The Pentatricopeptide Repeat Proteins TANG2 and ORGANELLE TRANSCRIPT PROCESSING439 Are Involved in the Splicing of the Multipartite nad5 Transcript Encoding a Subunit of Mitochondrial Complex I

Mitochondrial Splicing Efficiency Is Lower in Holoparasites Than in Free-Living Plants

Mitochondria play a crucial role in eukaryotic organisms, housing their own genome with genes vital for oxidative phosphorylation. Coordination between nuclear and mitochondrial genomes is pivotal for organelle gene expression. Splicing, editing and processing of mitochondrial transcripts are regulated by nuclear-encoded factors. Splicing efficiency (SEf) of the many group II introns present in plant mitochondrial genes is critical for mitochondrial function since a splicing defect or splicing deficiency can severely impact plant growth and development. This study investigates SEf in free-living and holoparasitic plants, focusing on 25 group II introns from 15 angiosperm species. Our comparative analyses reveal distinctive splicing patterns with holoparasites exhibiting significantly lower SEf, potentially linked to their unique evolutionary trajectory. Given the preponderance of horizontal gene transfer (HGT) in parasitic plants, we investigated the effect of HGT on SEf, such as the presence of foreign introns or foreign nuclear-encoded splicing factors. Contrary to expectations, the SEf reductions do not correlate with HGT events, suggesting that other factors are at play, such as the loss of photosynthesis or the transition to a holoparasitic lifestyle. The findings of this study broaden our understanding of the molecular evolution in parasitic plants and shed light on the multifaceted factors influencing organelle gene expression.

Candida albicans PPR proteins are required for the expression of respiratory Complex I subunits

Pentatricopeptide repeat (PPR) proteins bind RNA and are present in mitochondria and chloroplasts of Eukaryota. In fungi, they are responsible for controlling mitochondrial genome expression, mainly on the posttranscriptional level. Candida albicans is a human opportunistic pathogen with a facultative anaerobic metabolism which, unlike the model yeast Saccharomyces cerevisiae, possesses mitochondrially encoded respiratory Complex I (CI) subunits and does not tolerate loss of mtDNA. We characterized the function of 4 PPR proteins of C. albicans that lack orthologs in S. cerevisiae and found that they are required for the expression of mitochondrially encoded CI subunits. We demonstrated that these proteins localize to mitochondria and are essential to maintain the respiratory capacity of cells. Deletion of genes encoding these PPR proteins results in changes in steady-state levels of mitochondrial RNAs and proteins. We demonstrated that C. albicans cells lacking CaPpr4, CaPpr11, and CaPpr13 proteins show no CI assembly, whereas the lack of CaPpr7p results in a decreased CI activity. CaPpr13p is required to maintain the bicistronic NAD4L-NAD5 mRNA, whereas the other 3 PPR proteins are likely involved in translation-related assembly of mitochondrially encoded CI subunits. In addition, we show that CaAep3p, which is an ortholog of ScAep3p, performs the evolutionary conserved function of controlling expression of the ATP8-ATP6 mRNA. We also show that C. albicans cells lacking PPR proteins express a higher level of the inducible alternative oxidase (AOX2) which likely rescues respiratory defects and compensates for oxidative stress.

Mitochondrial atp1 mRNA knockdown by a custom-designed pentatricopeptide repeat protein alters ATP synthase

Spontaneous mutations are rare in mitochondria and the lack of mitochondrial transformation methods has hindered genetic analyses. We show that a custom-designed RNA-binding pentatricopeptide repeat (PPR) protein binds and specifically induces cleavage of ATP synthase subunit1 (atp1) mRNA in mitochondria, significantly decreasing the abundance of the Atp1 protein and the assembled F1Fo ATP synthase in Arabidopsis (Arabidopsis thaliana). The transformed plants are characterized by delayed vegetative growth and reduced fertility. Five-fold depletion of Atp1 level was accompanied by a decrease in abundance of other ATP synthase subunits and lowered ATP synthesis rate of isolated mitochondria, but no change to mitochondrial electron transport chain complexes, adenylates, or energy charge in planta. Transcripts for amino acid transport and a variety of stress response processes were differentially expressed in lines containing the PPR protein, indicating changes to achieve cellular homeostasis when ATP synthase was highly depleted. Leaves of ATP synthase-depleted lines showed higher respiratory rates and elevated steady-state levels of numerous amino acids, most notably of the serine family. The results show the value of using custom-designed PPR proteins to influence the expression of specific mitochondrial transcripts to carry out reverse genetic studies on mitochondrial gene functions and the consequences of ATP synthase depletion on cellular functions in Arabidopsis.

Chloroplast C-to-U editing, regulated by a PPR protein BoYgl-2, is important for chlorophyll biosynthesis in cabbage

Leaf color is an important agronomic trait in cabbage (Brassica oleracea L. var. capitata), but the detailed mechanism underlying leaf color formation remains unclear. In this study, we characterized a Brassica oleracea yellow-green leaf 2 (BoYgl-2) mutant 4036Y, which has significantly reduced chlorophyll content and abnormal chloroplasts during early leaf development. Genetic analysis revealed that the yellow-green leaf trait is controlled by a single recessive gene. Map-based cloning revealed that BoYgl-2 encodes a novel nuclear-targeted P-type PPR protein, which is absent in the 4036Y mutant. Functional complementation showed that BoYgl-2 from the normal-green leaf 4036G can rescue the yellow-green leaf phenotype of 4036Y. The C-to-U editing efficiency and expression levels of atpF, rps14, petL and ndhD were significantly reduced in 4036Y than that in 4036G, and significantly increased in BoYgl-2 overexpression lines than that in 4036Y. The expression levels of many plastid- and nuclear-encoded genes associated with chloroplast development in BoYgl-2 mutant were also significantly altered. These results suggest that BoYgl-2 participates in chloroplast C-to-U editing and development, which provides rare insight into the molecular mechanism underlying leaf color formation in cabbage.

H2O2 negatively regulates aluminum resistance via oxidation and degradation of the transcription factor STOP1

Aluminum (Al) stress triggers the accumulation of hydrogen peroxide (H2O2) in roots. However, whether H2O2 plays a regulatory role in aluminum resistance remains unclear. In this study, we show that H2O2 plays a crucial role in regulation of Al resistance, which is modulated by the mitochondrion-localized pentatricopeptide repeat protein REGULATION OF ALMT1 EXPRESSION 6 (RAE6). Mutation in RAE6 impairs the activity of complex I of the mitochondrial electron transport chain, resulting in the accumulation of H2O2 and increased sensitivity to Al. Our results suggest that higher H2O2 concentrations promote the oxidation of SENSITIVE TO PROTON RHIZOTOXICITY 1 (STOP1), an essential transcription factor that promotes Al resistance, thereby promoting its degradation by enhancing the interaction between STOP1 and the F-box protein RAE1. Conversely, decreasing H2O2 levels or blocking the oxidation of STOP1 leads to greater STOP1 stability and increased Al resistance. Moreover, we show that the thioredoxin TRX1 interacts with STOP1 to catalyze its chemical reduction. Thus, our results highlight the importance of H2O2 in Al resistance and regulation of STOP1 stability in Arabidopsis (Arabidopsis thaliana).

Research Progress of Group II Intron Splicing Factors in Land Plant Mitochondria

Mitochondria are important organelles that provide energy for the life of cells. Group II introns are usually found in the mitochondrial genes of land plants. Correct splicing of group II introns is critical to mitochondrial gene expression, mitochondrial biological function, and plant growth and development. Ancestral group II introns are self-splicing ribozymes that can catalyze their own removal from pre-RNAs, while group II introns in land plant mitochondria went through degenerations in RNA structures, and thus they lost the ability to self-splice. Instead, splicing of these introns in the mitochondria of land plants is promoted by nuclear- and mitochondrial-encoded proteins. Many proteins involved in mitochondrial group II intron splicing have been characterized in land plants to date. Here, we present a summary of research progress on mitochondrial group II intron splicing in land plants, with a major focus on protein splicing factors and their probable functions on the splicing of mitochondrial group II introns.

Alteration of Mitochondrial Transcript Expression in Arabidopsis thaliana Using a Custom-Made Library of Pentatricopeptide Repeat Proteins

Pentatricopeptide repeat (PPR) proteins are considered a potential tool for manipulating organelle gene expression in plants because they can recognise a wide range of different RNA sequences, and the molecular basis for this sequence recognition is partially known and understood. A library of redesigned PPR proteins related to restorer-of-fertility proteins was created and transformed into plants in order to target mitochondrial transcripts. Ninety different variants tested in vivo showed a wide range of phenotypes. One of these lines, which displayed slow growth and downward curled leaves, showed a clear reduction in complex V. The phenotype was due to a specific cleavage of atp1 transcripts induced by a modified PPR protein from the library, validating the use of this library as a source of mitochondrial 'mutants'. This study is a step towards developing specific RNA targeting tools using PPR proteins that can be aimed at desired targets.

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.

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.

CFM6 is an Essential CRM Protein Required for the Splicing of nad5 Transcript in Arabidopsis Mitochondria

Plant chloroplast RNA splicing and ribosome maturation (CRM)-domain-containing proteins are capable of binding RNA to facilitate the splicing of group I or II introns in chloroplasts, but their functions in mitochondria are less clear. In the present study, Arabidopsis thaliana CFM6, a protein with a single CRM domain, was expressed in most plant tissues, particularly in flower tissues, and restricted to mitochondria. Mutation of CFM6 causes severe growth defects, including stunted growth, curled leaves, delayed embryogenesis and pollen development. CFM6 functions specifically in the splicing of group II intron 4 of nad5, which encodes a subunit of mitochondrial complex I, as evidenced by the loss of nad5 intron 4 splicing and high accumulation of its pretranscripts in cfm6 mutants. The phenotypic and splicing defects of cfm6 were rescued in transgenic plants overexpressing 35S::CFM6-YFP. Splicing failure in cfm6 also led to the loss of complex I activity and to its improper assembly. Moreover, dysfunction of complex I induced the expression of proteins or genes involved in alternative respiratory pathways in cfm6. Collectively, CFM6, a previously uncharacterized CRM domain-containing protein, is specifically involved in the cis-splicing of nad5 intron 4 and plays a pivotal role in mitochondrial complex I biogenesis and normal plant growth.

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.

OsNBL3, a mitochondrion-localized pentatricopeptide repeat protein, is involved in splicing nad5 intron 4 and its disruption causes lesion mimic phenotype with enhanced resistance to biotic and abiotic stresses

Lesion mimic mutants are used to elucidate mechanisms controlling plant responses to pathogen attacks and environmental stresses. Although dozens of genes had been functionally demonstrated to be involved in lesion mimic phenotype in several plant species, the molecular mechanisms underlying the hypersensitive response are largely unknown. Here, a rice (Oryza sativa) lesion mimic mutant natural blight leaf 3 (nbl3) was identified from T-DNA insertion lines. The causative gene, OsNBL3, encodes a mitochondrion-localized pentatricopeptide repeat (PPR) protein. The nbl3 mutant exhibited spontaneous cell death response and H2 O2 accumulation, and displayed enhanced resistance to the fungal and bacterial pathogens Magnaporthe oryzae and Xanthomonas oryzae pv. oryzae. This resistance was consistent with the up-regulation of several defence-related genes; thus, defence responses were induced in nbl3. RNA interference lines of OsNBL3 exhibited enhanced disease resistance similar to that of nbl3, while the disease resistance in overexpression lines did not differ from that of the wild type. In addition, nbl3 displayed improved tolerance to salt, accompanied by up-regulation of several salt-associated marker genes. OsNBL3 was found to mainly participate in the splicing of mitochondrial gene nad5 intron 4. Disruption of OsNBL3 leads to the reduction in complex I activity, the elevation of alternative respiratory pathways and the destruction of mitochondrial morphology. Overall, the results demonstrated that the PPR protein-coding gene OsNBL3 is essential for mitochondrial development and functions, and its disruption causes the lesion mimic phenotype and enhances disease resistance and tolerance to salt in rice.

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.

Cotton Fiber Development Requires the Pentatricopeptide Repeat Protein GhIm for Splicing of Mitochondrial nad7 mRNA

Pentatricopeptide repeat (PPR) proteins encoded by nuclear genomes can bind to organellar RNA and are involved in the regulation of RNA metabolism. However, the functions of many PPR proteins remain unknown in plants, especially in polyploidy crops. Here, through a map-based cloning strategy and Clustered regularly interspaced short palindromic repeats/cas9 (CRISPR/cas9) gene editing technology, we cloned and verified an allotetraploid cotton immature fiber (im) mutant gene (GhImA) encoding a PPR protein in chromosome A03, that is associated with the non-fluffy fiber phenotype. GhImA protein targeted mitochondrion and could bind to mitochondrial nad7 mRNA, which encodes the NAD7 subunit of Complex I. GhImA and its homolog GhImD had the same function and were dosage-dependent. GhImA in the im mutant was a null allele with a 22 bp deletion in the coding region. Null GhImA resulted in the insufficient GhIm dosage, affected mitochondrial nad7 pre-mRNA splicing, produced less mature nad7 transcripts, and eventually reduced Complex I activities, up-regulated alternative oxidase metabolism, caused reactive oxygen species (ROS) burst and activation of stress or hormone response processes. This study indicates that the GhIm protein participates in mitochondrial nad7 splicing, affects respiratory metabolism, and further regulates cotton fiber development via ATP supply and ROS balance.

A RanBP2-type zinc finger protein functions in intron splicing in Arabidopsis mitochondria and is involved in the biogenesis of respiratory complex I

The RanBP2 zinc finger (Znf) domain is a prevalent domain that mediates protein interaction and RNA binding. In Arabidopsis, a clade of four RanBP2 Znf-containing proteins, named the Organelle Zinc (OZ) finger family, are known or predicted to be targeted to either the mitochondria or the plastids. Previously we reported that OZ1 is absolutely required for the editing of 14 sites in chloroplasts. We now have investigated the function of OZ2, whose null mutation is embryo lethal. We rescued the null mutant by expressing wild-type OZ2 under the control of the seed-specific ABSCISIC ACID-INSENSITIVE3 (ABI3) promoter. Rescued mutant plants exhibit severely delayed development and a distinctive morphological phenotype. Genetic and biochemical analyses demonstrated that OZ2 promotes the splicing of transcripts of several mitochondrial nad genes and rps3. The splicing defect of nad transcripts results in the destabilization of complex I, which in turn affects the respiratory ability of oz2 mutants, turning on the alternative respiratory pathway, and impacting the plant development. Protein-protein interaction assays demonstrated binding of OZ2 to several known mitochondrial splicing factors targeting the same splicing events. These findings extend the known functional repertoire of the RanBP2 zinc finger domain in nuclear splicing to include plant organelle splicing.

Rice FLOURY SHRUNKEN ENDOSPERM 5 Encodes a Putative Plant Organelle RNA Recognition Protein that Is Required for cis-Splicing of Mitochondrial nad4 Intron 1

BACKGROUND

The sequences of several important mitochondrion-encoded genes involved in respiration in higher plants are interrupted by introns. Many nuclear-encoded factors are involved in splicing these introns, but the mechanisms underlying this splicing remain unknown.

RESULTS

We isolated and characterized a rice mutant named floury shrunken endosperm 5 (fse5). In addition to having floury shrunken endosperm, the fse5 seeds either failed to germinate or produced seedlings which grew slowly and died ultimately. Fse5 encodes a putative plant organelle RNA recognition (PORR) protein targeted to mitochondria. Mutation of Fse5 hindered the splicing of the first intron of nad4, which encodes an essential subunit of mitochondrial NADH dehydrogenase complex I. The assembly and NADH dehydrogenase activity of complex I were subsequently disrupted by this mutation, and the structure of the mitochondria was abnormal in the fse5 mutant. The FSE5 protein was shown to interact with mitochondrial intron splicing factor 68 (MISF68), which is also a splicing factor for nad4 intron 1 identified previously via yeast two-hybrid (Y2H) assays.

CONCLUSION

Fse5 which encodes a PORR domain-containing protein, is essential for the splicing of nad4 intron 1, and loss of Fse5 function affects seed development and seedling growth.

OsPPR939, a nad5 splicing factor, is essential for plant growth and pollen development in rice

P-subfamily PPR protein OsPPR939, which can be phosphorylated by OsS6K1, regulates plant growth and pollen development by involving in the splicing of mitochondrial nad5 introns 1, 2, and 3. In land plants, pentatricopeptide repeat (PPR) proteins play key roles in mitochondrial group II intron splicing, but how these nucleus-encoded proteins are imported into mitochondria is unknown. To date, a few PPR proteins have been characterized in rice (Oryza sativa). Here, we demonstrate that the mitochondrion-localized P-subfamily PPR protein OsPPR939 is required for the splicing of nad5 introns 1, 2, and 3 in rice. Complete knockout or partial disruption of OsPPR939 function resulted in different degrees of growth retardation and pollen sterility. The dramatically reduced splicing efficiency of these introns in osppr939-4 and osppr939-5 led to reduced mitochondrial complex I abundance and activity and enhanced expression of alternative respiratory pathway genes. Complementation with OsPPR939 rescued the defective plant morphology of osppr939-4 and restored its decreased splicing efficiency of nad5 introns 1, 2, and 3. Therefore, OsPPR939 plays crucial roles in plant growth and pollen development by splicing mitochondrial nad5 introns 1, 2, and 3. More importantly, the 12th amino acid Ser in the N-terminal targeting sequence of OsPPR939 is phosphorylated by OsS6K1, and truncated OsPPR939 with a non-phosphorylatable S12A mutation in its presequence could not be imported into mitochondria, suggesting that phosphorylation of this amino acid plays an important role in the mitochondrial import of OsPPR939. To our knowledge, the 12th residue Ser on OsPPR939 is the first experimentally proven phosphorylation site in PPR proteins. Our results provide a basis for investigating the regulatory mechanism of PPR proteins at the post-translational level.

The Pentatricopeptide Repeat Protein MEF100 Is Required for the Editing of Four Mitochondrial Editing Sites in Arabidopsis

In Arabidopsis thaliana there are more than 600 C-to-U RNA editing events in the mitochondria and at least 44 in the chloroplasts. Pentatricopeptide repeat (PPR) proteins provide the specificity for these reactions. They recognize RNA sequences in a partially predictable fashion via key amino acids at the fifth and last position in each PPR motif that bind to individual ribonucleotides. A combined approach of RNA-Seq, mutant complementation, electrophoresis of mitochondrial protein complexes and Western blotting allowed us to show that MEF100, a PPR protein identified in a genetic screen for mutants resistant to an inhibitor of γ -glutamylcysteine synthetase, is required for the editing of nad1-493, nad4-403, nad7-698 and ccmFN2-356 sites in Arabidopsis mitochondria. The absence of editing in mef100 leads to a decrease in mitochondrial Complex I activity, which probably explains the physiological phenotype. Some plants have lost the requirement for MEF100 at one or more of these sites through mutations in the mitochondrial genome. We show that loss of the requirement for MEF100 editing leads to divergence in the MEF100 binding site.

Rerouting of ribosomal proteins into splicing in plant organelles

Production and expression of RNA requires the action of multiple RNA-binding proteins (RBPs). New RBPs are most often created by novel combinations of dedicated RNA-binding modules. However, recruiting existing genes to create new RBPs is also an important evolutionary strategy. In this report, we analyzed the eight-member uL18 ribosomal protein family in Arabidopsis uL18 proteins share a short structurally conserved domain that binds the 5S ribosomal RNA (rRNA) and allows its incorporation into ribosomes. Our results indicate that Arabidopsis uL18-Like proteins are targeted to either mitochondria or chloroplasts. While two members of the family are found in organelle ribosomes, we show here that two uL18-type proteins function as factors necessary for the splicing of certain mitochondrial and plastid group II introns. These two proteins do not cosediment with mitochondrial or plastid ribosomes but instead associate with the introns whose splicing they promote. Our study thus reveals that the RNA-binding capacity of uL18 ribosomal proteins has been repurposed to create factors that facilitate the splicing of organellar introns.

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.

Two Novel PLS-Class Pentatricopeptide Repeat Proteins Are Involved in the Group II Intron Splicing of Mitochondrial Transcripts in the Moss Physcomitrella patens

Pentatricopeptide repeat (PPR) proteins are RNA-binding proteins that function in posttranscriptional regulation as gene-specific regulators of RNA metabolism in plant organelles. Plant PPR proteins are divided into four classes: P, PLS, E and DYW. The E- and DYW-class proteins are mainly implicated in RNA editing, whereas most of the P-class proteins predominantly participate in RNA cleavage, splicing and stabilization. In contrast, the functions of PLS-class proteins still remain obscure. Here, we report the function of PLS-class PpPPR_31 and PpPPR_9 in Physcomitrella patens. The knockout (KO) mutants of PpPPR_31 and PpPPR_9 exhibited slower protonema growth compared to the wild type. The PpPPR_31 KO mutants showed a considerable reduction in the splicing of nad5 intron 3 and atp9 intron 1. The PpPPR_9 KO mutants displayed severely reduced splicing of cox1 intron 3. An RNA electrophoresis mobility shift assay showed that the recombinant PpPPR_31 protein bound to the 5' region of nad5 exon 4 and the bulged A region in domain VI of atp9 group II intron 1 while the recombinant PpPPR_9 bound to the translated region of ORF622 in cox1 intron 3. These results suggest that a certain set of PLS-class PPR proteins may influence the splicing efficiency of mitochondrial group II introns.

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.

One C-to-U RNA Editing Site and Two Independently Evolved Editing Factors: Testing Reciprocal Complementation with DYW-Type PPR Proteins from the Moss Physcomitrium (Physcomitrella) patens and the Flowering Plants Macadamia integrifolia and Arabidopsis

Cytidine-to-uridine RNA editing is a posttranscriptional process in plant organelles, mediated by specific pentatricopeptide repeat (PPR) proteins. In angiosperms, hundreds of sites undergo RNA editing. By contrast, only 13 sites are edited in the moss Physcomitrium (Physcomitrella) patens Some are conserved between the two species, like the mitochondrial editing site nad5eU598RC. The PPR proteins assigned to this editing site are known in both species: the DYW-type PPR protein PPR79 in P. patens and the E+-type PPR protein CWM1 in Arabidopsis (Arabidopsis thaliana). CWM1 also edits sites ccmCeU463RC, ccmBeU428SL, and nad5eU609VV. Here, we reciprocally expressed the P. patens and Arabidopsis editing factors in the respective other genetic environment. Surprisingly, the P. patens editing factor edited all target sites when expressed in the Arabidopsis cwm1 mutant background, even when carboxy-terminally truncated. Conversely, neither Arabidopsis CWM1 nor CWM1-PPR79 chimeras restored editing in P. patens ppr79 knockout plants. A CWM1-like PPR protein from the early diverging angiosperm macadamia (Macadamia integrifolia) features a complete DYW domain and fully rescued editing of nad5eU598RC when expressed in P. patens. We conclude that (1) the independently evolved P. patens editing factor PPR79 faithfully operates in the more complex Arabidopsis editing system, (2) truncated PPR79 recruits catalytic DYW domains in trans when expressed in Arabidopsis, and (3) the macadamia CWM1-like protein retains the capacity to work in the less complex P. patens editing environment.

Roles of Organellar RNA-Binding Proteins in Plant Growth, Development, and Abiotic Stress Responses

Organellar gene expression (OGE) in chloroplasts and mitochondria is primarily modulated at post-transcriptional levels, including RNA processing, intron splicing, RNA stability, editing, and translational control. Nucleus-encoded Chloroplast or Mitochondrial RNA-Binding Proteins (nCMRBPs) are key regulatory factors that are crucial for the fine-tuned regulation of post-transcriptional RNA metabolism in organelles. Although the functional roles of nCMRBPs have been studied in plants, their cellular and physiological functions remain largely unknown. Nevertheless, existing studies that have characterized the functions of nCMRBP families, such as chloroplast ribosome maturation and splicing domain (CRM) proteins, pentatricopeptide repeat (PPR) proteins, DEAD-Box RNA helicase (DBRH) proteins, and S1-domain containing proteins (SDPs), have begun to shed light on the role of nCMRBPs in plant growth, development, and stress responses. Here, we review the latest research developments regarding the functional roles of organellar RBPs in RNA metabolism during growth, development, and abiotic stress responses in plants.

Mitochondrial Pentatricopeptide Repeat Protein, EMB2794, Plays a Pivotal Role in NADH Dehydrogenase Subunit nad2 mRNA Maturation in Arabidopsis thaliana

The Arabidopsis genome encodes >450 proteins containing the pentatricopeptide repeat (PPR) motif. The PPR proteins are classified into two groups, termed as P and P Long-Short (PLS) classes. Typically, the PLS subclass proteins are mainly involved in the RNA editing of mitochondrial and chloroplast transcripts, whereas most of the analyzed P subclass proteins have been mainly implicated in RNA metabolism, such as 5' or 3' transcript stabilization and processing, splicing and translation. Mutations of PPR genes often result in embryogenesis and altered seedling developmental defect phenotypes, but only a limited number of ppr mutants have been characterized in detail. In this report, we show that null mutations in the EMB2794 gene result in embryo arrest, due to altered splicing of nad2 transcripts in the Arabidopsis mitochondria. In angiosperms, nad2 has five exons that are transcribed individually from two mitochondrial DNA regions. Biochemical and in vivo analyses further indicate that recombinant or transgenic EMB2794 proteins bind to the nad2 pre-mRNAs in vitro as well as in vivo, suggesting a role for this protein in trans-splicing of nad2 intron 2 and possibly in the stability of the second pre-mRNA of nad2. Homozygous emb2794 lines, showing embryo-defective phenotypes, can be partially rescued by the addition of sucrose to the growth medium. Mitochondria of rescued homozygous mutant plants contain only traces of respiratory complex I, which lack the NADH-dehydrogenase activity.

Seed comparative genomics in three coffee species identify desiccation tolerance mechanisms in intermediate seeds

In contrast to desiccation-tolerant 'orthodox' seeds, so-called 'intermediate' seeds cannot survive complete drying and are short-lived. All species of the genus Coffea produce intermediate seeds, but they show a considerable variability in seed desiccation tolerance (DT), which may help to decipher the molecular basis of seed DT in plants. We performed a comparative transcriptome analysis of developing seeds in three coffee species with contrasting desiccation tolerance. Seeds of all species shared a major transcriptional switch during late maturation that governs a general slow-down of metabolism. However, numerous key stress-related genes, including those coding for the late embryogenesis abundant protein EM6 and the osmosensitive calcium channel ERD4, were up-regulated during DT acquisition in the two species with high seed DT, C. arabica and C. eugenioides. By contrast, we detected up-regulation of numerous genes involved in the metabolism, transport, and perception of auxin in C. canephora seeds with low DT. Moreover, species with high DT showed a stronger down-regulation of the mitochondrial machinery dedicated to the tricarboxylic acid cycle and oxidative phosphorylation. Accordingly, respiration measurements during seed dehydration demonstrated that intermediate seeds with the highest DT are better prepared to cease respiration and avoid oxidative stresses.

Pentatricopeptide repeat protein MID1 modulates nad2 intron 1 splicing and Arabidopsis development

As one of the best-studied RNA binding proteins in plant, pentatricopeptide repeats (PPRs) protein are mainly targeted to mitochondria and/or chloroplasts for RNA processing to regulate the biogenesis and function of the organelles, but its molecular mechanism and role in development remain to be further revealed. Here, we identified a mitochondria-localized P-type small PPR protein, MITOCHONDRION-MEDIATED GROWTH DEFECT 1 (MID1) that is crucial for Arabidopsis development. Mutation in MID1 causes retarded embryo development and stunted plant growth with defects in cell expansion and proliferation. Molecular experiments showed that MID1 is required for the splicing of the nad2 intron 1 in mitochondria. Consistently, mid1 plants display significant reduction in the abundance and activity of mitochondrial respiration complex I, accompanied by abnormal mitochondrial morphology and energy metabolism. Furthermore, MID1 is associated with other trans-factors involved in NICOTINAMIDE ADENINE DINUCLEOTIDE HYDROGEN (NADH) DEHYDROGENASE SUBUNIT 2 (nad2) intron 1 splicing, and interacts directly with itself and MITOCHONDRIAL STABILITY FACTOR 1 (MTSF1). This suggests that MID1 most likely functions as a dimer for nad2 intron 1 splicing. Together, we characterized a novel PPR protein MID1 for nad2 intron 1 splicing.

Two Pentatricopeptide Repeat Proteins Are Required for the Splicing of nad5 Introns in Maize

Mitochondrial genes in flowering plants contain predominantly group II introns that require precise splicing before translation into functional proteins. Splicing of these introns is facilitated by various nucleus-encoded splicing factors. Due to lethality of mutants, functions of many splicing factors have not been revealed. Here, we report the function of two P-type PPR proteins PPR101 and PPR231, and their role in maize seed development. PPR101 and PPR231 are targeted to mitochondria. Null mutation of PPR101 and PPR231 arrests embryo and endosperm development, generating empty pericarp and small kernel phenotype, respectively, in maize. Loss-of-function in PPR101 abolishes the splicing of nad5 intron 2, and reduces the splicing of nad5 intron 1. Loss-of-function in PPR231 reduces the splicing of nad5 introns 1, 2, 3 and nad2 intron 3. The absence of Nad5 protein eliminates assembly of complex I, and activates the expression of alternative oxidase AOX2. These results indicate that both PPR101 and PPR231 are required for mitochondrial nad5 introns 1 and 2 splicing, while PPR231 is also required for nad5 intron 3 and nad2 intron 3. Both genes are essential to complex I assembly, mitochondrial function, and maize seed development. This work reveals that the splicing of a single intron involves multiple PPRs.

PPR14 Interacts With PPR-SMR1 and CRM Protein Zm-mCSF1 to Facilitate Mitochondrial Intron Splicing in Maize

In plants, splicing of organellar group II introns involves numerous nucleus-encoded trans-factors. But, how these trans-factors function and interact is not well understood. Here we report the function of a pentatricopeptide repeat (PPR) protein PPR14 and its physical relationship with other splicing factors in mitochondria. Null mutations of PPR14 severely arrest the embryo and endosperm development, causing an empty pericarp phenotype. PPR14 is required for the splicing of NADH dehydrogenase 2 (nad2) intron 3 and nad7 introns 1 and 2 in mitochondria. The absence of nad2 and nad7 transcripts leads to disruption of the mitochondrial complex I assembly and abolishes its NADH dehydrogenase activity. This is accompanied with increased levels of other mitochondrial complexes and elevated expression of the alternative oxidase proteins. As the function of PPR14 overlaps with PPR-SMR1 and the CRM-domain containing protein Zm-mCSF1, we tested their interactions. Protein-protein interaction analysis indicated that PPR14 interacts with PPR-SMR1 and Zm-mCSF1, suggesting that these three proteins may form a complex. As PPR proteins and CRM-domain containing proteins have many members in mitochondria and chloroplasts, we propose that organellar group II intron splicing is probably mediated by a dynamic complex that includes different PPR and CRM proteins in plants.

Em pty Pericarp24 and Empty Pericarp25 Are Required for the Splicing of Mitochondrial Introns, Complex I Assembly, and Seed Development in Maize

RNA splicing is an essential post-transcriptional regulation in plant mitochondria and chloroplasts. As the mechanism of RNA splicing remains obscure, identification and functional elucidation of new splicing factors are necessary. Through a characterization of two maize mutants, we cloned Empty pericarp 24 (Emp24) and Empty pericarp 25 (Emp25). Both Emp24 and Emp25 encode mitochondrion-targeted P-type PPR proteins. EMP24 is required for the splicing of nad4 introns 1 and 3, which was reported (Ren Z. et al., 2019), and EMP25 functions in the splicing of nad5 introns 1, 2, and 3. Absence of either Nad4 or Nad5 proteins blocks the assembly of mitochondrial complex I, resulting in the formation of a sub-sized complex I of similar size in both mutants. Mass spectrometry identification revealed that the subcomplexes in both mutants lack an identical set of proteins of complex I. These results indicate that EMP24 and EMP25 function in the splicing of nad4 and nad5 introns, respectively, and are essential to maize kernel development. The identification of the subcomplexes provides genetic and molecular insights into the modular complex I assembly pathway in maize.

The coordinated action of PPR4 and EMB2654 on each intron half mediates trans-splicing of rps12 transcripts in plant chloroplasts

The pentatricopeptide repeat proteins PPR4 and EMB2654 have been shown to be required for the trans-splicing of plastid rps12 transcripts in Zea mays (maize) and Arabidopsis, respectively, but their roles in this process are not well understood. We investigated the functions of the Arabidopsis and Oryza sativa (rice) orthologs of PPR4, designated AtPPR4 (At5g04810) and OsPPR4 (Os4g58780). Arabidopsis atppr4 and rice osppr4 mutants are embryo-lethal and seedling-lethal 3 weeks after germination, respectively, showing that PPR4 is essential in the development of both dicot and monocot plants. Artificial microRNA-mediated mutants of AtPPR4 displayed a specific defect in rps12 trans-splicing, with pale-green, yellowish or albino phenotypes, according to the degree of knock-down of AtPPR4 expression. Comparison of RNA footprints in atppr4 and emb2654 mutants showed a similar concordant loss of extensive footprints at the 3' end of intron 1a and at the 5' end of intron 1b in both cases. EMB2654 is known to bind within the footprint region in intron 1a and we show that AtPPR4 binds to the footprint region in intron 1b, via its PPR motifs. Binding of both PPR4 and EMB2654 is essential to juxtapose the two intron halves and to maintain the RNAs in a splicing-competent structure for the efficient trans-splicing of rps12 intron 1, which is crucial for chloroplast biogenesis and plant development. The similarity of EMB2654 and PPR4 orthologs and their respective binding sites across land plant phylogeny indicates that their coordinate function in rps12 trans-splicing has probably been conserved for 500 million years.

Genome-Wide Analysis of the DYW Subgroup PPR Gene Family and Identification of GmPPR4 Responses to Drought Stress

Pentatricopeptide-repeat (PPR) proteins were identified as a type of nucleus coding protein that is composed of multiple tandem repeats. It has been reported that PPR genes play an important role in RNA editing, plant growth and development, and abiotic stresses in plants. However, the functions of PPR proteins remain largely unknown in soybean. In this study, 179 DYW subgroup PPR genes were identified in soybean genome (Glycine max Wm82.a2.v1). Chromosomal location analysis indicated that DYW subgroup PPR genes were mapped to all 20 chromosomes. Phylogenetic relationship analysis revealed that DYW subgroup PPR genes were categorized into three distinct Clusters (I to III). Gene structure analysis showed that most PPR genes were featured by a lack of intron. Gene duplication analysis demonstrated 30 PPR genes (15 pairs; ~35.7%) were segmentally duplicated among Cluster I PPR genes. Furthermore, we validated the mRNA expression of three genes that were highly up-regulated in soybean drought- and salt-induced transcriptome database and found that the expression levels of GmPPR4 were induced under salt and drought stresses. Under drought stress condition, GmPPR4-overexpressing (GmPPR4-OE) plants showed delayed leaf rolling; higher content of proline (Pro); and lower contents of H2O2, O2- and malondialdehyde (MDA) compared with the empty vector (EV)-control plants. GmPPR4-OE plants exhibited increased transcripts of several drought-inducible genes compared with EV-control plants. Our results provided a comprehensive analysis of the DYW subgroup PPR genes and an insight for improving the drought tolerance in soybean.

ZmSMK9, a pentatricopeptide repeat protein, is involved in the cis-splicing of nad5, kernel development and plant architecture in maize

Maize kernel size and weight are essential contributors to its yield. So the identification of the genes controlling kernel size and weight can give us a chance to gain the yield. Here, we identified a small kernel mutant, Zea mays small kernel 9 (Zmsmk9), in maize. Cytological observation showed that the development of the endosperm and embryo was delayed in Zmsmk9 mutants at the early stages, resulting in a small kernel phenotype. Interestingly, despite substantial variation in kernel size, the germination of Zmsmk9 seeds was comparable to that of WT, and could develop into normal plants with upright leaf architecture. We cloned Zmsmk9 via map-based cloning. ZmSMK9 encodes a P-type pentatricopeptide repeat protein that targets to mitochondria, and is involved in RNA splicing in mitochondrial NADH dehydrogenase5 (nad5) intron-1 and intron-4. Consistent with the delayed development phenotype, transcriptome analysis of 12-DAP endosperm showed that starch and zeins biosynthesis related genes were dramatically down regulated in Zmsmk9, while cell cycle and cell growth related genes were dramatically increased. As a result, ZmSMK9 is a novel gene required for the splicing of nad5 intron-1 and intron-4, kernel development, and plant architecture in maize.

Maize pentatricopeptide repeat protein DEK41 affects cis-splicing of mitochondrial nad4 intron 3 and is required for normal seed development

The splicing of organelle-encoded mRNA in plants requires proteins encoded in the nucleus. The mechanism of splicing and the factors involved are not well understood. Pentatricopeptide repeat (PPR) proteins are known to participate in such RNA-protein interactions. Maize defective kernel 41 (dek41) is a seedling-lethal mutant that causes developmental defects. In this study, the Dek41 gene was cloned by Mutator tag isolation and allelic confirmation, and was found to encode a P-type PPR protein that targets mitochondria. Analysis of the mitochondrial RNA transcript profile revealed that dek41 mutations cause reduced splicing efficiency of mitochondrial nad4 intron 3. Immature dek41 kernels exhibited severe reductions in complex I assembly and NADH dehydrogenase activity. Up-regulated expression of alternative oxidase genes and deformed inner cristae of mitochondria in dek41, as revealed by TEM, indicated that proper splicing of nad4 is essential for correct mitochondrial functioning and morphology. Consistent with this finding, differentially expressed genes in the dek41 endosperm included those related to mitochondrial function and activity. Our results indicate that DEK41 is a PPR protein that affects cis-splicing of mitochondrial nad4 intron 3 and is required for correct mitochondrial functioning and maize kernel development.

Comparative Transcriptome Analysis in Eggplant Reveals Selection Trends during Eggplant Domestication

Eggplant (Solanum melongena L.) is an economically and nutritionally important fruit crop of the Solanaceae family, which was domesticated in India and southern China. However, the genome regions subjected to selective sweeps in eggplant remain unknown. In the present study, we performed comparative transcriptome analysis of cultivated and wild eggplant species with emphasis on the selection pattern during domestication. In total, 44,073 (S. sisymbriifolium) to 58,677 (S. melongena cultivar S58) unigenes were generated for the six eggplant accessions with total lengths of 36.6-46 Mb. The orthologous genes were assessed using the ratio of nonsynonymous (K a) to synonymous (K s) nucleotide substitutions to characterize selective patterns during eggplant domestication. We identified 19 genes under positive selection across the phylogeny that were classified into four groups. The gene (OG12205) under positive selection was possibly associated with fruit-related traits in eggplant, which may have resulted from human manipulation. Eight positive selected genes were potentially involved in stress tolerance or disease resistance, suggesting that environmental changes and biotic stresses were important selective pressures in eggplant domestication. Taken together, our results shed light on the effects of artificial and natural selection on the transcriptomes of eggplant and its wild relatives. Identification of the selected genes will facilitate the understanding of genetic architecture of domesticated-related traits and provide resources for resistant breeding in eggplant.

Composition of Mitochondrial Complex I during the Critical Node of Seed Aging in Oryza sativa

Previous studies have documented mitochondrial dysfunction during the critical node (CN) of rice (Oryza sativa) seed aging, including a decrease in the capacity of NADH dependent O₂ consumption. This raises the hypothesis that changes in the activity of NADH:ubiquinone oxidoreductase (complex I) may play a role in seed aging. The composition and activity of complex I was investigated at the CN of aged rice seeds. Using BN-PAGE and SWATH-MS 52 complex I subunits were identified, nineteen for the first time to be experimentally detected in rice. The subunits of the matrix arm (N and Q modules) were reduced in abundance at the CN, in accordance with a reduction in the capacity to oxidise NADH, reducing substrate oxidation and increase ROS accumulation. In contrast, subunits in the P module increased in abundance that contains many mitochondrial encoded subunits. It is proposed that the changes in complex I abundance subunits may indicate a premature re-activation of mitochondrial biogenesis, as evidenced by the increase in mitochondrial encoded subunits. This premature activation of mitochondrial biogenesis may under-pin the decreased viability of aged seeds, as mitochondrial biogenesis is a crucial event in germination to drive growth before autotrophic growth of the seedling is established.

The mitochondrial pentatricopeptide repeat protein EMP12 is involved in the splicing of three nad2 introns and seed development in maize

Plant mitochondrial genes contain cis- and trans-group II introns that must be spliced before translation. The mechanism by which these introns are spliced is not well understood. Several families of proteins have been implicated in the intron splicing, of which the pentatricopeptide repeat (PPR) proteins are proposed to confer the substrate binding specificity. However, very few PPRs are characterized. Here, we report the function of a P-type PPR protein, EMP12, and its role in seed development. EMP12 is targeted to mitochondria. Loss-of-function mutation in Emp12 severely arrests embryo and endosperm development, causing embryo lethality. The trans-splicing of mitochondrial nad2 intron 2 and cis-splicing of nad2 intron 4 are abolished, whereas the cis-splicing of nad2 intron 1 is reduced in emp12 mutants. As a result, complex I assembly is disrupted, and its activity is strongly reduced in the mutants. The expression of the alternative oxidase and several components of other mitochondrial complexes is increased, possibly in response to the defective complex I. These results suggest that Emp12 is required for the trans-splicing of nad2 intron 2 and cis-splicing of nad2 introns 1 and 4, and is important to complex I biogenesis, and embryogenesis and endosperm development in maize.

Transformation of nad7 into the nuclear genome rescues the slow growth3 mutant in Arabidopsis

Plant pentatricopeptide repeat (PPR) proteins are mostly involved in chloroplast or mitochondrial RNA metabolism. However, direct evidence that correction of the molecular defects in the organelles can restore the plant phenotypes has yet to be demonstrated in a ppr mutant. Arabidopsis slow growth3 (slo3), a ppr mutant, is impaired in the splicing of mitochondrial nad7 intron 2. Here, we have used slo3 as an example to demonstrate that transformation of correctly spliced nad7 into the nuclear genome and targeting the Nad7 subunit into mitochondria can restore complex I activity and plant phenotypes in the mutant. These results provide direct evidence that the strong growth and developmental phenotypes of the slo3 mutant are caused by defects in mitochondrial nad7.

Three new pentatricopeptide repeat proteins facilitate the splicing of mitochondrial transcripts and complex I biogenesis in Arabidopsis

Group II introns are common features of most angiosperm mitochondrial genomes. Intron splicing is thus essential for the expression of mitochondrial genes and is facilitated by numerous nuclear-encoded proteins. However, the molecular mechanism and the protein cofactors involved in this complex process have not been fully elucidated. In this study, we characterized three new pentatricopeptide repeat (PPR) genes, called MISF26, MISF68, and MISF74, of Arabidopsis and showed they all function in group II intron splicing and plant development. The three PPR genes encode P-type PPR proteins that localize in the mitochondrion. Transcript analysis revealed that the splicing of a single intron is altered in misf26 mutants, while several mitochondrial intron splicing defects were detected in misf68 and misf74 mutants. To our knowledge, MISF68 and MISF74 are the first two PPR proteins implicated in the splicing of more than one intron in plant mitochondria, suggesting that they may facilitate splicing differently from other previously identified PPR splicing factors. The splicing defects in the misf mutants induce a significant decrease in complex I assembly and activity, and an overexpression of mRNAs of the alternative respiratory pathway. These results therefore reveal that nuclear encoded proteins MISF26, MISF68, and MISF74 are involved in splicing of a cohort of mitochondrial group II introns and thereby required for complex I biogenesis.

Targeted cleavage of nad6 mRNA induced by a modified pentatricopeptide repeat protein in plant mitochondria

Mitochondrial genes encode key components of the cellular energy machinery, but their genetic analysis is difficult or impossible in most organisms (including plants) because of the lack of viable transformation approaches. We report here a method to block the expression of the mitochondrial nad6 gene encoding a subunit of respiratory complex I in Arabidopsis thaliana, via the modification of the specificity of the RNA-binding protein RNA PROCESSING FACTOR 2 (RPF2). We show that the modified RPF2 binds and specifically induces cleavage of nad6 RNA, almost eliminating expression of the Nad6 protein and consequently complex I accumulation and activity. To our knowledge, this is the first example of a targeted block in expression of a specific mitochondrial transcript by a custom-designed RNA-binding protein. This opens the path to reverse genetics studies on mitochondrial gene functions and leads to potential applications in agriculture.

Catherine Colas des Francs-Small et al. used an engineered pentatricopeptide repeat protein to induce cleavage of nad6 mRNA in the mitochondria of Arabidopsis thaliana, eliminating its expression. The approach has potential for use in functional characterization of mitochondrial genes and future agricultural applications.

The pentatricopeptide repeat protein EMPTY PERICARP8 is required for the splicing of three mitochondrial introns and seed development in maize

Splicing of plant organellar group II introns is under accurate nuclear control that employs many nucleus-encoded protein cofactors from various families. For mitochondrial introns, only a few splicing factors have been characterized because disruption of their functions often causes embryo lethality. Here, we report the function of Empty Pericarp8 (Emp8) in the splicing of three group II introns in mitochondria, complex I biogenesis, and seed development in maize. Emp8 encodes a P subfamily pentatricopeptide repeat protein that localizes in mitochondria. The loss-of-function mutants of Emp8 are embryo lethal, showing severely arrested embryo and endosperm development in maize. The respiration rate in the emp8 mutants is reduced with substantially enhanced expression of alternative oxidases. Transcript analysis indicated that the trans-splicing of nad1 intron 4 and cis-splicing of nad4 intron 1 are abolished, and the cis-splicing of nad2 intron 1 is severely impaired in the emp8 mutants. These defects consequently lead to the disassembly of mitochondrial complex I and a dramatic reduction in its activity. Together, these results suggest that Emp8 is required for the trans-splicing of nad1 intron 4 and cis-splicing of nad4 intron 1 and nad2 intron 1, which is essential to mitochondrial complex I assembly and hence to embryogenesis and endosperm development in maize.

Maize Dek37 Encodes a P-type PPR Protein That Affects cis-Splicing of Mitochondrial nad2 Intron 1 and Seed Development

Mitochondrial group II introns require the participation of numerous nucleus-encoded general and specific factors to achieve efficient splicing in vivo Pentatricopeptide repeat (PPR) proteins have been implicated in assisting group II intron splicing. Here, we identified and characterized a new maize seed mutant, defective kernel 37 (dek37), which has significantly delayed endosperm and embryo development. Dek37 encodes a classic P-type PPR protein that targets mitochondria. The dek37 mutation causes no detectable DEK37 protein in mutant seeds. Mitochondrial transcripts analysis indicated that dek37 mutation decreases splicing efficiency of mitochondrial nad2 intron 1, leading to reduced assembly and NADH dehydrogenase activity of complex I. Transmission Electron Microscopy (TEM) revealed severe morphological defects of mitochondria in dek37 Transcriptome analysis of dek37 endosperm indicated enhanced expression in the alternative respiratory pathway and extensive differentially expressed genes related to mitochondrial function. These results indicated that Dek37 is involved in cis-splicing of mitochondrial nad2 intron 1 and is required for complex I assembly, mitochondrial function, and seed development in maize.

Peroxisome Mitochondria Inter-relations in Plants

A large amount of ultrastructural, biochemical and molecular analysis indicates that peroxisomes and mitochondria not only share the same subcellular space but also maintain considerable overlap in their proteins, responses and functions. Recent approaches using imaging of fluorescent proteins targeted to both organelles in living plant cells are beginning to show the dynamic nature of their interactivity. Based on the observations of living cells, mitochondria respond rapidly to stress by undergoing fission. Mitochondrial fission is suggested to release key membrane-interacting members of the FISSION1 and DYNAMIN RELATED PROTEIN3 families and appears to be followed by the formation of thin peroxisomal extensions called peroxules. In a model we present the peroxules as an intermediate state prior to the formation of tubular peroxisomes, which, in turn are acted upon by the constriction-related proteins released by mitochondria and undergo rapid constriction and fission to increase the number of peroxisomes in a cell. The fluorescent protein aided imaging of peroxisome-mitochondria interaction provides visual evidence for their cooperation in maintenance of cellular homeostasis in plants.

Analysis of the Roles of the Arabidopsis nMAT2 and PMH2 Proteins Provided with New Insights into the Regulation of Group II Intron Splicing in Land-Plant Mitochondria

Plant mitochondria are remarkable with respect to the presence of numerous group II introns which reside in many essential genes. The removal of the organellar introns from the coding genes they interrupt is essential for respiratory functions, and is facilitated by different enzymes that belong to a diverse set of protein families. These include maturases and RNA helicases related proteins that function in group II intron splicing in different organisms. Previous studies indicate a role for the nMAT2 maturase and the RNA helicase PMH2 in the maturation of different pre-RNAs in Arabidopsis mitochondria. However, the specific roles of these proteins in the splicing activity still need to be resolved. Using transcriptome analyses of Arabidopsis mitochondria, we show that nMAT2 and PMH2 function in the splicing of similar subsets of group II introns. Fractionation of native organellar extracts and pulldown experiments indicate that nMAT2 and PMH2 are associated together with their intron-RNA targets in large ribonucleoprotein particle in vivo. Moreover, the splicing efficiencies of the joint intron targets of nMAT2 and PMH2 are more strongly affected in a double nmat2/pmh2 mutant-line. These results are significant as they may imply that these proteins serve as components of a proto-spliceosomal complex in plant mitochondria.

The Mitochondrion-Targeted PENTATRICOPEPTIDE REPEAT78 Protein Is Required for nad5 Mature mRNA Stability and Seed Development in Maize

Pentatricopepetide repeat (PPR) proteins are a large family of RNA-binding proteins involved in RNA metabolism in plant organelles. Although many PPR proteins have been functionally studied, few of them are identified with a function in mitochondrial RNA stability. By using a reverse genetic approach, we characterized the role of the mitochondrion-targeted PPR78 protein in nad5 mature mRNA stability and maize (Zea mays) seed development. Loss of PPR78 function leads to a dramatic reduction in the steady-state level of mitochondrial nad5 mature mRNA, blocks the assembly of complex I in the electron transport chain, and causes an arrest in embryogenesis and endosperm development. Characterization of a second strong allele confirms the function of PPR78 in nad5 mRNA accumulation and maize seed development. The generation of mature nad5 requires the assembly of three distinct precursor RNAs via trans-splicing reactions, and the accumulation of nad5T1 precursor is reduced in the ppr78 mutants. However, it is the instability of mature nad5 rather than nad5T1 causing loss of the full-length nad5 transcript, and degradation of nad5 losing both translation start and stop codons is enriched in the mutant. Our data imply the assembly of mature nad5 mRNA precedes the protection of PPR78.

The pentatricopeptide repeat protein MTSF2 stabilizes a nad1 precursor transcript and defines the 3΄ end of its 5΄-half intron

RNA expression in plant mitochondria implies a large number of post-transcriptional events in which transcript processing and stabilization are essential. In this study, we analyzed the function of the Arabidopsis mitochondrial stability factor 2 gene (MTSF2) and show that the encoded pentatricopeptide repeat protein is essential for the accumulation of stable nad1 mRNA. The production of mature nad1 requires the assembly of three independent RNA precursors via two trans-splicing reactions. Genetic analyses revealed that the lack of nad1 in mtsf2 mutants results from the specific destabilization of the nad1 exons 2-3 precursor transcript. We further demonstrated that MTSF2 binds to its 3΄ extremity with high affinity, suggesting a protective action by blocking exoribonuclease progression. By defining the 3΄ end of nad1 exons 2-3 precursor, MTSF2 concomitantly determines the 3΄ extremity of the first half of the trans-intron found at the end of the transcript. Therefore, binding of the MTSF2 protein to nad1 exons 2-3 precursor evolved both to stabilize the transcript and to define a 3΄ extremity compatible with the trans-splicing reaction needed to reconstitute mature nad1. We thus reveal that the range of transcripts stabilized by association with protective protein on their 3΄ end concerns also mitochondrial precursor transcripts.

The mitochondrial pentatricopeptide repeat protein PPR19 is involved in the stabilization of NADH dehydrogenase 1 transcripts and is crucial for mitochondrial function and Arabidopsis thaliana development

Despite the importance of pentatricopeptide repeat (PPR) proteins in organellar RNA metabolism and plant development, the functions of many PPR proteins remain unknown. Here, we determined the role of a mitochondrial PPR protein (At1g52620) comprising 19 PPR motifs, thus named PPR19, in Arabidopsis thaliana. The ppr19 mutant displayed abnormal seed development, reduced seed yield, delayed seed germination, and retarded growth, indicating that PPR19 is indispensable for normal growth and development of Arabidopsis thaliana. Splicing pattern analysis of mitochondrial genes revealed that PPR19 specifically binds to the specific sequence in the 3'-terminus of the NADH dehydrogenase 1 (nad1) transcript and stabilizes transcripts containing the second and third exons of nad1. Loss of these transcripts in ppr19 leads to multiple secondary effects on accumulation and splicing of other nad1 transcripts, from which we can infer the order in which cis- and trans-spliced nad1 transcripts are normally processed. Improper splicing of nad1 transcripts leads to the absence of mitochondrial complex I and alteration of the nuclear transcriptome, notably influencing the alternative splicing of a variety of nuclear genes. Our results indicate that the mitochondrial PPR19 is an essential component in the splicing of nad1 transcripts, which is crucial for mitochondrial function and plant development.

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

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