RNA editing in mitochondrial trans-introns is required for splicing

Decoding RNA Editing Sites Through Transcriptome Analysis in Rice Under Alkaline Stress

Ribonucleic acid editing (RE) is a post-transcriptional process that altered the genetics of RNA which provide the extra level of gene expression through insertion, deletions, and substitutions. In animals, it converts nucleotide residues C-U. Similarly in plants, the role of RNA editing sites (RES) in rice under alkaline stress is not fully studied. Rice is a staple food for most of the world population. Alkaline stress cause reduction in yield. Here, we explored the effect of alkaline stress on RES in the whole mRNA from rice chloroplast and mitochondria. Ribonucleic acid editing sites in both genomes (3336 RESs) including chloroplast (345 RESs) and mitochondria (2991 RESs) with average RES efficiency ∼55% were predicted. Our findings showed that majority of editing events found in non-synonymous codon changes and change trend in amino acids was hydrophobic. Four types of RNA editing A-G (A-I), C-T (C-U), G-A, and T-C were identified in treated and untreated samples. Overall, RNA editing efficiency was increased in the treated samples. Analysis of Gene Ontology revealed that mapped genes were engaged in many biological functions and molecular processes. We also checked the expression of pentatricopeptide repeat (PPR), organelle zinc-finger (OZI), and multiple organellar RNA editing factors/RNA editing factor interacting proteins genes in control and treatment, results revealed upregulation of PPR and OZ1 genes in treated samples. This induction showed the role of these genes in RNA editing. The current findings report that RNA editing increased under alkaline stress which may contribute in adaptation for rice by changing amino acids in edited genes (88 genes). These findings will provide basis for identification of RES in other crops and also will be useful in alkaline tolerance development in rice.

Genome-wide investigation and functional analysis of RNA editing sites in wheat

Wheat is an important cereal and half of the world population consumed it. Wheat faces environmental stresses and different techniques (CRISPR, gene silencing, GWAS, etc.) were used to enhance its production but RNA editing (RESs) is not fully explored in wheat. RNA editing has a special role in controlling environmental stresses. The genome-wide identification and functional characterization of RESs in different types of wheat genotypes was done. We employed six wheat genotypes by RNA-seq analyses to achieve RESs. The findings revealed that RNA editing events occurred on all chromosomes equally. RNA editing sites were distributed randomly and 10–12 types of RESs were detected in wheat genotypes. Higher number of RESs were detected in drought-tolerant genotypes. A-to-I RNA editing (2952, 2977, 1916, 2576, 3422, and 3459) sites were also identified in six wheat genotypes. Most of the genes were found to be engaged in molecular processes after a Gene Ontology analysis. PPR (pentatricopeptide repeat), OZ1 (organelle zinc-finger), and MORF/RIP gene expression levels in wheat were also examined. Normal growth conditions diverge gene expression of these three different gene families, implying that normal growth conditions for various genotypes can modify RNA editing events and have an impact on gene expression levels. While the expression of PPR genes was not change. We used Variant Effect Predictor (VEP) to annotate RNA editing sites, and Local White had the highest RESs in the CDS region of the protein. These findings will be useful for prediction of RESs in other crops and will be helpful in drought tolerance development in wheat.

Full Length Transcriptome Highlights the Coordination of Plastid Transcript Processing

Plastid gene expression involves many post-transcriptional maturation steps resulting in a complex transcriptome composed of multiple isoforms. Although short-read RNA-Seq has considerably improved our understanding of the molecular mechanisms controlling these processes, it is unable to sequence full-length transcripts. This information is crucial, however, when it comes to understanding the interplay between the various steps of plastid gene expression. Here, we describe a protocol to study the plastid transcriptome using nanopore sequencing. In the leaf of Arabidopsis thaliana, with about 1.5 million strand-specific reads mapped to the chloroplast genome, we could recapitulate most of the complexity of the plastid transcriptome (polygenic transcripts, multiple isoforms associated with post-transcriptional processing) using virtual Northern blots. Even if the transcripts longer than about 2500 nucleotides were missing, the study of the co-occurrence of editing and splicing events identified 42 pairs of events that were not occurring independently. This study also highlighted a preferential chronology of maturation events with splicing happening after most sites were edited.

Organelle Genomes and Transcriptomes of Nymphaea Reveal the Interplay between Intron Splicing and RNA Editing

Posttranscriptional modifications, including intron splicing and RNA editing, are common processes during regulation of gene expression in plant organelle genomes. However, the intermediate products of intron-splicing, and the interplay between intron-splicing and RNA-editing were not well studied. Most organelle transcriptome analyses were based on the Illumina short reads which were unable to capture the full spectrum of transcript intermediates within an organelle. To fully investigate the intermediates during intron splicing and the underlying relationships with RNA editing, we used PacBio DNA-seq and Iso-seq, together with Illumina short reads genome and transcriptome sequencing data to assemble the chloroplast and mitochondrial genomes of Nymphaea 'Joey Tomocik' and analyze their posttranscriptional features. With the direct evidence from Iso-seq, multiple intermediates partially or fully intron-spliced were observed, and we also found that both cis- and trans-splicing introns were spliced randomly. Moreover, by using rRNA-depleted and non-Oligo(dT)-enrichment strand-specific RNA-seq data and combining direct SNP-calling and transcript-mapping methods, we identified 98 and 865 RNA-editing sites in the plastome and mitogenome of N. 'Joey Tomocik', respectively. The target codon preference, the tendency of increasing protein hydrophobicity, and the bias distribution of editing sites are similar in both organelles, suggesting their common evolutionary origin and shared editing machinery. The distribution of RNA editing sites also implies that the RNA editing sites in the intron and exon regions may splice synchronously, except those exonic sites adjacent to intron which could only be edited after being intron-spliced. Our study provides solid evidence for the multiple intermediates co-existing during intron-splicing and their interplay with RNA editing in organelle genomes of a basal angiosperm.

Organellar Introns in Fungi, Algae, and Plants

Introns are ubiquitous in eukaryotic genomes and have long been considered as ‘junk RNA’ but the huge energy expenditure in their transcription, removal, and degradation indicate that they may have functional significance and can offer evolutionary advantages. In fungi, plants and algae introns make a significant contribution to the size of the organellar genomes. Organellar introns are classified as catalytic self-splicing introns that can be categorized as either Group I or Group II introns. There are some biases, with Group I introns being more frequently encountered in fungal mitochondrial genomes, whereas among plants Group II introns dominate within the mitochondrial and chloroplast genomes. Organellar introns can encode a variety of proteins, such as maturases, homing endonucleases, reverse transcriptases, and, in some cases, ribosomal proteins, along with other novel open reading frames. Although organellar introns are viewed to be ribozymes, they do interact with various intron- or nuclear genome-encoded protein factors that assist in the intron RNA to fold into competent splicing structures, or facilitate the turn-over of intron RNAs to prevent reverse splicing. Organellar introns are also known to be involved in non-canonical splicing, such as backsplicing and trans-splicing which can result in novel splicing products or, in some instances, compensate for the fragmentation of genes by recombination events. In organellar genomes, Group I and II introns may exist in nested intronic arrangements, such as introns within introns, referred to as twintrons, where splicing of the external intron may be dependent on splicing of the internal intron. These nested or complex introns, with two or three-component intron modules, are being explored as platforms for alternative splicing and their possible function as molecular switches for modulating gene expression which could be potentially applied towards heterologous gene expression. This review explores recent findings on organellar Group I and II introns, focusing on splicing and mobility mechanisms aided by associated intron/nuclear encoded proteins and their potential roles in organellar gene expression and cross talk between nuclear and organellar genomes. Potential application for these types of elements in biotechnology are also discussed.

The U-to-C RNA editing affects the mRNA stability of nuclear genes in Arabidopsis thaliana

Cytidine-to-uridine (C-to-U) RNA editing has been generally observed in land plants; however, reverse (U-to-C) RNA editing is a rare phenomenon. In this study, we investigated the U-to-C RNA editing-related genes in Arabidopsis tissues and the effects on mRNA stability, with a special focus on PPR proteins. A previous study showed the extensive occurrence of U-to-C RNA editing in 12-day and 20-dayold Arabidopsis seedlings. Here, we have demonstrated the effects of this "reverse" RNA editing on the mRNA stability for all seven edited genes. We also identified U-to-C RNA editing in the nuclear PPR gene (AT2G19280) in 12-day-old seedlings of Arabidopsis thaliana. The U-to-C RNA editing sites were found in the untranslated region (3' UTR) of the mature mRNA and may affect its secondary structure. We also examined the correlation between U-to-C RNA editing-related genes and their mRNA abundance. Furthermore, we investigated the effects of U-to-C RNA editing in Arabidopsis using the transcription inhibitor actinomycin D (Act D). The addition of Act D to the seedlings of transgenic Arabidopsis generated by Agrobacterium-mediated transformation showed that single nucleotide base conversion adversely affected the mRNA secondary structure and stability.

Genome-Wide Identification of U-To-C RNA Editing Events for Nuclear Genes in Arabidopsis thaliana

Cytosine-to-Uridine (C-to-U) RNA editing involves the deamination phenomenon, which is observed in animal nucleus and plant organelles; however, it has been considered the U-to-C is confined to the organelles of limited non-angiosperm plant species. Although previous RNA-seq-based analysis implied U-to-C RNA editing events in plant nuclear genes, it has not been broadly accepted due to inadequate confirmatory analyses. Here we examined the U-to-C RNA editing in Arabidopsis tissues at different developmental stages of growth. In this study, the high-throughput RNA sequencing (RNA-seq) of 12-day-old and 20-day-old Arabidopsis seedlings was performed, which enabled transcriptome-wide identification of RNA editing sites to analyze differentially expressed genes (DEGs) and nucleotide base conversions. The results showed that DEGs were expressed to higher levels in 12-day-old seedlings than in 20-day-old seedlings. Additionally, pentatricopeptide repeat (PPR) genes were also expressed at higher levels, as indicated by the log₂FC values. RNA-seq analysis of 12-day- and 20-day-old Arabidopsis seedlings revealed candidates of U-to-C RNA editing events. Sanger sequencing of both DNA and cDNA for all candidate nucleotide conversions confirmed the seven U-to-C RNA editing sites. This work clearly demonstrated presence of U-to-C RNA editing for nuclear genes in Arabidopsis, which provides the basis to study the mechanism as well as the functions of the unique post-transcriptional modification.

The novel E-subgroup pentatricopeptide repeat protein DEK55 is responsible for RNA editing at multiple sites and for the splicing of nad1 and nad4 in maize

BACKGROUND

Pentatricopeptide repeat (PPR) proteins compose a large protein family whose members are involved in both RNA processing in organelles and plant growth. Previous reports have shown that E-subgroup PPR proteins are involved in RNA editing. However, the additional functions and roles of the E-subgroup PPR proteins are unknown.

RESULTS

In this study, we developed and identified a new maize kernel mutant with arrested embryo and endosperm development, i.e., defective kernel (dek) 55 (dek55). Genetic and molecular evidence suggested that the defective kernels resulted from a mononucleotide alteration (C to T) at + 449 bp within the open reading frame (ORF) of Zm00001d014471 (hereafter referred to as DEK55). DEK55 encodes an E-subgroup PPR protein within the mitochondria. Molecular analyses showed that the editing percentage of 24 RNA editing sites decreased and that of seven RNA editing sites increased in dek55 kernels, the sites of which were distributed across 14 mitochondrial gene transcripts. Moreover, the splicing efficiency of nad1 introns 1 and 4 and nad4 intron 1 significantly decreased in dek55 compared with the wild type (WT). These results indicate that DEK55 plays a crucial role in RNA editing at multiple sites as well as in the splicing of nad1 and nad4 introns. Mutation in the DEK55 gene led to the dysfunction of mitochondrial complex I. Moreover, yeast two-hybrid assays showed that DEK55 interacts with two multiple organellar RNA-editing factors (MORFs), i.e., ZmMORF1 (Zm00001d049043) and ZmMORF8 (Zm00001d048291).

CONCLUSIONS

Our results demonstrated that a mutation in the DEK55 gene affects the mitochondrial function essential for maize kernel development. Our results also provide novel insight into the molecular functions of E-subgroup PPR proteins involved in plant organellar RNA processing.

Molecular and functional diversity of organelle RNA editing mediated by RNA recognition motif-containing protein ORRM4 in tomato

Plant organellar RNA editing is a distinct type of post-transcriptional RNA modification that is critical for plant development. We showed previously that the RNA editing factor SlORRM4 is required for mitochondrial function and fruit ripening in tomato (Solanum lycopersicum). However, a comprehensive atlas of the RNA editing mediated by SlORRM4 is lacking. We observed that SlORRM4 is targeted to both chloroplasts and mitochondria, and its knockout results in pale-green leaves and delayed fruit ripening. Using high-throughput sequencing, we identified 12 chloroplast editing sites and 336 mitochondrial editing sites controlled by SlORRM4, accounting for 23% of chloroplast sites in leaves and 61% of mitochondrial sites in fruits, respectively. Analysis of native RNA immunoprecipitation sequencing revealed that SlORRM4 binds to 31 RNA targets; 19 of these targets contain SlORRM4-dependent editing sites. Large-scale analysis of putative SlORRM4-interacting proteins identified SlRIP1b, a RIP/MORF protein. Moreover, functional characterization demonstrated that SlRIP1b is involved in tomato fruit ripening. Our results indicate that SlORRM4 binds to RNA targets and interacts with SlRIP1b to broadly affect RNA editing in tomato organelles. These results provide insights into the molecular and functional diversity of RNA editing factors in higher plants.

DEK46 performs C-to-U editing of a specific site in mitochondrial nad7 introns that is critical for intron splicing and seed development in maize

The self-splicing of group II introns during RNA processing depends on their catalytic structure and is influenced by numerous factors that promote the formation of that structure through direct binding. Here we report that C-to-U editing at a specific position in two nad7 introns is essential to splicing, which also implies that the catalytic activity of non-functional group II introns could be restored by editing. We characterized a maize (Zea mays) mutant, dek46, with a defective kernel phenotype; Dek46 encodes a pentatricopeptide repeat DYW protein exclusively localized in mitochondria. Analyses of the coding regions of mitochondrial transcripts did not uncover differences in RNA editing between dek46 mutant and wild-type maize, but showed that splicing of nad7 introns 3 and 4 is severely reduced in the mutant. Furthermore, editing at nucleotide 22 of domain 5 (D5-C22) of both introns is abolished in dek46. We constructed chimeric introns by swapping D5 of P.li.LSUI2 with D5 of nad7 intron 3. In vitro splicing assays indicated that the chimeric intron containing D5-U22 can be self-spliced, but the one containing D5-C22 cannot. These results indicate that DEK46 functions in the C-to-U editing of D5-C22 of both introns, and the U base at this position is critical to intron splicing.

WSL9 Encodes an HNH Endonuclease Domain-Containing Protein that Is Essential for Early Chloroplast Development in Rice

BACKGROUND

The plant chloroplast is essential for photosynthesis and other cellular processes, but an understanding of the biological mechanisms of plant chloroplast development are incomplete.

RESULTS

A new temperature-sensitive white stripe leaf 9(wsl9) rice mutant is described. The mutant develops white stripes during early leaf development, but becomes green after the three-leaf stage under field conditions. The wsl9 mutant was albinic when grown at low temperature. Gene mapping of the WSL9 locus, together with complementation tests indicated that WSL9 encodes a novel protein with an HNH domain. WSL9 was expressed in various tissues. Under low temperature, the wsl9 mutation caused defects in splicing of rpl2, but increased the editing efficiency of rpoB. Expression levels of plastid genome-encoded genes, which are transcribed by plastid-coded RNA polymerase (PEP), chloroplast development genes and photosynthesis-related genes were altered in the wsl9 mutant.

CONCLUSION

WSL9 encodes an HNH endonuclease domain-containing protein that is essential for early chloroplast development. Our study provides opportunities for further research on regulatory mechanisms of chloroplast development in rice.

Plant organellar RNA editing: what 30 years of research has revealed

The central dogma in biology defines the flow of genetic information from DNA to RNA to protein. Accordingly, RNA molecules generally accurately follow the sequences of the genes from which they are transcribed. This rule is transgressed by RNA editing, which creates RNA products that differ from their DNA templates. Analyses of the RNA landscapes of terrestrial plants have indicated that RNA editing (in the form of C-U base transitions) is highly prevalent within organelles (that is, mitochondria and chloroplasts). Numerous C→U conversions (and in some plants also U→C) alter the coding sequences of many of the organellar transcripts and can also produce translatable mRNAs by creating AUG start sites or eliminating premature stop codons, or affect the RNA structure, influence splicing and alter the stability of RNAs. RNA-binding proteins are at the heart of post-transcriptional RNA expression. The C-to-U RNA editing process in plant mitochondria involves numerous nuclear-encoded factors, many of which have been identified as pentatricopeptide repeat (PPR) proteins that target editing sites in a sequence-specific manner. In this review we report on major discoveries on RNA editing in plant organelles, since it was first documented 30 years ago.

Dynamic response of RNA editing to temperature in grape by RNA deep sequencing

RNA editing is a post-transcriptional process of modifying genetic information on RNA molecules, which provides cells an additional level of gene expression regulation. Unlike mammals, in land plants, RNA editing converts C-to-U residues in organelles. However, its potential roles in response to different stressors (heat, salt, and so on) remains unclear. Grape is one of the most popular and economically important fruits in the world, and its production, like other crops, must deal with abiotic and biotic stresses, which cause reductions in yield and fruit quality. In our study, we tested the influence of the environmental factor temperature on RNA editing process in the whole mRNA from grape organelle. In total, we identified 122 and 627 RNA editing sites in chloroplast and mitochondria respectively with the average editing efficiency nearly ~ 60%. The analyses revealed that number of non-synonymous editing sites were higher than that of synonymous editing sites, and the amino acid substitution type tends to be hydrophobic. Additionally, the overall editing level decreased with the temperature rises, especially for several gene transcripts in chloroplast and mitochondria (matK, ndhB, etc.). We also found that the expression level of most PPR genes decreased with the temperature rises, which may contribute to the decline of RNA editing efficiency at high temperature. Our findings suggested that the RNA editing events were very sensitive to heat stress; the changes of amino acid in RNA editing genes may contribute to the stress adaption for grape.

Whole plastid transcriptomes reveal abundant RNA editing sites and differential editing status in Phalaenopsis aphrodite subsp. formosana

BACKGROUND

RNA editing is a process of post-transcriptional level of gene regulation by nucleotide modification. Previously, the chloroplast DNA of Taiwan endemic moth orchid, P. aphrodite subsp. formosana was determined, and 44 RNA editing sites were identified from 24 plastid protein-coding transcripts of leaf tissue via RT-PCR and then conventional Sanger sequencing. However, the RNA editing status of whole-plastid transcripts in leaf and other distinct tissue types in moth orchids has not been addressed. To sensitively and extensively examine the plastid RNA editing status of moth orchid, RNA-Seq was used to investigate the editing status of whole-plastid transcripts from leaf and floral tissues by mapping the sequence reads to the corresponding cpDNA template. With the threshold of at least 5% C-to-U or U-to-C conversion events observed in sequence reads considered as RNA editing sites.

RESULTS

In total, 137 edits with 126 C-to-U and 11 U-to-C conversions, including 93 newly discovered edits, were identified in plastid transcripts, representing an average of 0.09% of the nucleotides examined in moth orchid. Overall, 110 and 106 edits were present in leaf and floral tissues, respectively, with 79 edits in common. As well, 79 edits were involved in protein-coding transcripts, and the 58 nucleotide conversions caused the non-synonymous substitution. At least 32 edits showed significant (≧20%) differential editing between leaf and floral tissues. Finally, RNA editing in trnM is required for the formation of a standard clover-leaf structure.

CONCLUSIONS

We identified 137 edits in plastid transcripts of moth orchid, the highest number reported so far in monocots. The consequence of RNA editing in protein-coding transcripts mainly cause the amino acid change and tend to increase the hydrophobicity as well as conservation among plant phylogeny. RNA editing occurred in non-protein-coding transcripts such as tRNA, introns and untranslated regulatory regions could affect the formation and stability of secondary structure, which might play an important role in the regulation of gene expression. Furthermore, some unidentified tissue-specific factors might be required for regulating RNA editing in moth orchid.

ORRM5, an RNA recognition motif-containing protein, has a unique effect on mitochondrial RNA editing

Plants have an RNA editing mechanism that prevents deleterious organelle mutations from resulting in impaired proteins. A typical flowering plant modifies about 40 cytidines in chloroplast transcripts and many hundreds of cytidines in mitochondrial transcripts. The plant editosome, the molecular machinery responsible for this process, contains members of several protein families, including the organelle RNA recognition motif (ORRM)-containing family. ORRM1 and ORRM6 are chloroplast editing factors, while ORRM2, ORRM3, and ORRM4 are mitochondrial editing factors. Here we report the identification of organelle RRM protein 5 (ORRM5) as a mitochondrial editing factor with a unique mode of action. Unlike other ORRM editing factors, the absence of ORRM5 in orrm5 mutant plants results in an increase of the editing extent in 14% of the mitochondrial sites surveyed. The orrm5 mutant also exhibits a reduced splicing efficiency of the first nad5 intron and slower growth and delayed flowering time. ORRM5 contains an RNA recognition motif (RRM) and a glycine-rich domain at the C terminus. The RRM provides the editing activity of ORRM5 and is able to complement the splicing but not the morphological defects.

Group II introns in wheat mitochondria have degenerate structural features and varied splicing pathways

Mitochondrial introns in flowering plant genes are virtually all classified as members of the group II ribozyme family although certain structural features have degenerated to varying degrees over evolutionary time. We are interested in the impact that unconventional intron architecture might have on splicing biochemistry in vivo and we have focused in particular on intronic domains V and VI, which for self-splicing introns provide a key component of the catalytic core and the bulged branchpoint adenosine, respectively. Notably, the two transesterification steps in classical group II splicing are the same as for nuclear spliceosomal introns and release the intron as a lariat. Using RT-PCR and circularized RT-PCR, we had previously demonstrated that several wheat mitochondrial introns which lack a branchpoint adenosine have atypical splicing pathways, and we have now extended this analysis to the full set of wheat introns, namely six trans-splicing and sixteen cis-splicing ones. A number of introns are excised using non-lariat pathways and interestingly, we find that several introns which do have a conventional domain VI also use pathways that appear to exploit other internal or external nucleophiles, with the lariat form being relatively minor. Somewhat surprisingly, several introns with weakly-structured domain V/VI helices still exhibit classical lariat splicing, suggesting that accessory factors aid in restoring a splicing-competent conformation. Our observations illustrate that the loss of conventional group II features during evolution is correlated with altered splicing biochemistry in an intron-distinctive manner.

Non-coding RNA may be associated with cytoplasmic male sterility in Silene vulgaris

Cytoplasmic male sterility (CMS) is a widespread phenomenon in flowering plants caused by mitochondrial (mt) genes. CMS genes typically encode novel proteins that interfere with mt functions and can be silenced by nuclear fertility-restorer genes. Although the molecular basis of CMS is well established in a number of crop systems, our understanding of it in natural populations is far more limited. To identify CMS genes in a gynodioecious plant, Silene vulgaris, we constructed mt transcriptomes and compared transcript levels and RNA editing patterns in floral bud tissue from female and hermaphrodite full siblings. The transcriptomes from female and hermaphrodite individuals were very similar overall with respect to variation in levels of transcript abundance across the genome, the extent of RNA editing, and the order in which RNA editing and intron splicing events occurred. We found only a single genomic region that was highly overexpressed and differentially edited in females relative to hermaphrodites. This region is not located near any other transcribed elements and lacks an open-reading frame (ORF) of even moderate size. To our knowledge, this transcript would represent the first non-coding mt RNA associated with CMS in plants and is, therefore, an important target for future functional validation studies.

The RNA Editing Factor WSP1 Is Essential for Chloroplast Development in Rice

Although the multiple organellar RNA editing factors (MORFs) in the plastids of Arabidopsis thaliana have been extensively studied, molecular details underlying how MORFs affect plant development in other species, particularly in rice, remain largely unknown. Here we describe the characterization of wsp1, a rice mutant with white-stripe leaves and panicles. Notably, wsp1 exhibited nearly white immature panicles at the heading stage. Transmission electron microscopy analysis and chlorophyll content measurement revealed a chloroplast developmental defect and reduced chlorophyll accumulation in wsp1. Positional cloning of WSP1 found a point mutation in Os04g51280, whose putative product shares high sequence similarity with MORF proteins. Complementation experiments demonstrated that WSP1 was responsible for the variegated phenotypes of wsp1. WSP1 is localized to chloroplasts and the point mutation in wsp1 affected the editing of multiple organellar RNA sites. Owing to the defect in plastid RNA editing, chloroplast ribosome biogenesis and ndhA splicing were also impaired in wsp1, which may affect normal chloroplast development in the leaves and panicles at the heading stage. Together, our results demonstrate the importance of rice WSP1 protein in chloroplast development and broaden our knowledge about MORF family members in rice.

RNA Editing Responses to Oxidative Stress between a Wild Abortive Type Male-Sterile Line and Its Maintainer Line

RNA editing of mitochondrial gene transcripts plays a central role during plant development and evolutionary adaptation. RNA editing has previously been reported to differ between the rice cytoplasmic male sterile (CMS) line and its maintainer line, which has been suggested as a cause for their different performances under environmental stress. To specifically test this hypothesis, a wild abortive (WA) CMS line (Huhan-1A) and its maintainer line (Huhan-1B) were utilized to investigate performances in response to oxidative stress, as well as RNA editing efficiencies on transcripts of six selected mitochondrial genes. Compared to the maintainer line, Huhan-1A represented both lower plant height and total antioxidant capacity, possessed higher total soluble protein and chlorophyll contents, accumulated less H₂O₂ content on the 3rd day after treatment (DAT), and exhibited higher survival ratio after re-watering. Furthermore, a total of 90 editing sites were detected on transcripts of six mitochondrial genes (atp9, nad2, nad7, nad9, ccmB, and ccmC) in both Huhan-1A and Huhan-1B on the 0, 1st, and 3rd DAT. Forty-eight sites were furthermore determined as stress-responsive sites (SRS). Generally, in response to oxidative stress, SRS in Huhan-1A increased the resulting editing efficiencies, while SRS in Huhan-1B decreased the resulting editing efficiencies. In addition, 33 and 22 sites at ccmB and ccmC were differentially edited between Huhan-1A and Huhan-1B, respectively, on the 0, 1st, and 3rd DAT. Editing efficiencies of ccmB and ccmC were generally lower in Huhan-1A (ccmB, 37.3-47.8%; ccmC, 41.2-52.3%) than those in Huhan-1B (ccmB, 82.6-86.5%; ccmC, 81.0-82.9%). Deficiencies of RNA editing in Huhan-1A at ccmB and ccmC could lead to the loss of transmembrane domains in their protein structures. Consequently, differences in RNA editing at ccmB and ccmC between the WA-CMS line and its maintainer line partially explained their different performances under stress. Moreover, we detected differences in expressions of pentatricopeptide repeat (PPR) genes between both lines, as well as significant correlations with RNA editing. Our study indicated potential associations of RNA editing and PPR genes in rice tolerance to abiotic stresses. However, the underlying molecular mechanisms of stress-adaptation, which are attributed to RNA editing on transcripts of mitochondrial genes, require further investigation.

Electroporation of DNA into Physarum polycephalum Mitochondria: Effects on Transcription and RNA Editing in Isolated Organelles

Mitochondrial RNAs in the acellular slime mold Physarum polycephalum contain nucleotides that are not encoded in the mitochondrial genes from which they are transcribed. These site-specific changes are quite extensive, comprising ~4% of the residues within mRNAs and ~2% of rRNAs and tRNAs. These “extra” nucleotides are added co-transcriptionally, but the means by which this is accomplished have not been elucidated. The cox1 mRNA also contains four sites of C to U changes, which occur post-transcriptionally, most likely via targeted deamination. The currently available in vitro systems for studying P. polycephalum editing are limited in that the template is the entire ~63,000 bp mitochondrial genome. This presents a significant challenge when trying to define the signals that specify editing sites. In an attempt to overcome this issue, a method for introducing DNA into isolated P. polycephalum mitochondria via electroporation has been developed. Exogenous DNA is expressed, but the transcripts synthesized from these templates are not edited under the conditions tested. However, transcripts derived from the mitochondrial genome are accurately edited after electroporation, indicating that the editing machinery is still functional. These findings suggest that this method may ultimately provide a feasible approach to elucidating editing signals.

Monilophyte mitochondrial rps1 genes carry a unique group II intron that likely originated from an ancient paralog in rpl2

Intron patterns in plant mitochondrial genomes differ significantly between the major land plant clades. We here report on a new, clade-specific group II intron in the rps1 gene of monilophytes (ferns). This intron, rps1i25g2, is strikingly similar to rpl2i846g2 previously identified in the mitochondrial rpl2 gene of seed plants, ferns, and the lycophyte Phlegmariurus squarrosus Although mitochondrial ribosomal protein genes are frequently subject to endosymbiotic gene transfer among plants, we could retrieve the mitochondrial rps1 gene in a taxonomically wide sampling of 44 monilophyte taxa including basal lineages such as the Ophioglossales, Psilotales, and Marattiales with the only exception being the Equisetales (horsetails). Introns rps1i25g2 and rpl2i846g2 were likewise consistently present with only two exceptions: Intron rps1i25g2 is lost in the genus Ophioglossum and intron rpl2i846g2 is lost in Equisetum bogotense Both intron sequences are moderately affected by RNA editing. The unprecedented primary and secondary structure similarity of rps1i25g2 and rpl2i846g2 suggests an ancient retrotransposition event copying rpl2i846g2 into rps1, for which we suggest a model. Our phylogenetic analysis adding the new rps1 locus to a previous data set is fully congruent with recent insights on monilophyte phylogeny and further supports a sister relationship of Gleicheniales and Hymenophyllales.

Convergent Evolution of Fern-Specific Mitochondrial Group II Intron atp1i361g2 and Its Ancient Source Paralogue rps3i249g2 and Independent Losses of Intron and RNA Editing among Pteridaceae

Mitochondrial intron patterns are highly divergent between the major land plant clades. An intron in the atp1 gene, atp1i361g2, is an example for a group II intron specific to monilophytes (ferns). Here, we report that atp1i361g2 is lost independently at least 4 times in the fern family Pteridaceae. Such plant organelle intron losses have previously been found to be accompanied by loss of RNA editing sites in the flanking exon regions as a consequence of genomic recombination of mature cDNA. Instead, we now observe that RNA editing events in both directions of pyrimidine exchange (C-to-U and U-to-C) are retained in atp1 exons after loss of the intron in Pteris argyraea/biaurita and in Actiniopteris and Onychium We find that atp1i361g2 has significant similarity with intron rps3i249g2 present in lycophytes and gymnosperms, which we now also find highly conserved in ferns. We conclude that atp1i361g2 may have originated from the more ancestral rps3i249g2 paralogue by a reverse splicing copy event early in the evolution of monilophytes. Secondary structure elements of the two introns, most characteristically their domains III, show strikingly convergent evolution in the monilophytes. Moreover, the intron paralogue rps3i249g2 reveals relaxed evolution in taxa where the atp1i361g2 paralogue is lost. Our findings may reflect convergent evolution of the two related mitochondrial introns exerted by co-evolution with an intron-binding protein simultaneously acting on the two paralogues.

Multiple splicing pathways of group II trans-splicing introns in wheat mitochondria

Trans-splicing of discontinuous introns in plant mitochondria requires the assembly of independently-transcribed precursor RNAs into splicing-competent structures, and they are expected to be excised as Y-branched molecules ("broken lariats") because these introns belong to the group II ribozyme family. We now demonstrate that this is just one of several trans-splicing pathways for wheat mitochondrial nad1 intron 4 and nad5 intron 2; they also use a hydrolytic pathway and the liberated 5'-half-intron linear molecules are unexpectedly abundant in the RNA population. We also observe a third productive splicing pathway for nad5 intron 2 that yields full-length excised introns in which the termini are joined in vivo and possess non-encoded nucleotides. In the case of trans-splicing nad1 intron 1, which has a weakly-structured and poorly-conserved core sequence, excision appears to be solely through a hydrolytic pathway. When wheat embryos are germinated in the cold rather than at room temperature, an increased complexity in trans-splicing products is seen for nad1 intron 4, suggesting that there can be environmental effects on the RNA folding of bipartite introns. Our observations provide insights into intron evolution and the complexity of RNA processing events in plant mitochondria.

The SLOW GROWTH3 Pentatricopeptide Repeat Protein Is Required for the Splicing of Mitochondrial NADH Dehydrogenase Subunit7 Intron 2 in Arabidopsis

Mitochondria play an important role in maintaining metabolic and energy homeostasis in the cell. In plants, impairment in mitochondrial functions usually has detrimental effects on growth and development. To study genes that are important for plant growth, we have isolated a collection of slow growth (slo) mutants in Arabidopsis (Arabidopsis thaliana). One of the slo mutants, slo3, has a significant reduction in mitochondrial complex I activity. The slo3 mutant has a four-nucleotide deletion in At3g61360 that encodes a pentatricopeptide repeat (PPR) protein. The SLO3 protein contains nine classic PPR domains belonging to the P subfamily. The small deletion in the slo3 mutant changes the reading frame and creates a premature stop codon in the first PPR domain. We demonstrated that the SLO3-GFP is localized to the mitochondrion. Further analysis of mitochondrial RNA metabolism revealed that the slo3 mutant was defective in splicing of NADH dehydrogenase subunit7 (nad7) intron 2. This specific splicing defect led to a dramatic reduction in complex I activity in the mutant as revealed by blue native gel analysis. Complementation of slo3 by 35S:SLO3 or 35S:SLO3-GFP restored the splicing of nad7 intron 2, the complex I activity, and the growth defects of the mutant. Together, these results indicate that the SLO3 PPR protein is a splicing factor of nad7 intron 2 in Arabidopsis mitochondria.

AEF1/MPR25 is implicated in RNA editing of plastid atpF and mitochondrial nad5, and also promotes atpF splicing in Arabidopsis and rice

RNA editing is an essential mechanism that modifies target cytidines to uridine in both mitochondrial and plastid mRNA. Target sites are recognized by pentatricopeptide repeat (PPR) proteins. Using bioinformatics predictions based on the code describing sequence recognition by PPR proteins, we have identified an Arabidopsis editing factor required for editing of atpF in plastids. A loss-of-function mutation in ATPF EDITING FACTOR 1 (AEF1, AT3G22150) results in severe variegation, presumably due to decreased plastid ATP synthase levels. Loss of editing at the atpF site is coupled with a large decrease in splicing of the atpF transcript, even though the editing site is within an exon and 53 nucleotides distant from the splice site. The rice orthologue of AEF1, MPR25, has been reported to be required for editing of a site in mitochondrial nad5 transcripts, and we confirm that editing of the same site is affected in the Arabidopsis aef1 mutant. We also show that splicing of chloroplast atpF transcripts is affected in the rice mpr25 mutant. AEF1 is thus highly unusual for an RNA editing specificity factor in that it has functions in both organelles.

Variable frequency of plastid RNA editing among ferns and repeated loss of uridine-to-cytidine editing from vascular plants

The distinct distribution and abundance of C-to-U and U-to-C RNA editing among land plants suggest that these two processes originated and evolve independently, but the paucity of information from several key lineages limits our understanding of their evolution. To examine the evolutionary diversity of RNA editing among ferns, we sequenced the plastid transcriptomes from two early diverging species, Ophioglossum californicum and Psilotum nudum. Using a relaxed automated approach to minimize false negatives combined with manual inspection to eliminate false positives, we identified 297 C-to-U and three U-to-C edit sites in the O. californicum plastid transcriptome but only 27 C-to-U and no U-to-C edit sites in the P. nudum plastid transcriptome. A broader comparison of editing content with the leptosporangiate fern Adiantum capillus-veneris and the hornwort Anthoceros formosae uncovered large variance in the abundance of plastid editing, indicating that the frequency and type of RNA editing is highly labile in ferns. Edit sites that increase protein conservation among species are more abundant and more efficiently edited than silent and non-conservative sites, suggesting that selection maintains functionally important editing. The absence of U-to-C editing from P. nudum plastid transcripts and other vascular plants demonstrates that U-to-C editing loss is a recurrent phenomenon in vascular plant evolution.

Chloroplast RNA editing going extreme: more than 3400 events of C-to-U editing in the chloroplast transcriptome of the lycophyte Selaginella uncinata

RNA editing in chloroplasts and mitochondria of land plants differs significantly in abundance. For example, some 200-500 sites of cytidine-to-uridine RNA editing exist in flowering plant mitochondria as opposed to only 30-50 such C-to-U editing events in their chloroplasts. In contrast, we predicted significantly more chloroplast RNA editing for the protein-coding genes in the available complete plastome sequences of two species of the spike moss genus Selaginella (Lycopodiophyta). To evaluate these predictions we investigated the Selaginella uncinata chloroplast transcriptome. Our exhaustive cDNA studies identified the extraordinary number of 3415 RNA-editing events, exclusively of the C-to-U type, in the 74 mRNAs encoding intact reading frames in the S. uncinata chloroplast. We find the overwhelming majority (61%) of the 428 silent editing events leaving codon meanings unaltered directly neighboring other editing events, possibly suggesting a sterically more flexible RNA-editing deaminase activity in Selaginella. No evidence of RNA editing was found for tRNAs or rRNAs but we identified a total of 74 editing sites in cDNA sequences of four group II introns (petBi6g2, petDi8g2, ycf3i124g2, and ycf3i354g2) retained in partially matured transcripts, which strongly contribute to improved base-pairing in the intron secondary structures as a likely prerequisite for their splicing.

RNA metabolism in plant mitochondria

Mitochondria are essential for the eukaryotic cell and are derived from the endosymbiosis of an α-proteobacterial ancestor. Compared to other eukaryotes, RNA metabolism in plant mitochondria is complex and combines bacterial-like traits with novel features that evolved in the host cell. These complex RNA processes are regulated by families of nucleus-encoded RNA-binding proteins. Transcription is particularly relaxed and is initiated from multiple promoters covering the entire genome. The variety of RNA precursors accumulating in mitochondria highlights the importance of post-transcriptional processes to determine the size and abundance of transcripts. Here we review RNA metabolism in plant mitochondria, from RNA transcription to translation, with a special focus on their unique features that are controlled by trans-factors.

The life of plant mitochondrial complex I

The mitochondrial NADH dehydrogenase complex (complex I) of the respiratory chain has several remarkable features in plants: (i) particularly many of its subunits are encoded by the mitochondrial genome, (ii) its mitochondrial transcripts undergo extensive maturation processes (e.g. RNA editing, trans-splicing), (iii) its assembly follows unique routes, (iv) it includes an additional functional domain which contains carbonic anhydrases and (v) it is, indirectly, involved in photosynthesis. Comprising about 50 distinct protein subunits, complex I of plants is very large. However, an even larger number of proteins are required to synthesize these subunits and assemble the enzyme complex. This review aims to follow the complete "life cycle" of plant complex I from various molecular perspectives. We provide arguments that complex I represents an ideal model system for studying the interplay of respiration and photosynthesis, the cooperation of mitochondria and the nucleus during organelle biogenesis and the evolution of the mitochondrial oxidative phosphorylation system.

Possible formation of mitochondrial-RNA containing chimeric or trimeric RNA implies a post-transcriptional and post-splicing mechanism for RNA fusion

Human cells are known to express many chimeric RNAs, i.e. RNAs containing two genes' sequences. Wondering whether there also is trimeric RNA, i.e. an RNA containing three genes' sequences, we wrote simple computer code to screen human expression sequence tags (ESTs) deposited in different public databases, and obtained hundreds of putative trimeric ESTs. We then used NCBI Blast and UCSC Blat browsers to further analyze their sequences, and identified 61 trimeric and two tetrameric ESTs (one EST containing four different sequences). We also identified 57 chimeric, trimeric or teterameric ESTs that contained both mitochondrial (mt) RNA and nuclear RNA (nRNA), i.e. were mtRNA-nRNA fusions. In some trimeric ESTs, the downstream partner was fused to the poly-A tail of the upstream partner, which, together with the mtRNA-nRNA fusions, suggests a possible new mechanism for RNA fusion that occurs after both transcription and splicing have been terminated, and possibly outside the nucleus, in contrast to the two current hypothetical mechanisms, trans-splicing and transcriptional-slippage, that occur in the nucleus. The mt-sequences in the mtRNA-nRNA fusions had pseudogenes in the nucleus but, surprisingly, localized mainly in chromosomes 1 and 5. In some mtRNA-nRNA fusions, as well as in some ESTs that were derived only from mtRNA, the mt-sequences might be cis- or trans-spliced. Actually, we cloned a new cis-spliced mtRNA, coined as 16SrRNA-s. Hence, mtDNA may not always be intron-less. Fusion of three or more RNAs to one, fusion of nRNA to mtRNA, and cis- or trans-splicing of mtRNA should all enlarge the cellular RNA repertoire, in turn enlarging the cellular functions. Therefore, future experimental verification of the existence of these novel classes of fusion RNAs and spliced mtRNAs in human cells should significantly advance our understanding of biology and medicine.

Impact of low temperature on splicing of atypical group II introns in wheat mitochondria

To investigate the impact of cold on group II intron splicing, we compared the physical forms of excised mitochondrial introns from wheat embryos germinated at room temperature and 4°C. For introns which deviate from the conventional branchpoint structure, we observed predominantly heterogeneous circularized introns in the cold rather than linear polyadenylated forms arising from a hydrolytic pathway as seen at room temperature. In addition, intron-containing precursors are elevated relative to mature mRNAs upon cold treatment. Our findings indicate that low temperature growth not only reduces splicing efficiency, but also shifts the splicing biochemistry of atypical group II introns to novel, yet productive, pathways.