeologous A, B and D Fd-GOGAT genes from hexaploid wheat (Triticum aestivum) and their localization and characterization. The gene is comprised of 33 exons and 32 introns for all the three homoeologues genes. The three genes show the same exon/intron number and size, with the only exception of a series of indels in intronic regions. The partial sequence of the Fd-GOGAT gene located on A genome was determined in two durum wheat (Triticum turgidum ssp. durum) cvs Ciccio and Svevo, characterized by different grain protein content. Genomic differences allowed the gene mapping in the centromeric region of chromosome 2A. QTL analysis was conducted in the Svevo×Ciccio RIL mapping population, previously evaluated in 5 different environments. The study co-localized the Fd-GOGAT-A gene with the marker GWM-339, identifying a significant major QTL for GPC.
The wheat Fd-GOGAT genes are highly conserved; both among the three homoeologous hexaploid wheat genes and in comparison with other plants. In durum wheat, an association was shown between the Fd-GOGAT allele of cv Svevo with increasing GPC - potentially useful in breeding programs.
Carotenoids are isoprenoid pigments, essential for photosynthesis and photoprotection in plants. The enzyme phytoene synthase (PSY) plays an essential role in mediating condensation of two geranylgeranyl diphosphate molecules, the first committed step in carotenogenesis. PSY are nuclear enzymes encoded by a small gene family consisting of three paralogous genes (PSY1-3) that have been widely characterized in rice, maize and sorghum.
In wheat, for which yellow pigment content is extremely important for flour colour, only PSY1 has been extensively studied because of its association with QTLs reported for yellow pigment whereas PSY2 has been partially characterized. Here, we report the isolation of bread wheat PSY3 genes from a Renan BAC library using Brachypodium as a model genome for the Triticeae to develop Conserved Orthologous Set markers prior to gene cloning and sequencing. Wheat PSY3 homoeologous genes were sequenced and annotated, unravelling their novel structure associated with intron-loss events and consequent exonic fusions. A wheat PSY3 promoter region was also investigated for the presence of cis-acting elements involved in the response to abscisic acid (ABA), since carotenoids also play an important role as precursors of signalling molecules devoted to plant development and biotic/abiotic stress responses. Expression of wheat PSYs in leaves and roots was investigated during ABA treatment to confirm the up-regulation of PSY3 during abiotic stress.
We investigated the structural and functional determinisms of PSY genes in wheat. More generally, among eudicots and monocots, the PSY gene family was found to be associated with differences in gene copy numbers, allowing us to propose an evolutionary model for the entire PSY gene family in Grasses.
Carotenoids; Phytoene synthase; Wheat; Intron loss; Abiotic stress; Evolution
The ~17 Gb hexaploid bread wheat genome is a high priority and a major technical challenge for genomic studies. In particular, the D sub-genome is relatively lacking in genetic diversity, making it both difficult to map genetically, and a target for introgression of agriculturally useful traits. Elucidating its sequence and structure will therefore facilitate wheat breeding and crop improvement.
We generated shotgun sequences from each arm of flow-sorted Triticum aestivum chromosome 5D using 454 FLX Titanium technology, giving 1.34× and 1.61× coverage of the short (5DS) and long (5DL) arms of the chromosome respectively. By a combination of sequence similarity and assembly-based methods, ~74% of the sequence reads were classified as repetitive elements, and coding sequence models of 1314 (5DS) and 2975 (5DL) genes were generated. The order of conserved genes in syntenic regions of previously sequenced grass genomes were integrated with physical and genetic map positions of 518 wheat markers to establish a virtual gene order for chromosome 5D.
The virtual gene order revealed a large-scale chromosomal rearrangement in the peri-centromeric region of 5DL, and a concentration of non-syntenic genes in the telomeric region of 5DS. Although our data support the large-scale conservation of Triticeae chromosome structure, they also suggest that some regions are evolving rapidly through frequent gene duplications and translocations.
EBI European Nucleotide Archive, Study no. ERP002330
Electronic supplementary material
The online version of this article (doi:10.1186/1471-2164-15-1080) contains supplementary material, which is available to authorized users.
Wheat genome; Chromosome sorting; Triticum aestivum; Genome zipper; Triticeae genome; Chromosome arm shotgun; Comparative grass genomics
A genome-wide assessment of nucleotide diversity in a polyploid species must minimize the inclusion of homoeologous sequences into diversity estimates and reliably allocate individual haplotypes into their respective genomes. The same requirements complicate the development and deployment of single nucleotide polymorphism (SNP) markers in polyploid species. We report here a strategy that satisfies these requirements and deploy it in the sequencing of genes in cultivated hexaploid wheat (Triticum aestivum, genomes AABBDD) and wild tetraploid wheat (Triticum turgidum ssp. dicoccoides, genomes AABB) from the putative site of wheat domestication in Turkey. Data are used to assess the distribution of diversity among and within wheat genomes and to develop a panel of SNP markers for polyploid wheat.
Nucleotide diversity was estimated in 2114 wheat genes and was similar between the A and B genomes and reduced in the D genome. Within a genome, diversity was diminished on some chromosomes. Low diversity was always accompanied by an excess of rare alleles. A total of 5,471 SNPs was discovered in 1791 wheat genes. Totals of 1,271, 1,218, and 2,203 SNPs were discovered in 488, 463, and 641 genes of wheat putative diploid ancestors, T. urartu, Aegilops speltoides, and Ae. tauschii, respectively. A public database containing genome-specific primers, SNPs, and other information was constructed. A total of 987 genes with nucleotide diversity estimated in one or more of the wheat genomes was placed on an Ae. tauschii genetic map, and the map was superimposed on wheat deletion-bin maps. The agreement between the maps was assessed.
In a young polyploid, exemplified by T. aestivum, ancestral species are the primary source of genetic diversity. Low effective recombination due to self-pollination and a genetic mechanism precluding homoeologous chromosome pairing during polyploid meiosis can lead to the loss of diversity from large chromosomal regions. The net effect of these factors in T. aestivum is large variation in diversity among genomes and chromosomes, which impacts the development of SNP markers and their practical utility. Accumulation of new mutations in older polyploid species, such as wild emmer, results in increased diversity and its more uniform distribution across the genome.
Bread wheat (Triticum aestivum) is an important staple food. However, wheat gluten proteins cause celiac disease (CD) in 0.5 to 1% of the general population. Among these proteins, the α-gliadins contain several peptides that are associated to the disease.
We obtained 230 distinct α-gliadin gene sequences from severaldiploid wheat species representing the ancestral A, B, and D genomes of the hexaploid bread wheat. The large majority of these sequences (87%) contained an internal stop codon. All α-gliadin sequences could be distinguished according to the genome of origin on the basis of sequence similarity, of the average length of the polyglutamine repeats, and of the differences in the presence of four peptides that have been identified as T cell stimulatory epitopes in CD patients through binding to HLA-DQ2/8. By sequence similarity, α-gliadins from the public database of hexaploid T. aestivum could be assigned directly to chromosome 6A, 6B, or 6D. T. monococcum (A genome) sequences, as well as those from chromosome 6A of bread wheat, almost invariably contained epitope glia-α9 and glia-α20, but never the intact epitopes glia-α and glia-α2. A number of sequences from T. speltoides, as well as a number of sequences fromchromosome 6B of bread wheat, did not contain any of the four T cell epitopes screened for. The sequences from T. tauschii (D genome), as well as those from chromosome 6D of bread wheat, were found to contain all of these T cell epitopes in variable combinations per gene. The differences in epitope composition resulted mainly from point mutations. These substitutions appeared to be genome specific.
Our analysis shows that α-gliadin sequences from the three genomes of bread wheat form distinct groups. The four known T cell stimulatory epitopes are distributed non-randomly across the sequences, indicating that the three genomes contribute differently to epitope content. A systematic analysis of all known epitopes in gliadins and glutenins will lead to better understanding of the differences in toxicity among wheat varieties. On the basis of such insight, breeding strategies can be designed to generate less toxic varieties of wheat which may be tolerated by at least part of the CD patient population.
Chromosome pairing, recombination and DNA repair are essential processes during meiosis in sexually reproducing organisms. Investigating the bread wheat (Triticum aestivum L.) Ph2 (Pairing homoeologous) locus has identified numerous candidate genes that may have a role in controlling such processes, including TaMSH7, a plant specific member of the DNA mismatch repair family.
Sequencing of the three MSH7 genes, located on the short arms of wheat chromosomes 3A, 3B and 3D, has revealed no significant sequence divergence at the amino acid level suggesting conservation of function across the homoeogroups. Functional analysis of MSH7 through the use of RNAi loss-of-function transgenics was undertaken in diploid barley (Hordeum vulgare L.). Quantitative real-time PCR revealed several T0 lines with reduced MSH7 expression. Positive segregants from two T1 lines studied in detail showed reduced MSH7 expression when compared to transformed controls and null segregants. Expression of MSH6, another member of the mismatch repair family which is most closely related to the MSH7 gene, was not significantly reduced in these lines. In both T1 lines, reduced seed set in positive segregants was observed.
Results presented here indicate, for the first time, a distinct functional role for MSH7 in vivo and show that expression of this gene is necessary for wild-type levels of fertility. These observations suggest that MSH7 has an important function during meiosis and as such remains a candidate for Ph2.
Both single gene and whole genome duplications (WGD) have recurred in angiosperm evolution. However, the evolutionary effects of different modes of gene duplication, especially regarding their contributions to genetic novelty or redundancy, have been inadequately explored.
In Arabidopsis thaliana and Oryza sativa (rice), species that deeply sample botanical diversity and for which expression data are available from a wide range of tissues and physiological conditions, we have compared expression divergence between genes duplicated by six different mechanisms (WGD, tandem, proximal, DNA based transposed, retrotransposed and dispersed), and between positional orthologs. Both neo-functionalization and genetic redundancy appear to contribute to retention of duplicate genes. Genes resulting from WGD and tandem duplications diverge slowest in both coding sequences and gene expression, and contribute most to genetic redundancy, while other duplication modes contribute more to evolutionary novelty. WGD duplicates may more frequently be retained due to dosage amplification, while inferred transposon mediated gene duplications tend to reduce gene expression levels. The extent of expression divergence between duplicates is discernibly related to duplication modes, different WGD events, amino acid divergence, and putatively neutral divergence (time), but the contribution of each factor is heterogeneous among duplication modes. Gene loss may retard inter-species expression divergence. Members of different gene families may have non-random patterns of origin that are similar in Arabidopsis and rice, suggesting the action of pan-taxon principles of molecular evolution.
Gene duplication modes differ in contribution to genetic novelty and redundancy, but show some parallels in taxa separated by hundreds of millions of years of evolution.