Origin of wheat B-genome chromosomes inferred from RNA sequencing analysis of leaf transcripts from section Sitopsis species of Aegilops

Wild grass-derived alleles represent a genetic architecture for the resilience of modern common wheat to stresses

This review explores the integration of wild grass-derived alleles into modern bread wheat breeding to tackle the challenges of climate change and increasing food demand. With a focus on synthetic hexaploid wheat, this review highlights the potential of genetic variability in wheat wild relatives, particularly Aegilops tauschii, for improving resilience to multifactorial stresses like drought, heat, and salinity. The evolutionary journey of wheat (Triticum spp.) from diploid to hexaploid species is examined, revealing significant genetic contributions from wild grasses. We also emphasize the importance of understanding incomplete lineage sorting in the genomic evolution of wheat. Grasping this information is crucial as it can guide breeders in selecting the appropriate alleles from the gene pool of wild relatives to incorporate into modern wheat varieties. This approach improves the precision of phylogenetic relationships and increases the overall effectiveness of breeding strategies. This review also addresses the challenges in utilizing the wheat wild genetic resources, such as the linkage drag and cross-compatibility issues. Finally, we culminate the review with future perspectives, advocating for a combined approach of high-throughput phenotyping tools and advanced genomic techniques to comprehensively understand the genetic and regulatory architectures of wheat under stress conditions, paving the way for more precise and efficient breeding strategies.

Evolution of wheat blast resistance gene Rmg8 accompanied by differentiation of variants recognizing the powdery mildew fungus

Wheat blast, a devastating disease having spread recently from South America to Asia and Africa, is caused by Pyricularia oryzae (synonym of Magnaporthe oryzae) pathotype Triticum, which first emerged in Brazil in 1985. Rmg8 and Rmg7, genes for resistance to wheat blast found in common wheat and tetraploid wheat, respectively, recognize the same avirulence gene, AVR-Rmg8. Here we show that an ancestral resistance gene, which had obtained an ability to recognize AVR-Rmg8 before the differentiation of Triticum and Aegilops, has expanded its target pathogens. Molecular cloning revealed that Rmg7 was an allele of Pm4, a gene for resistance to wheat powdery mildew on 2AL, whereas Rmg8 was its homoeologue on 2BL ineffective against wheat powdery mildew. Rmg8 variants with the ability to recognize AVR-Rmg8 were distributed not only in Triticum spp. but also in Aegilops speltoides, Aegilops umbellulata and Aegilops comosa. This result suggests that the origin of resistance gene(s) recognizing AVR-Rmg8 dates back to the time before differentiation of A, B, S, U and M genomes, that is, ~5 Myr before the emergence of its current target, the wheat blast fungus. Phylogenetic analyses suggested that, in the evolutionary process thereafter, some of their variants gained the ability to recognize the wheat powdery mildew fungus and evolved into genes controlling dual resistance to wheat powdery mildew and wheat blast.

Haplotype-resolved genome of Prunus zhengheensis provides insight into its evolution and low temperature adaptation in apricot

Prunus zhengheensis, an extremely rare population of apricots, originated in warm South-East China and is an excellent material for genetic breeding. However, most apricots and two related species (P. sibirica, P. mandshurica) are found in the cold northern regions in China and the mechanism of their distribution is still unclear. In addition, the classification status of P. zhengheensis is controversial. Thus, we generated a high-quality haplotype-resolved genome for P. zhengheensis, exploring key genetic variations in its adaptation and the causes of phylogenetic incongruence. We found extensive phylogenetic discordances between the nuclear and organelle phylogenies of P. zhengheensis, which could be explained by incomplete lineage sorting. A 242.22-Mb pan-genome of the Armeniaca section was developed with 13 chromosomal genomes. Importantly, we identified a 566-bp insertion in the promoter of the HSFA1d gene in apricot and showed that the activity of the HSFA1d promoter increased under low temperatures. In addition, HSFA1d overexpression in Arabidopsis thaliana indicated that HSFA1d positively regulated plant growth under chilling. Therefore, we hypothesized that the insertion in the promoter of HSFA1d in apricot improved its low-temperature adaptation, allowing it to thrive in relatively cold locations. The findings help explain the weather adaptability of Armeniaca plants.

Cloning of the wheat leaf rust resistance gene Lr47 introgressed from Aegilops speltoides

Leaf rust, caused by Puccinia triticina Eriksson (Pt), is one of the most severe foliar diseases of wheat. Breeding for leaf rust resistance is a practical and sustainable method to control this devastating disease. Here, we report the identification of Lr47, a broadly effective leaf rust resistance gene introgressed into wheat from Aegilops speltoides. Lr47 encodes a coiled-coil nucleotide-binding leucine-rich repeat protein that is both necessary and sufficient to confer Pt resistance, as demonstrated by loss-of-function mutations and transgenic complementation. Lr47 introgression lines with no or reduced linkage drag are generated using the Pairing homoeologous1 mutation, and a diagnostic molecular marker for Lr47 is developed. The coiled-coil domain of the Lr47 protein is unable to induce cell death, nor does it have self-protein interaction. The cloning of Lr47 expands the number of leaf rust resistance genes that can be incorporated into multigene transgenic cassettes to control this devastating disease.

Identification of drought-tolerant hub genes in Iranian KC-2226 genotype of Aegilops tauschii using transcriptomic analysis

Aegilops tauschii, as a donor of D genome to the bread wheat with a valuable source of resistance to different biotic and abiotic stresses, is used to improve the quality of wheat cultivars. Every genotype has a specific genetic content, the investigation of which can lead to the identification of useful genes such as stress tolerance genes, including drought. Therefore, 23 genotypes of Ae. tauschii were selected to evaluate their morphological and physiological traits under greenhouse conditions. Among them, a superior tolerant genotype (KC-2226) was chosen for transcriptomic analysis. Our result showed that 5007 and 3489 genes were deferentially up- and downregulated, respectively. Upregulated genes were involved in photosynthesis, glycolysis/gluconeogenesis, and amino acid biosynthesis whereas downregulated genes were often engaged in DNA synthesis, replication, repair and topological changes. The result of protein-protein interaction network analysis showed that AT1G76550 (1.46), AT1G20950 (1.42), IAR4 (1.19), and PYD2 (1.16) among upregulated genes and THY-1 (44), PCNA1 (41) and TOPII (22) among down-regulated genes had the highest interactions with other genes. In conclusion, Ae. tauschii employs elevated transcription of specific genes involved in photosynthesis, glycolysis and gluconeogenesis and amino acid biosynthesis pathways rather than genes active in DNA synthesis and repair to provide the energy needed for the plant to survive under stress conditions.

Genome sequencing of Sitopsis species provides insights into their contribution to the B subgenome of bread wheat

Wheat (Triticum aestivum, BBAADD) is an allohexaploid species that originated from two polyploidization events. The progenitors of the A and D subgenomes have been identified as Triticum urartu and Aegilops tauschii, respectively. Current research suggests that Aegilops speltoides is the closest but not the direct ancestor of the B subgenome. However, whether Ae. speltoides has contributed genomically to the wheat B subgenome and which chromosome regions are conserved between Ae. speltoides and the B subgenome remain unclear. Here, we assembled a high-quality reference genome for Ae. speltoides, resequenced 53 accessions from seven species (Aegilops bicornis, Aegilops longissima, Aegilops searsii, Aegilops sharonensis, Ae. speltoides, Aegilops mutica [syn. Amblyopyrum muticum], and Triticum dicoccoides) and revealed their genomic contributions to the wheat B subgenome. Our results showed that centromeric regions were particularly conserved between Aegilops and Triticum and revealed 0.17 Gb of conserved blocks between Ae. speltoides and the B subgenome. We classified five groups of conserved and non-conserved genes between Aegilops and Triticum, revealing their biological characteristics, differentiation in gene expression patterns, and collinear relationships between Ae. speltoides and the wheat B subgenome. We also identified gene families that expanded in Ae. speltoides during its evolution and 789 genes specific to Ae. speltoides. These genes can serve as genetic resources for improvement of adaptability to biotic and abiotic stress. The newly constructed reference genome and large-scale resequencing data for Sitopsis species will provide a valuable genomic resource for wheat genetic improvement and genomic studies.

Aegilops sharonensis genome-assisted identification of stem rust resistance gene Sr62

The wild relatives and progenitors of wheat have been widely used as sources of disease resistance (R) genes. Molecular identification and characterization of these R genes facilitates their manipulation and tracking in breeding programmes. Here, we develop a reference-quality genome assembly of the wild diploid wheat relative Aegilops sharonensis and use positional mapping, mutagenesis, RNA-Seq and transgenesis to identify the stem rust resistance gene Sr62, which has also been transferred to common wheat. This gene encodes a tandem kinase, homologues of which exist across multiple taxa in the plant kingdom. Stable Sr62 transgenic wheat lines show high levels of resistance against diverse isolates of the stem rust pathogen, highlighting the utility of Sr62 for deployment as part of a polygenic stack to maximize the durability of stem rust resistance.

Aegilops sharonensis is a wild diploid relative of wheat. Here, the authors assemble the genome of Ae. sharonensis and use the assembly as an aid to clone the Ae. sharonensis-derived stem rust resistance gene Sr62 in the allohexaploid genome of wheat.

Genome sequences of five Sitopsis species of Aegilops and the origin of polyploid wheat B subgenome

Common wheat (Triticum aestivum, BBAADD) is a major staple food crop worldwide. The diploid progenitors of the A and D subgenomes have been unequivocally identified; that of B, however, remains ambiguous and controversial but is suspected to be related to species of Aegilops, section Sitopsis. Here, we report the assembly of chromosome-level genome sequences of all five Sitopsis species, namely Aegilops bicornis, Ae. longissima, Ae. searsii, Ae. sharonensis, and Ae. speltoides, as well as the partial assembly of the Amblyopyrum muticum (synonym Aegilops mutica) genome for phylogenetic analysis. Our results reveal that the donor of the common wheat B subgenome is a distinct, and most probably extinct, diploid species that diverged from an ancestral progenitor of the B lineage to which the still extant Ae. speltoides and Am. muticum belong. In addition, we identified interspecific genetic introgressions throughout the evolution of the Triticum/Aegilops species complex. The five Sitopsis species have various assembled genome sizes (4.11-5.89 Gb) with high proportions of repetitive sequences (85.99%-89.81%); nonetheless, they retain high collinearity with other genomes or subgenomes of species in the Triticum/Aegilops complex. Differences in genome size were primarily due to independent post-speciation amplification of transposons. We also identified a set of Sitopsis genes pertinent to important agronomic traits that can be harnessed for wheat breeding. These newly assembled genome resources provide a new roadmap for evolutionary and genetic studies of the Triticum/Aegilops complex, as well as for wheat improvement.

Transcriptome based identification and validation of heat stress transcription factors in wheat progenitor species Aegilops speltoides

Wheat, one of the major cereal crops worldwide, get adversely affected by rising global temperature. We have identified the diploid B genome progenitor of wheat, Aegilops speltoides (SS), as a potential donor for heat stress tolerance. Therefore, the present work was planned to study the total transcriptome profile of heat stress-tolerant Ae. speltoides accession pau3809 (AS3809) and compare with that of tetraploid and hexaploid wheat cultivars PDW274 and PBW725, respectively. The comparative transcriptome was utilized to identify and validate heat stress transcription factors (HSFs), the key genes involved in imparting heat stress tolerance. Transcriptome analysis led to the identification of a total of 74 K, 68 K, and 76 K genes in AS3809, PDW274, and PBW725, respectively. There was a high uniformity of GO profiles under the biological, molecular, and cellular functions across the three wheat transcriptomes, suggesting the conservation of gene function. Twelve HSFs having the highest FPKM value were identified in the AS3809 transcriptome data, while six of these HSFs namely HSFA3, HSFA5, HSFA9, HSFB2a, HSFB2b, and HSFC1b, were validated with qRT PCR. These six HSFs were identified as an important component of thermotolerance in AS3809 as evident from their comparative higher expression under heat stress.

Selection and Validation of Reference Genes for RT-qPCR Analysis in Aegilops tauschii (Coss.) under Different Abiotic Stresses

Aegilops tauschii (Coss.) is an aggressive and serious annual grass weed in China. Its DD genome is a rich source of genetic material and performs better under different abiotic stress conditions (salinity, drought, temperature, etc.). Reverse-transcribed quantitative polymerase chain reaction (RT-qPCR) is a reliable technique for reference gene selection and validation. This work aimed to evaluate the stability of reference gene expression in Ae. tauschii under different abiotic stresses (salinity, drought, hot, and cold) and developmental stages (seedling and development). The results show that the ubiquitin-conjugating enzyme E2 36-like (UBC36) and protein microrchidia 2-like (HSP) are the most stable genes under control and salinity conditions, respectively. Under drought stress conditions, UBC36 is more stable as compared with others. Glyceraldehyde-3-phosphate dehydrogenase (GADPH) is the most stable reference gene during heat stress conditions and thioredoxin-like protein (YLS) under cold stress condition. Phosphate2A serine/threonine-protein phosphatase 2A (PP2A) and eukaryotic translation initiation factor 3 (ETIF3) are the most stable genes at seedling and developmental stages. Intracellular transport protein (CAC) is recommended as the most stable gene under different abiotic stresses and at developmental stages. Furthermore, the relative expression levels of NHX1 and DREB under different levels of salinity and drought stress conditions varied with the most (HSP and UBC36) and least (YLS and ACT) stable genes. This study provides reliable reference genes for understanding the tolerance mechanisms in Ae. tauschii under different abiotic stress conditions.

RNA-Seq-based DNA marker analysis of the genetics and molecular evolution of Triticeae species

The release of high-quality chromosome-level genome sequences of members of the Triticeae tribe has greatly facilitated genetic and genomic analyses of important crops such as wheat (Triticum aestivum) and barley (Hordeum vulgare). Due to the large diploid genome size of Triticeae plants (ca. 5 Gbp), transcript analysis is an important method for identifying genetic and genomic differences among Triticeae species. In this review, we summarize our results of RNA-Seq analyses of diploid wheat accessions belonging to the genera Aegilops and Triticum. We also describe studies of the molecular relationships among these accessions and provide insight into the evolution of common hexaploid wheat. DNA markers based on polymorphisms within species can be used to map loci of interest. Even though the genome sequence of diploid Aegilops tauschii, the D-genome donor of common wheat, has been released, the diploid barley genome continues to provide key information about the physical structures of diploid wheat genomes. We describe how a series of RNA-Seq analyses of wheat relatives has helped uncover the structural and evolutionary features of genomic and genetic systems in wild and cultivated Triticeae species.

High-resolution mapping of the Mov-1 locus in wheat by combining radiation hybrid (RH) and recombination-based mapping approaches

This work reports a quick method that integrates RH mapping and genetic mapping to map the dominant Mov-1 locus to a 1.1-Mb physical interval with a small number of candidate genes. Bread wheat is an important crop for global human population. Identification of genes and alleles controlling agronomic traits is essential toward sustainably increasing crop production. The unique multi-ovary (MOV) trait in wheat holds potential for improving yields and is characterized by the formation of 2-3 grains per spikelet. The genetic basis of the multi-ovary trait is known to be monogenic and dominant in nature. Its precise mapping and functional characterization is critical to utilizing this trait in a feasible manner. Previous mapping efforts of the locus controlling multiple ovary/pistil formation in the hexaploid wheat have failed to produce a consensus for a particular chromosome. We describe a mapping strategy integrating radiation hybrid mapping and high-resolution genetic mapping to locate the chromosomal position of the Mov-1 locus in hexaploid wheat. We used RH mapping approach using a panel of 188 lines to map the Mov-1 locus in the terminal part of long arm of wheat chromosome 2D with a map resolution of 1.67 Mb/cR₁₅₀₀. Then using a genetic population of MOV × Synthetic wheat of F₂ lines, we delineated the Mov-1 locus to a 1.1-Mb physical region with a small number of candidate genes. This demonstrates the value of this integrated strategy to mapping dominant genes in wheat.

Wheat omics: Classical breeding to new breeding technologies

Wheat is an important cereal crop, and its significance is more due to compete for dietary products in the world. Many constraints facing by the wheat crop due to environmental hazardous, biotic, abiotic stress and heavy matters factors, as a result, decrease the yield. Understanding the molecular mechanism related to these factors is significant to figure out genes regulate under specific conditions. Classical breeding using hybridization has been used to increase the yield but not prospered at the desired level. With the development of newly emerging technologies in biological sciences i.e., marker assisted breeding (MAB), QTLs mapping, mutation breeding, proteomics, metabolomics, next-generation sequencing (NGS), RNA_sequencing, transcriptomics, differential expression genes (DEGs), computational resources and genome editing techniques i.e. (CRISPR cas9; Cas13) advances in the field of omics. Application of new breeding technologies develops huge data; considerable development is needed in bioinformatics science to interpret the data. However, combined omics application to address physiological questions linked with genetics is still a challenge. Moreover, viroid discovery opens the new direction for research, economics, and target specification. Comparative genomics important to figure gene of interest processes are further discussed about considering the identification of genes, genomic loci, and biochemical pathways linked with stress resilience in wheat. Furthermore, this review extensively discussed the omics approaches and their effective use. Integrated plant omics technologies have been used viroid genomes associated with CRISPR and CRISPR-associated Cas13a proteins system used for engineering of viroid interference along with high-performance multidimensional phenotyping as a significant limiting factor for increasing stress resistance in wheat.

The unique disarticulation layer formed in the rachis of Aegilops longissima probably results from the spatial co-expression of Btr1 and Btr2

BACKGROUND AND AIMS

The brittle rachis trait is a feature of many wild grasses, particularly within the tribe Triticeae. Wild Hordeum and Triticum species form a disarticulation layer above the rachis node, resulting in the production of wedge-type dispersal units. In Aegilops longissima, only one or two of the nodes in the central portion of its rachis are brittle. In Triticeae species, the formation of a disarticulation layer above the rachis node requires the co-transcription of the two dominant and complementary genes Btr1 and Btr2. This study aims to establish whether homologues of Btr1 and/or Btr2 underlie the unusual brittle rachis phenotype observed in Ae. longissima.

METHODS

Scanning electron microscopy was used to examine the disarticulation surfaces. Quantitative RT-PCR and RNA in situ hybridization experiments were used to identify gene expression in the immature inflorescence.

KEY RESULTS

Analysis based on scanning electron microscopy was able to demonstrate that the disarticulation surfaces formed in the Ae. longissima rachis are morphologically indistinguishable from those formed in the rachises of wild Hordeum and Triticum species. RNA in situ hybridization showed that in the immature Ae. longissima inflorescence, the intensity of Btr1 transcription varied from high at the rachis base to low at its apex, while that of Btr2 was limited to the nodes in the central to distal portion of the rachis.

CONCLUSIONS

The disarticulation pattern shown by Ae. longissima results from the limitation of Btr1 and Btr2 co-expression to nodes lying in the centre of the rachis.

Genome evolution during bread wheat formation unveiled by the distribution dynamics of SSR sequences on chromosomes using FISH

BACKGROUND

During the bread wheat speciation by polyploidization, a series of genome rearrangement and sequence recombination occurred. Simple sequence repeat (SSR) sequences, predominately located in heterochromatic regions of chromosomes, are the effective marker for tracing the genomic DNA sequence variations. However, to date the distribution dynamics of SSRs on chromosomes of bread wheat and its donors, including diploid and tetraploid Triticum urartu, Aegilops speltoides, Aegilops tauschii, Triticum turgidum ssp. dicocoides, reflecting the genome evolution events during bread wheat formation had not been comprehensively investigated.

RESULTS

The genome evolution was studied by comprehensively comparing the distribution patterns of (AAC)_n, (AAG)_n, (AGC)_n and (AG)_n in bread wheat Triticum aestivum var. Chinese Spring and its progenitors T. urartu, A. speltoides, Ae. tauschii, wild tetroploid emmer wheat T. dicocoides, and cultivated emmer wheat T. dicoccum. Results indicated that there are specific distribution patterns in different chromosomes from different species for each SSRs. They provided efficient visible markers for identification of some individual chromosomes and SSR sequence evolution tracing from the diploid progenitors to hexaploid wheat. During wheat speciation, the SSR sequence expansion occurred predominately in the centromeric and pericentromeric regions of B genome chromosomes accompanied by little expansion and elimination on other chromosomes. This result indicated that the B genome might be more sensitive to the "genome shock" and more changeable during wheat polyplodization.

CONCLUSIONS

During the bread wheat evolution, SSRs including (AAC)_n, (AAG)_n, (AGC)_n and (AG)_n in B genome displayed the greatest changes (sequence expansion) especially in centromeric and pericentromeric regions during the polyploidization from Ae. speltoides S genome, the most likely donor of B genome. This work would enable a better understanding of the wheat genome formation and evolution and reinforce the viewpoint that B genome was originated from S genome.

Production of synthetic wheat lines to exploit the genetic diversity of emmer wheat and D genome containing Aegilops species in wheat breeding

Due to the accumulation of various useful traits over evolutionary time, emmer wheat (Triticum turgidum subsp. dicoccum and dicoccoides, 2n = 4x = 28; AABB), durum wheat (T. turgidum subsp. durum, 2n = 4x = 28; AABB), T. timopheevii (2n = 4x = 28; AAGG) and D genome containing Aegilops species offer excellent sources of novel variation for the improvement of bread wheat (T. aestivum L., AABBDD). Here, we made 192 different cross combinations between diverse genotypes of wheat and Aegilops species including emmer wheat × Ae. tauschii (2n = DD or DDDD), durum wheat × Ae. tauschii, T. timopheevii × Ae. tauschii, Ae. crassa × durum wheat, Ae. cylindrica × durum wheat and Ae. ventricosa × durum wheat in the field over three successive years. We successfully recovered 56 different synthetic hexaploid and octaploid F₂ lines with AABBDD, AABBDDDD, AAGGDD, D¹D¹X^crX^crAABB, D^cD^cC^cC^cAABB and D^vD^vN^vN^vAABB genomes via in vitro rescue of F₁ embryos and spontaneous production of F₂ seeds on the F_l plants. Cytogenetic analysis of F₂ lines showed that the produced synthetic wheat lines were generally promising stable amphiploids. Contribution of D genome bearing Aegilops and the less-investigated emmer wheat genotypes as parents in the crosses resulted in synthetic amphiploids which are a valuable resource for bread wheat breeding.

The Mitochondrial Genome of Eleusine indica and Characterization of Gene Content within Poaceae

Plant mitochondrial (mt) genome assembly provides baseline data on size, structure, and gene content, but resolving the sequence of these large and complex organelle genomes remains challenging due to fragmentation, frequent recombination, and transfers of DNA from neighboring plastids. The mt genome for Eleusine indica (Poaceae: goosegrass) is comprehensibly analyzed here, providing key reference data for an economically significant invasive species that is also the maternal parent of the allotetraploid crop Finger millet (Eleusine coracana). The assembled E. indica genome contains 33 protein coding genes, 6 rRNA subunits, 24 tRNA, 8 large repetitive regions 15 kb of transposable elements across a total of 520,691 bp. Evidence of RNA editing and loss of rpl2, rpl5, rps14, rps11, sdh4, and sdh3 genes is evaluated in the context of an updated survey of mt genomic gene content across the grasses through an analysis of publicly available data. Hypothesized patterns of Poaceae mt gene loss are examined in a phylogenetic context to clarify timing, showing that rpl2 was transferred to the nucleus from the mitochondrion prior to the origin of the PACMAD clade.

Transcriptome Analysis Reveals Important Candidate Genes Related to Nutrient Reservoir, Carbohydrate Metabolism, and Defence Proteins during Grain Development of Hexaploid Bread Wheat and Its Diploid Progenitors

Wheat grain development after anthesis is an important biological process, in which major components of seeds are synthesised, and these components are further required for germination and seed vigour. We have made a comparative RNA-Seq analysis between hexaploid wheat and its individual diploid progenitors to know the major differentially expressed genes (DEGs) involved during grain development. Two libraries from each species were generated with an average of 55.63, 55.23, 68.13, and 103.81 million reads, resulting in 79.3K, 113.7K, 90.6K, and 121.3K numbers of transcripts in AA, BB, DD, and AABBDD genome species respectively. Number of expressed genes in hexaploid wheat was not proportional to its genome size, but marginally higher than that of its diploid progenitors. However, to capture all the transcripts in hexaploid wheat, sufficiently higher number of reads was required. Functional analysis of DEGs, in all the three comparisons, showed their predominance in three major classes of genes during grain development, i.e., nutrient reservoirs, carbohydrate metabolism, and defence proteins; some of them were subsequently validated through real time quantitative Reverse Transcription Polymerase Chain Reaction (qRT-PCR). Further, developmental stage-specific gene expression showed most of the defence protein genes expressed during initial developmental stages in hexaploid contrary to the diploids at later stages. Genes related to carbohydrates anabolism expressed during early stages, whereas catabolism genes expressed at later stages in all the species. However, no trend was observed in case of different nutrient reservoirs gene expression. This data could be used to study the comparative gene expression among the three diploid species and homeologue-specific expression in hexaploid.

Appraisal of wheat genomics for gene discovery and breeding applications: a special emphasis on advances in Asia

We discussed the most recent efforts in wheat functional genomics to discover new genes and their deployment in breeding with special emphasis on advances in Asian countries. Wheat research community is making significant progress to bridge genotype-to-phenotype gap and then applying this knowledge in genetic improvement. The advances in genomics and phenomics have intrigued wheat researchers in Asia to make best use of this knowledge in gene and trait discovery. These advancements include, but not limited to, map-based gene cloning, translational genomics, gene mapping, association genetics, gene editing and genomic selection. We reviewed more than 57 homeologous genes discovered underpinning important traits and multiple strategies used for their discovery. Further, the complementary advancements in wheat phenomics and analytical approaches to understand the genetics of wheat adaptability, resilience to climate extremes and resistance to pest and diseases were discussed. The challenge to build a gold standard reference genome sequence of bread wheat is now achieved and several de novo reference sequences from the cultivars representing different gene pools will be available soon. New pan-genome sequencing resources of wheat will strengthen the foundation required for accelerated gene discovery and provide more opportunities to practice the knowledge-based breeding.

Identification and characterization of large-scale genomic rearrangements during wheat evolution

Following allopolyploidization, nascent polyploid wheat species react with massive genomic rearrangements, including deletion of transposable element-containing sequences. While such massive rearrangements are considered to be a prominent process in wheat genome evolution and speciation, their structure, extent, and underlying mechanisms remain poorly understood. In this study, we retrieved ~3500 insertions of a specific variant of Fatima, one of the most dynamic gypsy long-terminal repeat retrotransposons in wheat from the recently available high-quality genome drafts of Triticum aestivum (bread wheat) and Triticum turgidum ssp. dicoccoides or wild emmer, the allotetraploid mother of all modern wheats. The dynamic nature of Fatima facilitated the identification of large (i.e., up to ~ 1 million bases) Fatima-containing insertions/deletions (indels) upon comparison of bread wheat and wild emmer genomes. We characterized 11 such indels using computer-assisted analysis followed by PCR validation, and found that they might have occurred via unequal intra-strand recombination or double-strand break (DSB) events. Additionally, we observed one case of introgression of novel DNA fragments from an unknown source into the wheat genome. Our data thus indicate that massive large-scale DNA rearrangements might play a prominent role in wheat speciation.

Phenotypic effects of the U-genome variation in nascent synthetic hexaploids derived from interspecific crosses between durum wheat and its diploid relative Aegilops umbellulata

Aegilops umbellulata is a wild diploid wheat species with the UU genome that is an important genetic resource for wheat breeding. To exploit new synthetic allohexaploid lines available as bridges for wheat breeding, a total of 26 synthetic hexaploid lines were generated through crossing between the durum wheat cultivar Langdon and 26 accessions of Ae. umbellulata. In nascent synthetic hexaploids with the AABBUU genome, the presence of the set of seven U-genome chromosomes was confirmed with U-genome chromosome-specific markers developed based on RNA-seq-derived data from Ae. umbellulata. The AABBUU synthetic hexaploids showed large variations in flowering- and morphology-related traits, and these large variations transmitted well from the parental Ae. umbellulata accessions. However, the variation ranges in most traits examined were reduced under the AABBUU hexaploid background compared with under the diploid parents. The AABBUU and AABBDD synthetic hexaploids were clearly discriminated by several morphological traits, and an increase of plant height and in the number of spikes and a decrease of spike length were commonly observed in the AABBUU synthetics. Thus, interspecific differences in several morphological traits between Ae. umbellulata and A. tauschii largely affected the basic plant architecture of the synthetic hexaploids. In conclusion, the AABBUU synthetic hexaploid lines produced in the present study are useful resources for the introgression of desirable genes from Ae. umbellulata to common wheat.

A roadmap for gene functional characterisation in crops with large genomes: Lessons from polyploid wheat

Understanding the function of genes within staple crops will accelerate crop improvement by allowing targeted breeding approaches. Despite their importance, a lack of genomic information and resources has hindered the functional characterisation of genes in major crops. The recent release of high-quality reference sequences for these crops underpins a suite of genetic and genomic resources that support basic research and breeding. For wheat, these include gene model annotations, expression atlases and gene networks that provide information about putative function. Sequenced mutant populations, improved transformation protocols and structured natural populations provide rapid methods to study gene function directly. We highlight a case study exemplifying how to integrate these resources. This review provides a helpful guide for plant scientists, especially those expanding into crop research, to capitalise on the discoveries made in Arabidopsis and other plants. This will accelerate the improvement of crops of vital importance for food and nutrition security.

Diploid genome differentiation conferred by RNA sequencing-based survey of genome-wide polymorphisms throughout homoeologous loci in Triticum and Aegilops

Background

Triticum and Aegilops diploid species have morphological and genetic diversity and are crucial genetic resources for wheat breeding. According to the chromosomal pairing-affinity of these species, their genome nomenclatures have been defined. However, evaluations of genome differentiation based on genome-wide nucleotide variations are still limited, especially in the three genomes of the genus Aegilops: Ae. caudata L. (CC genome), Ae. comosa Sibth. et Sm. (MM genome), and Ae. uniaristata Vis. (NN genome). To reveal the genome differentiation of these diploid species, we first performed RNA-seq-based polymorphic analyses for C, M, and N genomes, and then expanded the analysis to include the 12 diploid species of Triticum and Aegilops.

Results

Genetic divergence of the exon regions throughout the entire chromosomes in the M and N genomes was larger than that between A- and A^m-genomes. Ae. caudata had the second highest genetic diversity following Ae. speltoides, the putative B genome donor of common wheat. In the phylogenetic trees derived from the nuclear and chloroplast genome-wide polymorphism data, the C, D, M, N, U, and S genome species were connected with short internal branches, suggesting that these diploid species emerged during a relatively short period in the evolutionary process. The highly consistent nuclear and chloroplast phylogenetic topologies indicated that nuclear and chloroplast genomes of the diploid Triticum and Aegilops species coevolved after their diversification into each genome, accounting for most of the genome differentiation among the diploid species.

Conclusions

RNA-sequencing-based analyses successfully evaluated genome differentiation among the diploid Triticum and Aegilops species and supported the chromosome-pairing-based genome nomenclature system, except for the position of Ae. speltoides. Phylogenomic and epigenetic analyses of intergenic and centromeric regions could be essential for clarifying the mechanisms behind this inconsistency.

Introgression of chromosomal segments conferring early heading date from wheat diploid progenitor, Aegilops tauschii Coss., into Japanese elite wheat cultivars

The breeding of agriculturally useful genes from wild crop relatives must take into account recent and future climate change. In Japan, the development of early heading wheat cultivars without the use of any major gene controlling the heading date is desired to avoid overlap of the harvesting time before the rainy season. Here, we backcrossed two early heading lines of a synthetic hexaploid wheat, derived from a crossing between durum wheat and the wild wheat progenitor Aegilops tauschii, with four Japanese elite cultivars to develop early heading lines of bread wheat. In total, nine early heading lines that showed a heading date two to eight days earlier than their parental cultivars in field conditions were selected and established from the selfed progenies of the two- or three-times backcrossed populations. The whole appearance and spike shape of the selected early heading lines looked like their parental wheat cultivars. The mature grains of the selected lines had the parental cultivars’ characteristics, although the grains exhibited longer and narrower shapes. RNA sequencing-based genotyping was performed to detect single nucleotide polymorphisms between the selected lines and their parental wheat cultivars, which revealed the chromosomal regions transmitted from the parental synthetic wheat to the selected lines. The introgression regions could shorten wheat heading date, and their chromosomal positions were dependent on the backcrossed wheat cultivars. Therefore, early heading synthetic hexaploid wheat is useful for fine-tuning of the heading date through introgression of Ae. tauschii chromosomal regions.

Identification of New Leaf Rust Resistance Loci in Wheat and Wild Relatives by Array-Based SNP Genotyping and Association Genetics

Leaf rust caused by Puccinia triticina is the most widespread rust disease of wheat. As pathogen populations are constantly evolving, identification of novel sources of resistance is necessary to maintain disease resistance and stay ahead of this plant-pathogen evolutionary arms race. The wild genepool of wheat is a rich source of genetic diversity, accounting for 44% of the Lr genes identified. Here we performed a genome-wide association study (GWAS) on a diverse germplasm of 385 accessions, including 27 different Triticum and Aegilops species. Genetic characterization using the wheat 90 K array and subsequent filtering identified a set of 20,501 single nucleotide polymorphic (SNP) markers. Of those, 9,570 were validated using exome capture and mapped onto the Chinese Spring reference sequence v1.0. Phylogenetic analyses illustrated four major clades, clearly separating the wild species from the T. aestivum and T. turgidum species. GWAS was conducted using eight statistical models for infection types against six leaf rust isolates and leaf rust severity rated in field trials for 3-4 years at 2-3 locations in Canada. Functional annotation of genes containing significant quantitative trait nucleotides (QTNs) identified 96 disease-related loci associated with leaf rust resistance. A total of 21 QTNs were in haplotype blocks or within flanking markers of at least 16 known Lr genes. The remaining significant QTNs were considered loci that putatively harbor new Lr resistance genes. Isolation of these candidate genes will contribute to the elucidation of their role in leaf rust resistance and promote their usefulness in marker-assisted selection and introgression.

Genome-wide polymorphisms from RNA sequencing assembly of leaf transcripts facilitate phylogenetic analysis and molecular marker development in wild einkorn wheat

A survey of genome-wide polymorphisms between closely related species is required to understand the molecular basis of the evolutionary differentiation of their genomes. Two wild diploid wheat species, namely Triticum monococcum ssp. aegilopoides and T. urartu, are closely related and harbour the A^m and A genomes, respectively. The A-genome donor of tetraploid and common wheat is T. urartu, and T. monococcum ssp. monococcum is the cultivated form derived from the wild einkorn wheat subspecies aegilopoides. Although subspecies aegilopoides has been a useful genetic resource in wheat breeding, genome-wide molecular markers for this subspecies have not been sufficiently developed. Here, we describe the detection of genome-wide polymorphisms such as single-nucleotide polymorphisms (SNPs) and insertions/deletions (indels) from RNA sequencing (RNA-seq) data of leaf transcripts in 15 accessions of the two diploid wheat species. The SNPs and indels, detected using the A genome of common wheat as the reference genome, covered the entire chromosomes of these species. The polymorphism information facilitated a comparison of the genetic diversity of einkorn wheat with that of two related diploid Aegilops species, namely, Ae. tauschii and Ae. umbellulata. Cleaved amplified polymorphic sequence (CAPS) markers converted from the SNP data were efficiently developed to confirm the addition of aegilopoides subspecies chromosomes to tetraploid wheat in nascent allohexaploid lines with AABBA^mA^m genomes. In addition, the CAPS markers permitted linkage map construction in mapping populations of aegilopoides subspecies accessions. Therefore, these RNA-seq data provide information for further breeding of closely related species with no reference genome sequence data.