The deletion of ppr2 interferes iron sensing and leads to oxidative stress response in Schizosaccharomyces pombe

Pentatricopeptide repeat proteins are involved in mitochondrial both transcriptional and posttranscriptional regulation. Schizosaccharomyces pombe Ppr2 is a general mitochondrial translation factor that plays a critical role in the synthesis of all mitochondrial DNA-encoded oxidative phosphorylation subunits, which are essential for mitochondrial respiration. Our previous analysis showed that ppr2 deletion resulted in increased expression of iron uptake genes and caused ferroptosis-like cell death in S. pombe. In the present work, we showed that deletion of ppr2 reduced viability on glycerol- and galactose-containing media.Php4 is a transcription repressor that regulates iron homeostasis in fission yeast. We found that in the ppr2 deletion strain, Php4 was constitutively active and accumulated in the nucleus in the stationary phase. We also found that deletion of ppr2 decreased the ferroptosis-related protein Gpx1 in the mitochondria. Overexpression of Gpx1 improves the viability of Δppr2 cells. We showed that the deletion of ppr2 increased the production of ROS, downregulated heme synthesis and iron-sulfur cluster proteins, and induced stress proteins. Finally, we observed the nuclear accumulation of Pap1-GFP and Sty1-GFP, suggesting that Sty1 and Pap1 in response to cellular stress in the ppr2 deletion strain. These results suggest thatppr2 deletion may cause mitochondrial dysfunction, which is likely to lead to iron-sensing defect and iron starvation response, resulting in perturbation of iron homeostasis and increased hydroxyl radical production. The increased hydroxyl radical production triggers cellular responses in theppr2 deletion strain.

Sls1 and Mtf2 mediate the assembly of the Mrh5C complex required for activation of cox1 mRNA translation

Mitochondrial translation depends on mRNA-specific activators. In Schizosaccharomyces pombe, DEAD-box protein Mrh5, pentatricopeptide repeat (PPR) protein Ppr4, Mtf2, and Sls1 form a stable complex (designated Mrh5C) required for translation of mitochondrial DNA (mtDNA)-encoded cox1 mRNA, the largest subunit of the cytochrome c oxidase complex. However, how Mrh5C is formed and what role Mrh5C plays in cox1 mRNA translation have not been reported. To address these questions, we investigated the role of individual Mrh5C subunits in the assembly and function of Mrh5C. Our results revealed that Mtf2 and Sls1 form a subcomplex that serves as a scaffold to bring Mrh5 and Ppr4 together. Mrh5C binds to the small subunit of the mitoribosome (mtSSU), but each subunit could not bind to the mtSSU independently. Importantly, Mrh5C is required for the association of cox1 mRNA with the mtSSU. Finally, we investigated the importance of the signature DEAD-box in Mrh5. We found that the DEAD-box of Mrh5 is required for the association of Mrh5C and cox1 mRNA with the mtSSU. Unexpectedly, this motif is also required for the interaction of Mrh5 with other Mrh5C subunits. Altogether, our results suggest that Mrh5 and Ppr4 cooperate in activating the translation of cox1 mRNA. Our results also suggest that Mrh5C activates the translation of cox1 mRNA by promoting the recruitment of cox1 mRNA to the mtSSU.

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.

Cryo-EM structures of the plant plastid-encoded RNA polymerase

Chloroplasts are green plastids in the cytoplasm of eukaryotic algae and plants responsible for photosynthesis. The plastid-encoded RNA polymerase (PEP) plays an essential role during chloroplast biogenesis from proplastids and functions as the predominant RNA polymerase in mature chloroplasts. The PEP-centered transcription apparatus comprises a bacterial-origin PEP core and more than a dozen eukaryotic-origin PEP-associated proteins (PAPs) encoded in the nucleus. Here, we determined the cryo-EM structures of Nicotiana tabacum (tobacco) PEP-PAP apoenzyme and PEP-PAP transcription elongation complexes at near-atomic resolutions. Our data show the PEP core adopts a typical fold as bacterial RNAP. Fifteen PAPs bind at the periphery of the PEP core, facilitate assembling the PEP-PAP supercomplex, protect the complex from oxidation damage, and likely couple gene transcription with RNA processing. Our results report the high-resolution architecture of the chloroplast transcription apparatus and provide the structural basis for the mechanistic and functional study of transcription regulation in chloroplasts.

Young Leaf White Stripe encodes a P-type PPR protein required for chloroplast development

Pentatricopeptide repeat (PPR) proteins function in post-transcriptional regulation of organellar gene expression. Although several PPR proteins are known to function in chloroplast development in rice (Oryza sativa), the detailed molecular functions of many PPR proteins remain unclear. Here, we characterized a rice young leaf white stripe (ylws) mutant, which has defective chloroplast development during early seedling growth. Map-based cloning revealed that YLWS encodes a novel P-type chloroplast-targeted PPR protein with 11 PPR motifs. Further expression analyses showed that many nuclear- and plastid-encoded genes in the ylws mutant were significantly changed at the RNA and protein levels. The ylws mutant was impaired in chloroplast ribosome biogenesis and chloroplast development under low-temperature conditions. The ylws mutation causes defects in the splicing of atpF, ndhA, rpl2, and rps12, and editing of ndhA, ndhB, and rps14 transcripts. YLWS directly binds to specific sites in the atpF, ndhA, and rpl2 pre-mRNAs. Our results suggest that YLWS participates in chloroplast RNA group II intron splicing and plays an important role in chloroplast development during early leaf development.

OsPPR11 encoding P-type PPR protein that affects group II intron splicing and chloroplast development

OsPPR11 belongs to the P-type PPR protein family and can interact with OsCAF2 to regulate Group II intron splicing and affect chloroplast development in rice. Pentatricopeptide repeat (PPR) proteins participate in chloroplasts or mitochondria group II introns splicing in plants. The PPR protein family contains 491 members in rice, but most of their functions are unknown. In this study, we identified a nuclear gene encoding the P-type PPR protein OsPPR11 in chloroplasts. The qRT-PCR analysis demonstrated that OsPPR11 was expressed in all plant tissues, but leaves had the highest expression. The osppr11 mutants had yellowing leaves and a lethal phenotype that inhibited chloroplast development and photosynthesis-related gene expression and reduced photosynthesis-related protein accumulation in seedlings. Moreover, photosynthetic complex accumulation decreased significantly in osppr11 mutants. The OsPPR11 is required for ndhA, and ycf3-1 introns splicing and interact with CRM family protein OsCAF2, suggesting that these two proteins may form splicing complexes to regulate group II introns splicing. Further analysis revealed that OsCAF2 interacts with OsPPR11 through the N-terminus. These results indicate that OsPPR11 is essential for chloroplast development and function by affecting group II intron splicing in rice.

OsPPR11 encoding P-type PPR protein that affects group II intron splicing and chloroplast development

OsPPR11 belongs to the P-type PPR protein family and can interact with OsCAF2 to regulate Group II intron splicing and affect chloroplast development in rice. Pentatricopeptide repeat (PPR) proteins participate in chloroplasts or mitochondria group II introns splicing in plants. The PPR protein family contains 491 members in rice, but most of their functions are unknown. In this study, we identified a nuclear gene encoding the P-type PPR protein OsPPR11 in chloroplasts. The qRT-PCR analysis demonstrated that OsPPR11 was expressed in all plant tissues, but leaves had the highest expression. The osppr11 mutants had yellowing leaves and a lethal phenotype that inhibited chloroplast development and photosynthesis-related gene expression and reduced photosynthesis-related protein accumulation in seedlings. Moreover, photosynthetic complex accumulation decreased significantly in osppr11 mutants. The OsPPR11 is required for ndhA, and ycf3-1 introns splicing and interact with CRM family protein OsCAF2, suggesting that these two proteins may form splicing complexes to regulate group II introns splicing. Further analysis revealed that OsCAF2 interacts with OsPPR11 through the N-terminus. These results indicate that OsPPR11 is essential for chloroplast development and function by affecting group II intron splicing in rice.

Schizosaccharomyces pombe MAP kinase Sty1 promotes survival of Δppr10 cells with defective mitochondrial protein synthesis

Deletion of the Schizosaccharomyces pombe pentatricopeptide repeat gene ppr10 severely impairs mitochondrial translation, resulting in defective oxidative phosphorylation (OXPHOS). ppr10 deletion also induces iron starvation response, resulting in increased reactive oxygen species (ROS) production and reduced viability under fermentative conditions. S. pombe has two principal stress-response pathways, which are mediated by the mitogen-activated protein kinase Sty1 and the basic leucine zipper transcription factor Pap1, respectively. In this study, we examined the roles of Sty1 and Pap1 in the cellular response to the mitochondrial translation defect caused by ppr10 deletion. We found that ppr10 deletion resulted in two waves of stress protein activation. The early response occurred in exponential phase and resulted in the expression of a subset of stress proteins including Gst2 and Obr1. The upregulation of some of these stress proteins in Δppr10 cells in early response is dependent on the basal nuclear levels of Sty1 or Pap1. The late response occurred in early stationary phase and coincided with the stable localization of Sty1 and Pap1 in the nucleus, presumably resulting in persistent activation of a large set of stress proteins. Deletion of sty1 in Δppr10 cells caused severe defects in cell division and growth, and further impaired cell viability. Deletion of the mitochondrial superoxide dismutase gene sod2 whose expression is controlled by Sty1 severely inhibited the growth of Δppr10 cells. Overexpression of sod2 improves the viability of Δppr10 cells. Our results support an important role for Sty1 in counteracting stress induced by ppr10 deletion under fermentative growth conditions.

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.

A chloroplast-localized pentatricopeptide repeat protein involved in RNA editing and splicing and its effects on chloroplast development in rice

BACKGROUND

The chloroplast is the organelle responsible for photosynthesis in higher plants. The generation of functional chloroplasts depends on the precise coordination of gene expression in the nucleus and chloroplasts and is essential for the development of plants. However, little is known about nuclear-plastid regulatory mechanisms at the early stage of chloroplast generation in rice.

RESULTS

In this study, we identified a rice (Oryza sativa) mutant that exhibited albino and seedling-lethal phenotypes and named it ssa1(seedling stage albino1). Transmission electron microscopy (TEM) analysis indicated that the chloroplasts of ssa1 did not have organized thylakoid lamellae and that the chloroplast structure was destroyed. Genetic analysis revealed that the albino phenotypes of ssa1 were controlled by a pair of recessive nuclear genes. Map-based cloning experiments found that SSA1 encoded a pentapeptide repeat (PPR) protein that was allelic to OSOTP51,which was previously reported to participate in Photosystem I (PSI) assembly. The albino phenotype was reversed to the wild type (WT) phenotype when the normal SSA1 sequence was expressed in ssa1 under the drive of the actin promoter. Knockout experiments further created mutants ssa1-2/1-9, which had a phenotype similar to that of ssa1. SSA1 consisted of 7 pentatricopeptide repeat domains and two C-terminal LAGLIDADG tandem sequence motifs and was located in the chloroplast. GUS staining and qRT-PCR analysis showed that SSA1 was mainly expressed in young leaves and stems. In the ssa1 mutants, plastid genes transcribed by plastid-encoded RNA polymerase decreased, while those transcribed by nuclear-encoded RNA polymerase increased at the mRNA level. Loss-of-function SSA1 destroys RNA editing of ndhB-737 and intron splicing of atpF and ycf3-2 in the plastid genome. Yeast two-hybrid and BiFC assays revealed that SSA1 physically interacted with two new RNA editing partners, OsMORF8 and OsTRXz, which have potential functions in RNA editing and chloroplast biogenesis.

CONCLUSIONS

Rice SSA1 encodes a pentatricopeptide repeat protein, which is targeted to the chloroplast. SSA1 regulates early chloroplast development and plays a critical role in RNA editing and intron splicing in rice. These data will facilitate efforts to further elucidate the molecular mechanism of chloroplast biogenesis.

EMP80 mediates the C-to-U editing of nad7 and atp4 and interacts with ZmDYW2 in maize mitochondria

RNA C-to-U editing is important to the expression and function of organellar genes in plants. Although several families of proteins have been identified to participate in this process, the underlying mechanism is not fully understood. Here we report the function of EMP80 in the C-to-U editing at the nad7-769 and atp4-118 sites, and the potential recruitment of ZmDYW2 as a trans deaminase in maize (Zea mays) mitochondria. Loss of EMP80 function arrests embryogenesis and endosperm development in maize. EMP80 is a PPR-E+ protein localised to mitochondria. An absence of EMP80 abolishes the C-to-U RNA editing at nad7-769 and atp4-118 sites, resulting in a cysteine-to-arginine (Cys→Arg) change in Nad7 and Atp4 in the emp80 mutant. The amino acid change consequently reduces the assembly of complexes I and V, leading to an accumulation of the F₁ subcomplex of complex V. EMP80 was found to interact with atypical DYW-type PPR protein ZmDYW2, which interacts with ZmNUWA. Co-expression of ZmNUWA enhances the interaction between EMP80 and ZmDYW2, suggesting that EMP80 potentially recruits ZmDYW2 as a trans deaminase through protein-protein interaction, and ZmNUWA may function as an enhancer of this interaction.

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 pentatricopeptide repeat protein GEND1 is required for root development and high temperature tolerance in Arabidopsis thaliana

Pentatricopeptide repeat (PPR) proteins are a large family in land plants that play a role in organellular RNA processing, editing, and splicing. Here, we identify an Arabidopsis thaliana mutant, gend1-1, which exhibits a short root phenotype with reduced meristem size and cell numbers. Positional cloning of GEND1 revealed that it encodes a PPR protein, and functional analysis showed that GEND1 can bind and edit mitochondrial ccmFn-1 mRNA, causing gend1 mutants to have decreased levels of cytochrome C. GEND1 was up-regulated by high temperature conditions, to which gend1 mutants were hypersensitive. Analysis of a set of PPR mutants under high temperature showed that mutants with defects in cytochrome C had comparable temperature sensitivity to gend1. Collectively, these results suggest that cytochrome C plays an important role in root development and high temperature response in Arabidopsis.

The pentatricopeptide repeat protein EMP603 is required for the splicing of mitochondrial Nad1 intron 2 and seed development in maize

Intron splicing is an essential event in post-transcriptional RNA processing in plant mitochondria, which requires the participation of diverse nuclear-encoded splicing factors. However, it is presently unclear how these proteins cooperatively take part in the splicing of specific introns. In this study, we characterized a nuclear-encoded mitochondrial P-type pentatricopeptide repeat (PPR) protein named EMP603. This protein is essential for splicing of intron 2 in the Nad1 gene and interacts with the mitochondria-localized DEAD-box RNA helicase PMH2-5140, the RAD52-like proteins ODB1-0814 and ODB1-5061, and the CRM domain-containing protein Zm-mCSF1. Further study revealed that the N-terminal region of EMP603 interacts with the DEAD-box of PMH2-5140, the CRM domain of Zm-mCSF1, and OBD1-5061, but not with OBD1-0814, whereas the PPR domain of EMP603 can interact with ODB1-0814, ODB1-5061, and PMH2-5140, but not with Zm-mCSF1. Defects in EMP603 severely disrupt the assembly and activity of mitochondrial complex I, leading to impaired mitochondrial function, and delayed seed development. The interactions revealed between EMP603 and PMH2-5140, ODB1-0814, ODB1-5061, and Zm-mCSF1 indicate a possible involvement of a dynamic 'spliceosome-like' complex in intron splicing, and may accelerate the elucidation of the intron splicing mechanism in plant mitochondria.

Functions of PPR Proteins in Plant Growth and Development

Pentatricopeptide repeat (PPR) proteins form a large protein family in land plants, with hundreds of different members in angiosperms. In the last decade, a number of studies have shown that PPR proteins are sequence-specific RNA-binding proteins involved in multiple aspects of plant organellar RNA processing, and perform numerous functions in plants throughout their life cycle. Recently, computational and structural studies have provided new insights into the working mechanisms of PPR proteins in RNA recognition and cytidine deamination. In this review, we summarized the research progress on the functions of PPR proteins in plant growth and development, with a particular focus on their effects on cytoplasmic male sterility, stress responses, and seed development. We also documented the molecular mechanisms of PPR proteins in mediating RNA processing in plant mitochondria and chloroplasts.

A PPR Protein ACM1 Is Involved in Chloroplast Gene Expression and Early Plastid Development in Arabidopsis

Chloroplasts cannot develop normally without the coordinated action of various proteins and signaling connections between the nucleus and the chloroplast genome. Many questions regarding these processes remain unanswered. Here, we report a novel P-type pentatricopeptide repeat (PPR) factor, named Albino Cotyledon Mutant1 (ACM1), which is encoded by a nuclear gene and involved in chloroplast development. Knock-down of ACM1 transgenic plants displayed albino cotyledons but normal true leaves, while knock-out of the ACM1 gene in seedlings was lethal. Fluorescent protein analysis showed that ACM1 was specifically localized within chloroplasts. PEP-dependent plastid transcript levels and splicing efficiency of several group II introns were seriously affected in cotyledons in the RNAi line. Furthermore, denaturing gel electrophoresis and Western blot experiments showed that the accumulation of chloroplast ribosomes was probably damaged. Collectively, our results indicate ACM1 is indispensable in early chloroplast development in Arabidopsis cotyledons.

Functioning of PPR Proteins in Organelle RNA Metabolism and Chloroplast Biogenesis

The pentatricopeptide repeat (PPR) proteins constitute one of the largest nuclear-encoded protein families in higher plants, with over 400 members in most sequenced plant species. The molecular functions of these proteins and their physiological roles during plant growth and development have been widely studied. Generally, there is mounting evidence that PPR proteins are involved in the post-transcriptional regulation of chloroplast and/or mitochondrial genes, including RNA maturation, editing, intron splicing, transcripts' stabilization, and translation initiation. The cooperative action of RNA metabolism has profound effects on the biogenesis and functioning of both chloroplasts and mitochondria and, consequently, on the photosynthesis, respiration, and development of plants and their environmental responses. In this review, we summarize the latest research on PPR proteins, specifically how they might function in the chloroplast, by documenting their mechanism of molecular function, their corresponding RNA targets, and their specific effects upon chloroplast biogenesis and host organisms.

Schizosaccharomyces pombe Ppr10 is required for mitochondrial translation

The mitochondrial genome encodes key components of the oxidative phosphorylation (OXPHOS) system, whose expression is essential for mitochondrial functions. We have previously shown that deletion of the Schizosaccharomyces pombe ppr10 encoding a pentatricopeptide repeat protein severely reduces the mature levels of intron-containing mitochondrial transcripts cox1 and cob1, and severely impairs mitochondrial translation. In this study, we examined the possibility that the reduced levels of Cox1 and Cob1 proteins in cells were due to lowered levels of cox1 and cob1 mRNAs. We found that deletion of ppr10 did not affect the levels of mature cox1 and cob1 mRNAs in a mitochondrial intronless background. However, synthesis of Cox1 and Cob1 proteins were still severely affected by deletion of ppr10 in a mitochondrial intronless background. Consistent with this, we found that deletion of mitochondrial introns could not rescue the respiratory growth defect of Δppr10 cells. Our results reveal that Ppr10 is not required for the stability of cox1 and cob1 mRNAs, and provide further support for the idea that Ppr10 plays a critical role in mitochondrial translation.

The DYW-subgroup pentatricopeptide repeat protein PPR27 interacts with ZmMORF1 to facilitate mitochondrial RNA editing and seed development in maize

C-to-U RNA editing in plant mitochondria requires the participation of many nucleus-encoded factors, most of which are pentatricopeptide repeat (PPR) proteins. There is a large number of PPR proteins and the functions many of them are unknown. Here, we report a mitochondrion-localized DYW-subgroup PPR protein, PPR27, which functions in the editing of multiple mitochondrial transcripts in maize. The ppr27 mutant is completely deficient in C-to-U editing at the ccmFN-1357 and rps3-707 sites, and editing at six other sites is substantially reduced. The lack of editing at ccmFN-1357 causes a deficiency of CcmFN protein. As CcmFN functions in the maturation pathway of cytochrome proteins that are subunits of mitochondrial complex III, its deficiency results in an absence of cytochrome c1 and cytochrome c proteins. Consequently, the assembly of mitochondrial complex III and super-complex I+III2 is decreased, which impairs the electron transport chain and respiration, leading to arrests in embryogenesis and endosperm development in ppr27. In addition, PPR27 was found to physically interact with ZmMORF1, which interacts with ZmMORF8, suggesting that these three proteins may facilitate C-to-U RNA editing via the formation of a complex in maize mitochondria. This RNA editing is essential for complex III assembly and seed development in maize.

The Mitochondrial Pentatricopeptide Repeat Protein PPR18 Is Required for the cis-Splicing of nad4 Intron 1 and Essential to Seed Development in Maize

Pentatricopeptide repeat (PPR) protein comprises a large family, participating in various aspects of organellar RNA metabolism in land plants. There are approximately 600 PPR proteins in maize, but the functions of many PPR proteins remain unknown. In this study, we defined the function of PPR18 in the cis-splicing of nad4 intron 1 in mitochondria and seed development in maize. Loss function of PPR18 seriously impairs embryo and endosperm development, resulting in the empty pericarp (emp) phenotype in maize. PPR18 encodes a mitochondrion-targeted P-type PPR protein with 18 PPR motifs. Transcripts analysis indicated that the splicing of nad4 intron 1 is impaired in the ppr18 mutant, resulting in the absence of nad4 transcript, leading to severely reduced assembly and activity of mitochondrial complex I and dramatically reduced respiration rate. These results demonstrate that PPR18 is required for the cis-splicing of nad4 intron 1 in mitochondria, and critical to complex I assembly and seed development in maize.

MEF46 and MEF47 are novel specificity factors for RNA editing sites in mitochondrial nad transcripts

Terrestrial plants have C-to-U RNA editing in the transcripts of plastids and mitochondria. Target specificity for more than several hundred editing sites are governed by PLS (PPR, Long and Short) class Pentatricopeptide repeat (PPR) proteins with additional C-terminal domains. Half of these PPR proteins have DYW (Aspartate (D), Tyrosine (Y) and Tryptophan (W)) domains, which most likely harbour cytidine deaminase activity. The other half of them, E subclass and E+ subclass proteins, contain no or only a part of the DYW domain. Missing DYW domains in the E and E+ subclass PPR proteins are likely to be complemented by other DYW containing proteins. All target sites of so far characterized E+ subclass PPR proteins show defects in dyw2 mutants, suggesting that the DYW2 protein complements the missing DYW domains in the E+ subclass PPR proteins. Here we report two novel RNA editing factors, MEF46 and MEF47, which belong to E+ and E subclass, respectively. The defective editing site in mef46, nad5-1958, overlaps with the affected sites in dyw2 mutants, while that in mef47, nad3-64 and ccmC-614 do not, further supporting the specific functional connection between E+ subclass PPR proteins and DYW2.

Transcriptomic analysis of poco1, a mitochondrial pentatricopeptide repeat protein mutant in Arabidopsis thaliana

BACKGROUND

Flowering is a crucial stage during plant development. Plants may respond to unfavorable conditions by accelerating reproductive processes like flowering. In a recent study, we showed that PRECOCIOUS1 (POCO1) is a mitochondrial pentatricopeptide repeat (PPR) protein involved in flowering time and abscisic acid (ABA) signaling in Arabidopsis thaliana. Here, we use RNA-seq data to investigate global gene expression alteration in the poco1 mutant.

RESULTS

RNA-seq analysis was performed during different developmental stages for wild-type and poco1 plants. The most profound differences in gene expression were found when wild-type and poco1 plants of the same developmental stage were compared. Coverage analysis confirmed the T-DNA insertion in POCO1, which was concomitant with truncated transcripts. Many biological processes were found to be enriched. Several flowering-related genes such as FLOWERING LOCUS T (FT), which may be involved in the early-flowering phenotype of poco1, were differentially regulated. Numerous ABA-associated genes, including the core components of ABA signaling such as ABA receptors, protein phosphatases, protein kinases, and ABA-responsive element (ABRE) binding proteins (AREBs)/ABRE-binding factors (ABFs) as well as important genes for stomatal function, were mostly down-regulated in poco1. Drought and oxidative stress-related genes, including ABA-induced stress genes, were differentially regulated. RNA-seq analysis also uncovered differentially regulated genes encoding various classes of transcription factors and genes involved in cellular signaling. Furthermore, the expression of stress-associated nuclear genes encoding mitochondrial proteins (NGEMPs) was found to be altered in poco1. Redox-related genes were affected, suggesting that the redox state in poco1 might be altered.

CONCLUSION

The identification of various enriched biological processes indicates that complex regulatory mechanisms underlie poco1 development. Differentially regulated genes associated with flowering may contribute to the early-flowering phenotype of poco1. Our data suggest the involvement of POCO1 in the early ABA signaling process. The down-regulation of many ABA-related genes suggests an association of poco1 mutation with the ABA signaling deficiency. This condition further affects the expression of many stress-related, especially drought-associated genes in poco1, consistent with the drought sensitivity of poco1. poco1 mutation also affects the expression of genes associated with the cellular regulation, redox, and mitochondrial perturbation.

OsSLC1 Encodes a Pentatricopeptide Repeat Protein Essential for Early Chloroplast Development and Seedling Survival

BACKGROUND

The large family of pentatricopeptide repeat (PPR) proteins is widely distributed among land plants. Such proteins play vital roles in intron splicing, RNA editing, RNA processing, RNA stability and RNA translation. However, only a small number of PPR genes have been identified in rice.

RESULTS

In this study, we raised a mutant from tissue-culture-derived plants of Oryza sativa subsp. japonica 'Zhonghua 11', which exhibited a lethal chlorosis phenotype from germination to the third-leaf stage. The mutant was designated seedling-lethal chlorosis 1 (slc1). The slc1 mutant leaves showed extremely low contents of photosynthetic pigments and abnormal chloroplast development, and were severely defective in photosynthesis. Map-based cloning of OsSLC1 revealed that a single base (G) deletion was detected in the first exon of Os06g0710800 in the slc1 mutant, which caused a premature stop codon. Knockout and complementation experiments further confirmed that OsSLC1 is responsible for the seedling-lethal chlorosis phenotype in the slc1 mutant. OsSLC1 was preferentially expressed in green leaves, and encoded a chloroplast-localized PPR protein harboring 12 PPR motifs. Loss-of-function of OsSLC1 affected the intron splicing of multiple group II introns, and especially precluded the intron splicing of rps16, and resulted in significant increase in the transcript levels of 3 chloroplast ribosomal RNAs and 16 chloroplast development-related and photosynthesis-related genes, and in significant reduction in the transcript levels of 1 chloroplast ribosomal RNAs and 2 chloroplast development-related and photosynthesis-related genes.

CONCLUSION

We characterized a novel chloroplast-localized PPR protein, OsSLC1, which plays a vital role in the intron splicing of multiple group II introns, especially the rps16 intron, and is essential for early chloroplast development and seedling survival in rice.

Identification of new pentatricopeptide repeat proteins, MREF1 and 2, involved in mitochondrial RNA editing, using computational target RNA prediction

C-to-U RNA editing has been widely observed in mitochondrial and plastid RNAs in plants. The editing sites are known to be recognized by pentatricopeptide repeat (PPR) proteins, which belong to one of the largest protein families in vascular plants. PPR proteins are sequence-specific RNA-binding proteins that participate in various steps of organelle RNA metabolism, such as cleavage, stabilization, splicing, translation, and editing. Elucidating the underlying mechanisms of sequence-specific RNA recognition by PPR proteins expanded our understanding of the role of PPR proteins in plant organellar RNA editing and enabled the computational prediction of target RNA-editing sites for PPR proteins of interest. Combining computational prediction and experimental verification, we identified three new PPR proteins involved in mitochondrial RNA editing: At1g56570, known as PGN for RNA editing of nad6_leader_-73 and cox2_742, At4g04370 for RNA editing of nad5_242, and At2g41080 for atp1_1292. Therefore, At4g04370 and At2g41080 were designated as mitochondrial RNA-editing factor 1 (MREF1) and MREF2, respectively. This study supports the use of computational prediction in establishing connections between PPR proteins and specific RNA-editing sites, which are important for maintaining various physiological processes, such as plant development, embryogenesis, and biotic- and abiotic-stress responses.

Lose-of-Function of a Rice Nucleolus-Localized Pentatricopeptide Repeat Protein Is Responsible for the floury endosperm14 Mutant Phenotypes

BACKGROUND

The endosperm of rice (Oryza sativa) has been usually used for the study of starch synthesis. Although several related factors have been revealed, other unknown members remain to be identified, given that starch synthesis is a complicated and sophisticated process.

RESULTS

Here, we identified and characterized a new rice seed mutant, floury endosperm14 (flo14), which showed chalked endosperm and seed-lethal phenotypes. Map-based cloning indicated FLO14 encodes a novel P-family PPR protein which contains ten PPR motifs. Afterwards the gene was named OsNPPR3. Subcellular localization showed OsNPPR3 was targeted to nucleolus. Quantitative RT-PCR analysis demonstrated that OsNPPR3 was universally expressed in various tissues, with pronounced levels during rice endosperm development. Molecular analysis further suggested that OsNPPR3 was involved in the regulation of expression levels and splicing of a few genes in mitochondria.

CONCLUSION

The study demonstrates that the nucleolus-localized PPR protein is responsible for the flo14 mutant phenotypes through affecting nuclear and mitochondrial gene expression and splicing.

Disruption of ppr3, ppr4, ppr6 or ppr10 induces flocculation and filamentous growth in Schizosaccharomyces pombe

Pentatricopeptide repeat (PPR) proteins are major players in mitochondrial and chloroplast RNA metabolism, which is essential for normal organellar function. The fission yeast Schizosaccharomyces pombe has 10 PPR proteins. We have previously reported that loss of ppr3, ppr4, ppr6 or ppr10 perturbs iron homeostasis leading to accumulation of reactive oxygen species and apoptotic cell death. In the present study, we show that loss of ppr3, ppr4, ppr6 or ppr10 can cause non-sexual flocculation and filamentous growth of cells. Furthermore, expression of a number of genes encoding cell-surface flocculins and cell wall-remodeling enzymes are induced in these ppr-deletion mutants. We also show that Δppr10 cells, and, to a lesser extent, Δppr4 and Δppr6 cells, exhibited increased tolerance to H2O2 toxicity compared with the wild-type strain. Finally, we found that overexpression of genes involved in iron uptake and/or iron homeostasis could cause the flocculation of wild-type cells. Our findings suggest that an elevated level of intracellular iron in the mutant caused by loss of ppr3, ppr4, ppr6 or ppr10 may result in flocculation and filamentous growth in S. pombe.

SMK6 mediates the C-to-U editing at multiple sites in maize mitochondria

The recently identified PPR-E+/NVWA/DYW2 RNA editing complex provides insights into the mechanism of RNA editing in higher plant organelles. However, whether the complex works together with the previously identified editing factors RIPs/MORFs is unclear. In this paper, we identified a maize Smk6 gene, which encodes a mitochondrion-targeted PPR-E+protein with E1 and E2 domains at the C terminus. Loss of Smk6 function affects the C-to-U editing at nad1-740, nad4L-110, nad7-739, and mttB-138,139 sites, impairs mitochondrial activity and blocks embryogenesis and endosperm development. Genetic and molecular analysis indicated that SMK6 is the maize ortholog of the Arabidopsis SLO2, which is a component of the PPR-E+/NVWA/DYW2 editing complex. However, yeast two-hybrid analyses did not detect any interaction between SMK6 and any of the mitochondrion-targeted RIPs/MORFs, suggesting that RIPs/MORFs may not be a component of PPR-E+/NVWA/DYW2 RNA editing complex. Further analyses are required to provide evidence that RIP/MORFs and SMK6 do not physically interact in vivo.

Maize Empty Pericarp602 Encodes a P-Type PPR Protein That Is Essential for Seed Development

Pentatricopeptide repeat (PPR) proteins play crucial roles in intron splicing, which is important for RNA maturation. Identification of novel PPR protein with the function of intron splicing would help to understand the RNA splicing mechanism. In this study, we identified the maize empty pericarp602 (emp602) mutants, the mature kernels of which showed empty pericarp phenotype. We cloned the Emp602 gene from emp602 mutants and revealed that Emp602 encodes a mitochondrial-localized P-type PPR protein. We further revealed that Emp602 is specific for the cis-splicing of mitochondrial Nad4 intron 1 and intron 3, and mutation of Emp602 led to the loss of mature Nad4 transcripts. The loss of function of Emp602 nearly damaged the assembly and accumulation of complex I and arrested mitochondria formation, which arrested the seed development. The failed assembly of complex I triggers significant upregulation of Aox expression in emp602 mutants. Transcriptome analysis showed that the expression of mitochondrial-related genes, e.g. the genes associated with mitochondrial inner membrane presequence translocase complex and electron carrier activity, were extensively upregulated in emp602 mutant. These results demonstrate that EMP602 functions in the splicing of Nad4 intron 1 and intron 3, and the loss of function of Emp602 arrested maize seed development by disrupting the mitochondria complex I assembly.

Comprehensive Prediction of Target RNA Editing Sites for PLS-Class PPR Proteins in Arabidopsis thaliana

Hundreds of RNA editing events, that is conversion of cytidines (Cs) to uridines (Us), have been observed in the mitochondrial and plastid transcriptome in vascular plants. Defects of C-to-U RNA editing affect a wide variety of physiological processes. These editing sites are recognized by pentatricopeptide repeat (PPR) superfamily proteins. PPR proteins are sequence-specific RNA binding proteins that participate in multiple aspects of organellar RNA metabolism. They are categorized into P and PLS subclasses, where PLS-class proteins are largely identified as RNA editing PPRs. Elucidating the principle involved in PPR-RNA recognition, the so-called PPR code, has enhanced our understanding of the recognition of RNA editing sites, thereby enabling prediction of target RNA editing sites for uncharacterized PLS-class proteins. Computational PPR-RNA prediction in RNA editing can be applied to the study of PPR-deficient plants that are genetically isolated from physiological abnormalities. However, the use of PPR-RNA prediction in RNA editing is still restricted due to ambiguous procedures and prediction reliability. Here, we refined the PPR code dataset, and the reliability of the computational prediction was quantitatively evaluated using known RNA editing PPRs. With this knowledge, a computational analysis was conducted in the 'PPR-to-editing site' and 'editing site-to-PPR' directions, against 199 PLS-class proteins and 499 organelle RNA editing sites in Arabidopsis thaliana. We propose 52 plausible PPR-RNA pairs for uncharacterized proteins and editing sites. The presented data will facilitate the study of organellar RNA editing involved in diverse physiological processes in A. thaliana.

PALE-GREEN LEAF12 Encodes a Novel Pentatricopeptide Repeat Protein Required for Chloroplast Development and 16S rRNA Processing in Rice

Pentatricopeptide repeat (PPR) proteins regulate organellar gene expression in plants, through their involvement in organellar RNA metabolism. In rice (Oryza sativa), 477 genes are predicted to encode PPR proteins; however, the majority of their functions remain unknown. In this study, we identified and characterized a rice mutant, pale-green leaf12 (pgl12); at the seedling stage, pgl12 mutants had yellow-green leaves, which gradually turned pale green as the plants grew. The pgl12 mutant had significantly reduced Chl contents and increased sensitivity to changes in temperature. A genetic analysis revealed that the pgl12 mutation is recessive and located within a single nuclear gene. Map-based cloning of PGL12, including a transgenic complementation test, confirmed the presence of a base substitution (C to T), generating a stop codon, within LOC_Os12g10184 in the pgl12 mutant. LOC_Os12g10184 encodes a novel PLS-type PPR protein containing 17 PPR motifs and targeted to the chloroplasts. A quantitative real-time PCR analysis showed that PGL12 was expressed in various tissues, especially the leaves. We also showed that the transcript levels of several nuclear- and plastid-encoded genes associated with chloroplast development and photosynthesis were significantly altered in pgl12 mutants. The mutant exhibited defects in the 16S rRNA processing and splicing of the plastid transcript ndhA. Our results indicate that PGL12 is a new PLS-type PPR protein required for proper chloroplast development and 16S rRNA processing in rice.

PPR Protein BFA2 Is Essential for the Accumulation of the atpH/F Transcript in Chloroplasts

As a fascinating and complicated nanomotor, chloroplast ATP synthase comprises nine subunits encoded by both the nuclear and plastid genomes. Because of its uneven subunit stoichiometry, biogenesis of ATP synthase and expression of plastid-encoded ATP synthase genes requires assistance by nucleus-encoded factors involved in transcriptional, post-transcriptional, and translational steps. In this study, we report a P-class pentatricopeptide repeat (PPR) protein BFA2 (Biogenesis Factor required for ATP synthase 2) that is essential for accumulation of the dicistronic atpH/F transcript in Arabidopsis chloroplasts. A loss-of-function mutation in BFA2 results in a specific reduction of more than 3/4 of chloroplast ATP synthase, which is likely due to the absence of dicistronic atpH/F transcript. BFA2 protein contains 22 putative PPR motifs and exclusively localizes in the chloroplast. Bioinformatics and Electrophoretic Mobility Shift Assays (EMSA) analysis showed that BFA2 binds to the consensus sequence of the atpF-atpA intergenic region in a sequence-specific manner. However, translation initiation of the atpA was not affected in the bfa2 mutant. Thus, we propose that the chloroplast PPR protein BFA2 mainly acts as barrier to prevent the atpH/F transcript degradation by exoribonucleases by binding to the consensus sequence of the atpF-atpA intergenic region.

Stabilization and translation of synthetic operon-derived mRNAs in chloroplasts by sequences representing PPR protein-binding sites

The chloroplast is a prime target for genetic engineering in plants, offering various advantages over nuclear transformation. For example, chloroplasts allow the expression of polycistronic transcripts and thus to engineer complex metabolic pathways. Each cistron within such a longer transcript needs its own expression elements. Within the 5'-UTR, such expression elements are needed for stabilizing mRNAs and for translation of the downstream reading frame. One of the few effective expression elements used so far in transplastomic approaches is the intercistronic expression element (IEE). The IEE is derived from the psbT-psbH intergenic region and includes a target sequence of the RNA binding protein HCF107. We here show that excessive expression of the IEE can lead to specific defects of endogenous chloroplast mRNA stabilization, likely via depletion of HCF107. Key players in chloroplast transcript stabilization and translation are pentatricopeptide repeat (PPR) proteins, which are structurally related to HCF107. PPR proteins that stabilize mRNAs leave behind short RNA footprints that are indicators of their activity. We identified such sRNAs in tobacco, and demonstrate that they are sufficient to stabilize and stimulate translation of mRNAs from synthetic dicistronic transgenes in chloroplasts. Thus, minimal sequence elements are generally adequate to support key steps in chloroplast gene expression, i.e. RNA stability and translation. Furthermore, our analysis expands the repertoire of available expression elements to facilitate the assembly and expression of multi-gene ensembles in the chloroplast.

Mitochondrial-nuclear co-evolution leads to hybrid incompatibility through pentatricopeptide repeat proteins

Mitochondrial-nuclear incompatibility has a major role in reproductive isolation between species. However, the underlying mechanism and driving force of mitochondrial-nuclear incompatibility remain elusive. Here, we report a pentatricopeptide repeat-containing (PPR) protein, Ccm1, and its interacting partner, 15S rRNA, to be involved in hybrid incompatibility between two yeast species, Saccharomyces cerevisiae and Saccharomyces bayanus S. bayanus-Ccm1 has reduced binding affinity for S. cerevisiae-15S rRNA, leading to respiratory defects in hybrid cells. This incompatibility can be rescued by single mutations on several individual PPR motifs, demonstrating the highly evolvable nature of PPR proteins. When we examined other PPR proteins in the closely related Saccharomyces sensu stricto yeasts, about two-thirds of them showed detectable incompatibility. Our results suggest that fast co-evolution between flexible PPR proteins and their mitochondrial RNA substrates may be a common driving force in the development of mitochondrial-nuclear hybrid incompatibility.

The Rice Pentatricopeptide Repeat Gene TCD10 is Needed for Chloroplast Development under Cold Stress

BACKGROUND

Chloroplast plays a vital role in plant development and growth. The pentatricopeptide repeat (PPR) gene family is one of the largest gene families in plants. In addition, cold stress affects a broad spectrum of cellular components, e.g. chloroplast, and metabolism in plants. However, the regulatory mechanism for rice PPR genes on chloroplast development still remains elusive under cold stress.

RESULT

In this paper, we characterized a new rice PPR gene mutant tcd10 (thermo-sensitive chlorophyll-deficient mutant 10) that exhibits the albino phenotype, malformed chloroplast and could not survive after the 5-leaf stage when grown at 20 °C, but does the normal phenotype at 32 °C. Map-based cloning, followed by RNA interference and CRISPR/Cas9 genome editing techniques, revealed that TCD10 encoding a novel PPR protein, mainly localized to the chloroplasts, with 27 PPR motifs, is responsible for the mutant phenotype. In addition, TCD10 is specific expression in tissues. The disruption of TCD10 resulted in an evidently reduced expression of chloroplast-associated genes under cold stress (20 °C), whereas they did recovered to normal levels at high temperature (32 °C). These results showed an important role of TCD10 for chloroplast development under cold stress.

CONCLUSIONS

The TCD10 encodes a novel rice PPR protein, mainly located in chloroplasts, which is important for chloroplast development, growth and the maintenance of photosynthetic electron transport and its disorder would lead to an aberrant chloroplast and abnormal expressions in these genes for chloroplast development and photosynthesis in rice under cold stress.

Cloning and Expression Analysis of Eight Upland Cotton Pentatricopeptide Repeat Family Genes

The pentatricopeptide repeat (PPR) gene family is one of the largest gene families in plants. Most PPR genes are localized in mitochondria and chloroplasts functioning in regulation of plant growth and development, fertility restoration for cytoplasmic male sterility (CMS), and stress defense. In this study, using in silico cloning and PCR amplification with degenerate primers based on Arabidopsis PPR genes, we cloned eight new full-length PPR genes encoding protein sequences ranging from 458 to 875 amino acids, with 8 to 16 repetitive PPR elements in upland cotton and all of them lack introns. Expression analysis revealed that eight PPR genes were differently expressed in roots, stems, leaves, and floral buds. As for GhI12, its expression in floral buds at days 3-5 was significantly higher in line 777R (restorer line) than in line 777A (CMS line). Further tests with real-time PCR showed that GhI12 expression peaked at day 3 in 777R, followed by a gradual decline, while its expression fluctuated in 777A, peaking at day 5 and day 13. In addition, Gh155c17 and GhI12 were upregulated under salt stress. This is the first report of upland cotton PPR genes involved in salt stress response.

PPR protein PDM1/SEL1 is involved in RNA editing and splicing of plastid genes in Arabidopsis thaliana

After transcription, most chloroplast precursor RNAs undergo further post-transcriptional processing including cleavage, editing, and splicing. Previous investigation has shown that the cleavage of the rpoA transcript and most editing sites, including accD-1, are defective in the knockout mutant of PDM1/SEL1, a PLS-type PPR protein, and that PDM1 is associated with the rpoA transcript. In this work, we found that the splicing of group II introns in trnK and ndhA is also affected in pdm1. Co-immunoprecipitation mass spectrometry experiments were performed to identify proteins that are associated with PDM1. We obtained 126 non-redundant proteins, of which MORF9 was reported to be involved in RNA editing in chloroplast. Yeast two-hybrid assays showed that PDM1 interacts directly with MORF9, MORF2, and MORF8. RNA immunoprecipitation showed that PDM1 associates with the transcripts of trnK and ndhA, as well as accD-1, suggesting that PDM1 is involved in RNA editing and splicing. Therefore, PDM1 is an important protein for post-transcriptional regulation in chloroplast.

MEF13 Requires MORF3 and MORF8 for RNA Editing at Eight Targets in Mitochondrial mRNAs in Arabidopsis thaliana

RNA editing sites in plant mitochondria and plastids are addressed by pentatricopeptide repeat (PPR) proteins with E or E and DYW domains, which recognize a specific nucleotide motif upstream of the edited nucleotide. In addition, some sites require MORF proteins for efficient RNA editing. Here, we assign the novel E domain-containing PPR protein, MEF13, as being required for editing at eight sites in Arabidopsis thaliana. A SNP in ecotype C24 altering the editing level at only one of the eight target sites was located by genomic mapping. An EMS mutant allele of the gene for MEF13 was identified in a SNaPshot screen of a mutated plant population. At all eight target sites of MEF13, editing levels are reduced in both morf3 and morf8 mutants, but at only one site in morf1 mutants, suggesting that specific MEF13-MORF interactions are required. Yeast two-hybrid analyses detect solid connections of MEF13 with MORF1 and weak contact with MORF3 proteins. Yeast three-hybrid (Y3H) analysis shows that the presence of MORF8 enhances the connection between MEF13 and MORF3, suggesting that a MORF3-MORF8 heteromer may form stably or transiently to establish interaction with MEF13.

RNA editing in plants: Machinery and flexibility of site recognition

In plants, RNA editing is a process that deaminates specific cytidines (C) to uridines (U). PLS subfamily members of PPR proteins function in site recognition of the target C. In silico analysis has predicted the code used for PPR motif-nucleotide interaction, and the crystal structure of a protein-RNA complex supports this model. Despite progress in understanding the RNA-binding mechanism of PPR proteins, some of the flexibility of RNA recognition observed in trans-factors of RNA editing has not been fully explained. It is probably necessary to consider another unknown mechanism, and this consideration is related to the question of how PPR proteins have managed the creation of RNA editing sites during evolution. This question may be related to the mystery of the biological function of RNA editing in plants. MORF/RIP family members are required for RNA editing at multiple editing sites and are components of the RNA editosome in plants. The DYW domain has been a strong candidate for the C deaminase activity required for C-to-U conversion in RNA editing. So far, the activity of this enzyme has not been detected in recombinant DYW proteins, and several puzzling experimental results need to be explained to support the model. It is still difficult to resolve the entire image of the editosome in RNA editing in plants. This article is part of a Special Issue entitled: Chloroplast Biogenesis.

The pentatricopeptide repeat protein MEF26 participates in RNA editing in mitochondrial cox3 and nad4 transcripts

In angiosperms most members of the large nuclear-encoded family of pentatricopeptide repeat (PPR) proteins are predicted to play relevant roles in the maturation of organellar RNAs. Here we report the novel Mitochondrial Editing Factor 26, a DYW-PPR protein involved in RNA editing at two sites. While at one site, cox3-311, editing is abolished in the absence of MEF26, the other site, nad4-166, is still partially edited. These sites share similar cis-elements and application of the recently proposed amino acid code for RNA recognition by PPR proteins ranks them at first and second positions of the most probable targets.

A novel chloroplast-localized pentatricopeptide repeat protein involved in splicing affects chloroplast development and abiotic stress response in rice

Pentatricopeptide repeat (PPR) proteins comprise a large family in higher plants and modulate organellar gene expression by participating in various aspects of organellar RNA metabolism. In rice, the family contains 477 members, and the majority of their functions remain unclear. In this study, we isolated and characterized a rice mutant, white stripe leaf (wsl), which displays chlorotic striations early in development. Map-based cloning revealed that WSL encodes a newly identified rice PPR protein which targets the chloroplasts. In wsl mutants, PEP-dependent plastid gene expression was significantly down-regulated, and plastid rRNAs and translation products accumulate to very low levels. Consistently with the observations, wsl shows a strong defect in the splicing of chloroplast transcript rpl2, resulting in aberrant transcript accumulation and its product reduction in the mutant. The wsl shows enhanced sensitivity to ABA, salinity, and sugar, and it accumulates more H2O2 than wild-type. These results suggest the reduced translation efficiency may affect the response of the mutant to abiotic stress.

The Arabidopsis thaliana RNA editing factor SLO2, which affects the mitochondrial electron transport chain, participates in multiple stress and hormone responses

Recently, we reported that the novel mitochondrial RNA editing factor SLO2 is essential for mitochondrial electron transport, and vital for plant growth through regulation of carbon and energy metabolism. Here, we show that mutation in SLO2 causes hypersensitivity to ABA and insensitivity to ethylene, suggesting a link with stress responses. Indeed, slo2 mutants are hypersensitive to salt and osmotic stress during the germination stage, while adult plants show increased drought and salt tolerance. Moreover, slo2 mutants are more susceptible to Botrytis cinerea infection. An increased expression of nuclear-encoded stress-responsive genes, as well as mitochondrial-encoded NAD genes of complex I and genes of the alternative respiratory pathway, was observed in slo2 mutants, further enhanced by ABA treatment. In addition, H2O2 accumulation and altered amino acid levels were recorded in slo2 mutants. We conclude that SLO2 is required for plant sensitivity to ABA, ethylene, biotic, and abiotic stress. Although two stress-related RNA editing factors were reported very recently, this study demonstrates a unique role of SLO2, and further supports a link between mitochondrial RNA editing events and stress response.

The longest mitochondrial RNA editing PPR protein MEF12 in Arabidopsis thaliana requires the full-length E domain

Mitochondrial RNA editing factor 12 (MEF12) was identified in a screen for editing defects of a chemically mutated plant population in Arabidopsis thaliana. The MEF12 editing protein is required for the C to U change of nucleotide nad5-374. The MEF12 polypeptide is characterized by an exceptionally long stretch of 25 pentatricopeptide repeats (PPR) and a C-terminal extension domain. Editing is lost in mutant plants with a stop codon in the extending element. A T-DNA insertion substituting the 10 C-terminal amino acids of the extension domain reduces RNA editing to about 20% at the target site in a mutant plant. These results support the importance of the full-length extension module for functional RNA editing in plant mitochondria.

Architecture of the PPR gene family in the moss Physcomitrella patens

Pentatricopeptide repeat (PPR) proteins are widespread in eukaryotes and in particular, include several hundred members in land plants. The majority of PPR proteins are localized in mitochondria and plastids, where they play a crucial role in various aspects of RNA metabolism at the post-transcriptional level in gene expression. However, many of their functions remain to be characterized. In contrast to vascular plants, the moss Physcomitrella patens has only 105 PPR genes. This number may represent a minimum set of PPR proteins required for post-transcriptional regulation in plant organelles. Here, we review the overall structure of the P. patens PPR gene family and the current status of the functional characterization of moss PPR proteins.

The SLO1 PPR protein is required for RNA editing at multiple sites with similar upstream sequences in Arabidopsis mitochondria

In Arabidopsis, RNA editing changes more than 500 cytidines to uridines in mitochondrial transcripts. The editing enzyme and co-factors involved in these processes are largely unknown. We have identified a nuclear gene SLOW GROWTH1 (SLO1) encoding an E motif-containing pentatricopeptide repeat protein that is required for RNA editing of nad4 and nad9 in Arabidopsis mitochondria. The SLO1 protein is localized to the mitochondrion, and its absence gives rise to small plants with slow growth and delayed development. A survey of approximately 500 mitochondrial RNA editing sites in Arabidopsis reveals that the editing of two sites, nad4-449 and nad9-328, is abolished in the slo1 mutants. Sequence comparison in the upstream (from -1 to -15 bp) of nad4-449 and nad9-328 editing sites shows that nine of the 15 nucleotides are identical. In addition to RNA editing, we used RNA gel blot analysis to compare the abundance and banding patterns of mitochondrial transcripts between the wild type and slo1 mutants. Of the 79 genes and open reading frames examined, steady-state levels of 56 mitochondrial transcripts are increased in the slo1 mutants. These results suggest that the SLO1 protein may indirectly regulate plant growth and development via affecting mitochondrial RNA editing and gene expression.

Post-transcriptional control of chloroplast gene expression

Chloroplasts contain their own genome, organized as operons, which are generally transcribed as polycistronic transcriptional units. These primary transcripts are processed into smaller RNAs, which are further modified to produce functional RNAs. The RNA processing mechanisms remain largely unknown and represent an important step in the control of chloroplast gene expression. Such mechanisms include RNA cleavage of pre-existing RNAs, RNA stabilization, intron splicing, and RNA editing. Recently, several nuclear-encoded proteins that participate in diverse plastid RNA processing events have been characterised. Many of them seem to belong to the pentatricopeptide repeat (PPR) protein family that is implicated in many crucial functions including organelle biogenesis and plant development. This review will provide an overview of current knowledge of the post-transcriptional processing in chloroplasts.