Epigenetics in plants-vernalisation and hybrid vigour

DNA methylation remodeling in F1 hybrids

F1 hybrids derived from a cross between two inbred parental lines often display widespread changes in DNA methylation patterns relative to their parents. To which extent these changes drive non-additive gene expression levels and phenotypic heterosis in F1 individuals is not fully resolved. Current mechanistic models propose that DNA methylation remodeling in hybrids is the result of epigenetic interactions between parental alleles via small interfering RNA (sRNA). These models have strong empirical support but are limited to genomic regions where the two parental lines differ in DNA methylation status. However, most remodeling events occur in parental regions with similar methylation patterns, and seem to be strongly conditioned by distally acting factors, even in isogenic hybrid systems. The molecular basis of these distal interactions is currently unknown, and will likely emerge as an active area of research in the future. Despite these gaps in our molecular understanding, parental DNA methylation states are statistically associated with heterosis, independent of genetic information, and may serve as biomarkers in crop breeding.

Characteristics of Salvia miltiorrhiza methylome and the regulatory mechanism of DNA methylation in tanshinone biosynthesis

Salvia miltiorrhiza is a model medicinal plant with significant economic and medicinal value. Its roots produce a group of diterpenoid lipophilic bioactive components, termed tanshinones. Biosynthesis and regulation of tanshinones has attracted widespread interest. However, the methylome of S. miltiorrhiza has not been analysed and the regulatory mechanism of DNA methylation in tanshinone production is largely unknown. Here we report single-base resolution DNA methylomes from roots and leaves. Comparative analysis revealed differential methylation patterns for CG, CHG, and CHH contexts and the association between DNA methylation and the expression of genes and small RNAs. Lowly methylated genes always had higher expression levels and 24-nucleotide sRNAs could be key players in the RdDM pathway in S. miltiorrhiza. DNA methylation variation analysis showed that CHH methylation contributed mostly to the difference. Go enrichment analysis showed that diterpenoid biosynthetic process was significantly enriched for genes with downstream overlapping with hypoCHHDMR in July_root when comparing with those in March_root. Tanshinone biosynthesis-related enzyme genes, such as DXS2, CMK, IDI1, HMGR2, DXR, MDS, CYP76AH1, 2OGD25, and CYP71D373, were less CHH methylated in gene promoters or downstream regions in roots collected in July than those collected in March. Consistently, gene expression was up-regulated in S. miltiorrhiza roots collected in July compared with March and the treatment of DNA methylation inhibitor 5-azacytidine significantly promoted tanshinone production. It suggests that DNA methylation plays a significant regulatory role in tanshinone biosynthesis in S. miltiorrhiza through changing the levels of CHH methylation in promoters or downstreams of key enzyme genes.

Genome-wide analysis of transcriptome and histone modifications in Brassica napus hybrid

Although utilization of heterosis has largely improved the yield of many crops worldwide, the underlying molecular mechanism of heterosis, particularly for allopolyploids, remains unclear. Here, we compared epigenome and transcriptome data of an elite hybrid and its parental lines in three assessed tissues (seedling, flower bud, and silique) to explore their contribution to heterosis in allopolyploid B. napus. Transcriptome analysis illustrated that a small proportion of non-additive genes in the hybrid compared with its parents, as well as parental expression level dominance, might have a significant effect on heterosis. We identified histone modification (H3K4me3 and H3K27me3) variation between the parents and hybrid, most of which resulted from the differences between parents. H3K4me3 variations were positively correlated with gene expression differences among the hybrid and its parents. Furthermore, H3K4me3 and H3K27me3 were rather stable in hybridization and were mainly inherited additively in the B. napus hybrid. Together, our data revealed that transcriptome reprogramming and histone modification remodeling in the hybrid could serve as valuable resources for better understanding heterosis in allopolyploid crops.

A long non-coding RNA PelncRNA1 is involved in Phyllostachys edulis response to UV-B stress

Phyllostachys edulis (moso bamboo) is China's most widespread bamboo species, with significant economic and ecological values. Long non-coding RNA (lncRNA) is a type of regulatory RNA that is longer than 200 nucleotides and incapable of encoding proteins, and is frequently involved in regulating biotic and abiotic stress and plant development. However, the biological functions of lncRNA in moso bamboo are unknown. In this study, a lncRNA (named PelncRNA1) differentially expressed following UV-B treatment was discovered in the whole transcriptome sequencing database of moso bamboo. The target genes were filtered and defined by correlation analysis of PelncRNA1 and gene expression pattern. The expression levels of PelncRNA1 and its target genes were verified using qRT-PCR. The results demonstrated that the expression levels of PelncRNA1 and its target genes increased during UV-B treatment. In Arabidopsis transgenic seedlings and moso bamboo protoplasts, PelncRNA1 was discovered to influence the expression of its target genes when overexpressed. In addition, transgenic Arabidopsis showed higher tolerance to UV-B stress. These results suggest that PelncRNA1 and its target genes are involved in the response of moso bamboo to UV-B stress. The novel findings would contribute to our understanding of how lncRNAs regulate the response to abiotic stresses in moso bamboo.

Multiple origins, one evolutionary trajectory: gradual evolution characterizes distinct lineages of allotetraploid Brachypodium

The "genomic shock" hypothesis posits that unusual challenges to genome integrity such as whole genome duplication may induce chaotic genome restructuring. Decades of research on polyploid genomes have revealed that this is often, but not always the case. While some polyploids show major chromosomal rearrangements and derepression of transposable elements in the immediate aftermath of whole genome duplication, others do not. Nonetheless, all polyploids show gradual diploidization over evolutionary time. To evaluate these hypotheses, we produced a chromosome-scale reference genome for the natural allotetraploid grass Brachypodium hybridum, accession "Bhyb26." We compared 2 independently derived accessions of B. hybridum and their deeply diverged diploid progenitor species Brachypodium stacei and Brachypodium distachyon. The 2 B. hybridum lineages provide a natural timecourse in genome evolution because one formed 1.4 million years ago, and the other formed 140 thousand years ago. The genome of the older lineage reveals signs of gradual post-whole genome duplication genome evolution including minor gene loss and genome rearrangement that are missing from the younger lineage. In neither B. hybridum lineage do we find signs of homeologous recombination or pronounced transposable element activation, though we find evidence supporting steady post-whole genome duplication transposable element activity in the older lineage. Gene loss in the older lineage was slightly biased toward 1 subgenome, but genome dominance was not observed at the transcriptomic level. We propose that relaxed selection, rather than an abrupt genomic shock, drives evolutionary novelty in B. hybridum, and that the progenitor species' similarity in transposable element load may account for the subtlety of the observed genome dominance.

Additive and non-additive epigenetic signatures of natural hybridisation between fish species with different mating systems

Hybridization is a major source of evolutionary innovation. In plants, epigenetic mechanisms can help to stabilize hybrid genomes and contribute to reproductive isolation, but the relationship between genetic and epigenetic changes in animal hybrids is unclear. We analysed the relationship between genetic background and methylation patterns in natural hybrids of two genetically divergent fish species with different mating systems, Kryptolebias hermaphroditus (self-fertilizing) and K. ocellatus (outcrossing). Co-existing parental species displayed highly distinct genetic (SNPs) and methylation patterns (37,000 differentially methylated cytosines). Hybrids had predominantly intermediate methylation patterns (88.5% of the sites) suggesting additive effects, as expected from hybridization between genetically distant species. The large number of differentially methylated cytosines between hybrids and parental species (n = 5,800) suggests that hybridization may play a role in increasing genetic and epigenetic variation. Although most of the observed epigenetic variation was additive and had a strong genetic component, we also found a small percentage of non-additive, potentially stochastic, methylation differences that might act as an evolutionary bet-hedging strategy and increase fitness under environmental instability.

Progressive DNA demethylation in epigenetic hybrids between parental plants with and without methylation of the transgene promoter

Crosses of parents that differ in their DNA methylation states leads to progressive demethylation in the F₁ hybrids. In plant breeding research, hybrid vigor in F₁ hybrids is known to be a very important phenomenon. Hybrid vigor, or heterosis, refers to the fact that F₁ hybrids from crosses with a certain combination of parents have traits that are superior to those of the parents. In addition, DNA methylation is an important factor that affects gene expression in plant genomes and contributes to hybrid vigor. We introduced the 35S promoter sequence into the cucumber mosaic virus (CMV)-based vector and inoculated the GFP-expressing transgenic Nicotiana benthamiana line 16c with the recombinant virus specifically to induce DNA methylation on the 35S promoter. For plants that had transcriptional gene silencing (TGS) of GFP established by methylation of the 35S promoter (35S-TGS), TGS was fully maintained in their later self-pollinated generations. When the 35S-TGS plants were crossed with 16c, which does not contain DNA methylation in the 35S promoter, the F₁ hybrids unexpectedly became progressively DNA demethylated as the plants grew. We hypothesis that in F₁ hybrids that are produced by a cross between parents with extremely different gene methylation states, the methylation state of the genes in question may shift more and more to hypomethylation as the plants grow. This progressive demethylation phenomenon observed in this study may be important in plant breeding to reactivate the genes which were silenced by DNA methylation.

Small RNAs mediate transgenerational inheritance of genome-wide trans-acting epialleles in maize

Background

Hybridization and backcrossing are commonly used in animal and plant breeding to induce heritable variation including epigenetic changes such as paramutation. However, the molecular basis for hybrid-induced epigenetic memory remains elusive.

Results

Here, we report that hybridization between the inbred parents B73 and Mo17 induces trans-acting hypermethylation and hypomethylation at thousands of loci; several hundreds (~ 3%) are transmitted through six backcrossing and three selfing generations. Notably, many transgenerational methylation patterns resemble epialleles of the nonrecurrent parent, despite > 99% of overall genomic loci are converted to the recurrent parent. These epialleles depend on 24-nt siRNAs, which are eliminated in the isogenic hybrid Mo17xB73:mop1-1 that is defective in siRNA biogenesis. This phenomenon resembles paramutation-like events and occurs in both intraspecific (Mo17xB73) and interspecific (W22xTeosinte) hybrid maize populations. Moreover, siRNA abundance and methylation levels of these epialleles can affect expression of their associated epigenes, many of which are related to stress responses.

Conclusion

Divergent siRNAs between the hybridizing parents can induce trans-acting epialleles in the hybrids, while the induced epigenetic status is maintained for transgenerational inheritance during backcross and hybrid breeding, which alters epigene expression to enhance growth and adaptation. These genetic and epigenetic principles may apply broadly from plants to animals.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-022-02614-0.

Epigenetics for Crop Improvement in Times of Global Change

Epigenetics has emerged as an important research field for crop improvement under the on-going climatic changes. Heritable epigenetic changes can arise independently of DNA sequence alterations and have been associated with altered gene expression and transmitted phenotypic variation. By modulating plant development and physiological responses to environmental conditions, epigenetic diversity-naturally, genetically, chemically, or environmentally induced-can help optimise crop traits in an era challenged by global climate change. Beyond DNA sequence variation, the epigenetic modifications may contribute to breeding by providing useful markers and allowing the use of epigenome diversity to predict plant performance and increase final crop production. Given the difficulties in transferring the knowledge of the epigenetic mechanisms from model plants to crops, various strategies have emerged. Among those strategies are modelling frameworks dedicated to predicting epigenetically controlled-adaptive traits, the use of epigenetics for in vitro regeneration to accelerate crop breeding, and changes of specific epigenetic marks that modulate gene expression of traits of interest. The key challenge that agriculture faces in the 21st century is to increase crop production by speeding up the breeding of resilient crop species. Therefore, epigenetics provides fundamental molecular information with potential direct applications in crop enhancement, tolerance, and adaptation within the context of climate change.

Non-coding RNAs in the interaction between rice and Meloidogyne graminicola

Background

Root knot nematodes (RKN) are plant parasitic nematodes causing major yield losses of widely consumed food crops such as rice (Oryza sativa). Because non-coding RNAs, including small interfering RNAs (siRNA), microRNAs (miRNAs) and long non-coding RNAs (lncRNAs), are key regulators of various plant processes, elucidating their regulation during this interaction may lead to new strategies to improve crop protection. In this study, we aimed to identify and characterize rice siRNAs, miRNAs and lncRNAs responsive to early infection with RKN Meloidogyne graminicola (Mg), based on sequencing of small RNA, degradome and total RNA libraries from rice gall tissues compared with uninfected root tissues.

Results

We found 425 lncRNAs, 3739 siRNAs and 16 miRNAs to be differentially expressed between both tissues, of which a subset was independently validated with RT-qPCR. Functional prediction of the lncRNAs indicates that a large part of their potential target genes code for serine/threonine protein kinases and transcription factors. Differentially expressed siRNAs have a predominant size of 24 nts, suggesting a role in DNA methylation. Differentially expressed miRNAs are generally downregulated and target transcription factors, which show reduced degradation according to the degradome data.

Conclusions

To our knowledge, this work is the first to focus on small and long non-coding RNAs in the interaction between rice and Mg, and provides an overview of rice non-coding RNAs with the potential to be used as a resource for the development of new crop protection strategies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07735-7.

Perspectives for epigenetic editing in crops

Site-specific nucleases (SSNs) have drawn much attention in plant biotechnology due to their ability to drive precision mutagenesis, gene targeting or allele replacement. However, when devoid of its nuclease activity, the underlying DNA-binding activity of SSNs can be used to bring other protein functional domains close to specific genomic sites, thus expanding further the range of applications of the technology. In particular, the addition of functional domains encoding epigenetic effectors and chromatin modifiers to the CRISPR/Cas ribonucleoprotein complex opens the possibility to introduce targeted epigenomic modifications in plants in an easily programmable manner. Here we examine some of the most important agronomic traits known to be controlled epigenetically and review the best studied epigenetic catalytic effectors in plants, such as DNA methylases/demethylases or histone acetylases/deacetylases and their associated marks. We also review the most efficient strategies developed to date to functionalize Cas proteins with both catalytic and non-catalytic epigenetic effectors, and the ability of these domains to influence the expression of endogenous genes in a regulatable manner. Based on these new technical developments, we discuss the possibilities offered by epigenetic editing tools in plant biotechnology and their implications in crop breeding.

Exploitation of Heterosis in Pearl Millet: A Review

The phenomenon of heterosis has fascinated plant breeders ever since it was first described by Charles Darwin in 1876 in the vegetable kingdom and later elaborated by George H Shull and Edward M East in maize during 1908. Heterosis is the phenotypic and functional superiority manifested in the F1 crosses over the parents. Various classical complementation mechanisms gave way to the study of the underlying potential cellular and molecular mechanisms responsible for heterosis. In cereals, such as maize, heterosis has been exploited very well, with the development of many single-cross hybrids that revolutionized the yield and productivity enhancements. Pearl millet (Pennisetum glaucum (L.) R. Br.) is one of the important cereal crops with nutritious grains and lower water and energy footprints in addition to the capability of growing in some of the harshest and most marginal environments of the world. In this highly cross-pollinating crop, heterosis was exploited by the development of a commercially viable cytoplasmic male-sterility (CMS) system involving a three-lines breeding system (A-, B- and R-lines). The first set of male-sterile lines, i.e., Tift 23A and Tift18A, were developed in the early 1960s in Tifton, Georgia, USA. These provided a breakthrough in the development of hybrids worldwide, e.g., Tift 23A was extensively used by Punjab Agricultural University (PAU), Ludhiana, India, for the development of the first single-cross pearl millet hybrid, named Hybrid Bajra 1 (HB 1), in 1965. Over the past five decades, the pearl millet community has shown tremendous improvement in terms of cytoplasmic and nuclear diversification of the hybrid parental lines, which led to a progressive increase in the yield and adaptability of the hybrids that were developed, resulting in significant genetic gains. Lately, the whole genome sequencing of Tift 23D2B1 and re-sequencing of circa 1000 genomes by a consortium led by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) has been a significant milestone in the development of cutting-edge genetic and genomic resources in pearl millet. Recently, the application of genomics and molecular technologies has provided better insights into genetic architecture and patterns of heterotic gene pools. Development of whole-genome prediction models incorporating heterotic gene pool models, mapped traits and markers have the potential to take heterosis breeding to a new level in pearl millet. This review discusses advances and prospects in various fronts of heterosis for pearl millet.

Genome-wide identification and characterisation of Aquaporins in Nicotiana tabacum and their relationships with other Solanaceae species

BACKGROUND

Cellular membranes are dynamic structures, continuously adjusting their composition, allowing plants to respond to developmental signals, stresses, and changing environments. To facilitate transmembrane transport of substrates, plant membranes are embedded with both active and passive transporters. Aquaporins (AQPs) constitute a major family of membrane spanning channel proteins that selectively facilitate the passive bidirectional passage of substrates across biological membranes at an astonishing 108 molecules per second. AQPs are the most diversified in the plant kingdom, comprising of five major subfamilies that differ in temporal and spatial gene expression, subcellular protein localisation, substrate specificity, and post-translational regulatory mechanisms; collectively providing a dynamic transportation network spanning the entire plant. Plant AQPs can transport a range of solutes essential for numerous plant processes including, water relations, growth and development, stress responses, root nutrient uptake, and photosynthesis. The ability to manipulate AQPs towards improving plant productivity, is reliant on expanding our insight into the diversity and functional roles of AQPs.

RESULTS

We characterised the AQP family from Nicotiana tabacum (NtAQPs; tobacco), a popular model system capable of scaling from the laboratory to the field. Tobacco is closely related to major economic crops (e.g. tomato, potato, eggplant and peppers) and itself has new commercial applications. Tobacco harbours 76 AQPs making it the second largest characterised AQP family. These fall into five distinct subfamilies, for which we characterised phylogenetic relationships, gene structures, protein sequences, selectivity filter compositions, sub-cellular localisation, and tissue-specific expression. We also identified the AQPs from tobacco's parental genomes (N. sylvestris and N. tomentosiformis), allowing us to characterise the evolutionary history of the NtAQP family. Assigning orthology to tomato and potato AQPs allowed for cross-species comparisons of conservation in protein structures, gene expression, and potential physiological roles.

CONCLUSIONS

This study provides a comprehensive characterisation of the tobacco AQP family, and strengthens the current knowledge of AQP biology. The refined gene/protein models, tissue-specific expression analysis, and cross-species comparisons, provide valuable insight into the evolutionary history and likely physiological roles of NtAQPs and their Solanaceae orthologs. Collectively, these results will support future functional studies and help transfer basic research to applied agriculture.

The histone modification H3 lysine 27 tri-methylation has conserved gene regulatory roles in the triplicated genome of Brassica rapa L

Brassica rapa L. is an important vegetable and oilseed crop. We investigated the distribution of the histone mark tri-methylation of H3K27 (H3K27me3) in B. rapa and its role in the control of gene expression at two stages of development (2-day cotyledons and 14-day leaves) and among paralogs in the triplicated genome. H3K27me3 has a similar distribution in two inbred lines, while there was variation of H3K27me3 sites between tissues. Sites that are specific to 2-day cotyledons have increased transcriptional activity, and low levels of H3K27me3 in the gene body region. In 14-day leaves, levels of H3K27me3 were associated with decreased gene expression. In the triplicated genome, H3K27me3 is associated with paralogs that have tissue-specific expression. Even though B. rapa and Arabidopsis thaliana are not closely related within the Brassicaceae, there is conservation of H3K27me3-marked sites in the two species. Both B. rapa and A. thaliana require vernalization for floral initiation with FLC being the major controlling locus. In all four BrFLC paralogs, low-temperature treatment increases H3K27me3 at the proximal nucleation site reducing BrFLC expression. Following return to normal temperature growth conditions, H3K27me3 spreads along all four BrFLC paralogs providing stable repression of the gene.

Genome Triplication Leads to Transcriptional Divergence of FLOWERING LOCUS C Genes During Vernalization in the Genus Brassica

The genus Brassica includes oil crops, vegetables, condiments, fodder crops, and ornamental plants. Brassica species underwent a whole genome triplication event after speciation between ancestral species of Brassica and closely related genera including Arabidopsis thaliana. Diploid species such as Brassica rapa and Brassica oleracea have three copies of genes orthologous to each A. thaliana gene, although deletion in one or two of the three homologs has occurred in some genes. The floral transition is one of the crucial events in a plant's life history, and time of flowering is an important agricultural trait. There is a variation in flowering time within species of the genus Brassica, and this variation is largely dependent on a difference in vernalization requirements. In Brassica, like in A. thaliana, the key gene of vernalization is FLOWERING LOCUS C (FLC). In Brassica species, the vernalization response including the repression of FLC expression by cold treatment and the enrichment of the repressive histone modification tri-methylated histone H3 lysine 27 (H3K27me3) at the FLC locus is similar to A. thaliana. B. rapa and B. oleracea each have four paralogs of FLC, and the allotetraploid species, Brassica napus, has nine paralogs. The increased number of paralogs makes the role of FLC in vernalization more complicated; in a single plant, paralogs vary in the expression level of FLC before and after vernalization. There is also variation in FLC expression levels between accessions. In this review, we focus on the regulatory circuits of the vernalization response of FLC expression in the genus Brassica.

Long noncoding RNAs in Brassica rapa L. following vernalization

Brassica rapa L. is an important agricultural crop that requires a period of prolonged cold for flowering. This process is known as vernalization. Studies have shown that long noncoding RNAs (lncRNAs) play important roles in abiotic stress responses and several cold-responsive noncoding RNAs have been suggested to be involved in vernalization. We examined the transcriptome of the Chinese cabbage inbred line (B. rapa L. var. pekinensis) RJKB-T24, and identified 1,444 long intergenic noncoding RNAs (lincRNAs), 551 natural antisense transcripts (NATs), and 93 intronic noncoding RNAs (incRNAs); 549 of the 2,088 lncRNAs significantly altered their expression in response to four weeks of cold treatment. Most differentially expressed lncRNAs did not lead to a change of expression levels in mRNAs covering or near lncRNAs, suggesting that the transcriptional responses to four weeks of cold treatment in lncRNA and mRNA are independent. However, some differentially expressed mRNAs had NATs with expression altered in the same direction. These genes were categorized as having an abiotic stress response, suggesting that the paired-expression may play a role in the transcriptional response to vernalization or cold treatment. We also identified short-term cold treatment induced NATs in BrFLC and BrMAF genes, which are involved in vernalization. The lncRNAs we identified differed from those reported in Arabidopsis thaliana, suggesting the role of lncRNAs in vernalization differ between these two species.

Recent research on the mechanism of heterosis is important for crop and vegetable breeding systems

Heterosis or hybrid vigor is a phenomenon where hybrid progeny have superior performance compared to their parental inbred lines. This is important in the use of F₁ hybrid cultivars in many crops and vegetables. However, the molecular mechanism of heterosis is not clearly understood. Gene interactions between the two genomes such as dominance, overdominance, and epistasis have been suggested to explain the increased biomass and yield. Genetic analyses of F₁ hybrids in maize, rice, and canola have defined a large number of quantitative trait loci, which may contribute to heterosis. Recent molecular analyses of transcriptomes together with reference to the epigenome of the parents and hybrids have begun to uncover new facts about the generation of heterosis. These include the identification of gene expression changes in hybrids, which may be important for heterosis, the role of epigenetic processes in heterosis, and the development of stable high yielding lines.

Twenty-four-nucleotide siRNAs produce heritable trans-chromosomal methylation in F1 Arabidopsis hybrids

Hybrid Arabidopsis plants undergo epigenetic reprogramming producing decreased levels of 24-nt siRNAs and altered patterns of DNA methylation that can affect gene expression. Driving the changes in methylation are the processes trans-chromosomal methylation (TCM) and trans-chromosomal demethylation (TCdM). In TCM/TCdM the methylation state of one allele is altered to resemble the other allele. We show that Pol IV-dependent sRNAs are required to establish TCM events. The changes in DNA methylation and the associated changes in sRNA levels in the F1 hybrid can be maintained in subsequent generations and affect hundreds of regions in the F2 epigenome. The inheritance of these altered epigenetic states varies in F2 individuals, resulting in individuals with genetically identical loci displaying different epigenetic states and gene expression profiles. The change in methylation at these regions is associated with the presence of sRNAs. Loci without any sRNA activity can have altered methylation states, suggesting that a sRNA-independent mechanism may also contribute to the altered methylation state of the F1 and F2 generations.

Development of primer sets that can verify the enrichment of histone modifications, and their application to examining vernalization-mediated chromatin changes in Brassica rapa L

Epigenetic regulation is crucial for the development of plants and for adaptation to a changing environment. Recently, genome-wide profiles of histone modifications have been determined by a combination of chromatin immunoprecipitation (ChIP) and genomic tiling arrays (ChIP on chip) or ChIP and high-throughput sequencing (ChIP-seq) in species including Arabidopsis thaliana, rice and maize. Validation of ChIP analysis by PCR or qPCR using positive and negative regions of histone modification is necessary. In contrast, information about histone modifications is limited in Chinese cabbage, Brassica rapa. The aim of this study was to develop positive and negative control primer sets for H3K4me3 (trimethylation of the 4(th) lysine of H3), H3K9me2, H3K27me3 and H3K36me3 in B. rapa. The expression and histone modification of four FLC paralogs in B. rapa, before and after vernalization, were examined using the method developed here. After vernalization, expression of all four BrFLC genes was reduced, and accumulation of H3K27me3 was observed in three of them. As with A. thaliana, the vernalization response and stability of FLC repression correlated with the accumulation of H3K27me3. These results suggest that the epigenetic state during vernalization is important for high bolting resistance in B. rapa. The positive and negative control primer sets developed here revealed positive and negative histone modifications in B. rapa that can be used as a control for future studies.

Molecular and cellular characteristics of hybrid vigour in a commercial hybrid of Chinese cabbage

BACKGROUND

Heterosis or hybrid vigour is a phenomenon in which hybrid progeny exhibit superior performance compared to their parental inbred lines. Most commercial Chinese cabbage cultivars are F1 hybrids and their level of hybrid vigour is of critical importance and is a key selection criterion in the breeding system.

RESULTS

We have characterized the heterotic phenotype of one F1 hybrid cultivar of Chinese cabbage and its parental lines from early- to late-developmental stages of the plants. Hybrid cotyledons are larger than those of the parents at 4 days after sowing and biomass in the hybrid, determined by the fresh weight of leaves, is greater than that of the larger parent line by approximately 20% at 14 days after sowing. The final yield of the hybrid harvested at 63 days after sowing is 25% greater than the yield of the better parent. The larger leaves of the hybrid are a consequence of increased cell size and number of the photosynthetic palisade mesophyll cells and other leaf cells. The accumulation of plant hormones in the F1 was within the range of the parental levels at both 2 and 10 days after sowing. Two days after sowing, the expression levels of chloroplast-targeted genes in the cotyledon cells were upregulated in the F1 hybrid relative to their mid parent values. Shutdown of chlorophyll biosynthesis in the cotyledon by norflurazon prevented the increased leaf area in the F1 hybrid.

CONCLUSIONS

In the cotyledons of F1 hybrids, chloroplast-targeted genes were upregulated at 2 days after sowing. The increased activity levels of this group of genes suggested that their differential transcription levels could be important for establishing early heterosis but the increased transcription levels were transient. Inhibition of the photosynthetic process in the cotyledon reduced heterosis in later seedling stages. These observations suggest early developmental events in the germinating seedling of the hybrid may be important for later developmental vigour and yield advantage.

Genetic basis of sRNA quantitative variation analyzed using an experimental population derived from an elite rice hybrid

We performed a genetic analysis of sRNA abundance in flag leaf from an immortalized F₂ (IMF2) population in rice. We identified 53,613,739 unique sRNAs and 165,797 sRNA expression traits (s-traits). A total of 66,649 s-traits mapped 40,049 local-sQTLs and 30,809 distant-sQTLs. By defining 80,362 sRNA clusters, 22,263 sRNA cluster QTLs (scQTLs) were recovered for 20,249 of all the 50,139 sRNA cluster expression traits (sc-traits). The expression levels for most of s-traits from the same genes or the same sRNA clusters were slightly positively correlated. While genetic co-regulation between sRNAs from the same mother genes and between sRNAs and their mother genes was observed for a portion of the sRNAs, most of the sRNAs and their mother genes showed little co-regulation. Some sRNA biogenesis genes were located in distant-sQTL hotspots and showed correspondence with specific length classes of sRNAs suggesting their important roles in the regulation and biogenesis of the sRNAs.

DOI:http://dx.doi.org/10.7554/eLife.03913.001

Genes within the DNA of a plant or animal contain instructions to make molecules called RNAs. Some RNA molecules can be decoded to make proteins, whereas others have different roles. A single gene often contains the instructions to make both protein-coding RNAs and non-coding RNAs.

Molecules called small RNAs (or sRNAs) do not code for proteins. Instead, sRNAs can control protein-coding RNA molecules or chemically alter the DNA itself; this allows them to perform many different roles in living organisms. In plants, for example, these molecules affect how the plant grows, the shapes and structures it forms, and how likely it is to survive challenges such as drought and diseases. Often different plants of the same species have different amounts of sRNAs, but the reasons for this remain unclear.

Now, Wang, Yao et al. have made use of a technique called ‘expression quantitative locus’ analysis to look at how sRNAs in rice plants are controlled by additional information encoded within DNA. The analysis identified over 53 million sRNA molecules from a population of rice plants. Many of these sRNAs varied in their abundance between different plants within the population. Wang, Yao et al. also found many thousands of individual instructions within the DNA of the rice that can either increase or reduce the abundance of their associated sRNA.

Some of the abundant sRNAs were influenced by instructions within their own genes; some were influenced by instructions from other genes; and some were influenced by both. Wang, Yao et al. also found that the control of protein-coding RNAs was not necessarily related to the control of sRNAs encoded by the same gene. Further work is now needed to identify which specific DNA sequences regulate the abundance of sRNA molecules in plants and other organisms.

DOI:http://dx.doi.org/10.7554/eLife.03913.002

MSH1-induced non-genetic variation provides a source of phenotypic diversity in Sorghum bicolor

MutS Homolog 1 (MSH1) encodes a plant-specific protein that functions in mitochondria and chloroplasts. We showed previously that disruption or suppression of the MSH1 gene results in a process of developmental reprogramming that is heritable and non-genetic in subsequent generations. In Arabidopsis, this developmental reprogramming process is accompanied by striking changes in gene expression of organellar and stress response genes. This developmentally reprogrammed state, when used in crossing, results in a range of variation for plant growth potential. Here we investigate the implications of MSH1 modulation in a crop species. We found that MSH1-mediated phenotypic variation in Sorghum bicolor is heritable and potentially valuable for crop breeding. We observed phenotypic variation for grain yield, plant height, flowering time, panicle architecture, and above-ground biomass. Focusing on grain yield and plant height, we found some lines that appeared to respond to selection. Based on amenability of this system to implementation in a range of crops, and the scope of phenotypic variation that is derived, our results suggest that MSH1 suppression provides a novel approach for breeding in crops.

Intraspecific Arabidopsis hybrids show different patterns of heterosis despite the close relatedness of the parental genomes

Heterosis is important for agriculture; however, little is known about the mechanisms driving hybrid vigor. Ultimately, heterosis depends on the interactions of specific alleles and epialleles provided by the parents, which is why hybrids can exhibit different levels of heterosis, even within the same species. We characterize the development of several intraspecific Arabidopsis (Arabidopsis thaliana) F1 hybrids that show different levels of heterosis at maturity. We identify several phases of heterosis beginning during embryogenesis and culminating in a final phase of vegetative maturity and seed production. During each phase, the hybrids show different levels and patterns of growth, despite the close relatedness of the parents. For instance, during the vegetative phases, the hybrids develop larger leaves than the parents to varied extents, and they do so by exploiting increases in cell size and cell numbers in different ratios. Consistent with this finding, we observed changes in the expression of genes known to regulate leaf size in developing rosettes of the hybrids, with the patterns of altered expression differing between combinations. The data show that heterosis is dependent on changes in development throughout the growth cycle of the hybrid, with the traits of mature vegetative biomass and reproductive yield as cumulative outcomes of heterosis at different levels, tissues, and times of development.

What is crop heterosis: new insights into an old topic

Heterosis (or hybrid vigor) refers to a natural phenomenon whereby hybrid offspring of genetically diverse individuals out-perform their parents in multiple traits including yield, adaptability and resistances to biotic and abiotic stressors. Innovations in technology and research continue to clarify the mechanisms underlying crop heterosis, however the intrinsic relationship between the biological basis of heterosis remain unclear. In this review, we aim to provide insight into the molecular genetic basis of heterosis by presenting recent advances in the 'omics' of heterosis and the role of non-coding regions, particularly in relation to energy-use efficiency. We propose that future research should focus on integrating the expanding datasets from different species and hybrid combinations, to mine key heterotic genes and unravel interactive 'omics' networks associated with heterosis. Improved understanding of heterosis and the biological basis for its manipulation in agriculture should help to streamline its use in enhancing crop productivity.

Differential methylation during maize leaf growth targets developmentally regulated genes

DNA methylation is an important and widespread epigenetic modification in plant genomes, mediated by DNA methyltransferases (DMTs). DNA methylation is known to play a role in genome protection, regulation of gene expression, and splicing and was previously associated with major developmental reprogramming in plants, such as vernalization and transition to flowering. Here, we show that DNA methylation also controls the growth processes of cell division and cell expansion within a growing organ. The maize (Zea mays) leaf offers a great tool to study growth processes, as the cells progressively move through the spatial gradient encompassing the division zone, transition zone, elongation zone, and mature zone. Opposite to de novo DMTs, the maintenance DMTs were transcriptionally regulated throughout the growth zone of the maize leaf, concomitant with differential CCGG methylation levels in the four zones. Surprisingly, the majority of differentially methylated sequences mapped on or close to gene bodies and not to repeat-rich loci. Moreover, especially the 5' and 3' regions of genes, which show overall low methylation levels, underwent differential methylation in a developmental context. Genes involved in processes such as chromatin remodeling, cell cycle progression, and growth regulation, were differentially methylated. The presence of differential methylation located upstream of the gene anticorrelated with transcript expression, while gene body differential methylation was unrelated to the expression level. These data indicate that DNA methylation is correlated with the decision to exit mitotic cell division and to enter cell expansion, which adds a new epigenetic level to the regulation of growth processes.

Histone variants and chromatin assembly in plant abiotic stress responses

Genome organization into nucleosomes and higher-order chromatin structures has profound implications for the regulation of gene expression, DNA replication and repair. The structure of chromatin can be remodeled by several mechanisms; among others, nucleosome assembly/disassembly and replacement of canonical histones with histone variants constitute important ones. In this review, we provide a brief description on the current knowledge about histone chaperones involved in nucleosome assembly/disassembly and histone variants in Arabidopsis thaliana. We discuss recent advances in revealing crucial functions of histone chaperones, nucleosome assembly/disassembly and histone variants in plant response to abiotic stresses. It appears that chromatin structure remodeling may provide a flexible, global and stable means for the regulation of gene transcription to help plants more effectively cope with environmental stresses. This article is part of a Special Issue entitled: Histone chaperones and chromatin assembly.

Inheritance of Trans Chromosomal Methylation patterns from Arabidopsis F1 hybrids

Hybridization in plants leads to transinteractions between the parental genomes and epigenomes that can result in changes to both 24 nt siRNA and cytosine methylation ((m)C) levels in the hybrid. In Arabidopsis the principle processes altering the hybrid methylome are Trans Chromosomal Methylation (TCM) and Trans Chromosomal deMethylation (TCdM) in which the (m)C pattern of a genomic segment attains the same (m)C pattern of the corresponding segment on the other parental chromosome. We examined two loci that undergo TCM/TCdM in the Arabidopsis C24/Landsberg erecta (Ler) F1 hybrids, which show patterns of inheritance dependent on the properties of the particular donor and recipient chromosomal segments. At At1g64790 the TCM- and TCdM-derived (m)C patterns are maintained in the F2 generation but are transmitted in outcrosses or backcrosses only by the C24 genomic segment. At a region between and adjacent to At3g43340 and At3g43350, the originally unmethylated Ler genomic segment receives the C24 (m)C pattern in the F1, which is then maintained in backcross plants independent of the presence of the parental C24 segment. In backcrosses to an unmethylated Ler allele, the newly methylated F1 Ler segment may act as a TCM source in a process comparable to paramutation in maize. TCM-derived (m)C patterns are associated with reduced expression of both At3g43340 and At3g43350 in F1 and F2 plants, providing support for such events influencing the transcriptome. The inheritance of the F1 (m)C patterns and the segregation of other genetic and epigenetic determinants may contribute to the reduced hybrid vigor in the F2 and subsequent generations.

Landscaping plant epigenetics

The understanding of epigenetic mechanisms is necessary for assessing the potential impacts of epigenetics on plant growth, development and reproduction, and ultimately for the response of these factors to evolutionary pressures and crop breeding programs. This volume highlights the latest in laboratory and bioinformatic techniques used for the investigation of epigenetic phenomena in plants. Such techniques now allow genome-wide analyses of epigenetic regulation and help to advance our understanding of how epigenetic regulatory mechanisms affect cellular and genome function. To set the scene, we begin with a short background of how the field of epigenetics has evolved, with a particular focus on plant epigenetics. We consider what has historically been understood by the term "epigenetics" before turning to the advances in biochemistry, molecular biology, and genetics which have led to current-day definitions of the term. Following this, we pay attention to key discoveries in the field of epigenetics that have emerged from the study of unusual and enigmatic phenomena in plants. Many of these phenomena have involved cases of non-Mendelian inheritance and have often been dismissed as mere curiosities prior to the elucidation of their molecular mechanisms. In the penultimate section, consideration is given to how advances in molecular techniques are opening the doors to a more comprehensive understanding of epigenetic phenomena in plants. We conclude by assessing some opportunities, challenges, and techniques for epigenetic research in both model and non-model plants, in particular for advancing understanding of the regulation of genome function by epigenetic mechanisms.

The role of epigenetics in hybrid vigour

Hybrid vigour, or heterosis, refers to the increased yield and biomass of hybrid offspring relative to the parents. Although this has been exploited in plants for agriculture and horticulture, the molecular and cellular mechanisms underlying hybrid vigour are largely unknown. Genetic analyses show that there are a large number of quantitative trait loci (QTLs) that contribute to the heterotic phenotype, indicating that it is a complex phenomenon. Gene expression in hybrids is regulated by the interactions of the two parental epigenetic systems and the underlying genomes. Increasing understanding of the interplay of small RNA (sRNA) molecules, DNA methylation, and histone marks provides new opportunities to define the basis of hybrid vigour and to understand why F1 heterosis is not passed on to subsequent generations. We discuss recent findings that suggest the existence of several pathways that alter DNA methylation patterns, which may lead to transcriptional changes resulting in the heterotic phenotype.

Epigenetic and small RNA regulation of senescence

Leaf senescence is regulated through a complex regulatory network triggered by internal and external signals for the reprogramming of gene expression. In plants, the major developmental phase transitions and stress responses are under epigenetic control. In this review, the underlying molecular mechanisms are briefly discussed and evidence is shown that epigenetic processes are also involved in the regulation of leaf senescence. Changes in the chromatin structure during senescence, differential histone modifications determining active and inactive sites at senescence-associated genes and DNA methylation are addressed. In addition, the role of small RNAs in senescence regulation is discussed.

Reciprocal cross differences in Drosophila melanogaster longevity: an evidence for non-genomic effects in heterosis phenomenon?

Reciprocal cross effects (i.e., differences between reciprocal hybrids that are developed by reversing the strains from which the dam and the sire are taken) are commonly used as a measure of sex-linkage or maternal effects. However, the papers reporting parental effects on life span of experimental animals are scarce. In order to investigate the potential of parent-of-origin effects for the longevity of hybrids, we determined the life spans of the inbred lines of Drosophila melanogaster [Oregon-R (OR), Canton-S (CS) and Uman (Um)] that differ significantly in longevity, as well as the life span of the progeny from the reciprocal crosses among them. The hybridization caused the increase in both flies' mean and maximum life span mainly shifting the survival curves upward proportionally at all ages. This resulted in the reduction in the Gompertz intercept (frailty) whereas the Gompertz slope (the rate of aging) was predominantly unchanged. Better-parent heterosis was observed in hybrids between OR and Um inbred lines and the extent of heterosis was more pronounced in hybrids between CS and Um inbred lines if long-lived parent was used as the female parent, and short-lived parent was used as the male parent in the crossing scheme. Such discrepancy in life span between reciprocal crosses may indicate that non-chromosomal factors are significantly contributing to a heterotic response. Our data are in line with the previous reports suggesting the involvement of non-genomic factors, particularly epigenetic events attributed to hybridization, in the manifestation of heterosis.

Epigenetic regulation of adaptive responses of forest tree species to the environment

Epigenetic variation is likely to contribute to the phenotypic plasticity and adaptative capacity of plant species, and may be especially important for long-lived organisms with complex life cycles, including forest trees. Diverse environmental stresses and hybridization/polyploidization events can create reversible heritable epigenetic marks that can be transmitted to subsequent generations as a form of molecular "memory". Epigenetic changes might also contribute to the ability of plants to colonize or persist in variable environments. In this review, we provide an overview of recent data on epigenetic mechanisms involved in developmental processes and responses to environmental cues in plant, with a focus on forest tree species. We consider the possible role of forest tree epigenetics as a new source of adaptive traits in plant breeding, biotechnology, and ecosystem conservation under rapid climate change.

Progress toward understanding heterosis in crop plants

Although heterosis, or hybrid vigor, is widely exploited in agriculture, a complete description of its molecular underpinnings has remained elusive despite extensive investigation. It appears that there is not a single, simple explanation for heterosis. Instead, it is likely that heterosis arises in crosses between genetically distinct individuals as a result of a diversity of mechanisms. Heterosis generally results from the action of multiple loci, and different loci affect heterosis for different traits and in different hybrids. Hence, multigene models are likely to prove most informative for understanding heterosis. Complementation of allelic variation, as well as complementation of variation in gene content and gene expression patterns, is likely to be an important contributor to heterosis. Epigenetic variation has the potential to interact in hybrid genotypes via novel mechanisms. Several other intriguing hypotheses are also under investigation. In crops, heterosis must be considered within the context of the genomic impacts of prior selection for agronomic traits.

[Epigenetics of plant vernalization regulated by non-coding RNAs]

Many higher plants must experience a period of winter cold to accomplish the transition from vegetative to reproductive growth. This biological process is called vernalization. Some crops such as wheat (Triticum aestivum L.) and oilseed rape (Brassica napus L.) produce seeds as edible organs, and therefore special measures of rotation and cultivation are necessary for plants to go through an early vernalization for flower differentiation and development, whereas the other crops such as Chinese cabbage (B rapa ssp. pekinenesis) and cabbage (Brassica napus L.) produce leafy heads as edible organs, and additional practice should be taken to avoid vernalization for a prolonged and fully vegetative growth. Before vernalization, flowering is repressed by the action of a gene called Flowering Locus C (FLC). This paper reviewed the function of non-coding RNAs and some proteins including VRN1, VRN2, and VIN3 in epigenetic regulation of FLC during vernalization.

Trans-chromosomal methylation

The epigenome plays a vital role in helping to maintain and regulate cell functions in all organisms. Alleles with differing epigenetic marks in the same nucleus do not function in isolation but can interact in trans to modify the epigenetic state of one or both alleles. This is particularly evident when two divergent epigenomes come together in a hybrid resulting in thousands of alterations to the methylome. These changes mainly involve the methylation patterns at one allele being changed to resemble the methylation patterns of the other allele, in processes we have termed trans-chromosomal methylation (TCM) and trans-chromosomal demethylation (TCdM). These processes are primarily modulated by siRNAs and the RNA directed DNA methylation pathway. Drawing from other examples of trans-allelic interactions, we describe the process of TCM and TCdM and the effect such changes can have on genome activity. Trans-allelic epigenetic interactions may be a common occurrence in many biological systems.

Trans chromosomal methylation in Arabidopsis hybrids

The heterotic hybrid offspring of Arabidopsis accessions C24 and Landsberg erecta have altered methylomes. Changes occur most frequently at loci where parental methylation levels are different. There are context-specific biases in the nonadditive methylation patterns with (m)CG generally increased and (m)CHH decreased relative to the parents. These changes are a result of two main mechanisms, Trans Chromosomal Methylation and Trans Chromosomal deMethylation, where the methylation level of one parental allele alters to resemble that of the other parent. Regions of altered methylation are enriched around genic regions and are often correlated with changes in siRNA levels. We identified examples of genes with altered expression likely to be due to methylation changes and suggest that in crosses between the C24 and Ler accessions, epigenetic controls can be important in the generation of altered transcription levels that may contribute to the increased biomass of the hybrids.