RNA methylation in chloroplasts or mitochondria in plants

Exploring RNA modifications in infectious non-coding circular RNAs

Viroids, small circular non-coding RNAs, act as infectious pathogens in higher plants, demonstrating high stability despite consisting solely of naked RNA. Their dependence of replication on host machinery poses the question of whether RNA modifications play a role in viroid biology. Here, we explore RNA modifications in the avocado sunblotch viroid (ASBVd) and the citrus exocortis viroid (CEVd), representative members of viroids replicating in chloroplasts and the nucleus, respectively, using LC - MS and Oxford Nanopore Technology (ONT) direct RNA sequencing. Although no modification was detected in ASBVd, CEVd contained approximately one m6A per RNA molecule. ONT sequencing predicted three m6A positions. Employing orthogonal SELECT method, we confirmed m6A in two positions A353 and A360, which are highly conserved among CEVd variants. These positions are located in the left terminal region of the CEVd rod-like structure where likely RNA Pol II and and TFIIIA-7ZF bind, thus suggesting potential biological role of methylation in viroid replication.

The Roles of RNA Modifications in Regulating Chloroplast Performance and Photosynthesis Efficiency

The regulation of gene expression is crucial for maintaining cellular activities and responding to environmental stimuli. RNA molecules are central to this regulatory network, influencing transcription, post-transcriptional processing, and translation. Recent advancements have expanded our understanding of RNA modifications beyond the nucleus, highlighting their impact on chloroplast function and photosynthesis efficiency. Chloroplasts, essential for photosynthesis, rely on precise genetic regulation to adapt to environmental changes. RNA modifications, such as methylation and pseudouridylation, are critical in regulating chloroplast RNA stability, processing, and translation. This review summarizes current knowledge of how RNA modifications affect chloroplast function and photosynthesis. It discusses the roles of specific RNA modifications occurring in chloroplast RNA, including N6-methyladenosine (m6A), 5-methylcytosine (m5C), and pseudouridylation, as well as the enzymes which are known to be involved in these processes. This review also explores extrachloroplastic RNA modifications that influence chloroplast function, emphasizing the importance of m6A and m5C modifications and their associated enzymes.

Mitochondrial RNA maturation

The vast majority of oxygen-utilizing eukaryotes need to express their own mitochondrial genome, mtDNA, to survive. In comparison to size of their nuclear genome, mtDNA is minimal, even in the most exceptional examples. Having evolved from bacteria in an endosymbiotic event, it might be expected that the process of mtDNA expression would be relatively simple. The aim of this short review is to illustrate just how wrong this assumption is. The production of functional mitochondrial RNA across species evolved in many directions. Organelles use a dizzying array of RNA processing, modifying, editing, splicing and maturation events that largely require the import of nuclear-encoded proteins from the cytosol. These processes are sometimes driven by the unusual behaviour of the mitochondrial genome from which the RNA is originally transcribed, but in many examples the complex processes that are essential for the production of functional RNA in the organelle, are fascinating and bewildering.

METTL5-mediated 18S rRNA m6A modification promotes oncogenic mRNA translation and intrahepatic cholangiocarcinoma progression

Intrahepatic cholangiocarcinoma (ICC) is a deadly cancer with rapid tumor progression. While hyperactive mRNA translation caused by mis-regulated mRNA or tRNA modifications promotes ICC development, the role of rRNA modifications remains elusive. Here, we found that 18S rRNA m6A modification and its methyltransferase METTL5 were aberrantly upregulated in ICC and associated with poorer survival (log rank test, p < 0.05). We further revealed the critical role of METTL5-mediated 18S rRNA m6A modification in regulation of ICC cell growth and metastasis using loss- and gain-of function assays in vitro and in vivo. The oncogenic function of METTL5 is corroborated using liver-specific knockout and overexpression ICC mouse models. Mechanistically, METTL5 depletion impairs 18S rRNA m6A modification that hampers ribosome synthesis and inhibits translation of G-quadruplex-containing mRNAs that are enriched in the transforming growth factor (TGF)-β pathway. Our study uncovers the important role of METTL5-mediated 18S rRNA m6A modification in ICC and unravels the mechanism of rRNA m6A modification-mediated oncogenic mRNA translation control.

Beyond transcription: compelling open questions in plant RNA biology

The study of RNAs has become one of the most influential research fields in contemporary biology and biomedicine. In the last few years, new sequencing technologies have produced an explosion of new and exciting discoveries in the field but have also given rise to many open questions. Defining these questions, together with old, long-standing gaps in our knowledge, is the spirit of this article. The breadth of topics within RNA biology research is vast, and every aspect of the biology of these molecules contains countless exciting open questions. Here, we asked 12 groups to discuss their most compelling question among some plant RNA biology topics. The following vignettes cover RNA alternative splicing; RNA dynamics; RNA translation; RNA structures; R-loops; epitranscriptomics; long non-coding RNAs; small RNA production and their functions in crops; small RNAs during gametogenesis and in cross-kingdom RNA interference; and RNA-directed DNA methylation. In each section, we will present the current state-of-the-art in plant RNA biology research before asking the questions that will surely motivate future discoveries in the field. We hope this article will spark a debate about the future perspective on RNA biology and provoke novel reflections in the reader.

RNA methylation in plants: An overview

RNA methylation is an important post-transcriptional modification that influences gene regulation. Over 200 different types of RNA modifications have been identified in plants. In animals, the mystery of RNA methylation has been revealed, and its biological role and applications have become increasingly clear. However, RNA methylation in plants is still poorly understood. Recently, plant science research on RNA methylation has advanced rapidly, and it has become clear that RNA methylation plays a critical role in plant development. This review summarizes current knowledge on RNA methylation in plant development. Plant writers, erasers, and readers are highlighted, as well as the occurrence, methods, and software development in RNA methylation is summarized. The most common and abundant RNA methylation in plants is N6-methyladenosine (m6A). In Arabidopsis, mutations in writers, erasers, and RNA methylation readers have affected the plant's phenotype. It has also been demonstrated that methylated TRANSLATIONALLY CONTROLLED TUMOR PROTEIN 1-messenger RNA moves from shoot to root while unmethylated TCTP1-mRNA does not. Methylated RNA immunoprecipitation, in conjunction with next-generation sequencing, has been a watershed moment in plant RNA methylation research. This method has been used successfully in rice, Arabidopsis, Brassica, and maize to study transcriptome-wide RNA methylation. Various software or tools have been used to detect methylated RNAs at the whole transcriptome level; the majority are model-based analysis tools (for example, MACS2). Finally, the limitations and future prospects of methylation of RNA research have been documented.

Occurrence of RNA post-transcriptional modifications in plant viruses and viroids and their correlation with structural and functional features

Post-transcriptional modifications of RNA bases are widespread across all the tree of life and have been linked to RNA maturation, stability, and molecular interactions. RNA modifications have been extensively described in endogenous eukaryotic mRNAs, however, little is known about the presence of RNA modifications in plant viral and subviral RNAs. Here, we used a computational approach to infer RNA modifications in plant-pathogenic viruses and viroids using high-throughput annotation of modified ribonucleotides (HAMR), a software that predicts modified ribonucleotides using high-throughput RNA sequencing data. We analyzed datasets from representative members of different plant viruses and viroids and compared them to plant-endogenous mRNAs. Our approach was able to predict potential RNA chemical modifications (RCMs) in all analyzed pathogens. We found that both DNA and RNA viruses presented a wide range of RCM proportions while viroids had lowest values. Furthermore, we found that for viruses with segmented genomes, some genomic RNAs had a higher proportion of RCM. Interestingly, nuclear-replicating viroids showed most of the predicted modifications located in the pathogenesis region, pointing towards a possible functional role of RCMs in their infectious cycle. Thus, our results strongly suggest that plant viral and subviral RNAs might contain a variety of previously unreported RNA modifications, thus opening a new perspective in the multifaceted process of plant-pathogen interactions.

Beyond the Primary Structure of Nucleic Acids: Potential Roles of Epigenetics and Noncanonical Structures in the Regulations of Plant Growth and Stress Responses

Epigenetics deals with changes in gene expression that are not caused by modifications in the primary sequence of nucleic acids. These changes beyond primary structures of nucleic acids not only include DNA/RNA methylation, but also other reversible conversions, together with histone modifications or RNA interference. In addition, under particular conditions (such as specific ion concentrations or protein-induced stabilization), the right-handed double-stranded DNA helix (B-DNA) can form noncanonical structures commonly described as "non-B DNA" structures. These structures comprise, for example, cruciforms, i-motifs, triplexes, and G-quadruplexes. Their formation often leads to significant differences in replication and transcription rates. Noncanonical RNA structures have also been documented to play important roles in translation regulation and the biology of noncoding RNAs. In human and animal studies, the frequency and dynamics of noncanonical DNA and RNA structures are intensively investigated, especially in the field of cancer research and neurodegenerative diseases. In contrast, noncanonical DNA and RNA structures in plants have been on the fringes of interest for a long time and only a few studies deal with their formation, regulation, and physiological importance for plant stress responses. Herein, we present a review focused on the main fields of epigenetics in plants and their possible roles in stress responses and signaling, with special attention dedicated to noncanonical DNA and RNA structures.

AtRsmD Is Required for Chloroplast Development and Chloroplast Function in Arabidopsis thaliana

AtRsmD was recently demonstrated to be a chloroplast 16S rRNA methyltransferase (MTase) for the m²G915 modification in Arabidopsis. Here, its function of AtRsmD for chloroplast development and photosynthesis was further analyzed. The AtRsmD gene is highly expressed in green photosynthetic tissues. AtRsmD is associated with the thylakoid in chloroplasts. The atrsmd-2 mutant exhibited impaired photosynthetic efficiency in emerging leaves under normal growth conditions. A few thylakoid lamellas could be observed in the chloroplast from the atrsmd-2 mutant, and these thylakoids were loosely organized. Knockout of the AtRsmD gene had minor effects on chloroplast ribosome biogenesis and RNA loading on chloroplast ribosomes, but it reduced the amounts of chloroplast-encoded photosynthesis-related proteins in the emerging leaves, for example, D1, D2, CP43, and CP47, which reduced the accumulation of the photosynthetic complex. Nevertheless, knockout of the AtRsmD gene did not cause a general reduction in chloroplast-encoded proteins in Arabidopsis grown under normal growth conditions. Additionally, the atrsmd-2 mutant exhibited more sensitivity to lincomycin, which specifically inhibits the elongation of nascent polypeptide chains. Cold stress exacerbated the effect on chloroplast ribosome biogenesis in the atrsmd-2 mutant. All these data suggest that the AtRsmD protein plays distinct regulatory roles in chloroplast translation, which is required for chloroplast development and chloroplast function.

RsmD, a Chloroplast rRNA m2G Methyltransferase, Plays a Role in Cold Stress Tolerance by Possibly Affecting Chloroplast Translation in Arabidopsis

Ribosomal RNA (rRNA) methylation is a pivotal process in the assembly and activity of ribosomes, which in turn play vital roles in the growth, development and stress responses of plants. Although few methyltransferases responsible for rRNA methylation have been identified in plant chloroplasts, the nature and function of these enzymes in chloroplasts remain largely unknown. In this study, we characterized ArabidopsisRsmD (At3g28460), an ortholog of the methyltransferase responsible for N2-methylguanosine (m2G) modification of 16S rRNA in Escherichia coli. Confocal microscopic analysis of an RsmD- green fluorescent protein fusion protein revealed that RsmD is localized to chloroplasts. Primer extension analysis indicated that RsmD is responsible for m2G methylation at position 915 in the 16S rRNA of Arabidopsis chloroplasts. Under cold stress, rsmd mutant plants exhibited retarded growth, i.e. had shorter roots, lower fresh weight and pale-green leaves, compared with wild-type (WT) plants. However, these phenotypes were not detected in response to drought or salt stress. Notably, the rsmd mutant was hypersensitive to erythromycin or lincomycin and accumulated fewer chloroplast proteins compared with the WT, suggesting that RsmD influences translation in chloroplasts. Complementation lines expressing RsmD in the rsmd mutant background recovered WT phenotypes. Importantly, RsmD harbored RNA methyltransferase activity. Collectively, the findings of this study indicate that RsmD is a chloroplast 16S rRNA methyltransferase responsible for m2G915 modification that plays a role in the adaptation of Arabidopsisto cold stress.

The Chloroplast Epitranscriptome: Factors, Sites, Regulation, and Detection Methods

Modifications in nucleic acids are present in all three domains of life. More than 170 distinct chemical modifications have been reported in cellular RNAs to date. Collectively termed as epitranscriptome, these RNA modifications are often dynamic and involve distinct regulatory proteins that install, remove, and interpret these marks in a site-specific manner. Covalent nucleotide modifications-such as methylations at diverse positions in the bases, polyuridylation, and pseudouridylation and many others impact various events in the lifecycle of an RNA such as folding, localization, processing, stability, ribosome assembly, and translational processes and are thus crucial regulators of the RNA metabolism. In plants, the nuclear/cytoplasmic epitranscriptome plays important roles in a wide range of biological processes, such as organ development, viral infection, and physiological means. Notably, recent transcriptome-wide analyses have also revealed novel dynamic modifications not only in plant nuclear/cytoplasmic RNAs related to photosynthesis but especially in chloroplast mRNAs, suggesting important and hitherto undefined regulatory steps in plastid functions and gene expression. Here we report on the latest findings of known plastid RNA modifications and highlight their relevance for the post-transcriptional regulation of chloroplast gene expression and their role in controlling plant development, stress reactions, and acclimation processes.