Chloroplast Protein Synthesis Research Articles

Unveiling enhanced photosynthesis in tetraploid sour jujube through transcriptome analysis

ABSTRACT Polyploidization, which can potentially impact phenotypic traits and photosynthetic performance of plants, is an important goal in breeding. In this study, diploid sour jujube and autotetraploid ‘W219’ were examined to understand the effects of polyploidization on photosynthesis. By comparing the phenotypic and photosynthetic differences between ‘W219’ and diploid sour jujube, it was found that ‘W219’ exhibited increased leaf length, leaf width, leaf area, and plant height, as well as enhanced chlorophyll content and photosynthetic efficiency. To explore the molecular mechanisms underlying these variations, comparative transcriptome analysis was conducted between ‘W219’ and diploid, resulting in the identification of 2892 differentially expressed genes (DEGs), with 993 up-regulated and 1899 down-regulated in ‘W219’. GO enrichment analysis revealed that most DEGs were related to biological processes, primarily enriched in metabolic processes. Further KEGG pathway analysis revealed significant differences in photosynthetic pathways between ‘W219’ and diploid, mainly involving key processes such as photosystem II, photosynthetic electron transport, and chloroplast protein synthesis. This study revealed the internal mechanism of enhanced photosynthesis in autotetraploid sour jujube ‘W219’ through comprehensive analysis at the morphological, physiological, and transcriptomic levels, providing a basis for explaining its unique photosynthetic characteristics.

CLPB3 is required for the removal of chloroplast protein aggregates and for thermotolerance in Chlamydomonas.

In the cytosol of plant cells, heat-induced protein aggregates are resolved by ClpB/Hsp100 family member HSP101, which is essential for thermotolerance. For chloroplast family member CLPB3 this is less clear with controversial reports on its role in conferring thermotolerance. To shed light onto this issue, we have characterized two clpb3 mutants in Chlamydomonas reinhardtii. We show that chloroplast CLPB3 is required for resolving heat-induced protein aggregates containing stromal TIG1 and the small heat shock proteins HSP22E/F in vivo and for conferring thermotolerance under heat stress. Although CLPB3 accumulates to similarly high levels as stromal HSP70B under ambient conditions, we observed no prominent constitutive phenotypes. However, we found decreased accumulation of the ribosomal subunit PRPL1 and increased accumulation of the stromal protease DEG1C in the clpb3 mutants, suggesting that reduction in chloroplast protein synthesis capacity and increase in proteolytic capacity may compensate for loss of CLPB3 function. Under ambient conditions, CLPB3 was distributed throughout the chloroplast but reorganized into stromal foci upon heat stress, which mostly disappeared during recovery. CLPB3 foci were localized next to HSP22E/F, which accumulated largely to the thylakoid membrane occupied area. This suggests a possible role for CLPB3 in disentangling protein aggregates from the thylakoid membrane system.

Arabidopsis CIA2 and CIL have distinct and overlapping functions in regulating chloroplast and flower development.

Arabidopsis CHLOROPLAST IMPORT APPARATUS 2 (CIA2) and its paralogous protein CIA2‐LIKE (CIL) are nuclear transcription factors containing a C‐terminal CCT motif. CIA2 promotes the expression of nuclear genes encoding chloroplast‐localized translocons and ribosomal proteins, thereby increasing the efficiency of protein import and synthesis in chloroplasts. We have previously reported that CIA2 and CIL form a homodimer or heterodimer through their C‐terminal sequences and interact with other nuclear proteins, such as CONSTANS (CO), via their N‐terminal sequences, but the function of CIL had remained unclear. In this study, we verified through transgenic cia2 mutant plants expressing the CIL coding sequence that CIL is partially functionally redundant to CIA2 during vegetative growth. We also compared phenotypes and gene expression profiles of wildtype Col‐0, cia2, cil, and cia2/cil mutants. Our results indicate that CIA2 and CIL coordinate chloroplast biogenesis and function mainly by upregulating the expression of the nuclear factor GOLDEN2‐LIKE 1 (GLK1) and chloroplast transcription‐, translation‐, protein import‐, and photosynthesis‐related genes, with CIA2 playing a more crucial role. Furthermore, we compared flowering phenotypes in single, double, and triple mutant plants of co, cia2, and cil. We found that CIA2 and CIL participate in modulating long‐day floral development. Notably, CIA2 increases flower number and height of the inflorescence main axis, whereas CIL promotes flowering.

The versatile interactome of chloroplast ribosomes revealed by affinity purification mass spectrometry.

In plant cells, chloroplast gene expression is predominantly controlled through post-transcriptional regulation. Such fine-tuning is vital for precisely orchestrating protein complex assembly as for the photosynthesis machinery and for quickly responding to environmental changes. While regulation of chloroplast protein synthesis is of central importance, little is known about the degree and nature of the regulatory network, mainly due to challenges associated with the specific isolation of transient ribosome interactors. Here, we established a ribosome affinity purification method, which enabled us to broadly uncover putative ribosome-associated proteins in chloroplasts. Endogenously tagging of a protein of the large or small subunit revealed not only interactors of the holo complex, but also preferential interactors of the two subunits. This includes known canonical regulatory proteins as well as several new proteins belonging to the categories of protein and RNA regulation, photosystem biogenesis, redox control and metabolism. The sensitivity of the here applied screen was validated for various transiently interacting proteins. We further provided evidence for the existence of a ribosome-associated Nα-acetyltransferase in chloroplasts and its ability to acetylate substrate proteins at their N-terminus. The broad set of ribosome interactors underscores the potential to regulate chloroplast gene expression on the level of protein synthesis.

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 mechanism of regulation of photosystem I cross-section in the pennate diatom Phaeodactylum tricornutum.

Photosystems possess distinct fluorescence emissions at low (77K) temperature. PSI emits in the long-wavelength region at ~710-740 nm. In diatoms, a successful clade of marine primary producers, the contribution of PSI-associated emission (710-717 nm) has been shown to be relatively small. However, in the pennate diatom Phaeodactylum tricornutum, the source of the long-wavelength emission at ~710 nm (F710) remains controversial. Here, we addressed the origin and modulation of F710 fluorescence in this alga grown under continuous and intermittent light. The latter condition led to a strong enhancement in F710. Biochemical and spectral properties of the photosynthetic complexes isolated from thylakoid membranes were investigated for both culture conditions. F710 emission appeared to be associated with PSI regardless of light acclimation. To further assess whether PSII could also contribute to this emission, we decreased the concentration of PSII reaction centres and core antenna by growing cells with lincomycin, a chloroplast protein synthesis inhibitor. The treatment did not diminish F710 fluorescence. Our data suggest that F710 emission originates from PSI under the conditions tested and is enhanced in intermittent light-grown cells due to increased energy flow from the FCP antenna to PSI.

Red and blue light differentially impact retrograde signalling and photoprotection in rice.

Chloroplast-to-nucleus retrograde signalling (RS) is known to impact plant growth and development. In Arabidopsis, we and others have shown that RS affects seedling establishment by inhibiting deetiolation. In the presence of lincomycin, a chloroplast protein synthesis inhibitor that triggers RS, Arabidopsis light-grown seedlings display partial skotomorphogenesis with undeveloped plastids and closed cotyledons. By contrast, RS in monocotyledonous has been much less studied. Here, we show that emerging rice seedlings exposed to lincomycin do not accumulate chlorophyll but otherwise remain remarkably unaffected. However, by using high red (R) and blue (B) monochromatic lights in combination with lincomycin, we have uncovered a RS inhibition of length and a reduction in the B light-induced declination of the second leaf. Furthermore, we present data showing that seedlings grown in high B and R light display different non-photochemical quenching capacity. Our findings support the view that excess B and R light impact seedling photomorphogenesis differently to photoprotect and optimize the response to high-light stress. This article is part of the theme issue 'Retrograde signalling from endosymbiotic organelles'.

Singlet Oxygen in Plants: Generation, Detection, and Signaling Roles.

Singlet oxygen (1O2) refers to the lowest excited electronic state of molecular oxygen. It easily oxidizes biological molecules and, therefore, is cytotoxic. In plant cells, 1O2 is formed mostly in the light in thylakoid membranes by reaction centers of photosystem II. In high concentrations, 1O2 destroys membranes, proteins and DNA, inhibits protein synthesis in chloroplasts leading to photoinhibition of photosynthesis, and can result in cell death. However, 1O2 also acts as a signal relaying information from chloroplasts to the nucleus, regulating expression of nuclear genes. In spite of its extremely short lifetime, 1O2 can diffuse from the chloroplasts into the cytoplasm and the apoplast. As shown by recent studies, 1O2-activated signaling pathways depend not only on the levels but also on the sites of 1O2 production in chloroplasts, and can activate two types of responses, either acclimation to high light or programmed cell death. 1O2 can be produced in high amounts also in root cells during drought stress. This review summarizes recent advances in research on mechanisms and sites of 1O2 generation in plants, on 1O2-activated pathways of retrograde- and cellular signaling, and on the methods to study 1O2 production in plants.

Chloroplast genomic comparison of two sister species Allium macranthum and A. fasciculatum provides valuable insights into adaptive evolution.

Allium macranthum and Allium fasciculatum are two sister species and their natural populations are separated by high mountains and deep valleys with exact opposite habitat. The chloroplast genome in angiosperms has showed useful for investigating plant evolution and systematic studies. Comparative analysis of these genomes revealed potential markers and phylogenetic analysis, and discuss the influence of positive selected sites on adaptive evolution. Here, we sequenced the complete chloroplast genomes of these two species and analyzed the repeat sequences components, nucleotide diversity, selection pressure and the phylogeny relationships with related species. A typical quadripartite structure was detected with a genome size changed from 152,148 to 152,931bp. We identified 67 and 79 simple sequence repeats in A. macranthum and A. fasciculatum, in which the mono-nucleotide repeats A/T possess the highest percentage. Three mutational hotspots (rpl32, rps16 and matK) at the SSC and LSC regions were observed, which showed remarkably higher Pi value (> 0.03). Additionally, eight genes (rpoA, atpF, cemA, rps4, ccsA, rpoC2, rpl14 and clpP) exhibited elevated pairwise Ka/Ks ratios in alpine species. Phylogenetic analyses based on the CDS sequences and the whole complete genomes showed same topologies with high support, and A. macranthum was closely clustered with A. fasciculatum within the fourteen Amaryllidaceae species. Their coding proteins of these genes often functioned in chloroplast protein synthesis, gene transcription, energy transformation and regulation and photosynthesis. These results provide valuable insights into the alpine species adaptation and evolution.

PP7L is essential for MAIL1-mediated transposable element silencing and primary root growth.

The two paralogous Arabidopsis genes MAINTENANCE OF MERISTEMS (MAIN) and MAINTENANCE OF MERISTEMS LIKE1 (MAIL1) encode a conserved retrotransposon-related plant mobile domain and are known to be required for silencing of transposable elements (TE) and for primary root development. Loss of function of either MAIN or MAIL1 leads to release of heterochromatic TEs, reduced condensation of pericentromeric heterochromatin, cell death of meristem cells and growth arrest of the primary root soon after germination. Here, we show that they act in one protein complex that also contains the inactive isoform of PROTEIN PHOSPHATASE 7 (PP7), which is named PROTEIN PHOSPHATASE 7-LIKE (PP7L). PP7L was previously shown to be important for chloroplast biogenesis and efficient chloroplast protein synthesis. We show that loss of PP7L function leads to the same root growth phenotype as loss of MAIL1 or MAIN. In addition, pp7l mutants show similar silencing defects. Double mutant analyses confirmed that the three proteins act in the same molecular pathway. The primary root growth arrest, which is associated with cell death of stem cells and their daughter cells, is a consequence of genome instability. Our data demonstrate so far unrecognized functions of an inactive phosphatase isoform in a protein complex that is essential for silencing of heterochromatic elements and for maintenance of genome stability in dividing cells.

The zeaxanthin epoxidase is degraded along with the D1 protein during photoinhibition of photosystem II.

The xanthophyll zeaxanthin is synthesized in chloroplasts upon high light exposure of plants and serves central photoprotective functions. The reconversion of zeaxanthin to violaxanthin is catalyzed by the zeaxanthin epoxidase (ZEP). ZEP shows highest activity after short and moderate high light periods, but becomes gradually down‐regulated in response to increasing high light stress along with down‐regulation of photosystem II (PSII) activity. ZEP activity and ZEP protein levels were studied in response to high light stress in four plant species: Arabidopsis thaliana, Pisum sativum, Nicotiana benthamiana and Spinacia oleracea. In all species, ZEP protein was degraded during photoinhibition of PSII in parallel with the D1 protein of PSII. In the presence of streptomycin, an inhibitor of chloroplast protein synthesis, photoinhibition of PSII and ZEP activity as well as degradation of D1 and ZEP protein was strongly increased, indicating a close correlation of ZEP regulation with PSII photoinhibition and repair. The concomitant high light‐induced inactivation/degradation of ZEP and D1 prevents the reconversion of zeaxanthin during photoinhibition and repair of PSII. This regulation of ZEP activity supports a coordinated degradation of D1 and ZEP during photoinhibition/repair of PSII and an essential photoprotective function of zeaxanthin during the PSII repair cycle.

Cellulose defects in the Arabidopsis secondary cell wall promote early chloroplast development.

Lincomycin (LIN)-mediated inhibition of protein synthesis in chloroplasts prevents the greening of seedlings, represses the activity of photosynthesis-related genes in the nucleus, including LHCB1.2, and induces the phenylpropanoid pathway, resulting in the production of anthocyanins. In genomes uncoupled (gun) mutants, LHCB1.2 expression is maintained in the presence of LIN or other inhibitors of early chloroplast development. In a screen using concentrations of LIN lower than those employed to isolate gun mutants, we have identified happy on lincomycin (holi) mutants. Several holi mutants show an increased tolerance to LIN, exhibiting de-repressed LHCB1.2 expression and chlorophyll synthesis in seedlings. The mutations responsible were identified by whole-genome single-nucleotide polymorphism (SNP) mapping, and most were found to affect the phenylpropanoid pathway; however, LHCB1.2 expression does not appear to be directly regulated by phenylpropanoids, as indicated by the metabolic profiling of mutants. The most potent holi mutant is defective in a subunit of cellulose synthase encoded by IRREGULAR XYLEM3, and comparative analysis of this and other cell-wall mutants establishes a link between secondary cell-wall integrity and early chloroplast development, possibly involving altered ABA metabolism or sensing.

Relationship of GUN1 to FUG1 in chloroplast protein homeostasis.

GUN1 integrates retrograde signals in chloroplasts but the underlying mechanism is elusive. FUG1, a chloroplast translation initiation factor, and GUN1 are co-expressed at the transcriptional level, and FUG1 co-immunoprecipitates with GUN1. We used mutants of GUN1 (gun1-103) and FUG1 (fug1-3) to analyse their functional relationship at the physiological and system-wide level, the latter including transcriptome and proteome analyses. Absence of GUN1 aggravates the effects of decreased FUG1 levels on chloroplast protein translation, resulting in transiently more pronounced phenotypes regarding photosynthesis, leaf colouration, growth and cold acclimation. The gun1-103 mutation also enhances variegation in the var2 mutant, increasing the fraction of white sectors, while fug1-3 suppresses the var2 phenotype. The transcriptomes of fug1-3 and gun1-103 plants are very similar, but absence of GUN1 alone has almost no effect on protein levels, whereas steady-state levels of chloroplast proteins are markedly decreased in fug1-3. In fug1 gun1 double mutants, effects on transcriptomes and particularly on proteomes are enhanced. Our results show that GUN1 function becomes critical when chloroplast proteostasis is perturbed by decreased rates of synthesis (fug1) or degradation (var2) of chloroplast proteins, or by low temperatures. The functions of FUG1 and GUN1 appear to be related, corroborating the view that GUN1 helps to maintain chloroplast protein homeostasis (proteostasis).

Physiological and comparative proteomic analyses of saline-alkali NaHCO3-responses in leaves of halophyte Puccinellia tenuiflora

Soil alkalization imposes severe ion toxicity, osmotic stress, and high pH stress to plants, inhibiting their growth and productivity. NaHCO3 is a main component of alkaline soil. However, knowledge of the NaHCO3-responsive proteomic pattern of alkaligrass is still lacking. Alkaligrass (Puccinellia tenuiflora) is a monocotyledonous halophyte pasture widely distributed in the Songnen Plain in Northeastern China. This study aims to investigate the NaHCO3-responsive molecular mechanisms in the alkaligrass plants. An integrative approach including photosynthetic and redox physiology, and comparative proteomics was used. NaHCO3 decreased photosynthesis, but increased nonphotochemical quenching, increased membrane electrolyte leakage of alkaligrass, and increased proline and glycine betaine concentrations in leaves. In addition, the NaHCO3 stress increased Na+ concentration and decreased K+/Na+ ratio in leaves, while Ca2+ and Mg2+ concentrations were maintained, contributing to signaling and homeostasis of ion and enzyme activity. Furthermore, O2− generation rate and H2O2 concentration were increased, and the activities of ten antioxidant enzymes and antioxidant concentrations were changed in response to the NaHCO3 stress. Proteomics revealed 90 NaHCO3-responsive proteins, 54% of which were localized in chloroplasts. They were mainly involved in signaling, photosynthesis, stress and defense, carbohydrate and energy metabolism, as well as protein synthesis, processing and turnover. Some protein abundances did not correlate well with their activities, implying that the enzyme activities were affected by NaHCO3-induced post-translational modifications. To cope with the NaHCO3 stress, alkaligrass deployed multiple strategies, including triggering phospholipase D (PLD)-mediated Ca2+ signaling pathways, enhancing diverse reactive oxygen species (ROS) scavenging pathways, and regulating chloroplast protein synthesis and processing.

Quantitative Proteomics Reveals Common and Specific Responses of a Marine Diatom Thalassiosira pseudonana to Different Macronutrient Deficiencies.

Macronutrients such as nitrogen (N), phosphorus (P), and silicon (Si) are essential for the productivity and distribution of diatoms in the ocean. Responses of diatoms to a particular macronutrient deficiency have been investigated, however, we know little about their common or specific responses to different macronutrients. Here, we investigated the physiology and quantitative proteomics of a diatom Thalassiosira pseudonana grown in nutrient-replete, N-, P-, and Si-deficient conditions. Cell growth was ceased in all macronutrient deficient conditions while cell volume and cellular C content under P- and Si-deficiencies increased. Contents of chlorophyll a, protein and cellular N decreased in both N- and P-deficient cells but chlorophyll a and cellular N increased in the Si-deficient cells. Cellular P content increased under N- and Si-deficiencies. Proteins involved in carbon fixation and photorespiration were down-regulated under all macronutrient deficiencies while neutral lipid synthesis and carbohydrate accumulation were enhanced. Photosynthesis, chlorophyll biosynthesis, and protein biosynthesis were down-regulated in both N- and P-deficient cells, while Si transporters, light-harvesting complex proteins, chloroplastic ATP synthase, plastid transcription and protein synthesis were up-regulated in the Si-deficient cells. Our results provided insights into the common and specific responses of T. pseudonana to different macronutrient deficiencies and identified specific proteins potentially indicating a particular macronutrient deficiency.

Comparative Analyses of Chloroplast Genomes of Cucurbitaceae Species: Lights into Selective Pressures and Phylogenetic Relationships.

Cucurbitaceae is the fourth most important economic plant family with creeping herbaceous species mainly distributed in tropical and subtropical regions. Here, we described and compared the complete chloroplast genome sequences of ten representative species from Cucurbitaceae. The lengths of the ten complete chloroplast genomes ranged from 155,293 bp (C. sativus) to 158,844 bp (M. charantia), and they shared the most common genomic features. 618 repeats of three categories and 813 microsatellites were found. Sequence divergence analysis showed that the coding and IR regions were highly conserved. Three protein-coding genes (accD, clpP, and matK) were under selection and their coding proteins often have functions in chloroplast protein synthesis, gene transcription, energy transformation, and plant development. An unconventional translation initiation codon of psbL gene was found and provided evidence for RNA editing. Applying BI and ML methods, phylogenetic analysis strongly supported the position of Gomphogyne, Hemsleya, and Gynostemma as the relatively original lineage in Cucurbitaceae. This study suggested that the complete chloroplast genome sequences were useful for phylogenetic studies. It would also determine potential molecular markers and candidate DNA barcodes for coming studies and enrich the valuable complete chloroplast genome resources of Cucurbitaceae.

The DEAD-box RNA Helicase RH50 Is a 23S-4.5S rRNA Maturation Factor that Functionally Overlaps with the Plastid Signaling Factor GUN1.

DEAD-box RNA helicases (DBRHs) modulate RNA secondary structure, allowing RNA molecules to adopt the conformations required for interaction with their target proteins. RH50 is a chloroplast-located DBRH that colocalizes and is coexpressed with GUN1, a central factor in chloroplast-to-nucleus signaling. When combined with mutations that impair plastid gene expression (prors1-1, prpl11-1, prps1-1, prps21-1, prps17-1, and prpl24-1), rh50 and gun1 mutations evoke similar patterns of epistatic effects. These observations, together with the synergistic growth phenotype of the double mutant rh50-1 gun1-102, suggest that RH50 and GUN1 are functionally related and that this function is associated with plastid gene expression, in particular ribosome functioning. However, rh50-1 itself is not a gun mutant, although-like gun1-102-the rh50-1 mutation suppresses the down-regulation of nuclear genes for photosynthesis induced by the prors1-1 mutation. The RH50 protein comigrates with ribosomal particles, and is required for efficient translation of plastid proteins. RH50 binds to transcripts of the 23S-4.5S intergenic region and, in its absence, levels of the corresponding rRNA processing intermediate are strongly increased, implying that RH50 is required for the maturation of the 23S and 4.5S rRNAs. This inference is supported by the finding that loss of RH50 renders chloroplast protein synthesis sensitive to erythromycin and exposure to cold. Based on these results, we conclude that RH50 is a plastid rRNA maturation factor.

Quantitative proteomics of a B12 -dependent alga grown in coculture with bacteria reveals metabolic tradeoffs required for mutualism.

Summary The unicellular green alga Lobomonas rostrata requires an external supply of vitamin B12 (cobalamin) for growth, which it can obtain in stable laboratory cultures from the soil bacterium Mesorhizobium loti in exchange for photosynthate. We investigated changes in protein expression in the alga that allow it to engage in this mutualism.We used quantitative isobaric tagging (iTRAQ) proteomics to determine the L. rostrata proteome grown axenically with B12 supplementation or in coculture with M. loti. Data are available via ProteomeXchange (PXD005046).Using the related Chlamydomonas reinhardtii as a reference genome, 588 algal proteins could be identified. Enzymes of amino acid biosynthesis were higher in coculture than in axenic culture, and this was reflected in increased amounts of total cellular protein and several free amino acids. A number of heat shock proteins were also elevated. Conversely, photosynthetic proteins and those of chloroplast protein synthesis were significantly lower in L. rostrata cells in coculture. These observations were confirmed by measurement of electron transfer rates in cells grown under the two conditions.The results indicate that, despite the stability of the mutualism, L. rostrata experiences stress in coculture with M. loti, and must adjust its metabolism accordingly.

Interference with plastome gene expression and Clp protease activity in Arabidopsis triggers a chloroplast unfolded protein response to restore protein homeostasis.

Disruption of protein homeostasis in chloroplasts impairs the correct functioning of essential metabolic pathways, including the methylerythritol 4-phosphate (MEP) pathway for the production of plastidial isoprenoids involved in photosynthesis and growth. We previously found that misfolded and aggregated forms of the first enzyme of the MEP pathway are degraded by the Clp protease with the involvement of Hsp70 and Hsp100/ClpC1 chaperones in Arabidopsis thaliana. By contrast, the combined unfolding and disaggregating actions of Hsp70 and Hsp100/ClpB3 chaperones allow solubilization and hence reactivation of the enzyme. The repair pathway is promoted when the levels of ClpB3 proteins increase upon reduction of Clp protease activity in mutants or wild-type plants treated with the chloroplast protein synthesis inhibitor lincomycin (LIN). Here we show that LIN treatment rapidly increases the levels of aggregated proteins in the chloroplast, unleashing a specific retrograde signaling pathway that up-regulates expression of ClpB3 and other nuclear genes encoding plastidial chaperones. As a consequence, folding capacity is increased to restore protein homeostasis. This sort of chloroplast unfolded protein response (cpUPR) mechanism appears to be mediated by the heat shock transcription factor HsfA2. Expression of HsfA2 and cpUPR-related target genes is independent of GUN1, a central integrator of retrograde signaling pathways. However, double mutants defective in both GUN1 and plastome gene expression (or Clp protease activity) are seedling lethal, confirming that the GUN1 protein is essential for protein homeostasis in chloroplasts.

Lethal albinic seedling, encoding a threonyl-tRNA synthetase, is involved in development of plastid protein synthesis system in rice.

An albinic rice is caused by mutation of threonyl-tRNA synthetase, which is essential for plant development by stabilizing of NEP and PEP gene expressions and chloroplast protein synthesis. Chloroplast biogenesis and development depend on complex genetic mechanisms. Apart from their function in translation, aminoacyl-tRNA synthetases (aaRSs) play additional role in gene expression regulation, RNA splicing, and cytokine activity. However, their detailed functions in plant development are still poorly understood. We isolated a lethal albinic seedling (las) mutant in rice. Physiological and ultrastructural analysis of las mutant plants revealed weak chlorophyll fluorescence, negligible chlorophyll accumulation, and defective thylakoid membrane development. By map based cloning we determined that the LAS allele gene encodes threonyl-tRNA synthetase (ThrRS). LAS was constitutively expressed with relatively high level in leaves. NEP-dependent gene transcripts accumulated in the developing chloroplasts, while PEP-dependent transcripts were reduced in the las mutant. This result indicated that PEP activity was impaired. Chloroplast-encoded protein levels were sharply reduced in the las mutant. Biogenesis of chloroplast rRNAs (16S and 23S rRNA) was arrested, leading to impaired translation and protein synthesis. Together, our findings indicated that LAS is essential not only for chloroplast development by stabilizing the NEP and PEP gene expression, but also for protein synthesis and construction of the ribosome system in rice chloroplasts.