Frequent numerical and structural chromosome changes in early generations of synthetic hexaploid wheat

Modern hexaploid wheat (Triticum aestivum L.; AABBDD) evolved from a hybrid of tetraploid wheat (closely related to Triticum turgidum L. ssp. durum (Desf.) Husn., AABB) and goatgrass (Aegilops tauschii Coss., DD). Variations in chromosome structure and ploidy played important roles in wheat evolution. How these variations occurred and their role in expanding the genetic diversity in modern wheat is mostly unknown. Synthetic hexaploid wheat (SHW) can be used to investigate chromosome variation that occurs during the early generations of existence. SHW lines derived by crossing durum wheat ‘Langdon’ with twelve Ae. tauschii accessions were analyzed using oligonucelotide probe multiplex fluorescence in situ hybridization (FISH) to metaphase chromosomes and SNP markers. Cluster analysis based on SNP markers categorized them into three groups. Among 702 plants from the S8 and S9 generations, 415 (59.12%) carried chromosome variations involving all 21 chromosomes but with different frequencies for each chromosome and sub-genome. Total chromosome variation frequencies varied between lines, but there was no significant difference among the three groups. The non-random chromosome variations in SHW lines detected in this research may be an indication that similar variations occurred in the early stages of wheat polyploidization and played important roles in wheat evolution.

Genome-wide identification of ZF-HD gene family in Triticum aestivum: Molecular evolution mechanism and function analysis

ZF-HD family genes play important roles in plant growth and development. Studies about the whole genome analysis of ZF-HD gene family have been reported in some plant species. In this study, the whole genome identification and expression profile of the ZF-HD gene family were analyzed for the first time in wheat. A total of 37 TaZF-HD genes were identified and divided into TaMIF and TaZHD subfamilies according to the conserved domain. The phylogeny tree of the TaZF-HD proteins was further divided into six groups based on the phylogenetic relationship. The 37 TaZF-HDs were distributed on 18 of 21 chromosomes, and almost all the genes had no introns. Gene duplication and Ka/Ks analysis showed that the gene family may have experienced powerful purification selection pressure during wheat evolution. The qRT-PCR analysis showed that TaZF-HD genes had significant expression patterns in different biotic stress and abiotic stress. Through subcellular localization experiments, we found that TaZHD6-3B was located in the nucleus, while TaMIF4-5D was located in the cell membrane and nucleus. Our research contributes to a comprehensive understanding of the TaZF-HD family, provides a new perspective for further research on the biological functions of TaZF-HD genes in wheat.

Genome-Wide Identification and Characterization of the Cystatin Gene Family in Bread Wheat (Triticum aestivum L.)

Cystatins, as reversible inhibitors of papain-like and legumain proteases, have been identified in several plant species. Although the cystatin family plays crucial roles in plant development and defense responses to various stresses, this family in wheat (Triticum aestivum L.) is still poorly understood. In this study, 55 wheat cystatins (TaCystatins) were identified. All TaCystatins were divided into three groups and both the conserved gene structures and peptide motifs were relatively conserved within each group. Homoeolog analysis suggested that both homoeolog retention percentage and gene duplications contributed to the abundance of the TaCystatin family. Analysis of duplication events confirmed that segmental duplications played an important role in the duplication patterns. The results of codon usage pattern analysis showed that TaCystatins had evident codon usage bias, which was mainly affected by mutation pressure. TaCystatins may be regulated by cis-acting elements, especially abscisic acid and methyl jasmonate responsive elements. In addition, the expression of all selected TaCystatins was significantly changed following viral infection and cold stress, suggesting potential roles in response to biotic and abiotic challenges. Overall, our work provides new insights into TaCystatins during wheat evolution and will help further research to decipher the roles of TaCystatins under diverse stress conditions.

Dynamic and reversible DNA methylation changes induced by genome separation and merger of polyploid wheat

Background Wheat is a powerful genetic model for studying polyploid evolution and crop domestication. Hexaploid bread wheat was formed by two rounds of interspecific hybridization and polyploidization, processes which are often accompanied by genetic and epigenetic changes, including DNA methylation. However, the extent and effect of such changes during wheat evolution, particularly from tetraploid-to-hexaploid wheat, are currently elusive. Results Here we report genome-wide DNA methylation landscapes in extracted tetraploid wheat (ETW, AABB), natural hexaploid wheat (NHW, AABBDD), resynthesized hexaploid wheat (RHW, AABBDD), natural tetraploid wheat (NTW, AABB), and diploid (DD). In the endosperm, levels of DNA methylation, especially in CHG (H=A, T, or C) context, were dramatically decreased in the ETW relative to natural hexaploid wheat; hypo-differentially methylated regions (DMRs) (850,832) were 24-fold more than hyper-DMRs (35,111). Interestingly, those demethylated regions in ETW were remethylated in the resynthesized hexaploid wheat after the addition of the D genome. In ETW, hypo-DMRs correlated with gene expression, and TEs were demethylated and activated, which could be silenced in the hexaploid wheat. In NHW, groups of TEs were dispersed in genic regions of three subgenomes, which may regulate the expression of TE-associated genes. Further, hypo-DMRs in ETW were associated with reduced H3K9me2 levels and increased expression of histone variant genes, suggesting concerted epigenetic changes after separation from the hexaploid. Conclusion Genome merger and separation provoke dynamic and reversible changes in chromatin and DNA methylation. These changes correlate with altered gene expression and TE activity, which may provide insights into polyploid genome and wheat evolution.

Breeding selection imposed a differential selective pressure on the wheat root-associated microbiome

ABSTRACT Plants-microbiome associations are the result of millions of years of co-evolution. Due to breeding-accelerated plant evolution in non-native and highly managed soil, plant-microbe links could have been lost. We hypothesized that post-domestication breeding of wheat changed the root-associated microbiome. To test this, we analyzed root-associated fungal and bacterial communities shortly after emergence of seedlings representing a transect of wheat evolution including modern wheat, landraces and ancestors. Numbers of observed microbial taxa were highest in landraces bred in low-input agricultural systems, and lowest in ancestors that had evolved in native soils. The microbial communities of modern cultivars were different from those of landraces and ancestors. Old wheat accessions enriched Acidobacteria and Actinobacteria, while modern cultivars enriched OTUs from Candidatus Saccharibacteria, Verrucomicrobia and Firmicutes. The fungal pathogens Fusarium, Neoascochyta and Microdochium enriched in modern cultivars. Both bacterial and fungal communities followed a neutral assembly model when bulk soil was considered as the source community, but accessions of the ancient Triticum turgidum and T. monococcum created a more isolated environment in their roots. In conclusion, wheat root-associated microbiomes have dramatically changed through a transect of breeding history.

Wheat domestication has dramatically affected the root-associated microbiome

Background: Plants-microbiome associations are the results of millions of years of co-evolution. Due to the accelerated plant evolution during domestication of crops and aided by cultivation in non-native highly managed soils, plant-microbe links created through co-evolution could have been lost. Therefore, we hypothesize that dramatic effects on the root-associated microbiome occurred during domestication of wheat.Results: To uncover domestication effects we analyzed root associated fungal and bacterial communities in a transect of wheat evolution including modern wheat cultivars, landraces, Triticum aestivum ssp. spelta and ancestors of wheat including T. turgidum ssp. dicoccum , T. monococcum ssp. monococcum and T. monococcum ssp. aegilopoides at three growth phases shortly after emergence of seedlings. We found that numbers of observed microbial taxa were highest in the landraces, which had been domesticated in low-input agricultural systems, and lowest in wheat ancestors that evolved in native soils. The root-associated microbial community of modern cultivars was significantly different from that of landraces and ancestors of wheat. Old wheat accessions enriched Acidobacteria and Actinobacteria , while modern cultivars enriched OTUs from Candidatus Saccharibacteria , Verrucomicrobia and Firmicutes . The fungal pathogens Fusarium , Neoascochyta and Microdochium were enriched in modern cultivars. The composition of root-associated microbial communities of modern wheat cultivars significantly followed patterns predicted by the neutral community assembly model. Our observations allowed us to suggest that a stronger selective pressure drives the root-associated microbiome of ancient wheat accessions than that of modern wheat cultivars.Conclusions: Here we demonstrate that the wheat root-associated microbiome has dramatically changed through a transect of evolution from wheat ancestors over landraces to modern cultivars. Colonization of roots of ancient accessions was slower than in modern cultivars, and the root-associated microbiome of ancient wheat accessions was driven by stronger selective pressure than that of the modern wheat cultivars. We identified several taxa including Acidobacteria and Actinobacteria enriched in old cultivars and fungal wheat pathogens that were enriched in modern cultivars.