Identification of plastid envelope proteins required for import of protochlorophyllide oxidoreductase A into the chloroplast of barley

Regulation of chlorophyll biosynthesis by light-dependent acetylation of NADPH:protochlorophyll oxidoreductase A in Arabidopsis

Chlorophylls are the major pigments that harvest light energy during photosynthesis in plants. Although reactions in chlorophyll biogenesis have been largely known, little attention has been paid to the post-translational regulation mechanism of this process. In this study, we found that four lysine sites (K128/340/350/390) of NADPH:protochlorophyllide oxidoreductase A (PORA), which catalyzes the only light-triggered step in chlorophyll biosynthesis, were acetylated after dark-grown seedlings transferred to light via acetylomics analysis. Etiolated seedlings with K390 mutation of PORA had a lower greening rate and decreased PORA acetylation after illumination. Importantly, K390 of PORA was found extremely conserved in plants and cyanobacteria via bioinformatics analysis. We further demonstrated that the acetylation level of PORA was increased by exposing the dark-grown seedlings to the histone deacetylase (HDAC) inhibitor TSA. Thus, the HDACs probably regulate the acetylation of PORA, thereby controlling this non-histone substrate to catalyze the reduction of Pchlide to produce chlorophyllide, which provides a novel regulatory mechanism by which the plant actively tunes chlorophyll biosynthesis during the conversion from skotomorphogenesis to photomorphogenesis.

Photocatalysis as the 'master switch' of photomorphogenesis in early plant development

Enzymatic photocatalysis is seldom used in biology. Photocatalysis by light-dependent protochlorophyllide oxidoreductase (LPOR)-one of only a few natural light-dependent enzymes-is an exception, and is responsible for the conversion of protochlorophyllide to chlorophyllide in chlorophyll biosynthesis. Photocatalysis by LPOR not only regulates the biosynthesis of the most abundant pigment on Earth but it is also a 'master switch' in photomorphogenesis in early plant development. Following illumination, LPOR promotes chlorophyll production, plastid membranes are transformed and the photosynthetic apparatus is established. Given these remarkable, light-induced pigment and morphological changes, the LPOR-catalysed reaction has been extensively studied from catalytic, physiological and plant development perspectives, highlighting vital, and multiple, cellular roles of this intriguing enzyme. Here, we offer a perspective in which the link between LPOR photocatalysis and plant photomorphogenesis is explored. Notable breakthroughs in LPOR structural biology have uncovered the structural-mechanistic basis of photocatalysis. These studies have clarified how photon absorption by the pigment protochlorophyllide-bound in a ternary LPOR-protochlorophyllide-NADPH complex-triggers photocatalysis and a cascade of complex molecular and cellular events that lead to plant morphological changes. Photocatalysis is therefore the master switch responsible for early-stage plant development and ultimately life on Earth.

Responses of unicellular predators to cope with the phototoxicity of photosynthetic prey

Feeding on unicellular photosynthetic organisms by unicellular eukaryotes is the base of the aquatic food chain and evolutionarily led to the establishment of photosynthetic endosymbionts/organelles. Photosynthesis generates reactive oxygen species and damages cells; thus, photosynthetic organisms possess several mechanisms to cope with the stress. Here, we demonstrate that photosynthetic prey also exposes unicellular amoebozoan and excavates predators to photosynthetic oxidative stress. Upon illumination, there is a commonality in transcriptomic changes among evolutionarily distant organisms feeding on photosynthetic prey. One of the genes commonly upregulated is a horizontally transferred homolog of algal and plant genes for chlorophyll degradation/detoxification. In addition, the predators reduce their phagocytic uptake while accelerating digestion of photosynthetic prey upon illumination, reducing the number of photosynthetic cells inside the predator cells, as this also occurs in facultative endosymbiotic associations upon certain stresses. Thus, some mechanisms in predators observed here probably have been necessary for evolution of endosymbiotic associations.

Photosynthesis generates reactive oxygen species that can damage cells. Here, the authors show that unicellular predators of photosynthetic prey have shared responses to photosynthetic oxidative stress and these may also have been important for the evolution of endosymbiosis.

A Protochlorophyllide (Pchlide) a Oxygenase for Plant Viability

Higher plants contain a small, 5-member family of Rieske non-heme oxygenases that comprise the inner plastid envelope protein TIC55, phaeophorbide a oxygenasee (PAO), chlorophyllide a oxygenase (CAO), choline monooxygenase, and a 52 kDa protein (PTC52) associated with the precursor NADPH:protochlorophyllide (Pchlide) oxidoreductase A (pPORA) A translocon (PTC). Some of these chloroplast proteins have documented roles in chlorophyll biosynthesis (CAO) and degradation (PAO and TIC55), whereas the function of PTC52 remains unresolved. Biochemical evidence provided here identifies PTC52 as Pchlide a oxygenase of the inner plastid envelope linking Pchlide b synthesis to pPORA import. Protochlorophyllide b is the preferred substrate of PORA and its lack no longer allows pPORA import. The Pchlide b-dependent import pathway of pPORA thus operates in etiolated seedlings and is switched off during greening. Using dexamethasone-induced RNA interference (RNAi) we tested if PTC52 is involved in controlling both, pPORA import and Pchlide homeostasis in planta. As shown here, RNAi plants deprived of PTC52 transcript and PTC52 protein were unable to import pPORA and died as a result of excess Pchlide a accumulation causing singlet oxygen formation during greening. In genetic studies, no homozygous ptc52 knock-out mutants could be obtained presumably as a result of embryo lethality, suggesting a role for PTC52 in the initial greening of plant embryos. Phylogenetic studies identified PTC52-like genes amongst unicellular photosynthetic bacteria and higher plants, suggesting that the biochemical function associated with PTC52 may have an ancient evolutionary origin. PTC52 also harbors conserved motifs with bacterial oxygenases such as the terminal oxygenase component of 3-ketosteroid 9-alpha-hydroxylase (KshA) from Rhodococcus rhodochrous. 3D-modeling of PTC52 structure permitted the prediction of amino acid residues that contribute to the substrate specificity of this enzyme. In vitro-mutagenesis was used to test the predicted PTC52 model and provide insights into the reaction mechanism of this Rieske non-heme oxygenase.

Influence of light intensity on chloroplast development and pigment accumulation in the wild-type and etiolated mutant plants of Anthurium andraeanum 'Sonate'

Seedlings of wild-type and etiolate mutant plants of Anthurium andraeanum cultivar 'Sonate' were treated for 15 d with different light intensities (20, 100, and 400 µmol·m^-2·s^-1) to analyze leaf plastid development and pigment content. Significant changes appeared in treated seedlings, including in leaf color, plastid ultrastructure, chloroplast development gene AaGLK expression, chlorophyll and anthocyanin contents, and protoplast shape. Wild-type and etiolated plants exhibited different plastid structures under the same light condition. The results suggest that light intensity is a crucial environmental factor influencing plastid development and leaf color formation in the A. andraeanum cultivar 'Sonate'.

Roles and maturation of iron-sulfur proteins in plastids

One reason why iron is an essential element for most organisms is its presence in prosthetic groups such as hemes or iron–sulfur (Fe–S) clusters, which are notably required for electron transfer reactions. As an organelle with an intense metabolism in plants, chloroplast relies on many Fe–S proteins. This includes those present in the electron transfer chain which will be, in fact, essential for most other metabolic processes occurring in chloroplasts, e.g., carbon fixation, nitrogen and sulfur assimilation, pigment, amino acid, and vitamin biosynthetic pathways to cite only a few examples. The maturation of these Fe–S proteins requires a complex and specific machinery named SUF (sulfur mobilisation). The assembly process can be split in two major steps, (1) the de novo assembly on scaffold proteins which requires ATP, iron and sulfur atoms, electrons, and thus the concerted action of several proteins forming early acting assembly complexes, and (2) the transfer of the preformed Fe–S cluster to client proteins using a set of late-acting maturation factors. Similar machineries, having in common these basic principles, are present in the cytosol and in mitochondria. This review focuses on the currently known molecular details concerning the assembly and roles of Fe–S proteins in plastids.

Thermophilic bacteria are potential sources of novel Rieske non-heme iron oxygenases

Rieske non-heme iron oxygenases, which have a Rieske-type [2Fe-2S] cluster and a non-heme catalytic iron center, are an important family of oxidoreductases involved mainly in regio- and stereoselective transformation of a wide array of aromatic hydrocarbons. Though present in all domains of life, the most widely studied Rieske non-heme iron oxygenases are found in mesophilic bacteria. The present study explores the potential for isolating novel Rieske non-heme iron oxygenases from thermophilic sources. Browsing the entire bacterial genome database led to the identification of 45 homologs from thermophilic bacteria distributed mainly among Chloroflexi, Deinococcus-Thermus and Firmicutes. Thermostability, measured according to the aliphatic index, showed higher values for certain homologs compared with their mesophilic relatives. Prediction of substrate preferences indicated that a wide array of aromatic hydrocarbons could be transformed by most of the identified oxygenase homologs. Further identification of putative genes encoding components of a functional oxygenase system opens up the possibility of reconstituting functional thermophilic Rieske non-heme iron oxygenase systems with novel properties.

NADPH:protochlorophyllide oxidoreductase B (PORB) action in Arabidopsis thaliana revisited through transgenic expression of engineered barley PORB mutant proteins

NADPH:protochlorophyllide oxidoreductase (POR) is a key enzyme for the light-induced greening of etiolated angiosperm plants. It belongs to the 'RED' family of reductases, epimerases and dehydrogenases. All POR proteins characterized so far contain evolutionarily conserved cysteine residues implicated in protochlorophyllide (Pchlide)-binding and catalysis. cDNAs were constructed by site-directed mutagenesis that encode PORB mutant proteins with defined Cys→Ala exchanges. These cDNAs were expressed in transgenic plants of a PORB-deficient knock-out mutant (porB) of Arabidopsis thaliana. Results show that porB plants expressing PORB mutant proteins with Ala substitutions of Cys276 or Cys303 are hypersensitive to high-light conditions during greening. Hereby, failure to assemble higher molecular weight complexes of PORB with its twin isoenzyme, PORA, as encountered with (Cys303→Ala)-PORB plants, caused more severe effects than replacing Cys276 by an Ala residue in the active site of the enzyme, as encountered in (Cys276→Ala)-PORB plants. Our results are consistent with the presence of two distinct pigment binding sites in PORB, with Cys276 establishing the active site of the enzyme and Cys303 providing a second, low affinity pigment binding site that is essential for the assembly of higher molecular mass light-harvesting PORB::PORA complexes and photoprotection of etiolated seedlings. Failure to assemble such complexes provoked photodynamic damage through the generation of singlet oxygen. Together, our data highlight the importance of PORB for Pchlide homoeostasis and greening in Arabidopsis.

Dual Protein Localization to the Envelope and Thylakoid Membranes Within the Chloroplast

The chloroplast houses various metabolic processes essential for plant viability. This organelle originated from an ancestral cyanobacterium via endosymbiosis and maintains the three membranes of its progenitor. Among them, the outer envelope membrane functions mainly in communication with cytoplasmic components while the inner envelope membrane houses selective transport of various metabolites and the biosynthesis of several compounds, including membrane lipids. These two envelope membranes also play essential roles in import of nuclear-encoded proteins and in organelle division. The third membrane, the internal membrane system known as the thylakoid, houses photosynthetic electron transport and chemiosmotic phosphorylation. The inner envelope and thylakoid membranes share similar lipid composition. Specific targeting pathways determine their defined proteomes and, thus, their distinct functions. Nonetheless, several proteins have been shown to exist in both the envelope and thylakoid membranes. These proteins include those that play roles in protein transport, tetrapyrrole biosynthesis, membrane dynamics, or transport of nucleotides or inorganic phosphate. In this review, we summarize the current knowledge about proteins localized to both the envelope and thylakoid membranes in the chloroplast, discussing their roles in each membrane and potential mechanisms of their dual localization. Addressing the unanswered questions about these dual-localized proteins should help advance our understanding of chloroplast development, protein transport, and metabolic regulation.

Light intensity affects chlorophyll synthesis during greening process by metabolite signal from mitochondrial alternative oxidase in Arabidopsis

Although mitochondrial alternative oxidase (AOX) has been proposed to play essential roles in high light stress tolerance, the effects of AOX on chlorophyll synthesis are unclear. Previous studies indicated that during greening, chlorophyll accumulation was largely delayed in plants whose mitochondrial cyanide-resistant respiration was inhibited by knocking out nuclear encoded AOX gene. Here, we showed that this delay of chlorophyll accumulation was more significant under high light condition. Inhibition of cyanide-resistant respiration was also accompanied by the increase of plastid NADPH/NADP(+) ratio, especially under high light treatment which subsequently blocked the import of multiple plastidial proteins, such as some components of the photosynthetic electron transport chain, the Calvin-Benson cycle enzymes and malate/oxaloacetate shuttle components. Overexpression of AOX1a rescued the aox1a mutant phenotype, including the chlorophyll accumulation during greening and plastidial protein import. It thus suggests that light intensity affects chlorophyll synthesis during greening process by a metabolic signal, the AOX-derived plastidial NADPH/NADP(+) ratio change. Further, our results thus revealed a molecular mechanism of chloroplast-mitochondria interactions.

Common functions of the chloroplast and mitochondrial co-chaperones cpDnaJL (CDF1) and mtDnaJ (PAM16) in protein import and ROS scavenging in Arabidopsis thaliana

As semi-autonomous cell organelles that contain only limited coding information in their own DNA, chloroplasts and mitochondria must import the vast majority of their protein constituents from the cytosol. Respective protein import machineries have been identified that mediate the uptake of chloroplast and mitochondrial proteins and interact with molecular chaperones of the HEAT-SHOCK PROTEIN (HSP) 70 family operating as import motors. Recent work identified unexpected new functions of 2 DnaJ co-chaperones in mitochondrial and chloroplast protein translocation and suggest a common mechanism of reactive oxygen species (ROS) scavenging that shall be discussed here.

Cell growth defect factor 1 is crucial for the plastid import of NADPH:protochlorophyllide oxidoreductase A in Arabidopsis thaliana

Tetrapyrroles such as chlorophyll, heme, and bacteriochlorophyll play fundamental roles in the energy absorption and transduction of all photosynthetic organisms. They are synthesized via a complex pathway taking place in chloroplasts. Chlorophyll biosynthesis in angiosperms involves 16 steps of which only one is light-requiring and driven by the NADPH:protochlorophyllide oxidoreductase (POR). Three POR isoforms have been identified in Arabidopsis thaliana--designated PORA, PORB, and PORC--that are differentially expressed in etiolated, light-exposed, and light-adapted plants. All three isoforms are encoded by nuclear genes, are synthesized as larger precursors in the cytosol (pPORs), and are imported posttranslationally into the plastid compartment. Import of the precursor to the dark-specific isoform PORA (pPORA) is protochlorophyllide (Pchlide)-dependent and due to the operation of a unique translocon complex dubbed PTC (Pchlide-dependent translocon complex) in the plastid envelope. Here, we identified a ∼30-kDa protein that participates in pPORA import. The ∼30-kDa protein is identical to the previously identified CELL GROWTH DEFECT FACTOR 1 (CDF1) in Arabidopsis that is conserved in higher plants and Synechocystis. CDF1 operates in pPORA import and stabilization and hereby acts as a chaperone for PORA protein translocation. CDF1 permits tight interactions between Pchlide synthesized in the plastid envelope and the importing PORA polypeptide chain such that no photoexcitative damage occurs through the generation of singlet oxygen operating as a cell death inducer. Together, our results identify an ancient mechanism dating back to the endosymbiotic origin of chloroplasts as a key element of Pchlide-dependent pPORA import.

Three proteins mediate import of transit sequence-less precursors into the inner envelope of chloroplasts in Arabidopsis thaliana

A family of 17 putative preprotein and amino acid transporters designated PRAT has been identified in Arabidopsis thaliana, comprising PRAT proteins in mitochondria and chloroplasts. Although some PRAT proteins, such as the translocon of the mitochondrial inner membrane (TIM) proteins TIM22 and TIM23, play decisive roles for the translocation and import of mitochondrial inner membrane proteins, little is known about the role of the different PRAT members in chloroplasts. Here we report the identification of three distinct PRAT proteins as part of a unique protein import site. One of the identified PRAT proteins is identical with a previously characterized hypothetical protein (HP) of 20 kDa designated HP20 of the outer plastid envelope membrane. The second PRAT component is represented by HP30, and the third is identical to HP30-2, a close relative of HP30. Both HP30 and HP30-2 are inner plastid envelope membrane proteins of chloroplasts. Using biochemical, cell biological, and genetic approaches we demonstrate that all three PRAT proteins cooperate during import of transit sequence-less proteins, such as the quinone oxidoreductase homolog ceQORH used as model, into the inner chloroplast envelope membrane. Our data are reminiscent of findings reported for the TIM22 translocase, which is involved in the import of carrier proteins and other, hydrophobic membrane proteins lacking cleavable transit sequences into the inner mitochondrial membrane. Together our results establish the PRAT family as a widely used system of protein translocases in different membranes of endosymbiotic origin.

The Plastid Outer Envelope - A Highly Dynamic Interface between Plastid and Cytoplasm

Plastids are the defining organelles of all photosynthetic eukaryotes. They are the site of photosynthesis and of a large number of other essential metabolic pathways, such as fatty acid and amino acid biosyntheses, sulfur and nitrogen assimilation, and aromatic and terpenoid compound production, to mention only a few examples. The metabolism of plastids is heavily intertwined and connected with that of the surrounding cytosol, thus causing massive traffic of metabolic precursors, intermediates, and products. Two layers of biological membranes that are called the inner (IE) and the outer (OE) plastid envelope membranes bound the plastids of Archaeplastida. While the IE is generally accepted as the osmo-regulatory barrier between cytosol and stroma, the OE was considered to represent an unspecific molecular sieve, permeable for molecules of up to 10 kDa. However, after the discovery of small substrate specific pores in the OE, this view has come under scrutiny. In addition to controlling metabolic fluxes between plastid and cytosol, the OE is also crucial for protein import into the chloroplast. It contains the receptors and translocation channel of the TOC complex that is required for the canonical post-translational import of nuclear-encoded, plastid-targeted proteins. Further, the OE is a metabolically active compartment of the chloroplast, being involved in, e.g., fatty acid metabolism and membrane lipid production. Also, recent findings hint on the OE as a defense platform against several biotic and abiotic stress conditions, such as cold acclimation, freezing tolerance, and phosphate deprivation. Moreover, dynamic non-covalent interactions between the OE and the endomembrane system are thought to play important roles in lipid and non-canonical protein trafficking between plastid and endoplasmic reticulum. While proteomics and bioinformatics has provided us with comprehensive but still incomplete information on proteins localized in the plastid IE, the stroma, and the thylakoids, our knowledge of the protein composition of the plastid OE is far from complete. In this article, we report on the recent progress in discovering novel OE proteins to draw a conclusive picture of the OE. A "parts list" of the plastid OE will be presented, using data generated by proteomics of plastids isolated from various plant sources.

Implication of the oep16-1 mutation in a flu-independent, singlet oxygen-regulated cell death pathway in Arabidopsis thaliana

Singlet oxygen is a prominent form of reactive oxygen species in higher plants. It is easily formed from molecular oxygen by triplet-triplet interchange with excited porphyrin species. Evidence has been obtained from studies on the flu mutant of Arabidopsis thaliana of a genetically determined cell death pathway that involves differential changes at the transcriptome level. Here we report on a different cell death pathway that can be deduced from the analysis of oep16 mutants of A. thaliana. Pure lines of four independent OEP16-deficient mutants with different cell death properties were isolated. Two of the mutants overproduced free protochlorophyllide (Pchlide) in the dark because of defects in import of NADPH:Pchlide oxidoreductase A (pPORA) and died after illumination. The other two mutants avoided excess Pchlide accumulation. Using pulse labeling and polysome profiling studies we show that translation is a major site of cell death regulation in flu and oep16 plants. flu plants respond to photooxidative stress triggered by singlet oxygen by reprogramming their translation toward synthesis of key enzymes involved in jasmonic acid synthesis and stress proteins. In contrast, those oep16 mutants that were prone to photooxidative damage were unable to respond in this way. Together, our results show that translation is differentially affected in the flu and oep16 mutants in response to singlet oxygen.

Uptake and partitioning of amino acids and peptides

Plant growth, productivity, and seed yield depend on the efficient uptake, metabolism, and allocation of nutrients. Nitrogen is an essential macronutrient needed in high amounts. Plants have evolved efficient and selective transport systems for nitrogen uptake and transport within the plant to sustain development, growth, and finally reproduction. This review summarizes current knowledge on membrane proteins involved in transport of amino acids and peptides. A special emphasis was put on their function in planta. We focus on uptake of the organic nitrogen by the root, source-sink partitioning, and import into floral tissues and seeds.

Chlorophyll biosynthesis: spotlight on protochlorophyllide reduction

Photosynthetic organisms require chlorophyll or bacteriochlorophyll for their light trapping and energy transduction activities. The biosynthetic pathways of chlorophyll and bacteriochlorophyll are similar in most of their early steps, except for the reduction of protochlorophyllide (Pchlide) to chlorophyllide. Whereas angiosperms make use of a light-dependent enzyme, cyanobacteria, algae, bryophytes, pteridophytes and gymnosperms contain an additional, light-independent enzyme dubbed dark-operative Pchlide oxidoreductase (DPOR). Anoxygenic photosynthetic bacteria such as Rhodobacter capsulatus and Rhodobacter sphaeroides rely solely on DPOR. Recent atomic resolution of reductase and catalytic components of DPOR from R. sphaeroides and R. capsulatus, respectively, have revealed their similarity to nitrogenase components. In this review, we discuss the two fundamentally different mechanisms of Pchlide reduction in photosynthetic organisms.

Etioplast and etio-chloroplast formation under natural conditions: the dark side of chlorophyll biosynthesis in angiosperms

Chloroplast development is usually regarded as proceeding from proplastids. However, direct or indirect conversion pathways have been described in the literature, the latter involving the etioplast or the etio-chloroplast stages. Etioplasts are characterized by the absence of chlorophylls (Chl-s) and the presence of a unique inner membrane network, the prolamellar body (PLB), whereas etio-chloroplasts contain Chl-s and small PLBs interconnected with chloroplast thylakoids. As etioplast development requires growth in darkness for several days, this stage is generally regarded as a nonnatural pathway of chloroplast development occurring only under laboratory conditions. In this article, we have reviewed the data in favor of the involvement of etioplasts and etio-chloroplasts as intermediary stage(s) in chloroplast formation under natural conditions, the molecular aspects of PLB formation and we propose a dynamic model for its regulation.

Overexpression of the CBF2 transcriptional activator in Arabidopsis delays leaf senescence and extends plant longevity

Leaf senescence is a programmed developmental process governed by various endogenous and exogenous factors, such as the plant developmental stage, leaf age, phytohormone levels, darkness, and exposure to stresses. It was found that, in addition to its well-documented role in the enhancement of plant frost tolerance, overexpression of the C-repeat/dehydration responsive element binding factor 2 (CBF2) gene in Arabidopsis delayed the onset of leaf senescence and extended the life span of the plants by approximately 2 weeks. This phenomenon was exhibited both during developmental leaf senescence and during senescence of detached leaves artificially induced by either darkness or phytohormones. Transcriptome analysis using the Affymetrix ATH1 genome array revealed that overexpression of CBF2 significantly influenced the expression of 286 genes in mature leaf tissue. In addition to 30 stress-related genes, overexpression of CBF2 also affected the expression of 24 transcription factor (TF) genes, and 20 genes involved in protein metabolism, degradation, and post-translational modification. These results indicate that overexpression of CBF2 not only increases frost tolerance, but also affects other developmental processes, most likely through interactions with additional TFs and protein modification genes. The present findings shed new light on the crucial relationship between plant stress tolerance and longevity, as reported for other eukaryotic organisms.

In vivo studies on the roles of Tic55-related proteins in chloroplast protein import in Arabidopsis thaliana

The Tic55 (Translocon at the inner envelope membrane of chloroplasts, 55 kDa) protein was identified in pea as a putative regulator, possibly linking chloroplast protein import to the redox state of the photosynthetic machinery. Two Tic55 homologs have been proposed to exist in Arabidopsis: atTic55-II and AtPTC52 (Protochlorophyllide-dependent Translocon Component, 52 kDa; has also been called atTic55-IV). Our phylogenetic analysis shows that atTic55-II is an ortholog of psTic55 from pea (Pisum sativum), and that AtPTC52 is a more distant homolog of the two. AtPTC52 was included in this study to rule out possible functional links between the proteins in Arabidopsis. No detectable mutant phenotypes were found in two independent T-DNA knockout mutant plant lines for each Arabidopsis protein, when compared with wild-type: visible appearance, chlorophyll content, photosynthetic performance, and chloroplast protein import, for example, were all normal. Both wild-type and tic55-II mutant chloroplasts exhibited deficient protein import when treated with diethylpyrocarbonate, indicating that Tic55 is not the sole target of this reagent in relation to protein import. Furthermore, ptc52 mutant chloroplasts were not defective with respect to pPORA import, which was previously reported to involve PTC52 in barley. Thus, we conclude that atTic55-II and AtPTC52 are not strictly required for functional protein import in Arabidopsis.

A search for factors influencing etioplast-chloroplast transition

Chloroplast biogenesis in angiosperm plants requires the light-dependent transition from an etioplast stage. A key factor in this process is NADPH:protochlorophyllide oxidoreductase A (PORA), which catalyzes the light-dependent reduction of protochlorophyllide to chlorophyllide. In a recent study the chloroplast outer envelope channel OEP16 was described to be involved in etioplast to chloroplast transition by forming the translocation pore for the precursor protein of PORA [Pollmann et al. (2007) Proc Natl Acad Sci USA 104:2019-2023]. This hypothesis was based on the finding that a single OEP16.1 knockout mutant in Arabidopsis thaliana was severely affected during seedling de-etiolation and PORA protein was absent in etioplasts. In contrast, in our study the identical T-DNA insertion line greened normally and showed normal etioplast to chloroplast transition, and mature PORA was present in etioplasts [Philippar et al. (2007) Proc Natl Acad Sci USA 104:678-683]. To address these conflicting results regarding the function of OEP16.1 for PORA import, we analyzed several lines segregating from the original OEP16.1 T-DNA insertion line. Thereby we can unequivocally show that the loss of OEP16.1 neither correlates with impaired PORA import nor causes the observed de-etiolation phenotype. Furthermore, we found that the mutant line contains at least 2 additional T-DNA insertions in the genes for the extracellular polygalacturonase converter AroGP1 and the plastid-localized chorismate mutase CM1. However, detailed examination of the de-etiolation phenotype and a genomewide transcriptional analysis revealed no direct influence of these genes on etioplast to chloroplast transition in Arabidopsis cotyledons.

Revaluating the evolution of the Toc and Tic protein translocons

The origin of the plastid from a cyanobacterial endosymbiont necessitated the establishment of specialized molecular machines (translocons) to facilitate the import of nuclear-encoded proteins into the organelle. To improve our understanding of the evolution of the translocons at the outer and inner envelope membrane of chloroplasts (Toc and Tic, respectively), we critically reassess the prevalent notion that their subunits have a function exclusive to protein import. We propose that many translocon components are multifunctional, conserving ancestral pre-endosymbiotic properties that predate their recruitment into the primitive translocon (putatively composed of subunits Toc34, Toc75 and Tic110 and associated chaperones). Multifunctionality seems to be a hallmark of the Tic complex, in which protein import is integrated with a broad array of plastid processes.

A pentapeptide motif related to a pigment binding site in the major light-harvesting protein of photosystem II, LHCII, governs substrate-dependent plastid import of NADPH:protochlorophyllide oxidoreductase A

NADPH:protochlorophyllide (Pchlide) oxidoreductase (POR) A is the only known example thus far of a nucleus-encoded plastid protein that is imported to its final destination in a substrate-dependent, Pchlide-regulated manner. Previous work has shown that the cytosolic PORA precursor (pPORA) does not utilize the general import site but uses a distinct translocon designated the Pchlide-dependent translocon complex. Here we demonstrate that a pentapeptide motif, threonine-threonine-serine-proline-glycine (TTSPG) in pPORA's transit peptide (transA), is involved in Pchlide-dependent transport. Deletion of this motif from the COOH-terminal end of transA abolished both Pchlide binding and protein import. Incorporation of the TTSPG motif into normally non-Pchlide-responsive transit sequences conferred the pigment binding properties onto the engineered chimeric precursors but was insufficient to render protein import substrate dependent. An additional motif was identified in the NH(2)-terminal part of transA that was needed for binding of the precursor to the Pchlide-dependent translocon complex. Point mutations of the TTSPG motif identified glycine as the Pchlide binding site. By analogy to the major light-harvesting chlorophyll a/b binding protein of photosystem II, we propose that the peptidyl carbonyl oxygen of glycine may bind directly or via a water molecule to the central Mg atom of the pigment.

Loss of the N-terminal domain of chlorophyllide a oxygenase induces photodamage during greening of Arabidopsis seedlings

BACKGROUND

Chlorophyll b is a major photosynthetic pigment in green plants that is synthesized by chlorophyllide a oxygenase (CAO). The regulation of chlorophyll b biosynthesis is an important determinant for the antenna size of photosystems. Chlorophyll b synthesis is partly regulated on a transcriptional level by the expression of the CAO gene. In addition, the synthesis of chlorophyll b is strictly regulated on a protein level by the stability of the CAO enzyme. CAO consists of three domains, which are sequentially named from the N terminus as the A, B and C domains. The A domain of CAO participates in the regulation of the CAO protein stability.

RESULTS

In order to clarify the physiological function of the A domain, we constructed transgenic Arabidopsis (Arabidopsis thaliana) plants which either overexpressed the complete CAO or a truncated version of CAO lacking the A domain. The transgenic plants overexpressing the A-domain-deleted CAO accumulated an excess amount of chlorophyll b during greening. The transgenic plants which lacked the A domain either died or were obviously retarded when they were exposed to continuous light immediately after etiolation. In addition, the loss of the A domain in CAO impaired another step of chlorophyll biosynthesis, namely the conversion of divinyl-protochlorophyllide a to monovinyl protochlorophyllide a under dark conditions.

CONCLUSION

The A domain of CAO regulates the level of CAO, and thus prevents the excess accumulation of chlorophyll b. This function of the A domain is especially important during the greening stage of etiolated seedlings. At this stage, the plants are vulnerable to photodamages which could be caused by excessive chlorophyll b accumulation. In addition, de-regulation of the CAO level affects monovinyl-protochlorophyllide biosynthesis in darkness by unknown mechanisms. In conclusion, the A domain of CAO is essential in the control of chlorophyll biosynthesis and in the survival of seedlings during de-etiolation especially under strong illumination.

Recent overview of the Mg branch of the tetrapyrrole biosynthesis leading to chlorophylls

In plants, chlorophylls (chlorophyll a and chlorophyll b) are the most abundant tetrapyrrole molecules and are essential for photosynthesis. The first committed step of chlorophyll biosynthesis is the insertion of Mg(2+) into protoporphyrin IX, and thus subsequent steps of the biosynthesis are called the Mg branch. As the Mg branch in higher plants is complex, it was not until the last decade--after many years of intensive research--that most of the genes encoding the enzymes for the pathway were identified. Biochemical and molecular genetic analyses have certainly modified the classic metabolic map of tetrapyrrole biosynthesis, and only recently have the molecular mechanisms of regulatory pathways governing chlorophyll metabolism been elucidated. As a result, novel functions of tetrapyrroles and biosynthetic enzymes have been proposed. In this review, I summarize the recent findings on enzymes involved in the Mg branch, mainly in higher plants.

Three thioredoxin targets in the inner envelope membrane of chloroplasts function in protein import and chlorophyll metabolism

Thioredoxins (Trxs) are ubiquitous small proteins with a redox-active disulfide bridge. In their reduced form, they constitute very efficient protein disulfide oxidoreductases. In chloroplasts, two types of Trxs (f and m) coexist and play central roles in the regulation of the Calvin cycle and other processes. Here, we identified a class of Trx targets in the inner plastid envelope membrane of chloroplasts that share a CxxC motif approximately 73 aa from their carboxyl-terminal end. Members of this group belong to a superfamily of Rieske iron-sulfur proteins involved in protein translocation and chlorophyll metabolism. These proteins include the protein translocon protein TIC55, the precursor NADPH:protochlorophyllide oxidoreductase translocon protein PTC52, which operates as protochlorophyllide a-oxygenase, and the lethal leaf spot protein LLS1, which is identical with pheophorbide a oxygenase. The role of these proteins in dark/light regulation and oxidative control by the Trx system is discussed.

Toc159- and Toc75-independent import of a transit sequence-less precursor into the inner envelope of chloroplasts

Chloroplast envelope quinone oxidoreductase (ceQORH) is an inner plastid envelope protein that is synthesized without cleavable chloroplast transit sequence for import. In the present work, we studied the in vitro-import characteristics of Arabidopsis ceQORH. We demonstrate that ceQORH import requires ATP and is dependent on proteinaceous receptor components exposed at the outer plastid surface. Competition experiments using small subunit precursor of ribulose-bisphosphate carboxylase/oxygenase and precursor of ferredoxin, as well as antibody blocking experiments, revealed that ceQORH import does not involve the main receptor and translocation channel proteins Toc159 and Toc75, respectively, which operate in import of proteins into the chloroplast. Molecular dissection of the ceQORH amino acid sequence by site-directed mutagenesis and subsequent import experiments in planta and in vitro highlighted that ceQORH consists of different domains that act concertedly in regulating import. Collectively, our results provide unprecedented evidence for the existence of a specific import pathway for transit sequence-less inner plastid envelope membrane proteins into chloroplasts.

A substrate-independent, 14:3:3 protein-mediated plastid import pathway of NADPH:protochlorophyllide oxidoreductase A

Plastids are semiautonomous organelles that contain only limited coding information in their own DNA. Because most of their genome was transferred to the nucleus after their endosymbiotic origin, plastids must import the major part of their protein constituents from the cytosol. The exact role of cytosolic targeting factors in the regulation of plastid protein import has not been determined. Here, we report that the nucleus-encoded NADPH:protochlorophyllide (Pchlide) oxidoreductase A plastid precursor (pPORA) can use two different plastid import pathways that differ by the requirements for cytosolic 14:3:3 proteins and Hsp70. pPORA synthesized in a wheat germ lysate segregated into different precursor fractions. While import of free pPORA and only Hsp70-complexed pPORA was Pchlide-dependent and involved the previously identified Pchlide-dependent translocon, 14:3:3 protein- and Hsp70-complexed pPORA was transported into Pchlide-free chloroplasts through the Toc75-containing standard translocon at the outer chloroplast membrane/translocon at the inner chloroplast membrane machinery. A 14:3:3 protein binding site was identified in the mature region of the (35)S-pPORA, which governed 14:3:3 protein- and Hsp70-mediated, Pchlide-independent plastid import. Collectively, our results reveal that the import of pPORA into the plastids is tightly regulated and involves different cytosolic targeting factors and plastid envelope translocon complexes.

Chloroplast envelope membranes: a dynamic interface between plastids and the cytosol

Chloroplasts are bounded by a pair of outer membranes, the envelope, that is the only permanent membrane structure of the different types of plastids. Chloroplasts have had a long and complex evolutionary past and integration of the envelope membranes in cellular functions is the result of this evolution. Plastid envelope membranes contain a wide diversity of lipids and terpenoid compounds serving numerous biochemical functions and the flexibility of their biosynthetic pathways allow plants to adapt to fluctuating environmental conditions (for instance phosphate deprivation). A large body of knowledge has been generated by proteomic studies targeted to envelope membranes, thus revealing an unexpected complexity of this membrane system. For instance, new transport systems for metabolites and ions have been identified in envelope membranes and new routes for the import of chloroplast-specific proteins have been identified. The picture emerging from our present understanding of plastid envelope membranes is that of a key player in plastid biogenesis and the co-ordinated gene expression of plastid-specific protein (owing to chlorophyll precursors), of a major hub for integration of metabolic and ionic networks in cell metabolism, of a flexible system that can divide, produce dynamic extensions and interact with other cell constituents. Envelope membranes are indeed one of the most complex and dynamic system within a plant cell. In this review, we present an overview of envelope constituents together with recent insights into the major functions fulfilled by envelope membranes and their dynamics within plant cells.

A plant porphyria related to defects in plastid import of protochlorophyllide oxidoreductase A

The plastid envelope of higher plant chloroplasts is a focal point of plant metabolism. It is involved in numerous pathways, including tetrapyrrole biosynthesis and protein translocation. Chloroplasts need to import a large number of proteins from the cytosol because most are encoded in the nucleus. Here we report that a loss-of-function mutation in the outer plastid envelope 16-kDa protein (oep16) gene causes a conditional seedling lethal phenotype related to defects in import and assembly of NADPH:protochlorophyllide (Pchlide) oxidoreductase A. In the isolated knockout mutant of Arabidopsis thaliana, excess Pchlide accumulated in the dark operated as photosensitizer and provoked cell death during greening. Our results highlight the essential role of the substrate-dependent plastid import pathway of precursor Pchlide oxidoreductase A for seedling survival and the avoidance of developmentally programmed porphyria in higher plants.

Chloroplast biogenesis: the use of mutants to study the etioplast-chloroplast transition

In angiosperm plants, the etioplast-chloroplast transition is light-dependent. A key factor in this process is the protochlorophyllide oxidoreductase A (PORA), which catalyzes the light-induced reduction of protochlorophyllide to chlorophyllide. The import pathway of the precursor protein prePORA into chloroplasts was analyzed in vivo and in vitro by using homozygous loss-of-function mutants in genes coding for chlorophyllide a oxygenase (CAO) or for members of the outer-envelope solute-channel protein family of 16 kDa (OEP16), both of which have been implied to be key factors for the import of prePORA. Our in vivo analyses show that cao or oep16 mutants contain a normally structured prolamellar body that contains the protochlorophyllide holochrome. Furthermore, etioplasts from cao and oep16 mutants contain PORA protein as found by mass spectrometry. Our data demonstrate that both CAO and OEP16 are dispensable for chloroplast biogenesis and play no central role in the import of prePORA in vivo and in vitro as further indicated by protein import studies.

Characterization of the preprotein and amino acid transporter gene family in Arabidopsis

Seventeen loci encode proteins of the preprotein and amino acid transporter family in Arabidopsis (Arabidopsis thaliana). Some of these genes have arisen from recent duplications and are not in annotated duplicated regions of the Arabidopsis genome. In comparison to a number of other eukaryotic organisms, this family of proteins has greatly expanded in plants, with 24 loci in rice (Oryza sativa). Most of the Arabidopsis and rice genes are orthologous, indicating expansion of this family before monocot and dicot divergence. In vitro protein uptake assays, in vivo green fluorescent protein tagging, and immunological analyses of selected proteins determined either mitochondrial or plastidic localization for 10 and six proteins, respectively. The protein encoded by At5g24650 is targeted to both mitochondria and chloroplasts and, to our knowledge, is the first membrane protein reported to be targeted to mitochondria and chloroplasts. Three genes encoded translocase of the inner mitochondrial membrane (TIM)17-like proteins, three TIM23-like proteins, and three outer envelope protein16-like proteins in Arabidopsis. The identity of Arabidopsis TIM22-like proteins is most likely a protein encoded by At3g10110/At1g18320, based on phylogenetic analysis, subcellular localization, and complementation of a yeast (Saccharomyces cerevisiae) mutant and coexpression analysis. The lack of a preprotein and amino acid transporter domain in some proteins, localization in mitochondria, plastids, or both, variation in gene structure, and the differences in expression profiles indicate that the function of this family has diverged in plants beyond roles in protein translocation.

Recent advances in chlorophyll biosynthesis

The importance of chlorophyll (Chl) to the process of photosynthesis is obvious, and there is clear evidence that the regulation of Chl biosynthesis has a significant role in the regulation of assembly of the photosynthetic apparatus. The understanding of Chl biosynthesis has rapidly advanced in recent years. The identification of genetic loci associated with each of the biochemical steps has been accompanied by a greater appreciation of the role of Chl biosynthesis intermediates in intracellular signaling. The purpose of this review is to provide a source of information for all the steps in Chl and bacteriochlorophyll a biosynthesis, with an emphasis on steps that are believed to be key regulation points.

Gene duplication, exon gain and neofunctionalization of OEP16-related genes in land plants

OEP16, a channel protein of the outer membrane of chloroplasts, has been implicated in amino acid transport and in the substrate-dependent import of protochlorophyllide oxidoreductase A. Two major clades of OEP16-related sequences were identified in land plants (OEP16-L and OEP16-S), which arose by a gene duplication event predating the divergence of seed plants and bryophytes. Remarkably, in angiosperms, OEP16-S genes evolved by gaining an additional exon that extends an interhelical loop domain in the pore-forming region of the protein. We analysed the sequence, structure and expression of the corresponding Arabidopsis genes (atOEP16-S and atOEP16-L) and demonstrated that following duplication, both genes diverged in terms of expression patterns and coding sequence. AtOEP16-S, which contains multiple G-box ABA-responsive elements (ABREs) in the promoter region, is regulated by ABI3 and ABI5 and is strongly expressed during the maturation phase in seeds and pollen grains, both desiccation-tolerant tissues. In contrast, atOEP-L, which lacks promoter ABREs, is expressed predominantly in leaves, is induced strongly by low-temperature stress and shows weak induction in response to osmotic stress, salicylic acid and exogenous ABA. Our results indicate that gene duplication, exon gain and regulatory sequence evolution each played a role in the divergence of OEP16 homologues in plants.

Protein import into chloroplasts: an ever-evolving storyThis review is one of a selection of papers published in the Special Issue on Plant Cell Biology

Chloroplasts are but one type of a diverse group of essential organelles that distinguish plant cells and house many critical biochemical pathways, including photosynthesis. The biogenesis of plastids is essential to plant growth and development and relies on the targeting and import of thousands of nuclear-encoded proteins from the cytoplasm. The import of the vast majority of these proteins is dependent on translocons located in the outer and inner envelope membranes of the chloroplast, termed the Toc and Tic complexes, respectively. The core components of the Toc and Tic complexes have been identified within the last 12 years; however, the precise functions of many components are still being elucidated, and new components are still being identified. In Arabidopsis thaliana (and other species), many of the components are encoded by more than one gene, and it appears that the isoforms differentially associate with structurally distinct import complexes. Furthermore, it appears that these complexes represent functionally distinct targeting pathways, and the regulation of import by these separate pathways may play a role in the differentiation and specific functions of distinct plastid types during plant growth and development. This review summarizes these recent discoveries and emphasizes the mechanisms of differential Toc complex assembly and substrate recognition.

A role for chlorophyllide a oxygenase in the regulated import and stabilization of light-harvesting chlorophyll a/b proteins

The Arabidopsis CAO gene encodes a 52-kDa protein with predicted localization in the plastid compartment. Here, we report that CAO is an intrinsic Rieske iron-sulfur protein of the plastid-envelope inner and thylakoid membranes. Activity measurements revealed that CAO catalyzes chlorophyllide a to chlorophyllide b conversion in vitro and that the enzyme was only slightly active with protochlorophyllide a, the nonreduced precursor of chlorophyllide a. Protein import and organelle fractionation studies identified CAO to be distinct from Ptc52 in the substrate-dependent transport pathway of NADPH:protochlorophyllide oxidoreductase A but instead to be part of a separate translocon complex. This complex was involved in the regulated import and stabilization of the chlorophyllide b-binding light-harvesting proteins Lhcb1 (LHCII) and Lhcb4 (CP29) in chloroplasts. Together, our results provide insights into the plastid subcompartmentalization and evolution of chlorophyll precursor biosynthesis in relation to protein import in higher plants.

Chlorophyll degradation during senescence

The catabolic pathway of chlorophyll (Chl) during senescence and fruit ripening leads to the accumulation of colorless breakdown products (NCCs). This review updates an earlier review on Chl breakdown published here in 1999 ( 69 ). It summarizes recent advances in the biochemical reactions of the pathway and describes the characterization of new NCCs and their formation inside the vacuole. Furthermore, I focus on the recent molecular identification of three chl catabolic enzymes, chlorophyllase, pheophorbide a oxygenase (PAO), and red Chl catabolite reductase (RCCR). The analysis of Chl catabolic mutants demonstrates the importance of Chl breakdown for plant development and survival. Mutants defective in PAO or RCCR develop a lesion mimic phenotype, due to the accumulation of breakdown intermediates. Thus, Chl breakdown is a prerequisite to detoxify the potentially phototoxic pigment within the vacuoles in order to permit the remobilization of nitrogen from Chl-binding proteins to proceed during senescence.

Evolution of the general protein import pathway of plastids (review)

The evolutionary process that transformed a cyanobacterial endosymbiont into contemporary plastids involved not only inheritance but also invention. Because gram-negative bacteria lack a system for polypeptide import, the envelope translocon complex of the general protein import pathway was the most important invention of organelle evolution resulting in a pathway to import back into plastids those nuclear-encoded proteins supplemented with a transit peptide. Genome information of cyanobacteria, phylogenetically diverse plastids, and the nuclei of the first red alga, a diatom, and Arabidopsis thaliana allows us to trace back the evolutionary origin of the twelve currently known translocon components and to partly deduce their assembly sequence. Development of the envelope translocon was initiated by recruitment of a cyanobacterial homolog of the protein-import channel Toc75, which belongs to a ubiquitous and essential family of Omp85/D15 outer membrane proteins of gram-negative bacteria that mediate biogenesis of beta-barrel proteins. Likewise, three other translocon subunits (Tic20, Tic22, and Tic55) and several stromal chaperones have been inherited from the ancestral cyanobacterium and modified to take over the novel function of precursor import. Most of the remaining subunits seem to be of eukaryotic origin, recruited from pre-existing nuclear genes. The next subunits that joined the evolving protein import complex likely were Toc34 and Tic110, as indicated by the presence of homologous genes in the red alga Cyanidioschyzon merolae, followed by the stromal processing peptidase, members of the Toc159 receptor family, Toc64, Tic40, and finally some regulatory redox components (Tic62, Tic32), all of which were probably required to increase specificity and efficiency of precursor import.

Recognition and envelope translocation of chloroplast preproteins

Plastids are a diverse group of plant organelles that perform essential functions including important steps in many biosynthetic pathways. Chloroplasts are the best characterized type of plastid, and constitute the site of oxygenic photosynthesis in plants, a process essential to all higher life forms. It is well established that the majority (>90%) of chloroplast proteins are nucleus-encoded and must be post-translationally imported into these envelope-bound compartments. Most nucleus-encoded chloroplast proteins are translated in precursor form on cytosolic ribosomes, targeted to the chloroplast surface, and then imported across the double-membrane envelope by translocons in the outer and inner envelope membranes of the chloroplast, termed TOC and TIC, respectively. Recently, significant progress has been made in our understanding of how proteins are targeted to the chloroplast surface and translocated across the chloroplast envelope into the stroma. Evidence suggesting the existence of multiple import pathways at the outer envelope membrane for different classes of precursor proteins has been presented. These pathways appear to utilize similar TOC complexes equipped with different combinations of homologous GTPase receptors, providing preprotein recognition specificity.

Multiplicity of different cell- and organ-specific import routes for the NADPH-protochlorophyllide oxidoreductases A and B in plastids of Arabidopsis seedlings

The NADPH-dependent protochlorophyllide (Pchlide) oxidoreductase (POR) is a photoenzyme that requires light for its catalytic activity and uses Pchlide itself as a photoreceptor. In Arabidopsis there are three PORs denoted PORA, PORB and PORC. The PORA and PORB genes are strongly expressed early in seedling development. In contrast to PORB the import of PORA into plastids of cotyledons is substrate-dependent and organ-specific. These differences in the import reactions between PORA and PORB most likely are due to different import mechanisms that are responsible for the uptake of these proteins. The two major core constituents of the translocon of the outer plastid envelope, Toc159 and Toc34, have been implicated in the binding and recognition of precursors of nuclear-encoded plastid proteins. Their involvement in conferring substrate dependency and organ specificity of PORA import was analyzed in intact Arabidopsis seedlings of wild type and the three mutants ppi3, ppi1 and ppi2 that are deficient in atToc34, atToc33, a closely related isoform of atToc34, and atToc159. Whereas none of these three Toc constituents is required for maintaining the organ specificity and substrate dependency of PORA import, atToc33 is indispensable for the import of PORB in cotyledons and true leaves suggesting that in these parts of the plant translocation of PORA and PORB occurs via two distinct import pathways. The analysis of PORA and PORB import into plastids of intact seedlings revealed an unexpected multiplicity of import routes that differed by their substrate, cell, tissue and organ specificities. This versatility of pathways for protein targeting to plastids suggests that in intact seedlings not only the constituents of the core complex of import channels but also other factors are involved in mediating the import of nuclear-encoded plastid proteins.

A role of Toc33 in the protochlorophyllide-dependent plastid import pathway of NADPH:protochlorophyllide oxidoreductase (POR) A

NADPH:protochlorophyllide oxidoreductase (POR) A is a key enzyme of chlorophyll biosynthesis in angiosperms. It is nucleus-encoded, synthesized as a larger precursor in the cytosol and imported into the plastids in a substrate-dependent manner. Plastid envelope membrane proteins, called protochlorophyllide-dependent translocon proteins, Ptcs, have been identified that interact with pPORA during import. Among them are a 16-kDa ortholog of the previously characterized outer envelope protein Oep16 (named Ptc16) and a 33-kDa protein (Ptc33) related to the GTP-binding proteins Toc33 and Toc34 of Arabidopsis. In the present work, we studied the interactions and roles of Ptc16 and Ptc33 during pPORA import. Radiolabeled Ptc16/Oep16 was synthesized from a corresponding cDNA and imported into isolated Arabidopsis plastids. Crosslinking experiments revealed that import of 35S-Oep16/Ptc16 is stimulated by GTP. 35S-Oep16/Ptc16 forms larger complexes with Toc33 but not Toc34. Plastids of the ppi1 mutant of Arabidopsis lacking Toc33, were unable to import pPORA in darkness but imported the small subunit precursor of ribulose-1,5-bisphosphate carboxylase/oxygenase (pSSU), precursor ferredoxin (pFd) as well as pPORB which is a close relative of pPORA. In white light, partial suppressions of pSSU, pFd and pPORB import were observed. Our results unveil a hitherto unrecognized role of Toc33 in pPORA import and suggest photooxidative membrane damage, induced by excess Pchlide accumulating in ppi1 chloroplasts because of the lack of pPORA import, to be the cause of the general drop of protein import.

Solute transporters of the plastid envelope membrane

Plastids are metabolically extraordinarily active and versatile organelles that are found in all plant cells with the exception of angiosperm pollen grains. Many of the plastid-localized biochemical pathways depend on precursors from the cytosol and, in turn, many cytosolic pathways depend on the supply of precursor molecules from the plastid stroma. Hence, a massive traffic of metabolites occurs across the permeability barrier between plastids and cytosol that is called the plastid envelope membrane. Many of the known plastid envelope solute transporters have been identified by biochemical purification and peptide sequencing. This approach is of limited use for less abundant proteins and for proteins of plastid subtypes that are difficult to isolate in preparative amounts. Hence, the majority of plastid envelope membrane transporters are not yet identified at the molecular level. The availability of fully sequenced plant genomes, the progress in bioinformatics to predict membrane transporters localized in plastids, and the development of highly sensitive proteomics techniques open new avenues toward identifying additional, to date unknown, plastid envelope membrane transporters.

Recent advances in chlorophyll biosynthesis and breakdown in higher plants

Chlorophyll (Chl) has unique and essential roles in photosynthetic light-harvesting and energy transduction, but its biosynthesis, accumulation and degradation is also associated with chloroplast development, photomorphogenesis and chloroplast-nuclear signaling. Biochemical analyses of the enzymatic steps paved the way to the identification of their encoding genes. Thus, important progress has been made in the recent elucidation of almost all genes involved in Chl biosynthesis and breakdown. In addition, analysis of mutants mainly in Arabidopsis , genetically engineered plants and the application of photo-reactive herbicides contributed to the genetic and regulatory characterization of the formation and breakdown of Chl. This review highlights recent progress in Chl metabolism indicating highly regulated pathways from the synthesis of precursors to Chl and its degradation to intermediates, which are not longer photochemically active.

atToc159 is a selective transit peptide receptor for the import of nucleus-encoded chloroplast proteins

The members of the Toc159 family of GTPases act as the primary receptors for the import of nucleus-encoded preproteins into plastids. Toc159, the most abundant member of this family in chloroplasts, is required for chloroplast biogenesis (Bauer, J., K. Chen, A. Hiltbunner, E. Wehrli, M. Eugster, D. Schnell, and F. Kessler. 2000. Nature. 403:203-207) and has been shown to covalently cross-link to bound preproteins at the chloroplast surface (Ma, Y., A. Kouranov, S. LaSala, and D.J. Schnell. 1996. J. Cell Biol. 134:1-13; Perry, S.E., and K. Keegstra. 1994. Plant Cell. 6:93-105). These reports led to the hypothesis that Toc159 functions as a selective import receptor for preproteins that are required for chloroplast development. In this report, we provide evidence that Toc159 is required for the import of several highly expressed photosynthetic preproteins in vivo. Furthermore, we demonstrate that the cytoplasmic and recombinant forms of soluble Toc159 bind directly and selectively to the transit peptides of these representative photosynthetic preproteins, but not representative constitutively expressed plastid preproteins. These data support the function of Toc159 as a selective import receptor for the targeting of a set of preproteins required for chloroplast biogenesis.