Mapping of Functional Subdomains in the atALKBH9B m6A-Demethylase Required for Its Binding to the Viral RNA and to the Coat Protein of Alfalfa Mosaic Virus

Writers, readers, and erasers of N6-Methyladenosine (m6A) methylomes in oilseed rape: identification, molecular evolution, and expression profiling

BACKGROUND

m6A RNA modifications are the most prevalent internal modifications in eukaryotic mRNAs and are crucial for plant growth and development, as well as for responses to biotic or abiotic stresses. The modification is catalyzed by writers, removed by erasers, and decoded by various m6A-binding proteins, which are readers. Brassica napus is a major oilseed crop. The dynamic regulation of m6A modifications by writers, erasers, and readers offers potential targets for improving the quality of this crop.

RESULTS

In this study, we identified 92 m6A-regulatory genes in B. napus, including 13 writers, 29 erasers, and 50 readers. A phylogenetic analysis revealed that they could be further divided into four, three, and two clades, respectively. The distribution of protein motifs and gene structures among members of the same clade exhibited notable similarity. During the course of evolution, whole genome duplication (WGD) and segmental duplication were the primary drivers of the expansion of m6A-related gene families. The genes were subjected to rigorous purification selection. Additionally, several sites under positive selection were identified in the proteins. RNA-seq and quantitative real-time PCR (qRT-PCR) expression analyses revealed that the identified Bnam6As exhibit tissue-specific expression patterns, as well as their expression patterns in response to various abiotic and biotic stresses. The 2000 bp sequence upstream of Bnam6As contained a number of cis-acting elements that regulate plant growth and environmental response. Furthermore, the protein interaction network revealed their interactions with a number of proteins of significant functional importance.

CONCLUSION

The identification of m6A modifiers in oilseed rape and their molecular evolution and expression profiling have revealed potential functions and molecular mechanisms of m6A, thus establishing a foundation for further functional validation and molecular breeding.

Insights into the role of N6-methyladenosine (m6A) in plant-virus interactions

N6-methyladenosine (m6A) is a common and dynamic epitranscriptomic modification in eukaryotic RNAs, affecting stability, splicing, translation, and degradation. Recent technological advancements have revealed the complex nature of m6A modifications, highlighting their importance in plant and animal species. The m6A modification is a reversible process, with "writers" depositing methylation, "erasers" demethylating it, and "reader" proteins recognizing m6A and executing various biological functions. Studying the relationship between m6A methylation and viral infection is crucial. Animal viruses, including retroviruses, RNA viruses, and DNA viruses, often employ the host's m6A machinery to replicate or avoid immune responses. In plant viruses, host methyltransferases or demethylases can stabilize or degrade viral RNA, depending on the virus-host interaction. Additionally, viral infections can modify the host's m6A machinery, impacting the viral life cycle. This review examines the role of m6A modifications in plant viral pathogenesis, focussing on RNA viruses infecting crops like alfalfa, turnip, wheat, rice, and potato. Understanding the role of m6A in virus-host interactions can aid in studying plant viral disease development and discovering novel antiviral targets for crop protection. In this review, we summarize current information on m6A in RNA biology, focussing on its function in viral infections and plant-virus interactions.

Palmatine inhibits expression fat mass and obesity associated protein (FTO) and exhibits a curative effect in dextran sulfate sodium (DSS)-induced experimental colitis

BACKGROUND

Ulcerative colitis (UC) is an inflammatory disease whose pathogenesis and mechanisms have not been fully described. The m6A methylation modification is a general mRNA modification in mammalian cells and is closely associated with the onset and progression of inflammatory bowel disease (IBD). Palmatine (PAL) is a biologically active alkaloid with anti-inflammatory and protective effects in animal models of colitis. Accordingly, we examined the role of PAL on colitis by regulating N6-methyladenosine (m6A) methylation.

METHODS

A rat experimental colitis model was established by 5 % dextran sulfate sodium (DSS) in drinking water for seven days, then PAL treatment was administered for seven days. The colonic tissue pathology was assessed using hematoxylin-eosin (HE) and disease activity index (DAI). In in vitro studies, a human, spontaneously immortalized non-cancerous colon mucosal epithelial cell line (NCM460) was exposed to 2 % DSS and treated with PAL and cell viability was assayed using Cell Counting Kit-8 (CCK-8). The levels of tumor necrosis factor α (TNF-α), interleukin (IL)-1β, IL-6, and IL-8 were detected by enzyme-linked immunosorbent assay (ELISA) kits. The level of Zonula occludens-1 (ZO-1) was dectected by immunofluorescence. Transepithelial electrical resistance (TEER) of cells was also assessed. The methyltransferase-like 3 (METTL3), METTL14, AlkB homologate 5 (ALKBH5), and fat mass and obesity-associated protein (FTO) expression levels were assessed by western blotting. The localized expression of m6A was measured by immunofluorescence.

RESULTS

PAL significantly prevented bodyweight loss and shortening of the colon in experimental colitis rats, as well as decreasing the DAI and histological damage scores. Furthermore, PAL inhibited the levels of inflammatory factors (TNF-α, IL-6, IL-8, and IL-1β) in both DSS treated rats and NCM460 cells. In addition, PAL enhanced the expression level of ZO-1, and increased the transepithelial electrical resistance to repaire intestinal barrier dysfunction. Colitis occurred due to decreased m6A levels, and the increased FTO expression led to a colitis phenotype. PAL markedly enhanced the METTL3 and METTL14 expression levels while decreasing ALKBH5 and FTO expression levels.

CONCLUSIONS

The findings demonstrated that PAL improved DSS-induced experimental colitis. This effect was associated with inhibiting FTO expression and regulating m6A methylation.

m6A RNA demethylase AtALKBH9B promotes mobilization of a heat-activated long terminal repeat retrotransposon in Arabidopsis

Transposons are mobile and ubiquitous DNA molecules that can cause vast genomic alterations. In plants, it is well documented that transposon mobilization is strongly repressed by DNA methylation; however, its regulation at the posttranscriptional level remains relatively uninvestigated. Here, we suggest that transposon RNA is marked by m6A RNA methylation and can be localized in stress granules (SGs). Intriguingly, SG-localized AtALKBH9B selectively demethylates a heat-activated retroelement, Onsen, and thereby releases it from spatial confinement, allowing for its mobilization. In addition, we show evidence that m6A RNA methylation contributes to transpositional suppression by inhibiting virus-like particle assembly and extrachromosomal DNA production. In summary, this study unveils a previously unknown role for m6A in the suppression of transposon mobility and provides insight into how transposons counteract the m6A-mediated repression mechanism by hitchhiking the RNA demethylase of the host.

Pathogen-induced m6A dynamics affect plant immunity

Posttranscriptional regulation of mRNA mediated by methylation at the N6 position of adenine (N6-methyladenosine [m6A]) has profound effects on transcriptome regulation in plants. Focused studies across eukaryotes offer glimpses into the processes governed by m6A throughout developmental and disease states. However, we lack an understanding of the dynamics and the regulatory potential of m6A during biotic stress in plants. Here, we provide a comprehensive look into the effects of m6A on both the short-term and long-term responses to pathogen signaling in Arabidopsis (Arabidopsis thaliana). We demonstrate that m6A-deficient plants are more resistant to bacterial and fungal pathogen infections and have altered immune responses. Furthermore, m6A deposition is specifically coordinated on transcripts involved in defense and immunity prior to and proceeding the pathogen signal flagellin. Consequently, the dynamic modulation of m6A on specific stress-responsive transcripts is correlated with changes in abundance and cleavage of these transcripts. Overall, we show that the m6A methylome is regulated prior to and during simulated and active pathogen stress and functions in the coordination and balancing of normal growth and pathogen responses.

The Roles of N6-Methyladenosine Modification in Plant-RNA Virus Interactions

N6-methyladenosine (m6A) is a dynamic post-transcriptional RNA modification. Recently, its role in viruses has led to the study of viral epitranscriptomics. m6A has been observed in viral genomes and alters the transcriptomes of both the host cell and virus during infection. The effects of m6A modifications on host plant mRNA can either increase the likelihood of viral infection or enhance the resistance of the host to the virus. However, to date, the regulatory mechanisms of m6A in viral infection and host immune responses have not been fully elucidated. With the development of sequencing-based biotechnologies, the study of m6A in plant viruses has received increasing attention. In this mini review, we summarize the positive and negative consequences of m6A modification in different RNA viral infections. Given its increasingly important roles in multiple viruses, m6A represents a new potential target for antiviral defense.

Plant YTHDF proteins are direct effectors of antiviral immunity against an N6-methyladenosine-containing RNA virus

In virus-host interactions, nucleic acid-directed first lines of defense that allow viral clearance without compromising growth are of paramount importance. Plants use the RNA interference pathway as a basal antiviral immune system, but additional RNA-based mechanisms of defense also exist. The infectivity of a plant positive-strand RNA virus, alfalfa mosaic virus (AMV), relies on the demethylation of viral RNA by the recruitment of the cellular N6-methyladenosine (m6 A) demethylase ALKBH9B, but how demethylation of viral RNA promotes AMV infection remains unknown. Here, we show that inactivation of the Arabidopsis cytoplasmic YT521-B homology domain (YTH)-containing m6 A-binding proteins ECT2, ECT3, and ECT5 is sufficient to restore AMV infectivity in partially resistant alkbh9b mutants. We further show that the antiviral function of ECT2 is distinct from its previously demonstrated function in the promotion of primordial cell proliferation: an ect2 mutant carrying a small deletion in its intrinsically disordered region is partially compromised for antiviral defense but not for developmental functions. These results indicate that the m6 A-YTHDF axis constitutes a novel branch of basal antiviral immunity in plants.

m6A RNA methylation impairs gene expression variability and reproductive thermotolerance in Arabidopsis

Heat-imposed crop failure is often attributed to reduced thermotolerance of floral tissues; however, the underlying mechanism remains unknown. Here, we demonstrate that m⁶A RNA methylation increases in Arabidopsis flowers and negatively regulates gene expression variability. Stochastic gene expression provides flexibility to cope with environmental stresses. We find that reduced transcriptional fluctuation is associated with compromised activation of heat-responsive genes. Moreover, disruption of an RNA demethylase AtALKBH10B leads to lower gene expression variability, suppression of heat-activated genes, and strong reduction of plant fertility. Our work proposes a novel role for RNA methylation in the bet-hedging strategy of heat stress response.

N6-methyladenosine RNA modification promotes viral genomic RNA stability and infection

Molecular manipulation of susceptibility (S) genes that are antipodes to resistance (R) genes has been adopted as an alternative strategy for controlling crop diseases. Here, we show the S gene encoding Triticum aestivum m⁶A methyltransferase B (TaMTB) is identified by a genome-wide association study and subsequently shown to be a positive regulator for wheat yellow mosaic virus (WYMV) infection. TaMTB is localized in the nucleus, is translocated into the cytoplasmic aggregates by binding to WYMV NIb to upregulate the m⁶A level of WYMV RNA1 and stabilize the viral RNA, thus promoting viral infection. A natural mutant allele TaMTB-SNP176C is found to confer an enhanced susceptibility to WYMV infection through genetic variation analysis on 243 wheat varieties. Our discovery highlights this allele can be a useful target for the molecular wheat breeding in the future.

AlkB RNA demethylase homologues and N6 -methyladenosine are involved in Potyvirus infection

Proteins of the alkylation B (AlkB) superfamily show RNA demethylase activity removing methyl adducts from N6 -methyladenosine (m6 A). m6 A is a reversible epigenetic mark of RNA that regulates human virus replication but has unclear roles in plant virus infection. We focused on Potyvirus-the largest genus of plant RNA viruses-and report here the identification of AlkB domains within P1 of endive necrotic mosaic virus (ENMV) and an additional virus of a putative novel species within Potyvirus. We show that Nicotiana benthamiana m6 A levels are reduced by infection of plum pox virus (PPV) and potato virus Y (PVY). The two potyviruses lack AlkB and the results suggest a general involvement of RNA methylation in potyvirus infection and evolution. Methylated RNA immunoprecipitation sequencing of virus-infected samples showed that m6 A peaks are enriched in plant transcript 3' untranslated regions and in discrete internal and 3' terminal regions of PPV and PVY genomes. Down-regulation of N. benthamiana AlkB homologues of the plant-specific ALKBH9 clade caused a significant decrease in PPV and PVY accumulation. In summary, our study provides evolutionary and experimental evidence that supports the m6 A implication and the proviral roles of AlkB homologues in Potyvirus infection.

Impact of the Potential m6A Modification Sites at the 3′UTR of Alfalfa Mosaic Virus RNA3 in the Viral Infection

We have previously reported the presence of m6A in the AMV (Alfamovirus, Bromoviridae) genome. Interestingly, two of these putative m6A-sites are in hairpin (hp) structures in the 3’UTR of the viral RNA3. One site (2012AAACU2016) is in the loop of hpB, within the coat protein binding site 1 (CPB1), while the other (1900UGACC1904) is in the lower stem of hpE, a loop previously associated with AMV negative-strand RNA synthesis. In this work, we have performed in vivo experiments to assess the role of these two regions, containing the putative m6A-sites in the AMV cycle, by introducing compensatory point mutations to interfere with or abolish the m6A-tag of these sites. Our results suggest that the loop of hpB could be involved in viral replication/accumulation. Meanwhile, in the 1900UGACC1904 motif of the hpE, the maintenance of the adenosine residue and the lower stem hpE structure are necessary for in vivo plus-strand accumulation. These results extend our understanding of the requirements for hpE in the AMV infection cycle, indicating that both the residue identity and the base-pairing capacity in this structure are essential for viral accumulation.

Recent Acquisition of Functional m6A RNA Demethylase Domain in Orchid Ty3/Gypsy Elements

Long terminal repeats (LTR) retrotransposons are transposable elements (TEs) representing major components of most plant genomes. The fixation of additional conserved protein domains in their genomes is considered a rare event in the course of their evolution. Such changes can bring novel functions and increase their fitness by playing a role in the regulation of their replicative cycle or by affecting their integration landscape so that the detection of new domains can in turn reveal important aspects of host-TE interactions. We have mined angiosperm genomes for the presence of additional domains in LTR retrotransposons. We report a lineage of large (25 kbp) Gypsy-type elements in the genomes of Phalaenopsis orchids that contain an additional open reading frame containing a 2-ODD domain with close similarity to those responsible for m6A RNA demethylase activity in AlkB proteins. By performing in vitro assays, we demonstrate the RNA binding capability and the demethylase activity of the Gypsy-encoded AlkB protein, suggesting it could be functional against cognate TE mRNA or any cellular RNA in planta. In line with recent literature, we propose that the fixation of an RNA demethylase in this lineage of LTR retrotransposons may reflect an important role for epitranscriptomic control in host surveillance against TEs.

The m6A RNA Demethylase ALKBH9B Plays a Critical Role for Vascular Movement of Alfalfa Mosaic Virus in Arabidopsis

The N 6-methyladenosine (m6A) pathway has been widely described as a viral regulatory mechanism in animals. We previously reported that the capsid protein (CP) of alfalfa mosaic virus (AMV) interacts with the Arabidopsis m6A demethylase ALKBH9B regulating m6A abundance on viral RNAs (vRNAs) and systemic invasion of floral stems. Here, we analyze the involvement of other ALKBH9 proteins in AMV infection and we carry out a detailed evaluation of the infection restraint observed in alkbh9b mutant plants. Thus, via viral titer quantification experiments and in situ hybridization assays, we define the viral cycle steps that are altered by the absence of the m6A demethylase ALKBH9B in Arabidopsis. We found that ALKBH9A and ALKBH9C do not regulate the AMV cycle, so ALKBH9B activity seems to be highly specific. We also define that not only systemic movement is affected by the absence of the demethylase, but also early stages of viral infection. Moreover, our findings suggest that viral upload into the phloem could be blocked in alkbh9b plants. Overall, our results point to ALKBH9B as a possible new component of phloem transport, at least for AMV, and as a potential target to obtain virus resistance crops.