A MITE insertion into the 3′-UTR regulates the transcription of TaHSP16.9 in common wheat

Miniature inverted-repeat transposable elements(MITEs) are a type of DNA transposon frequently inserted into promoters, untranslated regions(UTR), introns, or coding sequences of genes. We found a 276-bp tourist-like MITE insertion in the 3′-UTR of a 16.9 k Da small heat shock protein gene(TaH SP16.9-3A) on chromosome 3A of common wheat. Haplotype analysis revealed two haplotypes, s HSP-W(wild type without MITE insertion) and s HSP-M(mutant with MITE insertion), present in wheat germplasm. Both semiquantitative PCR and quantitative real-time PCR analyses showed increased transcription levels of TaH SP16.9-3A in s HSP-M compared with those of s HSP-W after heat treatment at 42 °C. It appeared that the MITE insertion into the 3′-UTR enhances the transcription of TaH SP16.9-3A.

Genetic improvement of heat tolerance in wheat:Recent progress in understanding the underlying molecular mechanisms

As a cool season crop, wheat(Triticum aestivum L.) has an optimal daytime growing temperature of 15 ℃ during the reproductive stage. With global climate change, heat stress is becoming an increasingly severe constraint on wheat production. In this review, we summarize recent progress in understanding the molecular mechanisms of heat tolerance in wheat. We firstly describe the impact of heat tolerance on morphology and physiology and its potential effect on agronomic traits. We then review recent discoveries in determining the genetic and molecular factors affecting heat tolerance, including the effects of phytohormone signaling and epigenetic regulation. Finally, we discuss integrative strategies to improve heat tolerance by utilization of existing germplasm including modern cultivars, landraces and related species.

Changes in concentrations and transcripts of plant hormones in wheat seedling roots in response to Fusarium crown rot

Fusarium crown rot(FCR) is a soilborne disease causing severe yield losses in many wheat-growing areas of the world. Diseased plants show browning and necrosis of roots and stems causing white heads at maturity. Little is known about the molecular processes employed by wheat roots to respond to the disease. We characterized morphological, transcriptional and hormonal changes in wheat seedling roots following challenge with Fusarium pseudograminearum(Fp), the main pathogen of FCR. The pathogen inhibited root development to various extents depending on plants' resistance level. Many genes responsive to FCR infection in wheat roots were enriched in plant hormone pathways. The contents of compounds involved in biosynthesis and metabolism of jasmonic acid, salicylic acid, cytokinin and auxin were drastically changed in roots at five days post-inoculation. Presoaking seeds in methyl jasmonate for 24 h promoted FCR resistance, whereas presoaking with cytokinin 6-benzylaminopurine made plants more susceptible. Overexpression of TaOPR3, a gene involved in jasmonic acid biosynthesis, enhanced plant resistance as well as root and shoot growth during infection.

Characterization of a new hexaploid triticale 6D(6A) substitution line with increased grain weight and decreased spikelet number

Hexaploid triticale(×Triticosecale,AABBRR)is an important forage crop and a promising energy plant.Transferring D-genome chromosomes or segments from common wheat(Triticum aestivum)into hexaploid triticale is attractive in improving its economically important traits.Here,a hexaploid triticale 6D(6A)substitution line Lin 456 derived from the cross between the octoploid triticale line H400 and the hexaploid wheat Lin 56 was identified and analyzed by genomic in situ hybridization(GISH),fluorescence in situ hybridization(FISH),and molecular markers.The GISH analysis showed that Lin 456 is a hexaploid triticalewith 14 rye(Secale cereale)chromosomes and 28 wheat chromosomes,whereas non-denaturing fluorescence in situ hybridization(ND-FISH)and molecular marker analysis revealed that it is a 6D(6A)substitution line.In contrast to previous studies,the signal of Oligo-pSc119.2 was observed at the distal end of 6DL in Lin 456.The wheat chromosome 6D was associatedwith increased grain weight and decreased spikelet number using the genotypic data combined with the phenotypes of the F2 population in the three environments.The thousand-grain weight and grain width in the substitution individuals were significantly higher than those in the non-substitution individuals in the F2 population across the three environments.We propose that the hexaploid triticale 6D(6A)substitution line Lin 456 can be a valuable and promising donor stock for genetic improvement during triticale breeding.

Breeding design in wheat by combining the QTL information in a GWAS panel with a general genetic map and computer simulation

A large amount of genome-wide association study(GWAS)panels together with quantitative-trait locus(QTL)information associated with breeding-targeted traits have been described in wheat(Triticum aestivum L.).However,the application of mapping results from a GWAS panel to conventional wheat breeding remains a challenge.In this study,we first report a general genetic map which was constructed from 44 published linkage maps.It permits the estimation of genetic distances between any two genetic loci with physical map positions,thereby unifying the linkage relationships between QTL,genes,and genomic markers from multiple genetic populations.Second,we describe QTL mapping in a wheat GWAS panel of 688 accessions,identifying 77 QTL associated with 12 yield and grain-quality traits.Because these QTL have known physical map positions,they could be mapped onto the general map.Finally,we present a design approach to wheat breeding by using known QTL information and computer simulation.Potential crosses between parents in the GWAS panel may be evaluated by the relative frequency of the target genotype,trait correlations in simulated progeny populations,and genetic gain of selected progenies.It is possible to simultaneously improve yield and grain quality by suitable parental selection,progeny population size,and progeny selection scheme.Applying the design approach will allow identifying the most promising crosses and selection schemes in advance of the field experiment,increasing predictability and efficiency in wheat breeding.

Genome-wide linkage mapping of QTL for root hair length in a Chinese common wheat population

Root hairs are fast growing,ephemeral tubular extensions of the root epidermis that aid nutrient and water uptake.The aim of the present study was to identify QTL for root hair length(RHL)using 227 F8 recombinant inbred lines(RILs)derived from a cross of Zhou 8425 B(Z8425 B)and Chinese Spring(CS),and to develop convenient molecular markers for markerassisted breeding in wheat.Analysis of variance of root hair length showed significant differences(P<0.01)among RILs.The genetic map for QTL analysis consisted of 3389 unique SNP markers.Using composite interval mapping,four major QTL(LOD>2.5)for RHL were identified on chromosomes 1 B(2),2 D and 6 D and four putative QTL(2≤LOD≤2.5)were detected on chromosomes 1 A,3 A,6 B,and 7 B,explaining 3.32%–6.52%of the phenotypic variance.The positive alleles for increased RHL of QTL on chromosomes 2 D,6 B and 6 D(QRhl.cau-2 D,q Rhl.cau-6 B,and QRhl.cau-6 D)were contributed by Z8425 B,and CS contributed positive QTL alleles on chromosomes 1 A(q Rhl.cau-1 A),1 B(QRhl.cau-1 B.1 and QRhl.cau-1 B.2),3 A(q Rhl.cau-3 A)and 7 B(q Rhl.cau-7 B).STARP markers were developed for QRhl.cau-1 B.1,QRhl.cau-2 D,QRhl.cau-6 D,and q Rhl.cau-7 B.Haplotype and association analysis indicated that the positive allele of QRhl.cau-6 D had been strongly selected in Chinese wheat breeding programs.Collectively,the identified QTL for root hair length are likely to be useful for marker-assisted selection.

Genomic insights into the origin and evolution of spelt(Triticum spelta L.)as a valuable gene pool for modern wheat breeding

Spelt(Triticum aestivum ssp.spelta)is an important wheat subspecies mainly cultivated in Europe before the 20th century that has contributed to modern wheat breeding as a valuable genetic resource.However,relatively little is known about the origins and maintenance of spelt populations.Here,using resequencing data from 416 worldwide wheat accessions,including representative spelt wheat,we demonstrate that Eu-ropean spelt emerged when primitive hexaploid wheat spread to the west and hybridized with pre-settled domesticated emmer,the putative maternal donor.Genomic introgression regions from domesticated emmer confer spelt’s primitive morphological characters used for species taxonomy,such as tenacious glumes and laterflowering.We propose a haplotype-based"spelt index"to identify spelt-type wheat vari-eties and to quantify utilization of the spelt gene pool in modern wheat cultivars.This study reveals the ge-netic basis for the establishment of the spelt wheat subspecies in a specific ecological niche and the vital role of the spelt gene pool as a unique germplasm resource in modern wheat breeding.

A k-mer-based pangenome approach for cataloging seed-storage-protein genes in wheat to facilitate genotype-to-phenotype prediction and improvement of end-use quality

Wheat is a staple foodfor more than 35%of the world's population,with wheatflourused to make hundreds of baked goods.Superior end-use quality is a major breeding target;however,improving it is especially time-consuming and expensive.Furthermore,genes encoding seed-storage proteins(ssPs)form multigene families and are repetitive,with gaps commonplace in several genome assemblies.To overcome these barriers and efficiently identify superior wheat SSP alleles,we developed"PanSK"(Pan-SSP k-mer)for genotype-to-phenotype prediction based on an SsP-based pangenome resource.PanSK uses 29-mer sequences that represent each ssP gene at the pangenomic level to reveal untapped diversity across landraces and modern cultivars.Genome-wide association studies with k-mers identified 23 Ssp genes associated with end-use quality that represent novel targets for improvement.We evaluated the effect of rye secalin genes on end-use quality and found that removal of w-secalins from 1BL/1RS wheat translocation lines is associated with enhanced end-use quality.Finally,using machine-learning-based prediction inspired by PanSK,we predicted the quality phenotypes with high accuracy from genotypes alone.This study provides an effective approach for genome design based on ssP genes,enabling the breeding of wheat varieties with superior processing capabilities and improved end-use quality.

On the evolution and genetic diversity of the bread wheat D genome

Bread wheat(Triticum aestivum)became a globally dominant crop after incorporating the D genome from the donor species Aegilops tauschii,but the evolutionary history that shaped the D genome during this process remains to be clarified.Here,we propose a renewed evolutionary model linking Ae.tauschii and the hexaploid wheat D genome by constructing an ancestral haplotype map covering 762 Ae.tauschii and hexaploid wheat accessions.We dissected the evolutionary trajectories of Ae.tauschii lineages and reported a few independent intermediate accessions,demonstrating that low-frequency intersublineage gene flow had enriched the diversity of Ae.tauschii.We discovered that the D genome of hexaploid wheat was inherited from a unified ancestral template,but with a mosaic composition that was highly mixed and derived mainly from three Ae.tauschii L2 sublineages located in the Caspian coastal region.This result suggests that early agricultural activities facilitated innovations in D-genome composition and finalized the success of hexaploidization.We found that the majority(51.4%)of genetic diversity was attributed to novel mutations absent in Ae.tauschii,and we identified large Ae.tauschii introgressions from various lineages,which expanded the diversity of the wheat D genome and introduced beneficial alleles.This work sheds light on the process of wheat hexaploidization and highlights the evolutionary significance of the multi-layered genetic diversity of the bread wheat D genome.

Innovative computational tools provide new insights into the polyploid wheat genome

Bread wheat(Triticum aestivum)is an important crop and serves as a significant source of protein and calories for humans,worldwide.Nevertheless,its large and allopolyploid genome poses constraints on genetic improvement.The complex reticulate evolutionary history and the intricacy of genomic resources make the deciphering of the functional genome considerably more challenging.Recently,we have developed a comprehensive list of versatile computational tools with the integration of statistical models for dissecting the polyploid wheat genome.Here,we summarize the methodological innovations and applications of these tools and databases.A series of step-by-step examples illustrates how these tools can be utilized for dissecting wheat germplasm resources and unveiling functional genes associated with important agronomic traits.Furthermore,we outline future perspectives on new advanced tools and databases,taking into consideration the unique features of bread wheat,to accelerate genomic-assisted wheat breeding.

lcQTH:Rapid quantitative trait mapping by tracing parental haplotypes with ultra-low-coverage sequencing

Dear Editor,Gene cloning has a fundamental role in crop research but has long been hindered by high costs and labor requirements,which have limited the numbers of genes that have been functionally characterized,especially in wheat(Liang et al.,2021).Quantitative trait locus(QTL)mapping is the first step in gene cloning,enabling the localization of genomic loci that show significant associations with quantitative traits.One commonly used strategy is based on single-nucleotide polymorphism(SNP)arrays(Sun et al.,2020).

A Collinearity-Incorporating Homology Inference Strategy for Connecting Emerging Assemblies in the Triticeae Tribe as a Pilot Practice in the Plant Pangenomic Era

Plant genome sequencing has dramatically increased,and some species even have multiple high-quality reference versions.Demands for clade-specific homology inference and analysis have increased in the pangenomic era.Here we present a novel method,GeneTribe(https://chenym1.github.io/genetribe/),for homology inference among genetically similar genomes that incorporates gene collinearity and shows bet-ter performance than traditional sequence-similarity-based methods in terms of accuracy and scalability.The Triticeae tribe is a typical allopolyploid-rich clade with complex species relationships that includes many important crops,such as wheat,barley,and rye.We built Triticeae-GeneTribe(http://wheat.cau.edu.cn/TGT/),a homology database,by integrating 12 Triticeae genomes and 3 outgroup model genomes and implemented versatile analysis and visualization functions.With macrocollinearity analysis,we were able to construct a refined model illustrating the structural rearrangements of the 4A-5A-7B chromosomes in wheat as two major translocation events.With collinearity analysis at both the macro-and microscale,we illustrated the complex evolutionary history of homologs of the wheat vernalization gene Vm2,which evolved as a combined result of genome translocation,duplication,and polyploidization and gene loss events.Our work provides a useful practice for connecting emerging genome assemblies,with awareness of the extensive polyploidy in plants,and will help researchers efficiently exploit genome sequence re-sources.

Ectopic expression of VRT-A2 underlies the origin of Triticum polonicum and Triticum petropavlovskyi with long outer glumes and grains

Polish wheat (Triticum polonicum) is a unique tetraploid wheat species characterized by an elongated outer glume. The genetic control of the long-glume trait by a single semi-dominant locus, P1 (from Polish wheat), was established more than 100 years ago, but the underlying causal gene and molecular nature remain elusive. Here, we report the isolation of VRT-A2, encoding an SVP-clade MADS-box transcription factor, as the P1 candidate gene. Genetic evidence suggests that in T. polonicum, a naturally occurring sequence rearrangement in the intron-1 region of VRT-A2 leads to ectopic expression of VRT-A2 in floral organs where the long-glume phenotype appears. Interestingly, we found that the intron-1 region is a key ON/OFF molecular switch for VRT-A2 expression, not only because it recruits transcriptional repressors, but also because it confers intron-mediated transcriptional enhancement. Genotypic analyses using wheat accessions indicated that the P1 locus is likely derived from a single natural mutation in tetraploid wheat, which was subsequently inherited by hexaploid T. petropavlovskyi. Taken together, our findings highlight the promoter-proximal intron variation as a molecular basis for phenotypic differentiation, and thus species formation in Triticum plants.

Shaping polyploid wheat for success:Origins,domestication,and the genetic improvement of agronomic traits

Bread wheat(Triticum aestivum L.,AABBDD,2 n=6 x=42),which accounts for most of the cultivated wheat crop worldwide,is a typical allohexaploid with a genome derived from three diploid wild ancestors.Bread wheat arose and evolved via two sequential allopolyploidization events and was further polished through multiple steps of domestication.Today,cultivated allohexaploid bread wheat has numerous advantageous traits,including adaptive plasticity,favorable yield traits,and extended end-use quality,which have enabled its cultivation well beyond the ranges of its tetraploid and diploid progenitors to become a global staple food crop.In the past decade,rapid advances in wheat genomic research have considerably accelerated our understanding of the bases for the shaping of complex agronomic traits in this polyploid crop.Here,we summarize recent advances in characterizing major genetic factors underlying the origin,evolution,and improvement of polyploid wheats.We end with a brief discussion of the future prospects for the design of gene cloning strategies and modern wheat breeding.

A natural variation in Ribonuclease H-like gene underlies Rht8 to confer“Green Revolution”trait in wheat

Dear Editor,Introduction of gibberellin(GA)-insensitive Reduced height(Rht)genes,Rht-B1b and Rht-D1b,has resulted in the“Green Revolution”in modern wheat cultivars(Triticum aestivum)that has skyrocketed wheat grain yields worldwide since the 1960s(Peng et al.,1999;Velde et al.,2021).However,Rht-B1b/D1b also reduce coleoptiles,which is undesired in dryland regions where deep planting is essential for seedling establishment(Rebetzke et al.,1999,Rebetzke et al.,2001;Ellis et al.,2004).

A wheat integrative regulatory network from large-scale complementary functional datasets enables trait-associated gene discovery for crop improvement

Gene regulation is central to all aspects of organism growth,and understanding it using large-scale functional datasets can provide a whole view of biological processes controlling complex phenotypic traits in crops.However,the connection between massive functional datasets and trait-associated gene discovery for crop improvement is still lacking.In this study,we constructed a wheat integrative gene regulatory network(wGRN)by combining an updated genome annotation and diverse complementary functional datasets,including gene expression,sequence motif,transcription factor(TF)binding,chromatin accessibility,and evolutionarily conserved regulation.wGRN contains 7.2 million genome-wide interactions covering 5947 TFs and 127439 target genes,which were further verified using known regulatory relationships,condition-specific expression,gene functional information,and experiments.We used wGRN to assign genome-wide genes to 3891 specific biological pathways and accurately prioritize candidate genes associated with complex phenotypic traits in genome-wide association studies.In addition,wGRN was used to enhance the interpretation of a spike temporal transcriptome dataset to construct high-resolution networks.We further unveiled novel regulators that enhance the power of spike phenotypic trait prediction using machine learning and contribute to the spike phenotypic differences among modern wheat accessions.Finally,we developed an interactive webserver,wGRN(http://wheat.cau.edu.cn/wGRN),for the community to explore gene regulation and discover trait-associated genes.Collectively,this community resource establishes the foundation for using large-scale functional datasets to guide trait-associated gene discovery for crop improvement.

Deciphering the evolution and complexity of wheat germplasm from a genomic perspective

Bread wheat provides an essential fraction of the daily calorific intake for humanity.Due to its huge and complex genome,progress in studying on the wheat genome is substantially trailed behind those of the other two major crops,rice and maize,for at least a decade.With rapid advances in genome assembling and reduced cost of high-throughput sequencing,emerging de novo genome assemblies of wheat and whole-genome sequencing data are leading to a paradigm shift in wheat research.Here,we review recent progress in dissecting the complex genome and germplasm evolution of wheat since the release of the first high-quality wheat genome.New insights have been gained in the evolution of wheat germplasm during domestication and modern breeding progress,genomic variations at multiple scales contributing to the diversity of wheat germplasm,and complex transcriptional and epigenetic regulations of functional genes in polyploid wheat.Genomics databases and bioinformatics tools meeting the urgent needs of wheat ge-nomics research are also summarized.The ever-increasing omics data,along with advanced tools and well-structured databases,are expected to accelerate deciphering the germplasm and gene resources in wheat for future breeding advances.

Transcriptome Comparison of Susceptible and Resistant Wheat in Response to Powdery Mildew Infection

Powdery mildew （Pro） caused by the infection of Blumeria graminis f. sp. tritici （Bgt） is a worldwide crop disease resulting in significant loss of wheat yield. To profile the genes and pathways responding to the Bgt infection, here, using Affymetrix wheat microarrays, we compared the leaf transcriptomes before and after Bgt inoculation in two wheat genotypes, a Pm-susceptible cultivar Jingdong 8 （S） and its near-isogenic line （R） carrying a single Pm resistant gene Pm30. Our analysis showed that the original gene expression status in the S and R genotypes of wheat was almost identical before Bgt inoculation, since only 60 genes exhibited differential expression by P = 0.01 cutoff. However, 12 h after Bgt inoculation, 3014 and 2800 genes in the S and R genotype, respectively, responded to infec- tion. A wide range of pathways were involved, including cell wall fortification, flavonoid biosynthesis and metabolic processes. Further- more, for the first time, we show that sense-antisense pair genes might be participants in wheat-powdery mildew interaction. In addition, the results of qRT-PCR analysis on several candidate genes were consistent with the microarray data in their expression patterns. In summary, this study reveals leaf transcriptome changes before and after powdery mildew infection in wheat near-isogenic lines, suggest- ing that powdery mildew resistance is a highly complex systematic response involving a large amount of gene regulation.

The genetic and molecular basis for improving heat stress tolerance in wheat

Wheat production requires at least-2.4%increase per year rate by 2050 globally to meet food demands.However,heat stress results in serious yield loss of wheat worldwide.Correspondingly,wheat has evolved genetic basis and molecular mechanisms to protect themselves from heat-induced damage.Thus,it is very urgent to understand the underlying genetic basis and molecular mechanisms responsive to elevated temperatures to provide important strategies for heat-tolerant varieties breeding.In this review,we focused on the impact of heat stress on morphology variation at adult stage in wheat breeding programs.We also summarize the recent studies of genetic and molecular factors regulating heat tolerance,including identification of heat stress tolerance related QTLs/genes,and the regulation pathway in response to heat stress.In addition,we discuss the potential ways to improve heat tolerance by developing new technologies such as genome editing.This review of wheat responses to heat stress may shed light on the understanding heat-responsive mechanisms,although the regu-latory network of heat tolerance is still ambiguous in wheat.

Sequencing and comparative analyses of Aegilops tauschii chromosome arm 3DS reveal rapid evolution of Triticeae genomes

Bread wheat （Triticum aestivum, AABBDD） is an allohexaploid species derived from two rounds of interspecific hybridizations. A high-quality genome sequence assembly of diploid Aegilops tauschii, the donor of the wheat D genome, will provide a useful platform to study polyploid wheat evolution. A combined approach of BAC pooling and next-generation sequencing technology was employed to sequence the minimum tiling path （MTP） of 3176 BAC clones from the short arm ofAe. tauschii chromosome 3 （At3DS）. The final assembly of 135 super-scaffolds with an N50 of 4.2 Mb was used to build a 247-Mb pseudomolecule with a total of 2222 predicted protein-coding genes. Compared with the orthologous regions of rice, Brachypodium, and sorghum, At3DS contains 38.67% more genes. In comparison to At3DS, the short arm sequence of wheat chromosome 3B （Ta3BS） is 95-Mb large in size, which is primarily due to the expansion of the non-centromeric region, suggesting that transposable element （TE） bursts in Ta3B likely occurred there. Also, the size increase is accompanied by a proportional increase in gene number in Ta3BS. We found that in the sequence of short arm of wheat chromosome 3D （Ta3DS）, there was only less than 0.27% gene loss compared to At3DS. Our study reveals divergent evolution of grass genomes and provides new insights into sequence changes in the polyploid wheat genome.