The Expansion and Diversification of Pentatricopeptide Repeat RNA-Editing Factors in Plants

A Pentatricopeptide Repeat Protein Restores Fertility in Tadukan-Type Cytoplasmic Male Sterile Rice via the Cleavage of the Mitochondrial orf312 RNA

Cytoplasmic male sterility (CMS) is associated with the inhibition of pollen and/or anther development regulated by a CMS-causing gene in the mitochondrial genome; it is a useful trait for preventing self-pollination and producing F1 hybrids, which can boost crop yields. Pollen and/or anther development can be recovered by the action of the RESTORER OF FERTILITY (Rf) gene, a nuclear-encoded gene. Most reported Rf genes encode pentatricopeptide repeat (PPR) proteins, which bind to RNA and promote RNA processing of the respective CMS-causing gene. In this study, we report the map-based cloning of the Rf gene (Rfta) for Tadukan-type CMS (TA-CMS) in rice (Oryza sativa L.), with anther dehiscence and seed setting inhibited by the mitochondrial gene orf312. The Rfta locus was delimited to a region comprising 10 PPR genes forming a cluster on chromosome 10. The complementation test revealed that the introduction of a PPR gene, PPR796, into the TA-CMS line resulted in the recovery of the anther dehiscence and seed setting. RNA-gel blot analysis and the determination of 3' ends of the orf312 RNA confirmed the PPR796-mediated cleavage of the orf312 RNA in the transgenic TA-CMS line. Furthermore, RNA gel electrophoretic mobility shift assays revealed that the recombinant PPR796 protein bound to the 3' side of the orf312 RNA in vitro. We concluded that RFta/PPR796 binds to orf312 RNA and promotes RNA cleavage to restore fertility in TA-CMS.

De novo RNA base editing in plant organelles with engineered synthetic P-type PPR editing factors

In plant mitochondria and chloroplasts, cytidine-to-uridine RNA editing is necessary for the production of functional proteins. While natural PLS-type PPR proteins are specialized in this process, synthetic PPR proteins offer significant potential for targeted RNA editing. In this study, we engineered chimeric editing factors by fusing synthetic P-type PPR guides with the DYW cytidine deaminase domain of a moss mitochondrial editing factor, PPR56. These designer PPR editors (dPPRe) elicited efficient and precise de novo RNA editing in Escherichia coli as well as in the chloroplasts and mitochondria of Nicotiana benthamiana. Chloroplast transcriptome-wide analysis of the most efficient dPPRe revealed minimal off-target effects, with only three nontarget C sites edited due to sequence similarity with the intended target. This study introduces a novel and precise method for RNA base editing in plant organelles, paving the way for new approaches in gene regulation applicable to plants and potentially other organisms.

The molecular basis and evolution of the organellar RNA editosome by complementary DYW deaminases in seed plants

The DYW deaminase domain catalyzes the conversion of cytidines (C) to uridines (U) in RNA editing of plant organelles. While the DYW subgroup contains a complete DYW deaminase domain at the C-terminus, the E2 and E+ subgroups rely on complementary deaminases, in which catalytic activity depends on interactions with short DYW proteins, such as DYW1, DYW2, and MITOCHONDRIAL EDITING FACTOR 8 (MEF8)/MITOCHONDRIAL EDITING FACTOR 8 SIMILAR (MEF8S). Although orthogonal RNA editing in bacteria by a DYW subgroup pentatricopeptide repeat (PPR) has been reported, attempts to activate the DYW deaminase through molecular complementation in bacteria have been unsuccessful, leaving its molecular basis unresolved. In this study, we reconstituted the simplest editosome in Escherichia coli, composed of PPR56PPRE1E2-CRR4PG and DYW1 alone. Systematical mutational analysis of the PG-box of CHLORORESPIRATORY REDUCTION 4 (CRR4) in bacteria and in planta revealed the critical role of serine, isoleucine, and phenylalanine residues in DYW deaminase complementation and catalysis. CRR4-like PPR proteins, termed the "PG-type" characterized by the PG-box with these 3 key amino acid residues at the C-terminus, are minor in angiosperms but constitute one of the major subgroups in gymnosperms. Putative orthologs of Arabidopsis thaliana DYW1 are present in limited angiosperm species, suggesting that in other species, other short DYW proteins serve as the interaction partners for PG-type PPR proteins. Our findings reveal a minimal functional editosome module, shedding light on the conserved and diverse mechanisms of RNA editing in plant organelles.

Fine-tuning of the PPR protein directs the RNA editing activity toward C-to-U or U-to-C conversion

A subfamily of pentatricopeptide repeat (PPR) proteins, known as PPR-DYW:PG, catalyzes the cytidine to uridine (C-to-U) RNA editing in plant organelles. A related PPR subfamily, PPR-DYW:KP, catalyzes the uridine to cytidine (U-to-C) reaction, via a crosslinking mechanism involving a lysine residue. We demonstrate that Lys88 in the DYW:KP domain is essential for the U-to-C editing activity of PPR-DYW:KP proteins. Substituting Lys88 with other amino acids in designer proteins switches the protein activity to C-to-U and prevents crosslinking with the edited RNA. However, this mutation leads to C-to-U off-target editing downstream the targeted site. Finally, other modifications can modulate the catalytic activity and alter the type of reaction catalyzed by the DYW domain. Altogether, our results suggest that subtle modifications in the DYW domain can influence the position of the edited nucleotide and the type of RNA editing reaction.

Pentatricopeptide repeat proteins in plants: Cellular functions, action mechanisms, and potential applications

Pentatricopeptide repeat (PPR) proteins are involved in nearly all aspects of post-transcriptional processing in plant mitochondria and plastids, playing vital roles in plant growth, development, cytoplasmic male sterility restoration, and responses to biotic and abiotic stresses. Over the last three decades, significant advances have been made in understanding the functions of PPR proteins and the primary mechanisms through which they mediate post-transcriptional processing. This review aims to summarize these advancements, highlighting the mechanisms by which PPR proteins facilitate RNA editing, intron splicing, and RNA maturation in the context of organellar gene expression. We also present the latest progress in PPR engineering and discuss its potential as a biotechnological tool. Additionally, we discuss key challenges and questions that remain in PPR research.

Genome-Wide Identification of Pentatricopeptide Repeat (PPR) Gene Family and Multi-Omics Analysis Provide New Insights Into the Albinism Mechanism of Kandelia obovata Propagule Leaves

Pentatricopeptide repeat (PPR) gene family constitutes one of the largest gene families in plants, which mainly participate in RNA editing and RNA splicing of organellar RNAs, thereby affecting the organellar development. Recently, some evidence elucidated the important roles of PPR proteins in the albino process of plant leaves. However, the functions of PPR genes in the woody mangrove species have not been investigated. In this study, using a typical true mangrove Kandelia obovata, we systematically identified 298 PPR genes and characterized their general features and physicochemical properties, including evolutionary relationships, the subcellular localization, PPR motif type, the number of introns and PPR motifs, and isoelectric point, and so forth. Furthermore, we combined genome-wide association studies (GWAS) and transcriptome analysis to identify the genetic architecture and potential PPR genes associated with propagule leaves colour variations of K. obovata. As a result, we prioritized 16 PPR genes related to the albino phenotype using different strategies, including differentially expressed genes analysis and genetic diversity analysis. Further analysis discovered two genes of interest, namely Maker00002998 (PLS-type) and Maker00003187 (P-type), which were differentially expressed genes and causal genes detected by GWAS analysis. Moreover, we successfully predicted downstream target chloroplast genes (rps14, rpoC1 and rpoC2) bound by Maker00002998 PPR proteins. The experimental verification of RNA editing sites of rps14, rpoC1, and rpoC2 in our previous study and the verification of interaction between Maker00002998 and rps14 transcript using in vitro RNA pull-down assays revealed that Maker00002998 PPR protein might be involved in the post-transcriptional process of chloroplast genes. Our result provides new insights into the roles of PPR genes in the albinism mechanism of K. obovata propagule leaves.

Recent advances and biotechnological applications of RNA metabolism in plant chloroplasts and mitochondria

The chloroplast and mitochondrion are semi-autonomous organelles that play essential roles in cell function. These two organelles are embellished with prokaryotic remnants and contain many new features emerging from the co-evolution of organelles and the nucleus. A typical plant chloroplast or mitochondrion genome encodes less than 100 genes, and the regulation of these genes' expression is remarkably complex. The regulation of chloroplast and mitochondrion gene expression can be achieved at multiple levels during development and in response to environmental cues, in which, RNA metabolism, including: RNA transcription, processing, translation, and degradation, plays an important role. RNA metabolism in plant chloroplasts and mitochondria combines bacterial-like traits with novel features evolved in the host cell and is regulated by a large number of nucleus-encoded proteins. Among these, pentatricopeptide repeat (PPR) proteins are deeply involved in multiple aspects of the RNA metabolism of organellar genes. Research over the past decades has revealed new insights into different RNA metabolic events in plant organelles, such as the composition of chloroplast and mitochondrion RNA editosomes. We summarize and discuss the most recent knowledge and biotechnological implications of various RNA metabolism processes in plant chloroplasts and mitochondria, with a focus on the nucleus-encoded factors supporting them, to gain a deeper understanding of the function and evolution of these two organelles in plant cells. Furthermore, a better understanding of the role of nucleus-encoded factors in chloroplast and mitochondrion RNA metabolism will motivate future studies on manipulating the plant gene expression machinery with engineered nucleus-encoded factors.

A weak allele of AtECB2, a member of the pentatricopeptide repeat motif superfamily, causes leaf virescence in Arabidopsis

KEY MESSAGE

Through the study of a point mutation of AtECB2, it is reconfirmed that AtECB2 plays an important role in the early development of chloroplast. AtECB2(EARLY CHLOROPLAST BIOGENESIS 2, At1g15510), a member of the pentatricopeptide repeat motif proteins (PPR) superfamily, and its loss of function mutation ecb2-1causes seedling lethal, while a point mutation ecb2-2 causes delayed chloroplast development. Finding more AtECB2 weak alleles helps to understand the molecular mechanisms of AtECB2. In this study, a leaf virescence mutant was identified from ethylmethane sulfonate (EMS) treated Arabidopsis Col-0 M2 mutants library. The mutation of this mutant was first confirmed as a recessive mutation of one gene through the phenotype of F1 and its F2 phenotype segregation of this mutant crossed with Col-0. The mutation of G1931A of AtECB2 is identified as the cause of this leaf virescence phenotype sequentially through positional cloning, whole genome resequencing, Sanger sequencing and complementation. Therefore, we named this weak allele of AtECB2 as ecb2-3. The chlorophyll content and photosystem II maximum photochemical efficiency of ecb2-3 are obviously lower than that of Col-0 and its complementation lines, respectively. The chloroplast development of ecb2-3 is also inferior to that of Col-0 and its complementation line at the observed time points using the transmission electron microscope. The RNA editing efficiency of three chloroplast gene sites (accD C794 and C1568, ndhF C290) was observed much lower compared with that of Col-0 and its complementation line. In summary, AtECB2 plays an important role in early chloroplast biogenesis through related chloroplast gene editing regulation, and this weak mutant ecb2-3 may be useful material in dissecting the function of AtECB2 in the near future.

Seeing the unseen in characterizing RNA editome during rice endosperm development

Rice (Oryza sativa L.) endosperm is essential to provide nutrients for seed germination and determine grain yield. RNA editing, a post-transcriptional modification essential for plant development, unfortunately, is not fully characterized during rice endosperm development. Here, we perform systematic analyses to characterize RNA editome during rice endosperm development. We find that most editing sites are C-to-U CDS-recoding in mitochondria, leading to increased hydrophobic amino acids and changed structures of mitochondrial proteins. Comparative analysis of RNA editome reveals that CDS-recoding sites present higher editing frequencies with lower variabilities and their resultant recoded amino acids tend to exhibit stronger evolutionary conservation across many land plants. Furthermore, we classify mitochondrial genes into three groups, presenting distinct patterns in terms of CDS-recoding events. Besides, we conduct genome-wide screening to detect pentatricopeptide repeat (PPR) proteins and construct PPR-RNA binding profiles, yielding candidate PPR editing factors related to rice endosperm development. Taken together, our findings provide valuable insights for deciphering fundamental mechanisms of rice endosperm development underlying RNA editing machinery.

Transcriptome-wide Identification of Nine Tandem Repeat Protein Families in Roselle (Hibiscus sabdariffa L.)

Plants are rich in tandem repeats-containing proteins. It is postulated that the occurrence of tandem repeat gene families facilitates the adaptation and survival of plants in adverse environmental conditions. This study intended to identify the tandem repeats in the transcriptome of a high potential tropical horticultural plant, roselle (Hibiscus sabdariffa L.). A total of 92,974 annotated de novo assembled transcripts were analysed using in silico approach, and 6,541 transcripts that encoded proteins containing tandem repeats with length of 20-60 amino acid residues were identified. Domain analysis revealed a total of nine tandem repeat protein families in the transcriptome of roselle, which are the Ankyrin repeats (ANK), Armadillo repeats (ARM), elongation factor-hand domain repeats (EF-hand), Huntingtin, elongation factor 3, protein phosphatase 2A, yeast kinase TOR1 repeats (HEAT), Kelch repeats (Kelch), leucine rich repeats (LRR), pentatricopeptide repeats (PPR), tetratricopeptide repeats (TPR) and WD40 repeats (WD40). Functional annotation analysis further matched 6,236 transcripts to 1,045 known proteins that contained tandem repeats including proteins implicated in plant development, protein-protein interaction, immunity and abiotic stress responses. The findings provide new insights into the occurrence of tandem repeats in the transcriptome and lay the foundation to elucidate the functional associations between tandem peptide repeats (TRs) and proteins in roselle and facilitate the identification of novel biotic and abiotic response related tandem repeats genes that may be useful in breeding improved varieties.

Crosslinking of base-modified RNAs by synthetic DYW-KP base editors implicates an enzymatic lysine as the nitrogen donor for U-to-C RNA editing

Sequence-specific cytidine to uridine (C-to-U) and adenosine to inosine editing tools can alter RNA and DNA sequences and utilize a hydrolytic deamination mechanism requiring an active site zinc ion and a glutamate residue. In plant organelles, DYW-PG domain containing enzymes catalyze C-to-U edits through the canonical deamination mechanism. Proteins developed from consensus sequences of the related DYW-KP domain family catalyze what initially appeared to be uridine to cytidine (U-to-C) edits leading to this investigation into the U-to-C editing mechanism. The synthetic DYW-KP enzyme KP6 was found sufficient for C-to-U editing activity stimulated by the addition of carboxylic acids in vitro. Despite addition of putative amine/amide donors, U-to-C editing by KP6 could not be observed in vitro. C-to-U editing was found not to be concomitant with U-to-C editing, discounting a pyrimidine transaminase mechanism. RNAs containing base modifications were highly enriched in interphase fractions consistent with covalent crosslinks to KP6, KP2, and KP3 proteins. Mass spectrometry of purified KP2 and KP6 proteins revealed secondary peaks with mass shifts of 319 Da. A U-to-C crosslinking mechanism was projected to explain the link between crosslinking, RNA base changes, and the ∼319 Da mass. In this model, an enzymatic lysine attacks C4 of uridine to form a Schiff base RNA-protein conjugate. Sequenced RT-PCR products from the fern Ceratopteris richardii indicate U-to-C base edits do not preserve proteinaceous crosslinks in planta. Hydrolysis of a protonated Schiff base conjugate releasing cytidine is hypothesized to explain the completed pathway in plants.

Dissecting the molecular puzzle of the editosome core in Arabidopsis organelles

Over the last decade, the composition of the C-to-U RNA editing complex in embryophyte organelles has turned out to be much more complex than first expected. While PPR proteins were initially thought to act alone, significant evidences have clearly depicted a sophisticated mechanism with numerous protein-protein interaction involving PPR and non-PPR proteins. Moreover, the identification of specific functional partnership between PPRs also suggests that, in addition to the highly specific PPRs directly involved in the RNA target recognition, non-RNA-specific ones are required. Although some of them, such as DYW1 and DYW2, were shown to be the catalytic domains of the editing complex, the molecular function of others, such as NUWA, remains elusive. It was suggested that they might stabilize the complex by acting as a scaffold. We here performed functional complementation of the crr28-2 mutant with truncated CRR28 proteins mimicking PPR without the catalytic domain and show that they exhibit a specific dependency to one of the catalytic proteins DYW1 or DYW2. Moreover, we also characterized the role of the PPR NUWA in the editing reaction and show that it likely acts as a scaffolding factor. NUWA is no longer required for efficient editing of the CLB19 editing sites once this RNA specific PPR is fused to the DYW catalytic domain of its partner DYW2. Altogether, our results strongly support a flexible, evolutive and resilient editing complex in which RNA binding activity, editing activity and stabilization/scaffolding function can be provided by one or more PPRs.

Conquering new grounds: plant organellar C-to-U RNA editing factors can be functional in the plant cytosol

Plant mitochondrial and chloroplast transcripts are subject to numerous events of specific cytidine-to-uridine (C-to-U) RNA editing to correct genetic information. Key protein factors for this process are specific RNA-binding pentatricopeptide repeat (PPR) proteins, which are encoded in the nucleus and post-translationally imported into the two endosymbiotic organelles. Despite hundreds of C-to-U editing sites in the plant organelles, no comparable editing has been found for nucleo-cytosolic mRNAs raising the question why plant RNA editing is restricted to chloroplasts and mitochondria. Here, we addressed this issue in the model moss Physcomitrium patens, where all PPR-type RNA editing factors comprise specific RNA-binding and cytidine deamination functionalities in single proteins. To explore whether organelle-type RNA editing can principally also take place in the plant cytosol, we expressed PPR56, PPR65 and PPR78, three editing factors recently shown to also function in a bacterial setup, together with cytosolic co-transcribed native targets in Physcomitrium. While we obtained unsatisfying results upon their constitutive expression, we found strong cytosolic RNA editing under hormone-inducible expression. Moreover, RNA-Seq analyses revealed varying numbers of up to more than 900 off-targets in other cytosolic transcripts. We conclude that PPR-mediated C-to-U RNA editing is not per se incompatible with the plant cytosol but that its limited target specificity has restricted its occurrence to the much less complex transcriptomes of mitochondria and chloroplast in the course of evolution.

Insights into U-to-C RNA editing from the lycophyte Phylloglossum drummondii

The lycophyte Phylloglossum drummondii is the sole inhabitant of its genus in the Huperzioideae group and one of a small minority of plants which perform uridine to cytidine RNA editing. We assembled the P. drummondii chloroplast and mitochondrial genomes and used RNA sequence data to build a comprehensive profile of organellar RNA editing events. In addition to many C-to-U editing events in both organelles, we found just four U-to-C editing events in the mitochondrial transcripts cob, nad1, nad5 and rpl2. These events are conserved in related lycophytes in the genera Huperzia and Phlegmariurus. De novo transcriptomes for three of these lycophytes were assembled to search for putative U-to-C RNA editing enzymes. Four putative U-to-C editing factors could be matched to the four mitochondrial U-to-C editing sites. Due to the unusually few numbers of U-to-C RNA editing sites, P. drummondii and related lycophytes are useful models for studying this poorly understood mechanism.

Evolutionary assembly of the plant terrestrialization toolkit from protein domains

Land plants (embryophytes) came about in a momentous evolutionary singularity: plant terrestrialization. This event marks not only the conquest of land by plants but also the massive radiation of embryophytes into a diverse array of novel forms and functions. The unique suite of traits present in the earliest land plants is thought to have been ushered in by a burst in genomic novelty. Here, we asked the question of how these bursts were possible. For this, we explored: (i) the initial emergence and (ii) the reshuffling of domains to give rise to hallmark environmental response genes of land plants. We pinpoint that a quarter of the embryophytic genes for stress physiology are specific to the lineage, yet a significant portion of this novelty arises not de novo but from reshuffling and recombining of pre-existing domains. Our data suggest that novel combinations of old genomic substrate shaped the plant terrestrialization toolkit, including hallmark processes in signalling, biotic interactions and specialized metabolism.

GhCTSF1, a short PPR protein with a conserved role in chloroplast development and photosynthesis, participates in intron splicing of rpoC1 and ycf3-2 transcripts in cotton

Cotton is one of the most important textile fibers worldwide. As crucial agronomic traits, leaves play an essential role in the growth, disease resistance, fiber quality, and yield of cotton plants. Pentatricopeptide repeat (PPR) proteins are a large family of nuclear-encoded proteins involved in organellar or nuclear RNA metabolism. Using a virus-induced gene silencing assay, we found that cotton plants displayed variegated yellow leaf phenotypes with decreased chlorophyll content when expression of the PPR gene GhCTSF1 was silenced. GhCTSF1 encodes a chloroplast-localized protein that contains only two PPR motifs. Disruption of GhCTSF1 substantially reduces the splicing efficiency of rpoC1 intron 1 and ycf3 intron 2. Loss of function of the GhCTSF1 ortholog EMB1417 causes splicing defects in rpoC1 and ycf3-2, leading to impaired chloroplast structure and decreased photosynthetic rates in Arabidopsis. We also found that GhCTSF1 interacts with two splicing factors, GhCRS2 and GhWTF1. Defects in GhCRS2 and GhWTF1 severely affect intron splicing of rpoC1 and ycf3-2 in cotton, leading to defects in chloroplast development and a reduction in photosynthesis. Our results suggest that GhCTSF1 is specifically required for splicing rpoC1 and ycf3-2 in cooperation with GhCRS2 and GhWTF1.

Is RNA editing truly absent in the complex thalloid liverworts (Marchantiopsida)? Evidence of extensive RNA editing from Cyathodium cavernarum

RNA editing is a crucial modification in plants' organellar transcripts that converts cytidine to uridine (C-to-U; and sometimes uridine to cytidine) in RNA molecules. This post-transcriptional process is controlled by the PLS-class protein with a DYW domain, which belongs to the pentatricopeptide repeat (PPR) protein family. RNA editing is widespread in land plants; however, complex thalloid liverworts (Marchantiopsida) are the only group reported to lack both RNA editing and DYW-PPR protein. The liverwort Cyathodium cavernarum (Marchantiopsida, Cyathodiaceae), typically found in cave habitats, was newly found to have 129 C-to-U RNA editing sites in its chloroplast and 172 sites in its mitochondria. The Cyathodium genus, specifically C. cavernarum, has a large number of PPR editing factor genes, including 251 DYW-type PPR proteins. These DYW-type PPR proteins may be responsible for C-to-U RNA editing in C. cavernarum. Cyathodium cavernarum possesses both PPR DYW proteins and RNA editing. Our analysis suggests that the remarkable RNA editing capability of C. cavernarum may have been acquired alongside the emergence of DYW-type PPR editing factors. These findings provide insight into the evolutionary pattern of RNA editing in land plants.

Applications of Synthetic Pentatricopeptide Repeat Proteins

RNA-binding proteins play integral roles in the regulation of essential processes in cells and as such are attractive targets for engineering to manipulate gene expression at the RNA level. Expression of transcripts in chloroplasts and mitochondria is heavily regulated by pentatricopeptide repeat (PPR) proteins. The diverse roles of PPR proteins and their naturally modular architecture make them ideal candidates for engineering. Synthetic PPR proteins are showing great potential to become valuable tools for controlling the expression of plastid and mitochondrial transcripts. In this review, by 'synthetic', we mean both rationally modified natural PPR proteins and completely novel proteins designed using the principles learned from their natural counterparts. We focus on the many different applications of synthetic PPR proteins, covering both their use in basic research to learn more about protein-RNA interactions and their use to achieve specific outcomes in RNA processing and the control of gene expression. We describe the challenges associated with the design, construction and deployment of synthetic PPR proteins and provide perspectives on how they might be assembled and used in future biotechnology applications.

Multiple factors interact in editing of PPR-E+-targeted sites in maize mitochondria and plastids

RNA cytidine-to-uridine editing is essential for plant organellar gene expression. Pentatricopeptide repeat (PPR)-E+ proteins have been proposed to bind to target sites and recruit the cytidine deaminase AtDYW2, facilitated by AtNUWA. Here we analyze the function of ZmNUWA, ZmDYW2A, and ZmDYW2B and their relationships with other editing factors in maize. The zmdyw2a and zmdyw2b single mutants are normal, but the zmdyw2a::zmdyw2b and zmnuwa mutants are severely arrested in seed development. ZmNUWA, ZmDYW2A, and ZmDYW2B are dual localized in mitochondria and plastids. Loss of ZmNUWA decreases the editing at 99 mitochondrial sites and 8 plastid sites. Surprisingly, loss of ZmDYW2A:ZmDYW2B affects almost the same set of sites targeted by PPR-E+ proteins. ZmNUWA interacts with ZmDYW2A and ZmDYW2B, suggesting that ZmNUWA recruits ZmDYW2A/2B in the editing of PPR-E+-targeted sites in maize. Further protein interaction analyses show that ZmNUWA and ZmDYW2A/2B interact with ZmMORF1, ZmMORF8, ZmMORF2, and ZmMORF9 and that ZmOZ1 interacts with ZmORRM1, ZmDYW2A, ZmDYW2B, ZmMORF8, and ZmMORF9. These results suggest that the maize mitochondrial PPR-E+ editosome contains PPR-E+, ZmDYW2A/2B, ZmNUWA, and ZmMORF1/8, whereas the plastid PPR-E+ editosome is composed of PPR-E+, ZmDYW2A/2B, ZmNUWA, ZmMORF2/8/9, ZmORRM1, and ZmOZ1.

The dicot homolog of maize PPR103 carries a C-terminal DYW domain and may have a role in C-to-U editing of some chloroplast RNA transcripts

In plants, cytidine-to-uridine (C-to-U) editing is a crucial step in processing mitochondria- and chloroplast-encoded transcripts. This editing requires nuclear-encoded proteins including members of the pentatricopeptide (PPR) family, especially PLS-type proteins carrying the DYW domain. IPI1/emb175/PPR103 is a nuclear gene encoding a PLS-type PPR protein essential for survival in Arabidopsis thaliana and maize. Arabidopsis IPI1 was identified as likely interacting with ISE2, a chloroplast-localized RNA helicase associated with C-to-U RNA editing in Arabidopsis and maize. Notably, while the Arabidopsis and Nicotiana IPI1 orthologs possess complete DYW motifs at their C-termini, the maize homolog, ZmPPR103, lacks this triplet of residues which are essential for editing. In this study we examined the function of IPI1 in chloroplast RNA processing in N. benthamiana to gain insight into the importance of the DYW domain to the function of the EMB175/PPR103/ IPI1 proteins. Structural predictions suggest that evolutionary loss of residues identified as critical for catalyzing C-to-U editing in other members of this class of proteins, were likely to lead to reduced or absent editing activity in the Nicotiana and Arabidopsis IPI1 orthologs. Virus-induced gene silencing of NbIPI1 led to defects in chloroplast ribosomal RNA processing and changes to stability of rpl16 transcripts, revealing conserved function with its maize ortholog. NbIPI1-silenced plants also had defective C-to-U RNA editing in several chloroplast transcripts, a contrast from the finding that maize PPR103 had no role in editing. The results indicate that in addition to its role in transcript stability, NbIPI1 may contribute to C-to-U editing in N. benthamiana chloroplasts.

An RNA polymerase that became a Swiss army knife

RNA polymerases (RNAPs) control the first step of gene expression in all forms of life by transferring genetic information from DNA to RNA, a process known as transcription. In this issue of Cell, Webster et al. and Wu et al. report three-dimensional structures of RNAP complexes from chloroplasts.

Conservation of the moss RNA editing factor PPR78 despite the loss of its known cytidine-to-uridine editing sites is explained by a hidden extra target

Cytidine (C)-to-uridine (U) RNA editing in plant organelles relies on specific RNA-binding pentatricopeptide repeat (PPR) proteins. In the moss Physcomitrium patens, all such RNA editing factors feature a C-terminal DYW domain that acts as the cytidine deaminase for C-to-U conversion. PPR78 of Physcomitrium targets 2 mitochondrial editing sites, cox1eU755SL and rps14eU137SL. Remarkably, the latter is edited to highly variable degrees in different mosses. Here, we aimed to unravel the coevolution of PPR78 and its 2 target sites in mosses. Heterologous complementation in a Physcomitrium knockout line revealed that the variable editing of rps14eU137SL depends on the PPR arrays of different PPR78 orthologues but not their C-terminal domains. Intriguingly, PPR78 has remained conserved despite the simultaneous loss of editing at both known targets among Hypnales (feather mosses), suggesting it serves an additional function. Using a recently established RNA editing assay in Escherichia coli, we confirmed site-specific RNA editing by PPR78 in the bacterium and identified 4 additional off-targets in the bacterial transcriptome. Based on conservation profiles, we predicted ccmFNeU1465RC as a candidate editing target of PPR78 in moss mitochondrial transcriptomes. We confirmed editing at this site in several mosses and verified that PPR78 targets ccmFNeU1465RC in the bacterial editing system, explaining the conservation and functional adaptation of PPR78 during moss evolution.

Comparative Genomic Analysis of Asian Cultivated Rice and Its Wild Progenitor (Oryza rufipogon) Has Revealed Evolutionary Innovation of the Pentatricopeptide Repeat Gene Family through Gene Duplication

The pentatricopeptide repeat (PPR) gene family is one of the largest gene families in land plants. However, current knowledge about the evolution of the PPR gene family remains largely limited. In this study, we performed a comparative genomic analysis of the PPR gene family in O. sativa and its wild progenitor, O. rufipogon, and outlined a comprehensive landscape of gene duplications. Our findings suggest that the majority of PPR genes originated from dispersed duplications. Although segmental duplications have only expanded approximately 11.30% and 13.57% of the PPR gene families in the O. sativa and O. rufipogon genomes, we interestingly obtained evidence that segmental duplication promotes the structural diversity of PPR genes through incomplete gene duplications. In the O. sativa and O. rufipogon genomes, 10 (~33.33%) and 22 pairs of gene duplications (~45.83%) had non-PPR paralogous genes through incomplete gene duplication. Segmental duplications leading to incomplete gene duplications might result in the acquisition of domains, thus promoting functional innovation and structural diversification of PPR genes. This study offers a unique perspective on the evolution of PPR gene structures and underscores the potential role of segmental duplications in PPR gene structural diversity.

DYW cytidine deaminase domains have a long-range impact on RNA recognition by the PPR array of chimeric plant C-to-U RNA editing factors and strongly affect target selection

The protein factors for the specific C-to-U RNA editing events in plant mitochondria and chloroplasts possess unique arrays of RNA-binding pentatricopeptide repeats (PPRs) linked to carboxy-terminal cytidine deaminase DYW domains via the extension motifs E1 and E2. The E1 and E2 motifs have distant similarities to tetratricopeptide repeats known to mediate protein-protein interactions but their precise function is unclear. Here, we investigate the tolerance of PPR56 and PPR65, two functionally characterized RNA editing factors of the moss Physcomitrium patens, for the creation of chimeras by variably replacing their C-terminal protein regions. Making use of a heterologous RNA editing assay system in Escherichia coli we find that heterologous DYW domains can strongly restrict or widen the spectrum of off-targets in the bacterial transcriptome for PPR56. Surprisingly, our data suggest that these changes are not only caused by the preference of a given heterologous DYW domain for the immediate sequence environment of the cytidine to be edited but also by a long-range impact on the nucleotide selectivity of the upstream PPRs.

Chloroplast gene expression: Recent advances and perspectives

Chloroplasts evolved from an ancient cyanobacterial endosymbiont more than 1.5 billion years ago. During subsequent coevolution with the nuclear genome, the chloroplast genome has remained independent, albeit strongly reduced, with its own transcriptional machinery and distinct features, such as chloroplast-specific innovations in gene expression and complicated post-transcriptional processing. Light activates the expression of chloroplast genes via mechanisms that optimize photosynthesis, minimize photodamage, and prioritize energy investments. Over the past few years, studies have moved from describing phases of chloroplast gene expression to exploring the underlying mechanisms. In this review, we focus on recent advances and emerging principles that govern chloroplast gene expression in land plants. We discuss engineering of pentatricopeptide repeat proteins and its biotechnological effects on chloroplast RNA research; new techniques for characterizing the molecular mechanisms of chloroplast gene expression; and important aspects of chloroplast gene expression for improving crop yield and stress tolerance. We also discuss biological and mechanistic questions that remain to be answered in the future.

A Genome-Wide Analysis of the Pentatricopeptide Repeat Protein Gene Family in Two Kiwifruit Species with an Emphasis on the Role of RNA Editing in Pathogen Stress

Kiwifruit is a perennial fruit tree with high nutritional and economic value; however, various pathogen stresses have resulted in reductions in its yield and quality. Pentatricopeptide repeat proteins (PPRs), characterized by tandem repetitions of 35 amino acid motifs, play roles in RNA editing, mRNA stability, and splicing. They may also regulate plant development and growth. Nevertheless, the roles of PPRs in plant development and disease resistance remain unclear. In this study, we focused on the roles of PPRs in the fruit development and pathogen stress of kiwifruit and conducted a series of analyses of the PPR gene family in two representative kiwifruit species (Actinidia chinensis (Ach) and Actinidia eriantha (Ace)) with markedly different degrees of disease resistance. A total of 497 and 499 PPRs were identified in Ach and Ace, respectively. All the kiwifruit PPRs could be phylogenetically divided into four subfamilies. There were about 40.68% PPRs predicted to be localized to mitochondria or chloroplasts. A synteny analysis showed that the expansion of the kiwifruit PPRs mainly originated from segmental duplication. Based on RNA-seq data from the fruit over 12 periods of development and maturity, a weighted correlation network analysis suggested that two PPRs, Actinidia20495.t1 and Actinidia15159.t1, may be involved in fruit development and maturation. In addition, we observed different responses with respect to the expression of PPRs and RNA editing between resistant and susceptible kiwifruits following infection with pathogenic bacteria, indicating the regulatory role of PPRs in the stress response via the modulation of RNA editing. The differentially expressed upstream transcription factors of the PPRs were further identified; they may regulate resistance adaption by modulating the expression of the PPRs. Collectively, these results suggest that PPRs play roles in the development and disease resistance of kiwifruit and provide candidate genes for further clarifying the resistance mechanisms in kiwifruits.

The DYW domain of RARE1 plays an indispensable role in regulating accD-C794 RNA editing in Arabidopsis thaliana

The Arabidopsis pentatricopeptide repeat (PPR) proteins, required for accD RNA editing 1 (RARE1) and early chloroplast biogenesis 2 (AtECB2), each contain a DYW domain deemed essential for cytosine deamination at the accD-C794 RNA editing site in chloroplasts. Complementation assays using the rare1 mutant investigate the correlation between these PPRs and their respective DYW domain functions in RNA editing of accD-C794. The results demonstrate that the coding sequence of AtECB2 cannot replace that of RARE1. Moreover, rare1 mutants complemented with DYW-deleted RARE1 failed to recover the RNA editing of accD-C794 even in the presence of the highly similar DYW domain of the AtECB2 protein. These findings indicate that RARE1 and AtECB2 possess divergent roles in RNA editing, with specificity for accD-C794 directly attributable to DYW domain within RARE1. Structural modeling data suggest this functioning pertains to a local α-helical motif that residues slightly N-terminal to the consensus glutamate and CXXCH motif in the DYW domain for cytidine deamination during C-to-U editing by RARE1 that is absent within AtECB2.

An mTRAN-mRNA interaction mediates mitochondrial translation initiation in plants

Plant mitochondria represent the largest group of respiring organelles on the planet. Plant mitochondrial messenger RNAs (mRNAs) lack Shine-Dalgarno-like ribosome-binding sites, so it is unknown how plant mitoribosomes recognize mRNA. We show that "mitochondrial translation factors" mTRAN1 and mTRAN2 are land plant-specific proteins, required for normal mitochondrial respiration chain biogenesis. Our studies suggest that mTRANs are noncanonical pentatricopeptide repeat (PPR)-like RNA binding proteins of the mitoribosomal "small" subunit. We identified conserved Adenosine (A)/Uridine (U)-rich motifs in the 5' regions of plant mitochondrial mRNAs. mTRAN1 binds this motif, suggesting that it is a mitoribosome homing factor to identify mRNAs. We demonstrate that mTRANs are likely required for translation of all plant mitochondrial mRNAs. Plant mitochondrial translation initiation thus appears to use a protein-mRNA interaction that is divergent from bacteria or mammalian mitochondria.

Evolution and diversification of CaM/CML gene family in green plants

Calmodulin (CaM) and calmodulin-like (CML) proteins are crucial Ca2+ sensors, which are widely involved in different biological processes of plants, including their growth and development, and stress responses. However, the origin and evolution of the CaM/CML gene family in plants remain elusive. In this study, 2133 CaM and 23094 CML genes were identified from the 1000 plants project (1 KP) species and the sequenced plants, covering algae, mosses, monilophytes, lycophytes, flowering plants, and all other green plant branches. Analysis showed that the size of the CML subfamily was correlated with the genome size of corresponding plant species, as well as the total gene number in the genome. Moreover, with the evolution from algae to angiosperms, the number of CML genes in plants increased gradually which could have been driven mainly by genome-wide segmental duplication events, while the number of CaMs remained basically stable at 2-3. Phylogenetic analysis demonstrated that CaM first appeared in green algae, while CML appeared earlier and has already been presented in dinoflagellates. Further analysis showed that the number and sequence of EF-hand domain in CaMs are highly conserved, while those of CMLs are diverse among different plant taxa. Expression analysis revealed that the expression level of CaMs was generally higher than that of CMLs, indicating that the high-expression genes have essential functions, while the low-expression genes are the main reasons for the functional diversity of the CaM/CML gene family in plants. The results might contribute to understanding the evolution of CaM/CML genes as well as their molecular functions.

Beyond a PPR-RNA recognition code: Many aspects matter for the multi-targeting properties of RNA editing factor PPR56

The mitochondrial C-to-U RNA editing factor PPR56 of the moss Physcomitrium patens is an RNA-binding pentatricopeptide repeat protein equipped with a terminal DYW-type cytidine deaminase domain. Transferred into Escherichia coli, PPR56 works faithfully on its two native RNA editing targets, nad3eU230SL and nad4eU272SL, and also converts cytidines into uridines at over 100 off-targets in the bacterial transcriptome. Accordingly, PPR56 is attractive for detailed mechanistic studies in the heterologous bacterial setup, allowing for scoring differential RNA editing activities of many target and protein variants in reasonable time. Here, we report (i) on the effects of numerous individual and combined PPR56 protein and target modifications, (ii) on the spectrum of off-target C-to-U editing in the bacterial background transcriptome for PPR56 and two variants engineered for target re-direction and (iii) on combinations of targets in tandem or separately at the 5'- and 3'-ends of large mRNAs. The latter experimentation finds enhancement of RNA editing at weak targets in many cases, including cox3eU290SF as a new candidate mitogenome target. We conclude that C-to-U RNA editing can be much enhanced by transcript features also outside the region ultimately targeted by PPRs of a plant editing factor, possibly facilitated by its enrichment or scanning along transcripts.

Allelic haplotype combinations at the MS-P1 region, including P-class pentatricopeptide repeat family genes, influence wide phenotypic variation in pollen grain number through a cytoplasmic male sterility model in citrus

In citrus breeding programs, male sterility is an important trait for developing seedless varieties. Sterility associated with the male sterile cytoplasm of Kishu mandarin (Kishu-cytoplasm) has been proposed to fit the cytoplasmic male sterility (CMS) model. However, it remains undetermined whether CMS in citrus is controlled by interactions between sterile cytoplasm and nuclear restorer-of-fertility (Rf) genes. Accordingly, mechanisms underlying the control of the wide phenotypic variation in pollen number for breeding germplasm should be elucidated. This study aimed to identify complete linkage DNA markers responsible for male sterility at the MS-P1 region based on fine mapping. Two P-class pentatricopeptide repeat (PPR) family genes were identified as candidates for Rf based on predicted mitochondrial localization and higher expression in a male fertile variety/selected strain than in a male sterile variety. Eleven haplotypes (HT1-HT11) at the MS-P1 region were defined based on genotyping of DNA markers. Association analysis of diplotypes at the MS-P1 region and the number of pollen grains per anther (NPG) in breeding germplasms harboring Kishu-cytoplasm revealed that the diplotypes in this region influenced NPG. Among these haplotypes, HT1 is a non-functional restorer-of-fertility (rf) haplotype; HT2, a less-functional Rf; HT3-HT5 are semi-functional Rfs; and HT6 and HT7 are functional Rfs. However, the rare haplotypes HT8-HT11 could not be characterized. Therefore, P-class PPR family genes in the MS-P1 region may constitute the nuclear Rf genes within the CMS model, and a combination of the seven haplotypes could contribute to phenotypic variation in the NPG of breeding germplasms. These findings reveal the genomic mechanisms of CMS in citrus and will contribute to seedless citrus breeding programs by selecting candidate seedless seedlings using the DNA markers at the MS-P1 region.

Plant organellar RNA maturation

Plant organellar RNA metabolism is run by a multitude of nucleus-encoded RNA-binding proteins (RBPs) that control RNA stability, processing, and degradation. In chloroplasts and mitochondria, these post-transcriptional processes are vital for the production of a small number of essential components of the photosynthetic and respiratory machinery-and consequently for organellar biogenesis and plant survival. Many organellar RBPs have been functionally assigned to individual steps in RNA maturation, often specific to selected transcripts. While the catalog of factors identified is ever-growing, our knowledge of how they achieve their functions mechanistically is far from complete. This review summarizes the current knowledge of plant organellar RNA metabolism taking an RBP-centric approach and focusing on mechanistic aspects of RBP functions and the kinetics of the processes they are involved in.

Structural insight into the activation of an Arabidopsis organellar C-to-U RNA editing enzyme by active site complementation

RNA-binding pentatricopeptide repeat (PPR) proteins catalyze hundreds of cytidine to uridine RNA editing events in plant organelles; these editing events are essential for proper gene expression. More than half of the PPR-type RNA editing factors, however, lack the DYW cytidine deaminase domain. Genetic analyses have suggested that their cytidine deaminase activity arises by association with a family of DYW1-like proteins that contain an N-terminally truncated DYW domain, but their molecular mechanism has been unclear. Here, we report the crystal structure of the Arabidopsis thaliana DYW1 deaminase domain at 1.8 Å resolution. DYW1 has a cytidine deaminase fold lacking the PG box. The internal insertion within the deaminase fold shows an α-helical fold instead of the β-finger reported for the gating domain of the A. thaliana ORGANELLE TRANSCRIPT PROCESSING 86. The substrate-binding pocket is incompletely formed and appears to be complemented in the complex by the E2 domain and the PG box of the interacting PPR protein. In vivo RNA editing assays corroborate the activation model for DYW1 deaminase. Our study demonstrates the common activation mechanism of the DYW1-like proteins by molecular complementation of the DYW domain and reconstitution of the substrate-binding pocket.

What can hornworts teach us?

The hornworts are a small group of land plants, consisting of only 11 families and approximately 220 species. Despite their small size as a group, their phylogenetic position and unique biology are of great importance. Hornworts, together with mosses and liverworts, form the monophyletic group of bryophytes that is sister to all other land plants (Tracheophytes). It is only recently that hornworts became amenable to experimental investigation with the establishment of Anthoceros agrestis as a model system. In this perspective, we summarize the recent advances in the development of A. agrestis as an experimental system and compare it with other plant model systems. We also discuss how A. agrestis can help to further research in comparative developmental studies across land plants and to solve key questions of plant biology associated with the colonization of the terrestrial environment. Finally, we explore the significance of A. agrestis in crop improvement and synthetic biology applications in general.

The dicot homolog of maize PPR103 carries a C-terminal DYW domain and is required for C-to-U editing of chloroplast RNA transcripts

In plants, cytidine-to-uridine (C-to-U) editing is a crucial step in processing mitochondria and chloroplast-encoded transcripts. This editing requires nuclear-encoded proteins including members of the pentatricopeptide (PPR) family, especially PLS-type proteins carrying the DYW domain. IPI1/emb175/PPR103 is a nuclear gene encoding a PLS-type PPR protein essential for survival in Arabidopsis thaliana and maize. Arabidopsis IPI1 was identified as likely interacting with ISE2, a chloroplast-localized RNA helicase associated with C-to-U RNA editing in Arabidopsis and maize. Notably, while the Arabidopsis and Nicotiana IPI1 homologs possess complete DYW motifs at their C-termini, the maize homolog, ZmPPR103, lacks this triplet of residues which are essential for editing. We examined the function of ISE2 and IPI1 in chloroplast RNA processing in N. benthamiana. A combination of deep sequencing and Sanger sequencing revealed C-to-U editing at 41 sites in 18 transcripts, with 34 sites conserved in the closely related N. tabacum. Virus induced gene silencing of NbISE2 or NbIPI1 led to defective C-to-U revealed that they have overlapping roles at editing a site in the rpoB transcript but have distinct roles in editing other transcripts. This finding contrasts with maize ppr103 mutants that showed no defects in editing. The results indicate that NbISE2 and NbIPI1 are important for C-to-U editing in N. benthamiana chloroplasts, and they may function in a complex to edit specific sites while having antagonistic effects on editing others. That NbIPI1, carrying a DYW domain, is involved in organelle C-to-U RNA editing supports previous work showing that this domain catalyzes RNA editing.

Maize PPR-E proteins mediate RNA C-to-U editing in mitochondria by recruiting the trans deaminase PCW1

RNA C-to-U editing in organelles is essential for plant growth and development; however, the underlying mechanism is not fully understood. Here, we report that pentatricopeptide repeat (PPR)-E subclass proteins carry out RNA C-to-U editing by recruiting the trans deaminase PPR motifs, coiled-coil, and DYW domain-containing protein 1 (PCW1) in maize (Zea mays) mitochondria. Loss-of-function of bZIP and coiled-coil domain-containing PPR 1 (bCCP1) or PCW1 arrests seed development in maize. bCCP1 encodes a bZIP and coiled-coil domain-containing PPR protein, and PCW1 encodes an atypical PPR-DYW protein. bCCP1 is required for editing at 66 sites in mitochondria and PCW1 is required for editing at 102 sites, including the 66 sites that require bCCP1. The PCW1-mediated editing sites are exclusively associated with PPR-E proteins. bCCP1 interacts with PCW1 and the PPR-E protein Empty pericarp7 (EMP7). Two multiple organellar RNA editing factor (MORF) proteins, ZmMORF1 and ZmMORF8, interact with PCW1, EMP7, and bCCP1. ZmMORF8 enhanced the EMP7-PCW1 interaction in a yeast three-hybrid assay. C-to-U editing at the ccmFN-1553 site in maize required EMP7, bCCP1, and PCW1. These results suggest that PPR-E proteins function in RNA editing by recruiting the trans deaminase PCW1 and bCCP1, and MORF1/8 assist this recruitment through protein-protein interactions.

Insights into complex I assembly: Function of NDUFAF1 and a link with cardiolipin remodeling

Respiratory complex I is a ~1-MDa proton pump in mitochondria. Its structure has been revealed in great detail, but the structural basis of its assembly, in humans involving at least 15 assembly factors, is essentially unknown. We determined cryo-electron microscopy structures of assembly intermediates associated with assembly factor NDUFAF1 in a yeast model system. Subunits ND2 and NDUFC2 together with assembly factors NDUFAF1 and CIA84 form the nucleation point of the NDUFAF1-dependent assembly pathway. Unexpectedly, the cardiolipin remodeling enzyme tafazzin is an integral component of this core complex. In a later intermediate, all 12 subunits of the proximal proton pump module have assembled. NDUFAF1 locks the central ND3 subunit in an assembly-competent conformation, and major rearrangements of central subunits are required for complex I maturation.

Emergence of Novel RNA-Editing Sites by Changes in the Binding Affinity of a Conserved PPR Protein

RNA editing converts cytidines to uridines in plant organellar transcripts. Editing typically restores codons for conserved amino acids. During evolution, specific C-to-U editing sites can be lost from some plant lineages by genomic C-to-T mutations. By contrast, the emergence of novel editing sites is less well documented. Editing sites are recognized by pentatricopeptide repeat (PPR) proteins with high specificity. RNA recognition by PPR proteins is partially predictable, but prediction is often inadequate for PPRs involved in RNA editing. Here we have characterized evolution and recognition of a recently gained editing site. We demonstrate that changes in the RNA recognition motifs that are not explainable with the current PPR code allow an ancient PPR protein, QED1, to uniquely target the ndhB-291 site in Brassicaceae. When expressed in tobacco, the Arabidopsis QED1 edits 33 high-confident off-target sites in chloroplasts and mitochondria causing a spectrum of mutant phenotypes. By manipulating the relative expression levels of QED1 and ndhB-291, we show that the target specificity of the PPR protein depends on the RNA:protein ratio. Finally, our data suggest that the low expression levels of PPR proteins are necessary to ensure the specificity of editing site selection and prevent deleterious off-target editing.

The GENOMES UNCOUPLED1 protein has an ancient, highly conserved role but not in retrograde signalling

The pentatricopeptide repeat protein GENOMES UNCOUPLED1 (GUN1) is required for chloroplast-to-nucleus signalling when plastid translation becomes inhibited during chloroplast development in Arabidopsis thaliana, but its exact molecular function remains unknown. We analysed GUN1 sequences in land plants and streptophyte algae. We tested functional conservation by complementation of the Arabidopsis gun1 mutant with GUN1 genes from the streptophyte alga Coleochate orbicularis or the liverwort Marchantia polymorpha. We also analysed the transcriptomes of M. polymorpha gun1 knockout mutant lines during chloroplast development. GUN1 evolved within the streptophyte algal ancestors of land plants and is highly conserved among land plants but missing from the Rafflesiaceae that lack chloroplast genomes. GUN1 genes from C. orbicularis and M. polymorpha suppress the cold-sensitive phenotype of the Arabidopsis gun1 mutant and restore typical retrograde responses to treatments with inhibitors of plastid translation, even though M. polymorpha responds very differently to such treatments. Our findings suggest that GUN1 is an ancient protein that evolved within the streptophyte algal ancestors of land plants before the first plants colonized land more than 470 million years ago. Its primary role is likely to be in chloroplast gene expression and its role in chloroplast retrograde signalling probably evolved more recently.

A Comprehensive Evolutionary Study of Chloroplast RNA Editing in Gymnosperms: A Novel Type of G-to-A RNA Editing Is Common in Gymnosperms

Although more than 9100 plant plastomes have been sequenced, RNA editing sites of the whole plastome have been experimentally verified in only approximately 21 species, which seriously hampers the comprehensive evolutionary study of chloroplast RNA editing. We investigated the evolutionary pattern of chloroplast RNA editing sites in 19 species from all 13 families of gymnosperms based on a combination of genomic and transcriptomic data. We found that the chloroplast C-to-U RNA editing sites of gymnosperms shared many common characteristics with those of other land plants, but also exhibited many unique characteristics. In contrast to that noted in angiosperms, the density of RNA editing sites in ndh genes was not the highest in the sampled gymnosperms, and both loss and gain events at editing sites occurred frequently during the evolution of gymnosperms. In addition, GC content and plastomic size were positively correlated with the number of chloroplast RNA editing sites in gymnosperms, suggesting that the increase in GC content could provide more materials for RNA editing and facilitate the evolution of RNA editing in land plants or vice versa. Interestingly, novel G-to-A RNA editing events were commonly found in all sampled gymnosperm species, and G-to-A RNA editing exhibits many different characteristics from C-to-U RNA editing in gymnosperms. This study revealed a comprehensive evolutionary scenario for chloroplast RNA editing sites in gymnosperms, and reported that a novel type of G-to-A RNA editing is prevalent in gymnosperms.

U-to-C RNA editing by synthetic PPR-DYW proteins in bacteria and human culture cells

Programmable RNA editing offers significant therapeutic potential for a wide range of genetic diseases. Currently, several deaminase enzymes, including ADAR and APOBEC, can perform programmable adenosine-to-inosine or cytidine-to-uridine RNA correction. However, enzymes to perform guanosine-to-adenosine and uridine-to-cytidine (U-to-C) editing are still lacking to complete the set of transition reactions. It is believed that the DYW:KP proteins, specific to seedless plants, catalyze the U-to-C reactions in mitochondria and chloroplasts. In this study, we designed seven DYW:KP domains based on consensus sequences and fused them to a designer RNA-binding pentatricopeptide repeat (PPR) domain. We show that three of these PPR-DYW:KP proteins edit targeted uridine to cytidine in bacteria and human cells. In addition, we show that these proteins have a 5′ but not apparent 3′ preference for neighboring nucleotides. Our results establish the DYW:KP aminase domain as a potential candidate for the development of a U-to-C editing tool in human cells.

DYW:KP domains, designed on proteins found in the mitochondria and chloroplasts of seedless plants, are fused to a designer RNA-binding pentatricopeptide repeat (PPR) domain to edit targeted uridine to cytidine in bacteria and human cells.

Plant mitochondrial RNA editing factors can perform targeted C-to-U editing of nuclear transcripts in human cells

RNA editing processes are strikingly different in animals and plants. Up to thousands of specific cytidines are converted into uridines in plant chloroplasts and mitochondria whereas up to millions of adenosines are converted into inosines in animal nucleo-cytosolic RNAs. It is unknown whether these two different RNA editing machineries are mutually incompatible. RNA-binding pentatricopeptide repeat (PPR) proteins are the key factors of plant organelle cytidine-to-uridine RNA editing. The complete absence of PPR mediated editing of cytosolic RNAs might be due to a yet unknown barrier that prevents its activity in the cytosol. Here, we transferred two plant mitochondrial PPR-type editing factors into human cell lines to explore whether they could operate in the nucleo-cytosolic environment. PPR56 and PPR65 not only faithfully edited their native, co-transcribed targets but also different sets of off-targets in the human background transcriptome. More than 900 of such off-targets with editing efficiencies up to 91%, largely explained by known PPR-RNA binding properties, were identified for PPR56. Engineering two crucial amino acid positions in its PPR array led to predictable shifts in target recognition. We conclude that plant PPR editing factors can operate in the entirely different genetic environment of the human nucleo-cytosol and can be intentionally re-engineered towards new targets.

Evolution of mitochondrial RNA editing in extant gymnosperms

To unveil the evolution of mitochondrial RNA editing in gymnosperms, we characterized mitochondrial genomes (mitogenomes), plastid genomes, RNA editing sites, and pentatricopeptide repeat (PPR) proteins from 10 key taxa representing four of the five extant gymnosperm clades. The assembled mitogenomes vary in gene content due to massive gene losses in Gnetum and Conifer II clades. Mitochondrial gene expression levels also vary according to protein function, with the most highly expressed genes involved in the respiratory complex. We identified 9132 mitochondrial C-to-U editing sites, as well as 2846 P-class and 8530 PLS-class PPR proteins. Regains of editing sites were demonstrated in Conifer II rps3 transcripts whose corresponding mitogenomic sequences lack introns due to retroprocessing. Our analyses reveal that non-synonymous editing is efficient and results in more codons encoding hydrophobic amino acids. In contrast, synonymous editing, although performed with variable efficiency, can increase the number of U-ending codons that are preferentially utilized in gymnosperm mitochondria. The inferred loss-to-gain ratio of mitochondrial editing sites in gymnosperms is 2.1:1, of which losses of non-synonymous editing are mainly due to genomic C-to-T substitutions. However, such substitutions only explain a small fraction of synonymous editing site losses, indicating distinct evolutionary mechanisms. We show that gymnosperms have experienced multiple lineage-specific duplications in PLS-class PPR proteins. These duplications likely contribute to accumulated RNA editing sites, as a mechanistic correlation between RNA editing and PLS-class PPR proteins is statistically supported.

The evolution of extremely diverged plastomes in Selaginellaceae (lycophyte) is driven by repeat patterns and the underlying DNA maintenance machinery

Two factors are proposed to account for the unusual features of organellar genomes: the disruptions of organelle-targeted DNA replication, repair, and recombination (DNA-RRR) systems in the nuclear genome and repetitive elements in organellar genomes. Little is known about how these factors affect organellar genome evolution. The deep-branching vascular plant family Selaginellaceae is known to have a deficient DNA-RRR system and convergently evolved organellar genomes. However, we found that the plastid genome (plastome) of Selaginella sinensis has extremely accelerated substitution rates, a low GC content, pervasive repeat elements, a dynamic network structure, and it lacks direct or inverted repeats. Unexpectedly, its organelle DNA-RRR system is short of a plastid-targeted Recombinase A1 (RecA1) and a mitochondrion-targeted RecA3, in line with other explored Selaginella species. The plastome contains a large collection of short- and medium-sized repeats. Given the absence of RecA1 surveillance, we propose that these repeats trigger illegitimate recombination, accelerated mutation rates, and structural instability. The correlations between repeat quantity and architectural complexity in the Selaginella plastomes support these conclusions. We, therefore, hypothesize that the interplay of the deficient DNA-RRR system and the high repeat content has led to the extraordinary divergence of the S. sinensis plastome. Our study not only sheds new light on the mechanism of plastome divergence by emphasizing the power of cytonuclear integration, but it also reconciles the longstanding contradiction on the effects of DNA-RRR system disruption on genome structure evolution.

Chloroplastic pentatricopeptide repeat proteins (PPR) in albino plantlets of Agave angustifolia Haw. reveal unexpected behavior

BACKGROUND

Pentatricopeptide repeat (PPR) proteins play an essential role in the post-transcriptional regulation of genes in plastid genomes. Although important advances have been made in understanding the functions of these genes, there is little information available on chloroplastic PPR genes in non-model plants and less in plants without chloroplasts. In the present study, a comprehensive and multifactorial bioinformatic strategy was applied to search for putative PPR genes in the foliar and meristematic tissues of green and albino plantlets of the non-model plant Agave angustifolia Haw.

RESULTS

A total of 1581 PPR transcripts were identified, of which 282 were chloroplastic. Leaf tissue in the albino plantlets showed the highest levels of expression of chloroplastic PPRs. The search for hypothetical targets of 12 PPR sequences in the chloroplast genes of A. angustifolia revealed their action on transcripts related to ribosomes and translation, photosystems, ATP synthase, plastid-encoded RNA polymerase and RuBisCO.

CONCLUSIONS

Our results suggest that the expression of PPR genes depends on the state of cell differentiation and plastid development. In the case of the albino leaf tissue, which lacks functional chloroplasts, it is possible that anterograde and retrograde signaling networks are severely compromised, leading to a compensatory anterograde response characterized by an increase in the expression of PPR genes.

Harnessing the Algal Chloroplast for Heterologous Protein Production

Photosynthetic microbes are gaining increasing attention as heterologous hosts for the light-driven, low-cost production of high-value recombinant proteins. Recent advances in the manipulation of unicellular algal genomes offer the opportunity to establish engineered strains as safe and viable alternatives to conventional heterotrophic expression systems, including for their use in the feed, food, and biopharmaceutical industries. Due to the relatively small size of their genomes, algal chloroplasts are excellent targets for synthetic biology approaches, and are convenient subcellular sites for the compartmentalized accumulation and storage of products. Different classes of recombinant proteins, including enzymes and peptides with therapeutical applications, have been successfully expressed in the plastid of the model organism Chlamydomonas reinhardtii, and of a few other species, highlighting the emerging potential of transplastomic algal biotechnology. In this review, we provide a unified view on the state-of-the-art tools that are available to introduce protein-encoding transgenes in microalgal plastids, and discuss the main (bio)technological bottlenecks that still need to be addressed to develop robust and sustainable green cell biofactories.

Maize PPR278 Functions in Mitochondrial RNA Splicing and Editing

Pentatricopeptide repeat (PPR) proteins are a large protein family in higher plants and play important roles during seed development. Most reported PPR proteins function in mitochondria. However, some PPR proteins localize to more than one organelle; functional characterization of these proteins remains limited in maize (Zea mays L.). Here, we cloned and analyzed the function of a P-subfamily PPR protein, PPR278. Loss-function of PPR278 led to a lower germination rate and other defects at the seedling stage, as well as smaller kernels compared to the wild type. PPR278 was expressed in all investigated tissues. Furthermore, we determined that PPR278 is involved in the splicing of two mitochondrial transcripts (nad2 intron 4 and nad5 introns 1 and 4), as well as RNA editing of C-to-U sites in 10 mitochondrial transcripts. PPR278 localized to the nucleus, implying that it may function as a transcriptional regulator during seed development. Our data indicate that PPR278 is involved in maize seed development via intron splicing and RNA editing in mitochondria and has potential regulatory roles in the nucleus.

The Development of Mitochondrial Gene Editing Tools and Their Possible Roles in Crop Improvement for Future Agriculture

We are living in the era of genome editing. Nowadays, targeted editing of the plant nuclear DNA is prevalent in basic biological research and crop improvement since its first establishment a decade ago. However, achieving the same accomplishment for the plant mitochondrial genome has long been deemed impossible. Recently, the pioneer studies on editing plant mitogenome have been done using the mitochondria-targeted transcription activator-like effector nucleases (mitoTALENs) in rice, rapeseed, and Arabidopsis. It is well documented that mitochondria play essential roles in plant development and stress tolerance, particularly, in cytoplasmic male sterility widely used in production of hybrids. The success of mitochondrial genome editing enables studying the fundamentals of mitochondrial genome. Furthermore, mitochondrial RNA editing (mostly by nuclear-encoded pentatricopeptide repeat (PPR) proteins) in a sequence-specific manner can simultaneously change the production of translatable mitochondrial mRNA. Moreover, direct editing of the nuclear-encoding mitochondria-targeted factors required for plant mitochondrial genome dynamics and recombination may facilitate genetic manipulation of plant mitochondria. Here, the present state of knowledge on editing the plant mitochondrial genome is reviewed.

MISF2 Encodes an Essential Mitochondrial Splicing Cofactor Required for nad2 mRNA Processing and Embryo Development in Arabidopsis thaliana

Mitochondria play key roles in cellular energy metabolism in eukaryotes. Mitochondria of most organisms contain their own genome and specific transcription and translation machineries. The expression of angiosperm mtDNA involves extensive RNA-processing steps, such as RNA trimming, editing, and the splicing of numerous group II-type introns. Pentatricopeptide repeat (PPR) proteins are key players in plant organelle gene expression and RNA metabolism. In the present analysis, we reveal the function of the MITOCHONDRIAL SPLICING FACTOR 2 gene (MISF2, AT3G22670) and show that it encodes a mitochondria-localized PPR protein that is crucial for early embryo development in Arabidopsis. Molecular characterization of embryo-rescued misf2 plantlets indicates that the splicing of nad2 intron 1, and thus respiratory complex I biogenesis, are strongly compromised. Moreover, the molecular function seems conserved between MISF2 protein in Arabidopsis and its orthologous gene (EMP10) in maize, suggesting that the ancestor of MISF2/EMP10 was recruited to function in nad2 processing before the monocot-dicot divergence ~200 million years ago. These data provide new insights into the function of nuclear-encoded factors in mitochondrial gene expression and respiratory chain biogenesis during plant embryo development.