RNA immunoprecipitation and microarray analysis show a chloroplast Pentatricopeptide repeat protein to be associated with the 5' region of mRNAs whose translation it activates

MatK impacts differential chloroplast translation by limiting spliced tRNA-K(UUU) abundance

The protein levels of chloroplast photosynthetic genes and genes related to the chloroplast genetic apparatus vary to adapt to different conditions. However, the underlying mechanisms governing these variations remain unclear. The chloroplast intron Maturase K is encoded within the trnK intron and has been suggested to be required for splicing several group IIA introns, including the trnK intron. In this study, we used RNA immunoprecipitation followed by high-throughput sequencing (RIP-Seq) to identify MatK's preference for binding to group IIA intron domains I and VI within target transcripts. Importantly, these domains are crucial for splice site selection, and we discovered alternative 5'-splice sites in three MatK target introns. The resulting alternative trnK lariat structure showed increased accumulation during heat acclimation. The cognate codon of tRNA-K(UUU) is highly enriched in mRNAs encoding ribosomal proteins and a trnK-matK over-expressor exhibited elevated levels of the spliced tRNA-K(UUU). Ribosome profiling analysis of the overexpressor revealed a significant up-shift in the translation of ribosomal proteins compared to photosynthetic genes. Our findings suggest the existence of a novel regulatory mechanism linked to the abundance of tRNA-K(UUU), enabling the differential expression of functional chloroplast gene groups.

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

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

Chloroplast gene expression: Recent advances and perspectives

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

A review on extraction of polysaccharides from crustacean wastes and their environmental applications

Disposal of biodegradable waste of seashells leads to an environmental imbalance. A tremendous amount of wastes produced from flourishing shell fish industries while preparing crustaceans for human consumption can be directed towards proper utilization. The review of the present study focuses on these polysaccharides from crustaceans and a few important industrial applications. This review aimed to emphasize the current research on structural analyses and extraction of polysaccharides. The article summarises the properties of chitin, chitosan, and chitooligosaccharides and their derivatives that make them non-toxic, biodegradable, and biocompatible. Different extraction methods of chitin, chitosan, and chitooligosaccharides have been discussed in detail. Additionally, this information outlines possible uses for derivatives of chitin, chitosan, and chitooligosaccharides in the environmental, pharmaceutical, agricultural, and food industries. Additionally, it is essential to the textile, cosmetic, and enzyme-immobilization industries. This review focuses on new, insightful suggestions for raising the value of crustacean shell waste by repurposing a highly valuable material.

Tetrapyrrole biosynthesis pathway regulates plastid-to-nucleus signaling by controlling plastid gene expression in plants

Plastid-to-nucleus retrograde signaling coordinates nuclear gene expression with chloroplast developmental status and is essential for the photoautotrophic lifestyle of plants. Previous studies have established that tetrapyrrole biosynthesis (TPB) and plastid gene expression (PGE) play essential roles in plastid retrograde signaling during early chloroplast biogenesis; however, their functional relationship remains unknown. In this study, we generated a series of rice TPB-related gun (genome uncoupled) mutants and systematically analyzed their effects on nuclear and plastid gene expression under normal conditions or when subjected to treatments with norflurazon (NF; a noncompetitive inhibitor of carotenoid biosynthesis) and/or lincomycin (Lin; a specific inhibitor of plastid translation). We show that under NF treatment, expression of plastid-encoded polymerase (PEP)-transcribed genes is significantly reduced in the wild type but is derepressed in the TPB-related gun mutants. We further demonstrate that the derepressed expression of PEP-transcribed genes may be caused by increased expression of the PEP core subunit and nuclear-encoded sigma factors and by elevated copy numbers of plastid genome per haploid genome. In addition, we show that expression of photosynthesis-associated nuclear genes (PhANGs) and PEP-transcribed genes is correlated in the rice TPB-related gun mutants, with or without NF or Lin treatment. A similar correlation between PhANGs and PGE is also observed in the Arabidopsis gun4 and gun5 mutants. Moreover, we show that increased expression of PEP-transcribed plastid genes is necessary for the gun phenotype in NF-treated TPB-related gun mutants. Further, we provide evidence that these TPB-related GUN genes act upstream of GUN1 in the regulation of retrograde signaling. Taken together, our results suggest that the TPB-related GUN genes control retrograde plastid signaling by regulating the PGE-dependent retrograde signaling pathway.

An Overview of Pentatricopeptide Repeat (PPR) Proteins in the Moss Physcomitrium patens and Their Role in Organellar Gene Expression

Pentatricopeptide repeat (PPR) proteins are one type of helical repeat protein that are widespread in eukaryotes. In particular, there are several hundred PPR members in flowering plants. The majority of PPR proteins are localized in the plastids and mitochondria, where they play a crucial role in various aspects of RNA metabolism at the post-transcriptional and translational steps during gene expression. Among the early land plants, the moss Physcomitrium (formerly Physcomitrella) patens has at least 107 PPR protein-encoding genes, but most of their functions remain unclear. To elucidate the functions of PPR proteins, a reverse-genetics approach has been applied to P. patens. To date, the molecular functions of 22 PPR proteins were identified as essential factors required for either mRNA processing and stabilization, RNA splicing, or RNA editing. This review examines the P. patens PPR gene family and their current functional characterization. Similarities and a diversity of functions of PPR proteins between P. patens and flowering plants and their roles in the post-transcriptional regulation of organellar gene expression are discussed.

Physcomitrium patens pentatricopeptide repeat protein PpPPR_32 is involved in the accumulation of psaC mRNA encoding the iron sulfur protein of photosystem I

Pentatricopeptide repeat (PPR) proteins are involved in RNA metabolism and also play a role in posttranscriptional regulation during plant organellar gene expression. Although a hundred of PPR proteins exist in the moss Physcomitrium patens, their functions are not fully understood. Here, we report the function of P-class PPR protein PpPPR_32 in P. patens. A transient expression assay using green fluorescent protein demonstrated that the N-terminal region of PpPPR_32 functions as a chloroplast-targeting transit peptide, indicating that PpPPR_32 is localized in chloroplasts. PpPPR_32 knockout (KO) mutants grew autotrophically but with reduced protonema growth and the poor formation of photosystem I (PSI) complexes. Quantitative real-time reverse transcription-polymerase chain reaction and RNA gel blot hybridization analyses revealed a significant reduction in the transcript level of the psaC gene encoding the iron sulfur protein of PSI but no alteration to the transcript levels of other PSI genes. This suggests that PpPPR_32 is specifically involved in the expression level of the psaC gene. Our results indicate that PpPPR_32 is essential for the accumulation of psaC transcript and PSI complexes.

CAF Proteins Help SOT1 Regulate the Stability of Chloroplast ndhA Transcripts

Protein-mediated RNA stabilization plays profound roles in chloroplast gene expression. Genetic studies have indicated that chloroplast ndhA transcripts, encoding a key subunit of the NADH dehydrogenase-like complex that mediates photosystem I cyclic electron transport and facilitates chlororespiration, are stabilized by PPR53 and its orthologs, but the underlying mechanisms are unclear. Here, we report that CHLOROPLAST RNA SPLICING 2 (CRS2)-ASSOCIATED FACTOR (CAF) proteins activate SUPPRESSOR OF THYLAKOID FORMATION 1 (SOT1), an ortholog of PPR53 in Arabidopsis thaliana, enhancing their affinity for the 5' ends of ndhA transcripts to stabilize these molecules while inhibiting the RNA endonuclease activity of the SOT1 C-terminal SMR domain. In addition, we established that SOT1 improves the splicing efficiency of ndhA by facilitating the association of CAF2 with the ndhA intron, which may be due to the SOT1-mediated stability of the ndhA transcripts. Our findings shed light on the importance of PPR protein interaction partners in moderating RNA metabolism.

GUN control in retrograde signaling: How GENOMES UNCOUPLED proteins adjust nuclear gene expression to plastid biogenesis

Communication between cellular compartments is vital for development and environmental adaptation. Signals emanating from organelles, so-called retrograde signals, coordinate nuclear gene expression with the developmental stage and/or the functional status of the organelle. Plastids (best known in their green photosynthesizing differentiated form, the chloroplasts) are the primary energy-producing compartment of plant cells, and the site for the biosynthesis of many metabolites, including fatty acids, amino acids, nucleotides, isoprenoids, tetrapyrroles, vitamins, and phytohormone precursors. Signals derived from plastids regulate the accumulation of a large set of nucleus-encoded proteins, many of which localize to plastids. A set of mutants defective in retrograde signaling (genomes uncoupled, or gun) was isolated over 25 years ago. While most GUN genes act in tetrapyrrole biosynthesis, resolving the molecular function of GUN1, the proposed integrator of multiple retrograde signals, has turned out to be particularly challenging. Based on its amino acid sequence, GUN1 was initially predicted to be a plastid-localized nucleic acid-binding protein. Only recently, mechanistic information on the function of GUN1 has been obtained, pointing to a role in plastid protein homeostasis. This review article summarizes our current understanding of GUN-related retrograde signaling and provides a critical appraisal of the various proposed roles for GUNs and their respective pathways.

CFM1, a member of the CRM-domain protein family, functions in chloroplast group II intron splicing in Setaria viridis

The chloroplast RNA splicing and ribosome maturation (CRM) domain is a RNA-binding domain found in a plant-specific protein family whose characterized members play essential roles in splicing group I and group II introns in mitochondria and chloroplasts. Together, these proteins are required for splicing of the majority of the approximately 20 chloroplast introns in land plants. Here, we provide evidence from Setaria viridis and maize that an uncharacterized member of this family, CRM Family Member1 (CFM1), promotes the splicing of most of the introns that had not previously been shown to require a CRM domain protein. A Setaria mutant expressing mutated CFM1 was strongly disrupted in the splicing of three chloroplast tRNAs: trnI, trnV and trnA. Analyses by RNA gel blot and polysome association suggest that the tRNA deficiencies lead to compromised chloroplast protein synthesis and the observed whole-plant chlorotic phenotypes. Co-immunoprecipitation data demonstrate that the maize CFM1 ortholog is bound to introns whose splicing is disrupted in the cfm1 mutant. With these results, CRM domain proteins have been shown to promote the splicing of all but two of the introns found in angiosperm chloroplast genomes.

The OPR Protein MTHI1 Controls the Expression of Two Different Subunits of ATP Synthase CFo in Chlamydomonas reinhardtii

In the green alga Chlamydomonas (Chlamydomonas r einhardtii), chloroplast gene expression is tightly regulated posttranscriptionally by gene-specific trans-acting protein factors. Here, we report the identification of the octotricopeptide repeat protein MTHI1, which is critical for the biogenesis of chloroplast ATP synthase oligomycin-sensitive chloroplast coupling factor. Unlike most trans-acting factors characterized so far in Chlamydomonas, which control the expression of a single gene, MTHI1 targets two distinct transcripts: it is required for the accumulation and translation of atpH mRNA, encoding a subunit of the selective proton channel, but it also enhances the translation of atpI mRNA, which encodes the other subunit of the channel. MTHI1 targets the 5' untranslated regions of both the atpH and atpI genes. Coimmunoprecipitation and small RNA sequencing revealed that MTHI1 binds specifically a sequence highly conserved among Chlorophyceae and the Ulvale clade of Ulvophyceae at the 5' end of triphosphorylated atpH mRNA. A very similar sequence, located ∼60 nucleotides upstream of the atpI initiation codon, was also found in some Chlorophyceae and Ulvale algae species and is essential for atpI mRNA translation in Chlamydomonas. Such a dual-targeted trans-acting factor provides a means to coregulate the expression of the two proton hemi-channels.

The Chloroplast Ribonucleoprotein CP33B Quantitatively Binds the psbA mRNA

Chloroplast RNAs are stabilized and processed by a multitude of nuclear-encoded RNA-binding proteins, often in response to external stimuli like light and temperature. A particularly interesting RNA-based regulation occurs with the psbA mRNA, which shows light-dependent translation. Recently, the chloroplast ribonucleoprotein CP33B was identified as a ligand of the psbA mRNA. We here characterized the interaction of CP33B with chloroplast RNAs in greater detail using a combination of RIP-chip, quantitative dot-blot, and RNA-Bind-n-Seq experiments. We demonstrate that CP33B prefers psbA over all other chloroplast RNAs and associates with the vast majority of the psbA transcript pool. The RNA sequence target motif, determined in vitro, does not fully explain CP33B's preference for psbA, suggesting that there are other determinants of specificity in vivo.

The nuclear-localized PPR protein OsNPPR1 is important for mitochondrial function and endosperm development in rice

Pentatricopeptide repeat (PPR) proteins constitute one of the largest protein families in land plants. Recent studies revealed the functions of PPR proteins in organellar RNA metabolism and plant development, but the functions of most PPR proteins, especially PPRs localized in the nucleus, remain largely unknown. Here, we report the isolation and characterization of a rice mutant named floury and growth retardation1 (fgr1). fgr1 showed floury endosperm with loosely arranged starch grains, decreased starch and amylose contents, and retarded seedling growth. Map-based cloning showed that the mutant phenotype was caused by a single nucleotide substitution in the coding region of Os08g0290000. This gene encodes a nuclear-localized PPR protein, which we named OsNPPR1, that affected mitochondrial function. In vitro SELEX and RNA-EMSAs showed that OsNPPR1 was an RNA protein that bound to the CUCAC motif. Moreover, a number of retained intron (RI) events were detected in fgr1. Thus, OsNPPR1 was involved in regulation of mitochondrial development and/or functions that are important for endosperm development. Our results provide novel insights into coordinated interaction between nuclear-localized PPR proteins and mitochondrial function.

Ribonucleoprotein Capture by in Vivo Expression of a Designer Pentatricopeptide Repeat Protein in Arabidopsis

Pentatricopeptide repeat (PPR) proteins bind RNA via a mechanism that facilitates the customization of sequence specificity. However, natural PPR proteins have irregular features that limit the degree to which their specificity can be predicted and customized. We demonstrate here that artificial PPR proteins built from consensus PPR motifs selectively bind the intended RNA in vivo, and we use this property to develop a new tool for ribonucleoprotein characterization. We show by RNA coimmunoprecipitation sequencing (RIP-seq) that artificial PPR proteins designed to bind the Arabidopsis (Arabidopsis thaliana) chloroplast psbA mRNA bind with high specificity to psbA mRNA in vivo. Analysis of coimmunoprecipitating proteins by mass spectrometry showed the psbA translational activator HCF173 and two RNA binding proteins of unknown function (CP33C and SRRP1) to be highly enriched. RIP-seq revealed that these proteins are bound primarily to psbA RNA in vivo, and precise mapping of the HCF173 and CP33C binding sites placed them in different locations on psbA mRNA. These results demonstrate that artificial PPR proteins can be tailored to bind specific endogenous RNAs in vivo, add to the toolkit for characterizing native ribonucleoproteins, and open the door to other applications that rely on the ability to target a protein to a specified RNA sequence.

Unexpected functional versatility of the pentatricopeptide repeat proteins PGR3, PPR5 and PPR10

Pentatricopeptide repeat (PPR) proteins are a large family of helical repeat proteins that bind RNA in mitochondria and chloroplasts. Sites of PPR action have been inferred primarily from genetic data, which have led to the view that most PPR proteins act at a very small number of sites in vivo. Here, we report new functions for three chloroplast PPR proteins that had already been studied in depth. Maize PPR5, previously shown to promote trnG splicing, is also required for rpl16 splicing. Maize PPR10, previously shown to bind the atpI-atpH and psaJ-rpl33 intercistronic regions, also stabilizes a 3′-end downstream from psaI. Arabidopsis PGR3, shown previously to bind upstream of petL, also binds the rpl14-rps8 intercistronic region where it stabilizes a 3′-end and stimulates rps8 translation. These functions of PGR3 are conserved in maize. The discovery of new functions for three proteins that were already among the best characterized members of the PPR family implies that functional repertoires of PPR proteins are more complex than have been appreciated. The diversity of sequences bound by PPR10 and PGR3 in vivo highlights challenges of predicting binding sites of native PPR proteins based on the amino acid code for nucleotide recognition by PPR motifs.

A chloroplast-targeted pentatricopeptide repeat protein PPR287 is crucial for chloroplast function and Arabidopsis development

BACKGROUND

Even though the roles of pentatricopeptide repeat (PPR) proteins are essential in plant organelles, the function of many chloroplast-targeted PPR proteins remains unknown. Here, we characterized the function of a chloroplast-localized PPR protein (At3g59040), which is classified as the 287th PPR protein among the 450 PPR proteins in Arabidopsis ( http://ppr.plantenergy.uwa.edu.au ).

RESULTS

The homozygous ppr287 mutant with the T-DNA inserted into the last exon displayed pale-green and yellowish phenotypes. The microRNA-mediated knockdown mutants were generated to further confirm the developmental defect phenotypes of ppr287 mutants. All mutants had yellowish leaves, shorter roots and height, and less seed yield, indicating that PPR287 is crucial for normal Arabidopsis growth and development. The photosynthetic activity and chlorophyll content of ppr287 mutants were markedly reduced, and the chloroplast structures of the mutants were abnormal. The levels of chloroplast rRNAs were decreased in ppr287 mutants.

CONCLUSIONS

These results suggest that PPR287 plays an essential role in chloroplast biogenesis and function, which is crucial for the normal growth and development of Arabidopsis.

An RNA Chaperone-Like Protein Plays Critical Roles in Chloroplast mRNA Stability and Translation in Arabidopsis and Maize

A key characteristic of chloroplast gene expression is the predominance of posttranscriptional control via numerous nucleus-encoded RNA binding factors. Here, we explored the essential roles of the S1-domain-containing protein photosynthetic electron transfer B (petB)/ petD Stabilizing Factor (BSF) in the stabilization and translation of chloroplast mRNAs. BSF binds to the intergenic region of petB-petD, thereby stabilizing 3' processed petB transcripts and stimulating petD translation. BSF also binds to the 5' untranslated region of petA and activates its translation. BSF displayed nucleic-acid-melting activity in vitro, and its absence induces structural changes to target RNAs in vivo, suggesting that BSF functions as an RNA chaperone to remodel RNA structure. BSF physically interacts with the pentatricopeptide repeat protein Chloroplast RNA Processing 1 (AtCRP1) and the ribosomal release factor-like protein Peptide chain Release Factor 3 (PrfB3), whose established RNA ligands overlap with those of BSF. In addition, PrfB3 stimulated the RNA binding ability of BSF in vitro. We propose that BSF and PrfB3 cooperatively reduce the formation of secondary RNA structures within target mRNAs and facilitate AtCRP1 binding. The translation activation function of BSF for petD is conserved in Arabidopsis (Arabidopsis thaliana) and maize (Zea mays), but that for petA operates specifically in Arabidopsis. Our study sheds light on the mechanisms by which RNA binding proteins cooperatively regulate mRNA stability and translation in chloroplasts.

Magnetic Tracking of Protein Synthesis in Microfluidic Environments-Challenges and Perspectives

A novel technique to study protein synthesis is proposed that uses magnetic nanoparticles in combination with microfluidic devices to achieve new insights into translational regulation. Cellular protein synthesis is an energy-demanding process which is tightly controlled and is dependent on environmental and developmental requirements. Processivity and regulation of protein synthesis as part of the posttranslational nano-machinery has now moved back into the focus of cell biology, since it became apparent that multiple mechanisms are in place for fine-tuning of translation and conditional selection of transcripts. Recent methodological developments, such as ribosome foot printing, propel current research. Here we propose a strategy to open up a new field of labelling, separation, and analysis of specific polysomes using superparamagnetic particles following pharmacological arrest of translation during cell lysis and subsequent analysis. Translation occurs in polysomes, which are assemblies of specific transcripts, associated ribosomes, nascent polypeptides, and other factors. This supramolecular structure allows for unique approaches to selection of polysomes by targeting the specific transcript, ribosomes, or nascent polypeptides. Once labeled with functionalized superparamagnetic particles, such assemblies can be separated in microfluidic devices or magnetic ratchets and quantified. Insights into the dynamics of translation is obtained through quantifying large numbers of ribosomes along different locations of the polysome. Thus, an entire new concept for in vitro, ex vivo, and eventually single cell analysis will be realized and will allow for magnetic tracking of protein synthesis.

beRBP: binding estimation for human RNA-binding proteins

Identifying binding targets of RNA-binding proteins (RBPs) can greatly facilitate our understanding of their functional mechanisms. Most computational methods employ machine learning to train classifiers on either RBP-specific targets or pooled RBP-RNA interactions. The former strategy is more powerful, but it only applies to a few RBPs with a large number of known targets; conversely, the latter strategy sacrifices prediction accuracy for a wider application, since specific interaction features are inevitably obscured through pooling heterogeneous datasets. Here, we present beRBP, a dual approach to predict human RBP-RNA interaction given PWM of a RBP and one RNA sequence. Based on Random Forests, beRBP not only builds a specific model for each RBP with a decent number of known targets, but also develops a general model for RBPs with limited or null known targets. The specific and general models both compared well with existing methods on three benchmark datasets. Notably, the general model achieved a better performance than existing methods on most novel RBPs. Overall, as a composite solution overarching the RBP-specific and RBP-General strategies, beRBP is a promising tool for human RBP binding estimation with good prediction accuracy and a broad application scope.

The P-class pentatricopeptide repeat protein PpPPR_21 is needed for accumulation of the psbI-ycf12 dicistronic mRNA in Physcomitrella chloroplasts

Chloroplast gene expression is controlled by numerous nuclear-encoded RNA-binding proteins. Among these, pentatricopeptide repeat (PPR) proteins are known to be key players in post-transcriptional regulation in chloroplasts. However, the functions of many PPR proteins remain unknown. In this study, we characterized the function of a chloroplast-localized P-class PPR protein PpPPR_21 in Physcomitrella patens. Knockout (KO) mutants of PpPPR_21 exhibited reduced protonemata growth and lower photosynthetic activity. Immunoblot analysis and blue-native gel analysis showed a remarkable reduction of the photosystem II (PSII) reaction center protein and poor formation of the PSII supercomplexes in the KO mutants. To assess whether PpPPR_21 is involved in chloroplast gene expression, chloroplast genome-wide microarray analysis and Northern blot hybridization were performed. These analyses indicated that the psbI-ycf12 transcript encoding the low molecular weight subunits of PSII did not accumulate in the KO mutants while other psb transcripts accumulated at similar levels in wild-type and KO mutants. A complemented PpPPR_21KO moss transformed with the cognate full-length PpPPR_21cDNA rescued the level of accumulation of psbI-ycf12 transcript. RNA-binding experiments showed that the recombinant PpPPR_21 bound efficiently to the 5' untranslated and translated regions of psbImRNA. The present study suggests that PpPPR_21 may be essential for the accumulation of a stable psbI-ycf12mRNA.

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.

LOW PHOTOSYNTHETIC EFFICIENCY 1 is required for light-regulated photosystem II biogenesis in Arabidopsis

Photosystem II (PSII) reaction center protein D1 is encoded by chloroplast gene psbA and is crucial to the biogenesis and functional maintenance of PSII. D1 proteins are highly dynamic under varying light conditions and thus require efficient synthesis, but the mechanism remains poorly understood. We reported that Arabidopsis LPE1 directly binds to the 5′ UTR of psbA mRNA in a light-dependent manner through a redox-based mechanism and facilitates the association of HCF173 with psbA mRNA to regulate D1 translation. These findings fill a major gap in our understanding of the mechanism of light-regulated D1 synthesis in higher plants and imply that higher plants and primitive photosynthetic organisms share conserved mechanisms but use distinct regulators to regulate biogenesis of PSII subunits.

Photosystem II (PSII), a multisubunit protein complex of the photosynthetic electron transport chain, functions as a water-plastoquinone oxidoreductase, which is vital to the initiation of photosynthesis and electron transport. Although the structure, composition, and function of PSII are well understood, the mechanism of PSII biogenesis remains largely elusive. Here, we identified a nuclear-encoded pentatricopeptide repeat (PPR) protein LOW PHOTOSYNTHETIC EFFICIENCY 1 (LPE1; encoded by At3g46610) in Arabidopsis, which plays a crucial role in PSII biogenesis. LPE1 is exclusively targeted to chloroplasts and directly binds to the 5′ UTR of psbA mRNA which encodes the PSII reaction center protein D1. The loss of LPE1 results in less efficient loading of ribosome on the psbA mRNA and great synthesis defects in D1 protein. We further found that LPE1 interacts with a known regulator of psbA mRNA translation HIGH CHLOROPHYLL FLUORESCENCE 173 (HCF173) and facilitates the association of HCF173 with psbA mRNA. More interestingly, our results indicate that LPE1 associates with psbA mRNA in a light-dependent manner through a redox-based mechanism. This study enhances our understanding of the mechanism of light-regulated D1 synthesis, providing important insight into PSII biogenesis and the functional maintenance of efficient photosynthesis in higher plants.

RNA-stabilization factors in chloroplasts of vascular plants

In contrast to the cyanobacterial ancestor, chloroplast gene expression is predominantly governed on the post-transcriptional level such as modifications of the RNA sequence, decay rates, exo- and endonucleolytic processing as well as translational events. The concerted function of numerous chloroplast RNA-binding proteins plays a fundamental and often essential role in all these processes but our understanding of their impact in regulation of RNA degradation is only at the beginning. Moreover, metabolic processes and post-translational modifications are thought to affect the function of RNA protectors. These protectors contain a variety of different RNA-recognition motifs, which often appear as multiple repeats. They are required for normal plant growth and development as well as diverse stress responses and acclimation processes. Interestingly, most of the protectors are plant specific which reflects a fast-evolving RNA metabolism in chloroplasts congruent with the diverging RNA targets. Here, we mainly focused on the characteristics of known chloroplast RNA-binding proteins that protect exonuclease-sensitive sites in chloroplasts of vascular 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.

PALE CRESS binds to plastid RNAs and facilitates the biogenesis of the 50S ribosomal subunit

The plant-specific PALE CRESS (PAC) protein has previously been shown to be essential for photoautotrophic growth. Here we further investigated the molecular function of the PAC protein. PAC localizes to plastid nucleoids and forms large proteinaceous and RNA-containing megadalton complexes. It co-immunoprecipitates with a specific subset of chloroplast RNAs including psbK-psbI, ndhF, ndhD, and 23S ribosomal RNA (rRNA), as demonstrated by RNA immunoprecipitation in combination with high throughput RNA sequencing (RIP-seq) analyses. Furthermore, it co-migrates with premature 50S ribosomal particles and specifically binds to 23S rRNA in vitro. This coincides with severely reduced levels of 23S rRNA in pac leading to translational deficiencies and related alterations of plastid transcript patterns and abundance similar to plants treated with the translation inhibitor lincomycin. Thus, we conclude that deficiency in plastid ribosomes accounts for the pac phenotype. Moreover, the absence or reduction of PAC levels in the corresponding mutants induces structural changes of the 23S rRNA, as demonstrated by in vivo RNA structure probing. Our results indicate that PAC binds to the 23S rRNA to promote the biogenesis of the 50S subunit.

Adaptation of iCLIP to plants determines the binding landscape of the clock-regulated RNA-binding protein AtGRP7

Background

Functions for RNA-binding proteins in orchestrating plant development and environmental responses are well established. However, the lack of a genome-wide view of their in vivo binding targets and binding landscapes represents a gap in understanding the mode of action of plant RNA-binding proteins. Here, we adapt individual nucleotide resolution crosslinking and immunoprecipitation (iCLIP) genome-wide to determine the binding repertoire of the circadian clock-regulated Arabidopsis thaliana glycine-rich RNA-binding protein AtGRP7.

Results

iCLIP identifies 858 transcripts with significantly enriched crosslink sites in plants expressing AtGRP7-GFP that are absent in plants expressing an RNA-binding-dead AtGRP7 variant or GFP alone. To independently validate the targets, we performed RNA immunoprecipitation (RIP)-sequencing of AtGRP7-GFP plants subjected to formaldehyde fixation. Of the iCLIP targets, 452 were also identified by RIP-seq and represent a set of high-confidence binders. AtGRP7 can bind to all transcript regions, with a preference for 3′ untranslated regions. In the vicinity of crosslink sites, U/C-rich motifs are overrepresented. Cross-referencing the targets against transcriptome changes in AtGRP7 loss-of-function mutants or AtGRP7-overexpressing plants reveals a predominantly negative effect of AtGRP7 on its targets. In particular, elevated AtGRP7 levels lead to damping of circadian oscillations of transcripts, including DORMANCY/AUXIN ASSOCIATED FAMILY PROTEIN2 and CCR-LIKE. Furthermore, several targets show changes in alternative splicing or polyadenylation in response to altered AtGRP7 levels.

Conclusions

We have established iCLIP for plants to identify target transcripts of the RNA-binding protein AtGRP7. This paves the way to investigate the dynamics of posttranscriptional networks in response to exogenous and endogenous cues.

Electronic supplementary material

The online version of this article (doi:10.1186/s13059-017-1332-x) contains supplementary material, which is available to authorized users.

Transcriptome sequencing and characterization of Symbiodinium muscatinei and Elliptochloris marina, symbionts found within the aggregating sea anemone Anthopleura elegantissima

There is a growing body of literature using transcriptomic data to study how tropical cnidarians and their photosynthetic endosymbionts respond to environmental stressors and participate in metabolic exchange. Despite these efforts, our understanding of how essential genes function to facilitate symbiosis establishment and maintenance remains limited. The inclusion of taxonomically and ecologically diverse endosymbionts will enhance our understanding of these interactions. Here we characterize the transcriptomes of two very different symbionts found within the temperate sea anemone Anthopleura elegantissima: the chlorophyte Elliptochloris marina and the dinoflagellate Symbiodinium muscatinei. We use a multi-level approach to assess the diversity of genes found across S. muscatinei and E. marina transcriptomes, and compare their overall protein domains with other dinoflagellates and chlorophytes. Our analysis identified several genes that are potentially involved in mitigating stress response (e.g., heat shock proteins pathways for mediating reactive oxygen species) and metabolic exchange (e.g., ion transporters). Finally, we show that S. muscatinei and other Symbiodinium strains are equipped with a high salt peridinin-chl-protein (HSPCP) gene previously identified only in free-living dinoflagellates. The addition of these transcriptomes to the cnidarian-symbiont molecular toolkit will aid in understanding how these vitally important symbiotic relationships are established and maintained across a variety of environmental conditions.

Stable Membrane-Association of mRNAs in Etiolated, Greening and Mature Plastids

Chloroplast genes are transcribed as polycistronic precursor RNAs that give rise to a multitude of processing products down to monocistronic forms. Translation of these mRNAs is realized by bacterial type 70S ribosomes. A larger fraction of these ribosomes is attached to chloroplast membranes. This study analyzed transcriptome-wide distribution of plastid mRNAs between soluble and membrane fractions of purified plastids using microarray analyses and validating RNA gel blot hybridizations. To determine the impact of light on mRNA localization, we used etioplasts, greening plastids and mature chloroplasts from Zea mays as a source for membrane and soluble extracts. The results show that the three plastid types display an almost identical distribution of RNAs between the two organellar fractions, which is confirmed by quantitative RNA gel blot analyses. Furthermore, they reveal that different RNAs processed from polycistronic precursors show transcript-autonomous distribution between stroma and membrane fractions. Disruption of ribosomes leads to release of mRNAs from membranes, demonstrating that attachment is likely a direct consequence of translation. We conclude that plastid mRNA distribution is a stable feature of different plastid types, setting up rapid chloroplast translation in any plastid type.

RNA structure, binding, and coordination in Arabidopsis

From the moment of transcription, up through degradation, each RNA transcript is bound by an ever-changing cohort of RNA binding proteins. The binding of these proteins is regulated by both the primary RNA sequence, as well as the intramolecular RNA folding, or secondary structure, of the transcript. Thus, RNA secondary structure regulates many post-transcriptional processes. With the advent of next generation sequencing, several techniques have been developed to generate global landscapes of both RNA-protein interactions and RNA secondary structure. In this review, we describe the current state of the field detailing techniques to globally interrogate RNA secondary structure and/or RNA-protein interaction sites, as well as our current understanding of these features in the transcriptome of the model plant Arabidopsis thaliana. WIREs RNA 2017, 8:e1426. doi: 10.1002/wrna.1426 For further resources related to this article, please visit the WIREs website.

Cryptochrome 2 extensively regulates transcription of the chloroplast genome in tomato

Light plays a key role in the regulation of many physiological processes required for plant and chloroplast development. Plant cryptochromes (crys) play an important role in monitoring, capturing, and transmitting the light stimuli. In this study, we analyzed the effects of CRY2 overexpression on transcription of tomato chloroplast genome by a tiling array, containing about 90 000 overlapping probes (5‐nucleotide resolution). We profiled transcription in leaves of wild‐type and CRY2‐overexpressing plants grown in a diurnal cycle, to generate a comprehensive map of chloroplast transcription and to monitor potential specific modulations of the chloroplast transcriptome induced by the overexpression of CRY2. Our results demonstrate that CRY2 is a master gene of transcriptional regulation in the tomato chloroplast. In fact, it modulates the day/night mRNA abundance of about 58% of the 114 ORFs. The effect of CRY2 includes a differential extension of some transcripts at their 5′‐end, according to the period of the day. We observed that the influence of CRY2 on chloroplast transcription is not limited to coding RNA; a great number of putative noncoding micro RNA also showed differential accumulation pattern. To our knowledge, this is the first study that highlights how a photoreceptor affects the day/night transcription of the chloroplast genome.

CRP1 Protein: (dis)similarities between Arabidopsis thaliana and Zea mays

Biogenesis of chloroplasts in higher plants is initiated from proplastids, and involves a series of processes by which a plastid able to perform photosynthesis, to synthesize amino acids, lipids, and phytohormones is formed. All plastid protein complexes are composed of subunits encoded by the nucleus and chloroplast genomes, which require a coordinated gene expression to produce the correct concentrations of organellar proteins and to maintain organelle function. To achieve this, hundreds of nucleus-encoded factors are imported into the chloroplast to control plastid gene expression. Among these factors, members of the Pentatricopeptide Repeat (PPR) containing protein family have emerged as key regulators of the organellar post-transcriptional processing. PPR proteins represent a large family in plants, and the extent to which PPR functions are conserved between dicots and monocots deserves evaluation, in light of differences in photosynthetic metabolism (C3 vs. C4) and localization of chloroplast biogenesis (mesophyll vs. bundle sheath cells). In this work we investigated the role played in the process of chloroplast biogenesis by At5g42310, a member of the Arabidopsis PPR family which we here refer to as AtCRP1 (Chloroplast RNA Processing 1), providing a comparison with the orthologous ZmCRP1 protein from Zea mays. Loss-of-function atcrp1 mutants are characterized by yellow-albinotic cotyledons and leaves owing to defects in the accumulation of subunits of the thylakoid protein complexes. As in the case of ZmCRP1, AtCRP1 associates with the 5' UTRs of both psaC and, albeit very weakly, petA transcripts, indicating that the role of CRP1 as regulator of chloroplast protein synthesis has been conserved between maize and Arabidopsis. AtCRP1 also interacts with the petB-petD intergenic region and is required for the generation of petB and petD monocistronic RNAs. A similar role has been also attributed to ZmCRP1, although the direct interaction of ZmCRP1 with the petB-petD intergenic region has never been reported, which could indicate that AtCRP1 and ZmCRP1 differ, in part, in their plastid RNA targets.

The RNA recognition motif protein CP33A is a global ligand of chloroplast mRNAs and is essential for plastid biogenesis and plant development

Chloroplast RNA metabolism depends on a multitude of nuclear-encoded RNA-binding proteins (RBPs). Most known chloroplast RBPs address specific RNA targets and RNA-processing functions. However, members of the small chloroplast ribonucleoprotein family (cpRNPs) play a global role in processing and stabilizing chloroplast RNAs. Here, we show that the cpRNP CP33A localizes to a distinct sub-chloroplastic domain and is essential for chloroplast development. The loss of CP33A yields albino seedlings that exhibit aberrant leaf development and can only survive in the presence of an external carbon source. Genome-wide RNA association studies demonstrate that CP33A associates with all chloroplast mRNAs. For a given transcript, quantification of CP33A-bound versus free RNAs demonstrates that CP33A associates with the majority of most mRNAs analyzed. Our results further show that CP33A is required for the accumulation of a number of tested mRNAs, and is particularly relevant for unspliced and unprocessed precursor mRNAs. Finally, CP33A fails to associate with polysomes or to strongly co-precipitate with ribosomal RNA, suggesting that it defines a ribodomain that is separate from the chloroplast translation machinery. Collectively, these findings suggest that CP33A contributes to globally essential RNA processes in the chloroplasts of higher plants.

WHITE STRIPE LEAF4 Encodes a Novel P-Type PPR Protein Required for Chloroplast Biogenesis during Early Leaf Development

Pentatricopeptide repeat (PPR) proteins comprise a large family in higher plants and perform diverse functions in organellar RNA metabolism. Despite the rice genome encodes 477 PRR proteins, the regulatory effects of PRR proteins on chloroplast development remains unknown. In this study, we report the functional characterization of the rice white stripe leaf4 (wsl4) mutant. The wsl4 mutant develops white-striped leaves during early leaf development, characterized by decreased chlorophyll content and malformed chloroplasts. Positional cloning of the WSL4 gene, together with complementation and RNA-interference tests, reveal that it encodes a novel P-family PPR protein with 12 PPR motifs, and is localized to chloroplast nucleoids. Quantitative RT-PCR analyses demonstrate that WSL4 is a low temperature response gene abundantly expressed in young leaves. Further expression analyses show that many nuclear- and plastid-encoded genes in the wsl4 mutant are significantly affected at the RNA and protein levels. Notably, the wsl4 mutant causes defects in the splicing of atpF, ndhA, rpl2, and rps12. Our findings identify WSL4 as a novel P-family PPR protein essential for chloroplast RNA group II intron splicing during early leaf development in rice.

The Reverse Transcriptase/RNA Maturase Protein MatR Is Required for the Splicing of Various Group II Introns in Brassicaceae Mitochondria

Group II introns are large catalytic RNAs that are ancestrally related to nuclear spliceosomal introns. Sequences corresponding to group II RNAs are found in many prokaryotes and are particularly prevalent within plants organellar genomes. Proteins encoded within the introns themselves (maturases) facilitate the splicing of their own host pre-RNAs. Mitochondrial introns in plants have diverged considerably in sequence and have lost their maturases. In angiosperms, only a single maturase has been retained in the mitochondrial DNA: the matR gene found within NADH dehydrogenase 1 (nad1) intron 4. Its conservation across land plants and RNA editing events, which restore conserved amino acids, indicates that matR encodes a functional protein. However, the biological role of MatR remains unclear. Here, we performed an in vivo investigation of the roles of MatR in Brassicaceae. Directed knockdown of matR expression via synthetically designed ribozymes altered the processing of various introns, including nad1 i4. Pull-down experiments further indicated that MatR is associated with nad1 i4 and several other intron-containing pre-mRNAs. MatR may thus represent an intermediate link in the gradual evolutionary transition from the intron-specific maturases in bacteria into their versatile spliceosomal descendants in the nucleus. The similarity between maturases and the core spliceosomal Prp8 protein further supports this intriguing theory.

Analyses of RNA-Seq and sRNA-Seq data reveal a complex network of anti-viral defense in TCV-infected Arabidopsis thaliana

In order to identify specific plant anti-viral genes related to the miRNA regulatory pathway, RNA-Seq and sRNA-Seq were performed using Arabidopsis WT and dcl1-9 mutant line. A total of 5,204 DEGs were identified in TCV-infected WT plants. In contrast, only 595 DEGs were obtained in the infected dcl1-9 mutant plants. GO enrichment analysis of the shared DEGs and dcl1-9 unique DEGs showed that a wide range of biological processes were affected in the infected WT plants. In addition, miRNAs displayed different patterns between mock and infected WT plants. This is the first global view of dcl1-9 transcriptome which provides TCV responsive miRNAs data. In conclusion, our results indicated the significance of DCL1 and suggested that PPR genes may play an important role in plant anti-viral defense.

ChloroSeq, an Optimized Chloroplast RNA-Seq Bioinformatic Pipeline, Reveals Remodeling of the Organellar Transcriptome Under Heat Stress

Although RNA-Seq has revolutionized transcript analysis, organellar transcriptomes are rarely assessed even when present in published datasets. Here, we describe the development and application of a rapid and convenient method, ChloroSeq, to delineate qualitative and quantitative features of chloroplast RNA metabolism from strand-specific RNA-Seq datasets, including processing, editing, splicing, and relative transcript abundance. The use of a single experiment to analyze systematically chloroplast transcript maturation and abundance is of particular interest due to frequent pleiotropic effects observed in mutants that affect chloroplast gene expression and/or photosynthesis. To illustrate its utility, ChloroSeq was applied to published RNA-Seq datasets derived from Arabidopsis thaliana grown under control and abiotic stress conditions, where the organellar transcriptome had not been examined. The most appreciable effects were found for heat stress, which induces a global reduction in splicing and editing efficiency, and leads to increased abundance of chloroplast transcripts, including genic, intergenic, and antisense transcripts. Moreover, by concomitantly analyzing nuclear transcripts that encode chloroplast gene expression regulators from the same libraries, we demonstrate the possibility of achieving a holistic understanding of the nucleus-organelle system. ChloroSeq thus represents a unique method for streamlining RNA-Seq data interpretation of the chloroplast transcriptome and its regulators.

Morphological Structure and Transcriptome Comparison of the Cytoplasmic Male Sterility Line in Brassica napus (SaNa-1A) Derived from Somatic Hybridization and Its Maintainer Line SaNa-1B

SaNa-1A is a novel cytoplasmic male sterility (CMS) line in Brassica napus derived from progenies of somatic hybrids between B.napus and Sinapis alba, and SaNa-1B is the corresponding maintainer line. In this study, phenotypic differences of floral organs between CMS and the maintainer lines were observed. By microscope observation in different anther developmental stages of two lines, we found the anther development in SaNa-1A was abnormal since the tetrad stage, and microspore development was ceased during the uninucleate stage. Transcriptomic sequencing for floral buds of sterile and fertile plants were conducted to elucidate gene expression and regulation caused by the alien chromosome and cytoplasm from S. alba. Clean tags obtained were assembled into 195,568 unigenes, and 7811 unigenes distributed in the metabolic and protein synthesis pathways were identified with significant expression differences between two libraries. We also observed that genes participating in carbon metabolism, tricarboxylic acid cycle, oxidative phosphorylation, oxidation-reduction system, pentatricopeptide repeat, and anther development were downregulated in the sterile line. Some of them are candidates for researches on the sterility mechanism in the CMS material, fertility restoration, and improvement of economic traits in the maintainer line. Further research on the tags with expressional specificity in the fertile line would be helpful to explore desirable agronomic traits from wild species of rapeseed.

Dynamics of Chloroplast Translation during Chloroplast Differentiation in Maize

Chloroplast genomes in land plants contain approximately 100 genes, the majority of which reside in polycistronic transcription units derived from cyanobacterial operons. The expression of chloroplast genes is integrated into developmental programs underlying the differentiation of photosynthetic cells from non-photosynthetic progenitors. In C4 plants, the partitioning of photosynthesis between two cell types, bundle sheath and mesophyll, adds an additional layer of complexity. We used ribosome profiling and RNA-seq to generate a comprehensive description of chloroplast gene expression at four stages of chloroplast differentiation, as displayed along the maize seedling leaf blade. The rate of protein output of most genes increases early in development and declines once the photosynthetic apparatus is mature. The developmental dynamics of protein output fall into several patterns. Programmed changes in mRNA abundance make a strong contribution to the developmental shifts in protein output, but output is further adjusted by changes in translational efficiency. RNAs with prioritized translation early in development are largely involved in chloroplast gene expression, whereas those with prioritized translation in photosynthetic tissues are generally involved in photosynthesis. Differential gene expression in bundle sheath and mesophyll chloroplasts results primarily from differences in mRNA abundance, but differences in translational efficiency amplify mRNA-level effects in some instances. In most cases, rates of protein output approximate steady-state protein stoichiometries, implying a limited role for proteolysis in eliminating unassembled or damaged proteins under non-stress conditions. Tuned protein output results from gene-specific trade-offs between translational efficiency and mRNA abundance, both of which span a large dynamic range. Analysis of ribosome footprints at sites of RNA editing showed that the chloroplast translation machinery does not generally discriminate between edited and unedited RNAs. However, editing of ACG to AUG at the rpl2 start codon is essential for translation initiation, demonstrating that ACG does not serve as a start codon in maize chloroplasts.

Systematic analysis of plant mitochondrial and chloroplast small RNAs suggests organelle-specific mRNA stabilization mechanisms

Land plant organellar genomes encode a small number of genes, many of which are essential for respiration and photosynthesis. Organellar gene expression is characterized by a multitude of RNA processing events that lead to stable, translatable transcripts. RNA binding proteins (RBPs), have been shown to generate and protect transcript termini and eventually induce the accumulation of short RNA footprints. We applied knowledge of such RBP-derived footprints to develop software (sRNA miner) that enables identification of RBP footprints, or other clusters of small RNAs, in organelles. We used this tool to determine mitochondrial and chloroplast cosRNAs (clustered organellar sRNAs) in Arabidopsis. We found that in mitochondria, cosRNAs coincide with transcript 3'-ends, but are largely absent from 5'-ends. In chloroplasts this bias is absent, suggesting a different mode of 5' processing, possibly owing to different sets of RNases. Furthermore, we identified a large number of cosRNAs that represent silenced insertions of mitochondrial DNA in the nuclear genome of Arabidopsis. Steady-state RNA analyses demonstrate that cosRNAs display differential accumulation during development. Finally, we demonstrate that the chloroplast RBP PPR10 associates in vivo with its cognate cosRNA. A hypothetical role of cosRNAs as competitors of mRNAs for PPR proteins is discussed.

A Nucleus-Encoded Chloroplast Phosphoprotein Governs Expression of the Photosystem I Subunit PsaC in Chlamydomonas reinhardtii

The nucleo-cytoplasmic compartment exerts anterograde control on chloroplast gene expression through numerous proteins that intervene at posttranscriptional steps. Here, we show that the maturation of psaC mutant (mac1) of Chlamydomonas reinhardtii is defective in photosystem I and fails to accumulate psaC mRNA. The MAC1 locus encodes a member of the Half-A-Tetratricopeptide (HAT) family of super-helical repeat proteins, some of which are involved in RNA transactions. The Mac1 protein localizes to the chloroplast in the soluble fraction. MAC1 acts through the 5' untranslated region of psaC transcripts and is required for their stability. Small RNAs that map to the 5'end of psaC RNA in the wild type but not in the mac1 mutant are inferred to represent footprints of MAC1-dependent protein binding, and Mac1 expressed in bacteria binds RNA in vitro. A coordinate response to iron deficiency, which leads to dismantling of the photosynthetic electron transfer chain and in particular of photosystem I, also causes a decrease of Mac1. Overexpression of Mac1 leads to a parallel increase in psaC mRNA but not in PsaC protein, suggesting that Mac1 may be limiting for psaC mRNA accumulation but that other processes regulate protein accumulation. Furthermore, Mac 1 is differentially phosphorylated in response to iron availability and to conditions that alter the redox balance of the electron transfer chain.

A PPR protein in the PLS subfamily stabilizes the 5'-end of processed rpl16 mRNAs in maize chloroplasts

Pentatricopeptide repeat (PPR) proteins are a large family of helical-repeat proteins that bind RNA in mitochondria and chloroplasts. Precise RNA targets and functions have been assigned to only a small fraction of the >400 members of the PPR family in plants. We used the amino acid code governing the specificity of RNA binding by PPR repeats to infer candidate-binding sites for the maize protein PPR103 and its ortholog Arabidopsis EMB175. Genetic and biochemical data confirmed a predicted binding site in the chloroplast rpl16 5′UTR to be a site of PPR103 action. This site maps to the 5′ end of transcripts that fail to accumulate in ppr103 mutants. A small RNA corresponding to the predicted PPR103 binding site accumulates in a PPR103-dependent fashion, as expected of PPR103's in vivo footprint. Recombinant PPR103 bound specifically to this sequence in vitro. These observations imply that PPR103 stabilizes rpl16 mRNA by impeding 5′→3′ RNA degradation. Previously described PPR proteins with this type of function consist of canonical PPR motifs. By contrast, PPR103 is a PLS-type protein, an architecture typically associated with proteins that specify sites of RNA editing. However, PPR103 is not required to specify editing sites in chloroplasts.

Transcriptome Analysis of Flower Sex Differentiation in Jatropha curcas L. Using RNA Sequencing

Background

Jatropha curcas is thought to be a promising biofuel material, but its yield is restricted by a low ratio of instaminate / staminate flowers (1/10-1/30). Furthermore, valuable information about flower sex differentiation in this plant is scarce. To explore the mechanism of this process in J. curcas, transcriptome profiling of flower development was carried out, and certain genes related with sex differentiation were obtained through digital gene expression analysis of flower buds from different phases of floral development.

Results

After Illumina sequencing and clustering, 57,962 unigenes were identified. A total of 47,423 unigenes were annotated, with 85 being related to carpel and stamen differentiation, 126 involved in carpel and stamen development, and 592 functioning in the later development stage for the maturation of staminate or instaminate flowers. Annotation of these genes provided comprehensive information regarding the sex differentiation of flowers, including the signaling system, hormone biosynthesis and regulation, transcription regulation and ubiquitin-mediated proteolysis. A further expression pattern analysis of 15 sex-related genes using quantitative real-time PCR revealed that gibberellin-regulated protein 4-like protein and AMP-activated protein kinase are associated with stamen differentiation, whereas auxin response factor 6-like protein, AGAMOUS-like 20 protein, CLAVATA1, RING-H2 finger protein ATL3J, auxin-induced protein 22D, and r2r3-myb transcription factor contribute to embryo sac development in the instaminate flower. Cytokinin oxidase, Unigene28, auxin repressed-like protein ARP1, gibberellin receptor protein GID1 and auxin-induced protein X10A are involved in both stages mentioned above. In addition to its function in the differentiation and development of the stamens, the gibberellin signaling pathway also functions in embryo sac development for the instaminate flower. The auxin signaling pathway also participates in both stamen development and embryo sac development.

Conclusions

Our transcriptome data provide a comprehensive gene expression profile for flower sex differentiation in Jatropha curcas, as well as new clues and information for further study in this field.

The Propensity of Pentatricopeptide Repeat Genes to Evolve into Restorers of Cytoplasmic Male Sterility

Cytoplasmic male sterility (CMS) is a widespread phenotype in plants, which present a defect in the production of functional pollen. The male sterilizing factors usually consist of unusual genes or open reading frames encoded by the mitochondrial genome. CMS can be suppressed by specific nuclear genes called restorers of fertility (Rfs). In the majority of cases, Rf genes produce proteins that act directly on the CMS conferring mitochondrial transcripts by binding them specifically and promoting processing events. In this review, we explore the wide array of mechanisms guiding fertility restoration. PPR proteins represent the most frequent protein class among identified Rfs and they exhibit ideal characteristics to evolve into restorer of fertility when the mechanism of restoration implies a post-transcriptional action. Here, we review the literature that highlights those characteristics and help explain why PPR proteins are ideal for the roles they play as restorers of fertility.

The MTL1 Pentatricopeptide Repeat Protein Is Required for Both Translation and Splicing of the Mitochondrial NADH DEHYDROGENASE SUBUNIT7 mRNA in Arabidopsis

Mitochondrial translation involves a complex interplay of ancient bacteria-like features and host-derived functionalities. Although the basic components of the mitochondrial translation apparatus have been recognized, very few protein factors aiding in recruiting ribosomes on mitochondria-encoded messenger RNA (mRNAs) have been identified in higher plants. In this study, we describe the identification of the Arabidopsis (Arabidopsis thaliana) MITOCHONDRIAL TRANSLATION FACTOR1 (MTL1) protein, a new member of the Pentatricopeptide Repeat family, and show that it is essential for the translation of the mitochondrial NADH dehydrogenase subunit7 (nad7) mRNA. We demonstrate that mtl1 mutant plants fail to accumulate the Nad7 protein, even though the nad7 mature mRNA is produced and bears the same 5' and 3' extremities as in wild-type plants. We next observed that polysome association of nad7 mature mRNA is specifically disrupted in mtl1 mutants, indicating that the absence of Nad7 results from a lack of translation of nad7 mRNA. These findings illustrate that mitochondrial translation requires the intervention of gene-specific nucleus-encoded PPR trans-factors and that their action does not necessarily involve the 5' processing of their target mRNA, as observed previously. Interestingly, a partial decrease in nad7 intron 2 splicing was also detected in mtl1 mutants, suggesting that MTL1 is also involved in group II intron splicing. However, this second function appears to be less essential for nad7 expression than its role in translation. MTL1 will be instrumental to understand the multifunctionality of PPR proteins and the mechanisms governing mRNA translation and intron splicing in plant mitochondria.

Pentatricopeptide-repeat family protein RF6 functions with hexokinase 6 to rescue rice cytoplasmic male sterility

Cytoplasmic male sterility (CMS) has been extensively used for hybrid seed production in many major crops. Honglian CMS (HL-CMS) is one of the three major types of CMS in rice and has contributed greatly to food security worldwide. The HL-CMS trait is associated with an aberrant chimeric mitochondrial transcript, atp6-orfH79, which causes pollen sterility and can be rescued by two nonallelic restorer-of-fertility (Rf) genes, Rf5 or Rf6. Here, we report the identification of Rf6, which encodes a novel pentatricopeptide repeat (PPR) family protein with a characteristic duplication of PPR motifs 3-5. RF6 is targeted to mitochondria, where it physically associates with hexokinase 6 (OsHXK6) and promotes the processing of the aberrant CMS-associated transcript atp6-orfH79 at nucleotide 1238, which ensures normal pollen development and restores fertility. The duplicated motif 3 of RF6 is essential for RF6-OsHXK6 interactions, processing of the aberrant transcript, and restoration of fertility. Furthermore, reductions in the level of OsHXK6 result in atp6-orfH79 transcript accumulation and male sterility. Together these results reveal a novel mechanism for CMS restoration by which RF6 functions with OsHXK6 to restore HL-CMS fertility. The present study also provides insight into the function of hexokinase 6 in regulating mitochondrial RNA metabolism and may facilitate further exploitation of heterosis in rice.

A Member of the Arabidopsis Mitochondrial Transcription Termination Factor Family Is Required for Maturation of Chloroplast Transfer RNAIle(GAU)

Plastid gene expression is crucial for organelle function, but the factors that control it are still largely unclear. Members of the so-called mitochondrial transcription termination factor (mTERF) family are found in metazoans and plants and regulate organellar gene expression at different levels. Arabidopsis (Arabidopsis thaliana) mTERF6 is localized in chloroplasts and mitochondria, and its knockout perturbs plastid development and results in seedling lethality. In the leaky mterf6-1 mutant, a defect in photosynthesis is associated with reduced levels of photosystem subunits, although corresponding messenger RNA levels are unaffected, whereas translational capacity and maturation of chloroplast ribosomal RNAs (rRNAs) are perturbed in mterf6-1 mutants. Bacterial one-hybrid screening, electrophoretic mobility shift assays, and coimmunoprecipitation experiments reveal a specific interaction between mTERF6 and an RNA sequence in the chloroplast isoleucine transfer RNA gene (trnI.2) located in the rRNA operon. In vitro, recombinant mTERF6 bound to its plastid DNA target site can terminate transcription. At present, it is unclear whether disturbed rRNA maturation is a primary or secondary defect. However, it is clear that mTERF6 is required for the maturation of trnI.2. This points to an additional function of mTERFs.

Two Chlamydomonas OPR proteins stabilize chloroplast mRNAs encoding small subunits of photosystem II and cytochrome b6 f

In plants and algae, chloroplast gene expression is controlled by nucleus-encoded proteins that bind to mRNAs in a specific manner, stabilizing mRNAs or promoting their splicing, editing, or translation. Here, we present the characterization of two mRNA stabilization factors of the green alga Chlamydomonas reinhardtii, which both belong to the OctotricoPeptide Repeat (OPR) family. MCG1 is necessary to stabilize the petG mRNA, encoding a small subunit of the cytochrome b6 f complex, while MBI1 stabilizes the psbI mRNA, coding for a small subunit of photosystem II. In the mcg1 mutant, the small RNA footprint corresponding to the 5'-end of the petG transcript is reduced in abundance. In both cases, the absence of the small subunit perturbs assembly of the cognate complex. Whereas PetG is essential for formation of a functional cytochrome b6 f dimer, PsbI appears partly dispensable as a low level of PSII activity can still be measured in its absence. Thus, nuclear control of chloroplast gene expression is not only exerted on the major core subunits of the complexes, but also on small subunits with a single transmembrane helix. While OPR proteins have thus far been involved in translation or trans-splicing of plastid mRNAs, our results expand the potential roles of this repeat family to their stabilization.