Paternally imprinted LATE-FLOWERING2 transcription factor contributes to paternal-excess interploidy hybridization barriers in wheat.

Interploidy hybridization between hexaploid and tetraploid genotypes occurred repeatedly during genomic introgression events throughout wheat evolution, and is commonly employed in wheat breeding programs. Hexaploid wheat usually serves as maternal parent because the reciprocal cross generates progeny with severe defects and poor seed germination, but the underlying mechanism is poorly understood. Here, we performed detailed analysis of phenotypic variation in endosperm between two interploidy reciprocal crosses arising from tetraploid (Triticum durum, AABB) and hexaploid wheat (Triticum aestivum, AABBDD). In the paternal- versus the maternal-excess cross, the timing of endosperm cellularization was delayed and starch granule accumulation in the endosperm was repressed, causing reduced germination percentage. The expression profiles of genes involved in nutrient metabolism differed strongly between these endosperm types. Furthermore, expression patterns of parental alleles were dramatically disturbed in interploidy versus intraploidy crosses, leading to increased number of imprinted genes. The endosperm-specific TaLFL2 showed a paternally imprinted expression pattern in interploidy crosses partially due to allele-specific DNA methylation. Paternal TaLFL2 binds to and represses a nutrient accumulation regulator TaNAC019, leading to reduced storage protein and starch accumulation during endosperm development in paternal-excess cross, as confirmed by interploidy crosses between tetraploid wild-type and clustered regularly interspaced palindromic repeats (CRISPR) - CRISPR-associated protein 9 generated hexaploid mutants. These findings reveal a contribution of genomic imprinting to paternal-excess interploidy hybridization barriers during wheat evolution history and explains why experienced breeders preferentially exploit maternal-excess interploidy crosses in wheat breeding programs.

Stress granule-associated TaMBF1c confers thermotolerance through regulating specific mRNA translation in wheat (Triticum aestivum).

Heat stress is a major limiting factor for global wheat production and causes dramatic yield loss worldwide. The TaMBF1c gene is upregulated in response to heat stress in wheat. Understanding the molecular mechanisms associated with heat stress responses will pave the way to improve wheat thermotolerance. Through CRISPR/Cas9-based gene editing, polysome profiling coupled with RNA-sequencing analysis, and protein-protein interactions, we show that TaMBF1c conferred heat response via regulating a specific gene translation in wheat. The results showed that TaMBF1c is evolutionarily conserved in diploid, tetraploid and hexaploid wheat species, and its knockdown and knockout lines show increased heat sensitivity. TaMBF1c is colocalized with the stress granule complex and interacts with TaG3BP. TaMBF1c affects the translation efficiency of a subset of heat responsive genes, which are significantly enriched in the 'sequence-specific DNA binding' term. Moreover, gene expression network analysis demonstrated that TaMBF1c is closely associated with the translation of heat shock proteins. Our findings reveal a contribution of TaMBF1c in regulating the heat stress response via the translation process, and provide a new target for improving heat tolerance in wheat breeding programs.

Eddy Current Measurement for Planar Structures.

Eddy current (EC) testing has become one of the most common techniques for measuring metallic planar structures in various industrial scenarios such as infrastructures, automotive, manufacturing, and chemical engineering. There has been significant progress in measuring the geometry, electromagnetic properties, and defects of metallic planar structures based on electromagnetic principles. In this review, we summarize recent developments in EC computational models, systems, algorithms, and measurement approaches for planar structures. First, the computational models including analytical models, numerical methods, and plate property estimation algorithms are introduced. Subsequently, the impedance measurement system and probes are presented. In plate measurements, sensor signals are sensitive to probe lift-off, and various algorithms for reducing the lift-off effect are reviewed. These approaches can be used for measureing thickness and electromagnetic properties. Furthermore, defect detection for metallic plates is also discussed.

Changes in Alternative Splicing in Response to Domestication and Polyploidization in Wheat.

Alternative splicing (AS) occurs extensively in eukaryotes as an important mechanism for regulating transcriptome complexity and proteome diversity, but variation in the AS landscape in response to domestication and polyploidization in crops is unclear. Hexaploid wheat (AABBDD, Triticum aestivum) has undergone two separate allopolyploidization events, providing an ideal model for studying AS changes during domestication and polyploidization events. In this study, we performed high-throughput transcriptome sequencing of roots and leaves from wheat species with varied ploidies, including wild diploids (AbAb, Triticum boeoticum) and tetraploids (AABB, Triticum dicoccoides), domesticated diploids (AmAm, Triticum monococcum) and tetraploids (AABB, Triticum dicoccum), hexaploid wheat (AABBDD, T aestivum), as well as newly synthesized hexaploids together with their parents. Approximately 22.1% of genes exhibited AS, with the major AS type being intron retention. The number of AS events decreased after domestication in both diploids and tetraploids. Moreover, the frequency of AS occurrence tended to decrease after polyploidization, consistent with the functional sharing model that proposes AS and duplicated genes are complementary in regulating transcriptome plasticity in polyploid crops. In addition, the subgenomes exhibited biased AS responses to polyploidization, and ∼87.1% of homeologs showed AS partitioning in hexaploid wheat. Interestingly, substitution of the D-subgenome modified 42.8% of AS patterns of the A- and B-subgenomes, indicating subgenome interplay reprograms AS profiles at a genome-wide level, although the causal-consequence relationship requires further study. Conclusively, our study shows that AS variation occurs extensively after polyploidization and domestication in wheat species.

Genomic Imprinting Was Evolutionarily Conserved during Wheat Polyploidization.

Genomic imprinting is an epigenetic phenomenon that causes genes to be differentially expressed depending on their parent of origin. To evaluate the evolutionary conservation of genomic imprinting and the effects of ploidy on this process, we investigated parent-of-origin-specific gene expression patterns in the endosperm of diploid (Aegilops spp), tetraploid, and hexaploid wheat (Triticum spp) at various stages of development via high-throughput transcriptome sequencing. We identified 91, 135, and 146 maternally or paternally expressed genes (MEGs or PEGs, respectively) in diploid, tetraploid, and hexaploid wheat, respectively, 52.7% of which exhibited dynamic expression patterns at different developmental stages. Gene Ontology enrichment analysis suggested that MEGs and PEGs were involved in metabolic processes and DNA-dependent transcription, respectively. Nearly half of the imprinted genes exhibited conserved expression patterns during wheat hexaploidization. In addition, 40% of the homoeolog pairs originating from whole-genome duplication were consistently maternally or paternally biased in the different subgenomes of hexaploid wheat. Furthermore, imprinted expression was found for 41.2% and 50.0% of homolog pairs that evolved by tandem duplication after genome duplication in tetraploid and hexaploid wheat, respectively. These results suggest that genomic imprinting was evolutionarily conserved between closely related Triticum and Aegilops species and in the face of polyploid hybridization between species in these genera.

Wheat TaSPL8 Modulates Leaf Angle Through Auxin and Brassinosteroid Signaling.

In grass crops, leaf angle is determined by development of the lamina joint, the tissue connecting the leaf blade and sheath, and is closely related to crop architecture and yield. In this study, we identified a mutant generated by fast neutron radiation that exhibited an erect leaf phenotype caused by defects in lamina joint development. Map-based cloning revealed that the gene TaSPL8, encoding a SQUAMOSA PROMOTER BINDING-LIKE (SPL) protein, is deleted in this mutant. TaSPL8 knock-out mutants exhibit erect leaves due to loss of the lamina joint, compact architecture, and increased spike number especially in high planting density, suggesting similarity with its LIGULESS1 homologs in maize (Zea mays) and rice (Oryza sativa). Hence, LG1 could be a robust target for plant architecture improvement in grass species. Common wheat (Triticum aestivum, 2n = 6× = 42; BBAADD) is an allohexaploid containing A, B, and D subgenomes and the homeologous gene of TaSPL8 from the D subgenome contributes to the length of the lamina joint to a greater extent than that from the A and B subgenomes. Comparison of the transcriptome between the Taspl8 mutant and the wild type revealed that TaSPL8 is involved in the activation of genes related to auxin and brassinosteroid pathways and cell elongation. TaSPL8 binds to the promoters of the AUXIN RESPONSE FACTOR gene and of the brassinosteroid biogenesis gene CYP90D2 and activates their expression. These results indicate that TaSPL8 might regulate lamina joint development through auxin signaling and the brassinosteroid biosynthesis pathway.

Btr1-A Induces Grain Shattering and Affects Spike Morphology and Yield-Related Traits in Wheat.

Spike brittleness represents an important domestication trait in crops. Although the brittle rachis of wild wheat was cloned, however, the molecular mechanism underlying spike brittleness is yet to be elucidated. Here, we identified a single dominant brittle rachis gene Br-Ab on chromosome arm 3AbS using an F2 population of diploid wheat and designated Btr1-Ab. Sequence analysis of the Btr1-A gene in 40 diploid wheat accessions, 80 tetraploid wheat accessions and 38 hexaploid wheat accessions showed that two independent mutations (Ala119Thr for diploid and Gly97* for polyploids) in the Btr1-A coding region resulting in the nonbrittle rachis allele. Overexpression of Btr1-Ab in nonbrittle hexaploid wheat led to brittle rachis in transgenic plants. RNA-Seq analysis revealed that Btr1-A represses the expression of cell wall biosynthesis genes during wheat rachis development. In addition, we found that Btr1-A can modify spike morphology and reduce threshability, grain size and thousand grain weight in transgenic wheat. These results demonstrated that Btr1-A reduces cell wall synthesis in rachis nodes, resulting in natural spikelet shattering, and that the transition from Btr1-A to btr1-A during wheat domestication had profound effects on evolution of spike morphology and yield-related traits.

Use of near-isogenic lines to precisely map and validate a major QTL for grain weight on chromosome 4AL in bread wheat (Triticum aestivum L.).

KEY MESSAGE: This study precisely mapped and validated a major quantitative trait locus (QTL) on chromosome 4AL for thousand-grain weight in wheat using multiple near-isogenic lines. Thousand-grain weight (TGW) is an essential yield component. Following the previous identification of a major QTL for TGW within the interval of 15.7 cM (92.7-108.4 cM) on chromosome 4AL using the Nongda3338 (ND3338)/Jingdong6 (JD6) doubled haploid population, the aim of this study was to perform more precise mapping and validate the genetic effect of the QTL. Multiple near-isogenic lines (NILs) were developed using ND3338 as the recurrent parent through marker-assisted selection. Based on five independent BC3F3:4 segregating populations derived from BC3F3 plants with different heterozygous segments for the target QTL site and the results of genotyping analysis performed using the Wheat660 K SNP array, it was possible to delimit the QTL region to a physical interval of approximately 6.5 Mb (677.11-683.61 Mb, IWGSC Ref Seq v1.0). Field trials across multiple environments showed that NILsJD6 had a consistent effect on increasing the TGW by 5.16-27.48% and decreasing the grain number per spike (GNS) by 3.98-32.91% compared to the corresponding NILsND3338, which exhibited locus-specific TGW-GNS trade-offs. Moreover, by using RNA sequencing (RNA-Seq) of whole grains at 10 days after pollination stage of multiple NILs, we found that differentially expressed genes between the NIL pairs were significantly enriched for cell cycle and the replication of chromosome-related genes, hence affecting cell division and cell proliferation. Overall, our results provide a basis for map-based cloning of the major QTL and determining the mechanisms underlying TGW-GNS trade-offs in wheat, which would help to fine-tune these two components and maximize the grain yield for breeders.

Genetic architecture of inducible and constitutive metabolic profile related to drought resistance in qingke (Tibetan hulless barley).

Qingke (Tibetan hulless barley, Hordeum vulgare L. var. nudum) is the primary food crop on the Tibet Plateau, the long-term drought and other harsh environments makes qingke an important resource for the study of abiotic resistance. Here, we evaluated the drought sensitivity of 246 qingke varieties. Genome-wide association studies (GWAS) found that root-specific expressed gene CYP84 may be involved in the regulation of drought resistance. Based on widely targeted metabolic profiling, we identified 2,769 metabolites in qingke leaves, of which 302 were significantly changed in response to drought stress, including 4-aminobutyric acid (GABA), proline, sucrose and raffinose. Unexpectedly, these drought-induced metabolites changed more violently in drought-sensitive qingkes, while the constitutive metabolites that had little response to drought stress, such as C-glycosylflavonoids and some amino acids, accumulated excessively in drought-resistant qingkes. Combined with metabolite-based genome-wide association study (mGWAS), a total of 1,006 metabolites under optimal condition and 1,031 metabolites under mild drought stress had significant associated loci. As a marker metabolite induced by drought stress, raffinose was significantly associated with two conservatively adjacent α-galactosidase genes, qRT-PCR suggests that these two genes may jointly regulate the raffinose content in qingke. Besides, as constituent metabolites with stable differences between drought-sensitive and drought-resistant qingkes, a class of C-glycosylflavonoids are simultaneously regulated by a UDP-glucosyltransferase gene. Overall, we performed GWAS for sensitivity and widely targeted metabolites during drought stress in qingke for the first time, which provides new insights into the response mechanism of plant drought stress and drought resistance breeding.

QTL analysis of important agronomic traits and metabolites in foxtail millet (Setaria italica) by RIL population and widely targeted metabolome.

As a bridge between genome and phenotype, metabolome is closely related to plant growth and development. However, the research on the combination of genome, metabolome and multiple agronomic traits in foxtail millet (Setaria italica) is insufficient. Here, based on the linkage analysis of 3,452 metabolites via with high-quality genetic linkage maps, we detected a total of 1,049 metabolic quantitative trait loci (mQTLs) distributed in 11 hotspots, and 28 metabolite-related candidate genes were mined from 14 mQTLs. In addition, 136 single-environment phenotypic QTL (pQTLs) related to 63 phenotypes were identified by linkage analysis, and there were 12 hotspots on these pQTLs. We futher dissected 39 candidate genes related to agronomic traits through metabolite-phenotype correlation and gene function analysis, including Sd1 semidwarf gene, which can affect plant height by regulating GA synthesis. Combined correlation network and QTL analysis, we found that flavonoid-lignin pathway maybe closely related to plant architecture and yield in foxtail millet. For example, the correlation coefficient between apigenin 7-rutinoside and stem diameter reached 0.98, and they were co-located at 41.33-44.15 Mb of chromosome 5, further gene function analysis revealed that 5 flavonoid pathway genes, as well as Sd1, were located in this interval . Therefore, the correlation and co-localization between flavonoid-lignins and plant architecture may be due to the close linkage of their regulatory genes in millet. Besides, we also found that a combination of genomic and metabolomic for BLUP analysis can better predict plant agronomic traits than genomic or metabolomic data, independently. In conclusion, the combined analysis of mQTL and pQTL in millet have linked genetic, metabolic and agronomic traits, and is of great significance for metabolite-related molecular assisted breeding.

Origin and adaptation to high altitude of Tibetan semi-wild wheat.

Tibetan wheat is grown under environmental constraints at high-altitude conditions, but its underlying adaptation mechanism remains unknown. Here, we present a draft genome sequence of a Tibetan semi-wild wheat (Triticum aestivum ssp. tibetanum Shao) accession Zang1817 and re-sequence 245 wheat accessions, including world-wide wheat landraces, cultivars as well as Tibetan landraces. We demonstrate that high-altitude environments can trigger extensive reshaping of wheat genomes, and also uncover that Tibetan wheat accessions accumulate high-altitude adapted haplotypes of related genes in response to harsh environmental constraints. Moreover, we find that Tibetan semi-wild wheat is a feral form of Tibetan landrace, and identify two associated loci, including a 0.8-Mb deletion region containing Brt1/2 homologs and a genomic region with TaQ-5A gene, responsible for rachis brittleness during the de-domestication episode. Our study provides confident evidence to support the hypothesis that Tibetan semi-wild wheat is de-domesticated from local landraces, in response to high-altitude extremes.