Singlet oxygen induces cell wall thickening and stomatal density reducing by transcriptome reprogramming.

Singlet oxygen (1O2) has a very short half-life of 10-5 s; however, it is a strong oxidant that causes growth arrest and necrotic lesions on plants. Its signaling pathway remains largely unknown. The Arabidopsis flu (fluorescent) mutant accumulates a high level of 1O2 and shows drastic changes in nuclear gene expression. Only two plastid proteins, EX1 (executer 1) and EX2 (executer 2), have been identified in the singlet oxygen signaling. Here, we found that the transcription factor abscisic acid insensitive 4 (ABI4) binds the promoters of genes responsive to 1O2-signals. Inactivation of the ABI4 protein in the flu/abi4 double mutant was sufficient to compromise the changes of almost all 1O2-responsive-genes and rescued the lethal phenotype of flu grown under light/dark cycles, similar to the flu/ex1/ex2 triple mutant. In addition to cell death, we reported for the first time that 1O2 also induces cell wall thickening and stomatal development defect. Contrastingly, no apparent growth arrest was observed for the flu mutant under normal light/dim light cycles, but the cell wall thickening (doubled) and stomatal density reduction (by two-thirds) still occurred. These results offer a new idea for breeding stress tolerant plants.

tRNA-Dependent Import of a Transit Sequence-Less Aminoacyl-tRNA Synthetase (LeuRS2) into the Mitochondria of Arabidopsis.

Aminoacyl-tRNA synthetases (AaRS) charge tRNAs with amino acids for protein translation. In plants, cytoplasmic, mitochondrial, and chloroplast AaRS exist that are all coded for by nuclear genes and must be imported from the cytosol. In addition, only a few of the mitochondrial tRNAs needed for translation are encoded in mitochondrial DNA. Despite considerable progress made over the last few years, still little is known how the bulk of cytosolic AaRS and respective tRNAs are transported into mitochondria. Here, we report the identification of a protein complex that ties AaRS and tRNA import into the mitochondria of Arabidopsis thaliana. Using leucyl-tRNA synthetase 2 (LeuRS2) as a model for a mitochondrial signal peptide (MSP)-less precursor, a ≈30 kDa protein was identified that interacts with LeuRS2 during import. The protein identified is identical with a previously characterized mitochondrial protein designated HP30-2 (encoded by At3g49560) that contains a sterile alpha motif (SAM) similar to that found in RNA binding proteins. HP30-2 is part of a larger protein complex that contains with TIM22, TIM8, TIM9 and TIM10 four previously identified components of the translocase for MSP-less precursors. Lack of HP30-2 perturbed mitochondrial biogenesis and function and caused seedling lethality during greening, suggesting an essential role of HP30-2 in planta.

Serpin1 and WSCP differentially regulate the activity of the cysteine protease RD21 during plant development in Arabidopsis thaliana.

Proteolytic enzymes (proteases) participate in a vast range of physiological processes, ranging from nutrient digestion to blood coagulation, thrombosis, and beyond. In plants, proteases are implicated in host recognition and pathogen infection, induced defense (immunity), and the deterrence of insect pests. Because proteases irreversibly cleave peptide bonds of protein substrates, their activity must be tightly controlled in time and space. Here, we report an example of how nature evolved alternative mechanisms to fine-tune the activity of a cysteine protease dubbed RD21 (RESPONSIVE TO DESICCATION-21). One mechanism in the model plant Arabidopsis thaliana studied here comprises irreversible inhibition of RD21's activity by Serpin1, whereas the other mechanism is a result of the reversible inhibition of RD21 activity by a Kunitz protease inhibitor named water-soluble chlorophyll-binding protein (WSCP). Activity profiling, complex isolation, and homology modeling data revealed unique interactions of RD21 with Serpin1 and WSCP, respectively. Expression studies identified only partial overlaps in Serpin1 and WSCP accumulation that explain how RD21 contributes to the innate immunity of mature plants and arthropod deterrence of seedlings undergoing skotomorphogenesis and greening.

Substrate channeling in oxylipin biosynthesis through a protein complex in the plastid envelope of Arabidopsis thaliana.

Oxygenated membrane fatty acid derivatives termed oxylipins play important roles in plant defense against biotic and abiotic cues. Plants challenged by insect pests, for example, synthesize a blend of different defense compounds that include volatile aldehydes and jasmonic acid (JA), among others. Because all oxylipins are derived from the same pathway, we investigated how their synthesis might be regulated, focusing on two closely related atypical cytochrome P450 enzymes designated CYP74A and CYP74B, respectively, allene oxide synthase (AOS) and hydroperoxide lyase (HPL). These enzymes compete for the same substrate but give rise to different products: the final product of the AOS branch of the oxylipin pathway is JA, while those of the HPL branch comprise volatile aldehydes and alcohols. AOS and HPL are plastid envelope enzymes in Arabidopsis thaliana but accumulate at different locations. Biochemical experiments identified AOS as a constituent of complexes also containing lipoxygenase 2 (LOX2) and allene oxide cyclase (AOC), which catalyze consecutive steps in JA precursor biosynthesis, while excluding the concurrent HPL reaction. Based on published X-ray data, the structure of this complex was modelled and amino acids involved in catalysis and subunit interactions predicted. Genetic studies identified the microRNA 319-regulated clade of TCP (TEOSINTE BRANCHED/CYCLOIDEA/PCF) transcription factor genes and CORONATINE INSENSITIVE 1 (COI1) as controlling JA production through the LOX2-AOS-AOC2 complex. Together, our results define a molecular branch point in oxylipin biosynthesis that allows fine-tuning of the plant's defense machinery in response to biotic and abiotic stimuli.

Programmed chloroplast destruction during leaf senescence involves 13-lipoxygenase (13-LOX).

Leaf senescence is the terminal stage in the development of perennial plants. Massive physiological changes occur that lead to the shut down of photosynthesis and a cessation of growth. Leaf senescence involves the selective destruction of the chloroplast as the site of photosynthesis. Here, we show that 13-lipoxygenase (13-LOX) accomplishes a key role in the destruction of chloroplasts in senescing plants and propose a critical role of its NH2-terminal chloroplast transit peptide. The 13-LOX enzyme identified here accumulated in the plastid envelope and catalyzed the dioxygenation of unsaturated membrane fatty acids, leading to a selective destruction of the chloroplast and the release of stromal constituents. Because 13-LOX pathway products comprise compounds involved in insect deterrence and pathogen defense (volatile aldehydes and oxylipins), a mechanism of unmolested nitrogen and carbon relocation is suggested that occurs from leaves to seeds and roots during fall.

ALLENE OXIDE SYNTHASE and HYDROPEROXIDE LYASE, Two Non-Canonical Cytochrome P450s in Arabidopsis thaliana and Their Different Roles in Plant Defense.

The channeling of metabolites is an essential step of metabolic regulation in all living organisms. Multifunctional enzymes with defined domains for metabolite compartmentalization are rare, but in many cases, larger assemblies forming multimeric protein complexes operate in defined metabolic shunts. In Arabidopsis thaliana, a multimeric complex was discovered that contains a 13-lipoxygenase and allene oxide synthase (AOS) as well as allene oxide cyclase. All three plant enzymes are localized in chloroplasts, contributing to the biosynthesis of jasmonic acid (JA). JA and its derivatives act as ubiquitous plant defense regulators in responses to both biotic and abiotic stresses. AOS belongs to the superfamily of cytochrome P450 enzymes and is named CYP74A. Another CYP450 in chloroplasts, hydroperoxide lyase (HPL, CYP74B), competes with AOS for the common substrate. The products of the HPL reaction are green leaf volatiles that are involved in the deterrence of insect pests. Both enzymes represent non-canonical CYP450 family members, as they do not depend on O2 and NADPH-dependent CYP450 reductase activities. AOS and HPL activities are crucial for plants to respond to different biotic foes. In this mini-review, we aim to summarize how plants make use of the LOX2-AOS-AOC2 complex in chloroplasts to boost JA biosynthesis over volatile production and how this situation may change in plant communities during mass ingestion by insect pests.

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.

Water-soluble chlorophyll protein is involved in herbivore resistance activation during greening of Arabidopsis thaliana.

Water-soluble chlorophyll proteins (WSCPs) constitute a small family of unusual chlorophyll (Chl)-binding proteins that possess a Kunitz-type protease inhibitor domain. In Arabidopsis thaliana, a WSCP has been identified, named AtWSCP, that forms complexes with Chl and the Chl precursor chlorophyllide (Chlide) in vitro. AtWSCP exhibits a quite unexpected expression pattern for a Chl binding protein and accumulated to high levels in the apical hook of etiolated plants. AtWSCP expression was negatively light-regulated. Transgenic expression of AtWSCP fused to green fluorescent protein (GFP) revealed that AtWSCP is localized to cell walls/apoplastic spaces. Biochemical assays identified AtWSCP as interacting with RD21 (responsive to desiccation 21), a granulin domain-containing cysteine protease implicated in stress responses and defense. Reconstitution experiments showed tight interactions between RD21 and WSCP that were relieved upon Chlide binding. Laboratory feeding experiments with two herbivorous isopod crustaceans, Porcellio scaber (woodlouse) and Armadillidium vulgare (pillbug), identified the apical hook as Achilles' heel of etiolated plants and that this was protected by RD21 during greening. Because Chlide is formed in the apical hook during seedling emergence from the soil, our data suggest an unprecedented mechanism of herbivore resistance activation that is triggered by light and involves AtWSCP.

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.

JIP60-mediated, jasmonate- and senescence-induced molecular switch in translation toward stress and defense protein synthesis.

Two closely related genes encoding the jasmonate-induced protein 60 (JIP60) were identified in the barley genome. The gene on chromosome arm 4HL encodes the previously identified protein encoded by the cDNA X66376.1. This JIP60 protein is characterized here and shown to consist of two domains: an NH2-terminal domain related to ribosome-inactivating proteins and a COOH-terminal domain, which displays similarity to eukaryotic translation initiation factor 4E (eIF4E). JIP60 undergoes processing in vivo, as a result of which JIP60's COOH-terminal eIF4E domain is released and functions in recruiting a subset of cellular messengers for translation. This effect was observed for both MeJA-treated and naturally senescing plants. Because the JIP60 gene is in close proximity to several quantitative trait loci for both biotic and abiotic stress resistance, our results identify a unique target for future breeding programs.

HP30-2, a mitochondrial PRAT protein for import of signal sequence-less precursor proteins in Arabidopsis thaliana.

Chloroplasts and mitochondria contain a family of putative preprotein and amino acid transporters designated PRAT. Here, we analyzed the role of two previously characterized PRAT protein family members, encoded by At3g49560 (HP30) and At5g24650 (HP30-2), in planta using a combination of genetic, cell biological and biochemical approaches. Expression studies and green fluorescent protein tagging identified HP30-2 both in chloroplasts and mitochondria, whereas HP30 was located exclusively in chloroplasts. Biochemical evidence was obtained for an association of mitochondrial HP30-2 with two distinct protein complexes, one containing the inner membrane translocase TIM22 and the other containing an alternative NAD(P)H dehydrogenase subunit (NDC1) implicated in a respiratory complex 1-like electron transport chain. Through its association with TIM22, HP30-2 is involved in the uptake of carrier proteins and other, hydrophobic membrane proteins lacking cleavable NH2 -terminal presequences, whereas HP30-2's interaction with NDC1 may permit controlling mitochondrial biogenesis and activity.

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.

A Kunitz-type protease inhibitor regulates programmed cell death during flower development in Arabidopsis thaliana.

Flower development and fertilization are tightly controlled in Arabidopsis thaliana. In order to permit the fertilization of a maximum amount of ovules as well as proper embryo and seed development, a subtle balance between pollen tube growth inside the transmitting tract and pollen tube exit from the septum is needed. Both processes depend on a type of programmed cell death that is still poorly understood. Here, it is shown that a Kunitz protease inhibitor related to water-soluble chlorophyll proteins of Brassicaceae (AtWSCP, encoded by At1g72290) is involved in controlling cell death during flower development in A. thaliana. Genetic, biochemical, and cell biology approaches revealed that WSCP physically interacts with RD21 (RESPONSIVE TO DESICCATION) and that this interaction in turn inhibits the activity of RD21 as a pro-death protein. The regulatory circuit identified depends on the restricted expression of WSCP in the transmitting tract and the septum epidermis. In a respective Atwscp knock-out mutant, flowers exhibited precocious cell death in the transmitting tract and unnatural death of septum epidermis cells. As a consequence, apical-basal pollen tube growth, fertilization of ovules, as well as embryo development and seed formation were perturbed. Together, the data identify a unique mechanism of cell death regulation that fine-tunes pollen tube growth.

Singlet oxygen-dependent translational control in the tigrina-d.12 mutant of barley.

The tigrina (tig)-d.12 mutant of barley is impaired in the negative control limiting excess protochlorophyllide (Pchlide) accumulation in the dark. Upon illumination, Pchlide operates as photosensitizer and triggers singlet oxygen production and cell death. Here, we show that both Pchlide and singlet oxygen operate as signals that control gene expression and metabolite accumulation in tig-d.12 plants. In vivo labeling, Northern blotting, polysome profiling, and protein gel blot analyses revealed a selective suppression of synthesis of the small and large subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase (RBCSs and RBCLs), the major light-harvesting chlorophyll a/b-binding protein of photosystem II (LHCB2), as well as other chlorophyll-binding proteins, in response to singlet oxygen. In part, these effects were caused by an arrest in translation initiation of photosynthetic transcripts at 80S cytoplasmic ribosomes. The observed changes in translation correlated with a decline in the phosphorylation level of ribosomal protein S6. At later stages, ribosome dissociation occurred. Together, our results identify translation as a major target of singlet oxygen-dependent growth control and cell death in higher plants.

Targeting Induced Local Lesions in the Wheat DEMETER and DRE2 Genes, Responsible for Transcriptional Derepression of Wheat Gluten Proteins in the Developing Endosperm.

Wheat is a major source of energy and nutrition worldwide, but it is also a primary cause of frequent diet-induced health issues, specifically celiac disease, for which the only effective therapy so far is strict dietary abstinence from gluten-containing grains. Wheat gluten proteins are grouped into two major categories: high-molecular-weight glutenin subunits (HMWgs), vital for mixing and baking properties, and gliadins plus low-molecular-weight glutenin subunits (LMWgs) that contain the overwhelming majority of celiac-causing epitopes. We put forth a hypothesis that eliminating gliadins and LMWgs while retaining HMWgs might allow the development of reduced-immunogenicity wheat genotypes relevant to most gluten-sensitive individuals. This hypothesis stems from the knowledge that the molecular structures and regulatory mechanisms of the genes encoding the two groups of gluten proteins are quite different, and blocking one group's transcription, without affecting the other's, is possible. The genes for gliadins and LMWgs have to be de-methylated by 5-methylcytosine DNA glycosylase/lyase (DEMETER) and an iron-sulfur (Fe-S) cluster biogenesis enzyme (DRE2) early during endosperm development to permit their transcription. In this study, a TILLING (Targeting Induced Local Lesions IN Genomes) approach was undertaken to identify mutations in the homoeologous DEMETER (DME) and DRE2 genes in common and durum wheat. Lines with mutations in these genes were obtained that displayed reduced content of immunogenic gluten proteins while retaining essential baking properties. Although our data at first glance suggest new possibilities for treating celiac disease and are therefore of medical and agronomical interest, it also shows that inducing mutations in the DME and DRE2 genes analyzed here affected pollen viability and germination. Hence there is a need to develop other approaches in the future to overcome this undesired effect.

The outer chloroplast envelope protein OEP16-1 for plastid import of NADPH:protochlorophyllide oxidoreductase A in Arabidopsis thaliana.

The outer plastid envelope protein OEP16-1 was previously identified as an amino acid-selective channel protein and translocation pore for NADPH:protochlorophyllide oxidoreductase A (PORA). Reverse genetic approaches used to dissect these mutually not exclusive functions of OEP16-1 in planta have led to descriptions of different phenotypes resulting from the presence of several mutant lines in the SALK_024018 seed stock. In addition to the T-DNA insertion in the AtOEP16-1 gene, lines were purified that contain two additional T-DNA insertions and as yet unidentified point mutations. In a first attempt to resolve the genetic basis of four different lines in the SALK_024018 seed stock, we used genetic transformation with the OEP16-1 cDNA and segregation analyses after crossing out presumed point mutations. We show that AtOEP16-1 is involved in PORA precursor import and by virtue of this activity confers photoprotection onto etiolated seedlings during greening.

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.

Protein-induced excited-state dynamics of protochlorophyllide.

The light-driven NADPH:protochlorophyllide oxidoreductase (POR) is a key enzyme of chlorophyll biosynthesis in angiosperms. POR's unique requirement for light to become catalytically active makes the enzyme an attractive model to study the dynamics of enzymatic reactions in real time. Here, we use picosecond time-resolved fluorescence and femtosecond pump-probe spectroscopy to examine the influence of the protein environment on the excited-state dynamics of the substrate, protochlorophyllide (PChlide), in the enzyme/substrate (PChlide/POR) and pseudoternary complex including the nucleotide cofactor NADP(+) (PChlide/NADP(+)/ POR). In comparison with the excited-state processes of unbound PChlide, the lifetime of the thermally equilibrated S(1) excited state is lengthened from 3.4 to 4.4 and 5.4 ns in the PChlide/POR and PChlide/NADP(+)/POR complex, whereas the nonradiative rates are decreased by ∼30 and 40%, respectively. This effect is most likely due to the reduced probability of nonradiative decay into the triplet excited state, thus keeping the risk of photosensitized side reactions in the enzyme low. Further, the initial reaction path involves the formation of an intramolecular charge-transfer state (S(ICT)) as an intermediate product. From a strong blue shift in the excited-state absorption, it is concluded that the S(ICT) state is stabilized by local interactions with specific protein sites in the catalytic pocket. The possible relevance of this result for the catalytic reaction in the enzyme POR is discussed.

Photoprotective role of NADPH:protochlorophyllide oxidoreductase A.

A homology model of NADPH:protochlorophyllide (Pchlide) oxidoreductase A (POR; E.C. 1.3.33.1) of barley is developed and verified by site-directed mutagenesis. PORA is considered a globular protein consisting of nine alpha-helices and seven beta-strands. The model predicts the presence of two functionally distinctive Pchlide binding sites where the pigment is coordinated by cysteine residues. The pigment bound to the first, high-affinity Pchlide binding site is used for the formation of the photoactive state of the enzyme. The pigment bound to the second, low-affinity Pchlide binding site is involved in the PORA:PORB interaction, allowing for resonance energy transfer between the neighboring PORs in the complex. In the in vitro reconstituted light-harvesting POR:Pchlide complex (LHPP), light absorbed by PORA-bound Pchlide b is transferred to PORB-bound Pchlide a. That induces the conversion of Pchlide a to chlorophyllide (Chlide) a. This energy transfer eliminates the possibility of Pchlide b photoreduction and prevents that excited triplet states of either Pchlides a or b accumulate and provoke singlet oxygen production. Together, our results provide a photoprotective role of PORA during greening.

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.