Chloroplast Redox Regulatory Mechanisms in Plant Adaptation to Light and Darkness

The impact of light and thioredoxins on the plant thiol-disulfide proteome

Thiol-based redox regulation is a crucial posttranslational mechanism to acclimate plants to changing light availability. Here, we conducted a biotin switch-based redox proteomics study in Arabidopsis (Arabidopsis thaliana) to systematically investigate dynamics of thiol-redox networks in response to temporal changes in light availability and across genotypes lacking parts of the thioredoxin (Trx) or NADPH-Trx-reductase C (NTRC) systems in the chloroplast. Time-resolved dynamics revealed light led to marked decreases in the oxidation states of many chloroplast proteins with photosynthetic functions during the first 10 min, followed by their partial reoxidation after 2 to 6 h into the photoperiod. This involved f, m, and x-type Trx proteins showing similar light-induced reduction-oxidation dynamics, while NTRC, 2-Cys peroxiredoxins, and Trx y2 showed an opposing pattern, being more oxidized in light than dark. In Arabidopsis trxf1f2, trxm1m2, or ntrc mutants, most proteins showed increased oxidation states in the light compared to wild type, suggesting their light-dependent dynamics were related to NTRC/Trx networks. While NTRC deficiency had a strong influence in all light conditions, deficiencies in f- or m-type Trxs showed differential impacts on the thiol-redox proteome depending on the light environment, being higher in constant or fluctuating light, respectively. The results indicate plant redox proteomes are subject to dynamic changes in reductive and oxidative pathways to cooperatively fine-tune photosynthetic and metabolic processes in the light. The importance of the individual elements of the NTRC/Trx networks mediating these responses depend on the extent of light variability, with NTRC playing a crucial role to balance protein-redox states in rapidly fluctuating light.

SULTR2;1 Adjusts the Bolting Timing by Transporting Sulfate from Rosette Leaves to the Primary Stem

Sulfur (S) is an essential macronutrient for plant growth and metabolism. SULTR2;1 is a low-affinity sulfate transporter facilitating the long-distance transport of sulfate in Arabidopsis. The physiological function of SULTR2;1 in the plant life cycle still needs to be determined. Therefore, we analyzed the sulfate transport, S-containing metabolite accumulation and plant growth using Arabidopsis SULTR2;1 disruption lines, sultr2;1-1 and sultr2;1-2, from seedling to mature growth stages to clarify the metabolic and physiological roles of SULTR2;1. We observed that sulfate distribution to the stems was affected in sultr2;1 mutants, resulting in decreased levels of sulfate, cysteine, glutathione (GSH) and total S in the stems, flowers and siliques; however, the GSH levels increased in the rosette leaves. This suggested the essential role of SULTR2;1 in sulfate transport from rosette leaves to the primary stem. In addition, sultr2;1 mutants unexpectedly bolted earlier than the wild-type without affecting the plant biomass. Correlation between GSH levels in rosette leaves and the bolting timing suggested that the rosette leaf GSH levels or limited sulfate transport to the early stem can trigger bolting. Overall, this study demonstrated the critical roles of SULTR2;1 in maintaining the S metabolite levels in the aerial part and transitioning from the vegetative to the reproductive growth phase.

Ketocarotenoid production in tomato triggers metabolic reprogramming and cellular adaptation: The quest for homeostasis

Plants are sessile and therefore have developed an extraordinary capacity to adapt to external signals. Here, the focus is on the plasticity of the plant cell to respond to new intracellular cues. Ketocarotenoids are high-value natural red pigments with potent antioxidant activity. In the present study, system-level analyses have revealed that the heterologous biosynthesis of ketocarotenoids in tomato initiated a series of cellular and metabolic mechanisms to cope with the formation of metabolites that are non-endogenous to the plant. The broad multilevel changes were linked to, among others, (i) the remodelling of the plastidial membrane, where the synthesis and storage of ketocarotenoids occurs; (ii) the recruiting of core metabolic pathways for the generation of metabolite precursors and energy; and (iii) redox control. The involvement of the metabolites as regulators of cellular processes shown here reinforces their pivotal role suggested in the remodelled 'central dogma' concept. Furthermore, the role of metabolic reprogramming to ensure cellular homeostasis is proposed.

The integration of TRX/GRX systems and phytohormonal signalling pathways in plant stress and development

Plant acclimation to changing environmental conditions involves the interaction of different signalling molecules, including reactive oxygen species and hormones. Redox regulation exerted by thioredoxin (TRX) and glutaredoxin (GRX), two oxidoreductases, is emerging as a specific point of control mediating signal transduction pathways associated with plant growth and stress response. Phytohormones are messengers that coordinate plant cell activities to regulate growth, defence, and productivity, although their cross-talk with components of the redox system is less known. The present review focuses on our current knowledge of the interplay that occurs between TRX and GRX systems and phytohormonal signalling pathways in connection with the control of plant development and stress responses. Here, we consider the regulation that phytohormones exert on TRX and GRX systems, as well as the involvement of these redox proteins in the control of phytohormone-mediated signalling pathways.

More indications for redox-sensitive cysteine residues of the Arabidopsis 5-aminolevulinate dehydratase

Redox-dependent thiol-disulfide switches of cysteine residues are one of the significant posttranslational modifications of proteins to control rapidly their stability, activity, and protein interaction. Redox control also modulates the tetrapyrrole biosynthesis (TBS). Among the redox-dependent TBS enzymes, 5-aminolevulinic acid dehydratase (ALAD) was previously recognized to interact with reductants, such a thioredoxins or NADPH-dependent thioredoxin reductase C. In this report, we aim to verify the redox sensitivity of ALAD and identify the redox-reactive cysteine residues among the six cysteines of the mature protein form Arabidopsis. Based on structural modelling and comparative studies of wild-type ALAD and ALAD mutants with single and double Cys➔Ser substitutions under oxidizing and reducing conditions, we aim to predict the dimerization and oligomerisation of ALAD as well as the crucial Cys residues for disulfide bridge formation and enzyme activity. The Cys404Ser mutation led to a drastic inactivation of ALAD and redox-dependent properties of ALAD were severely impaired, when Cys71 was simultaneously mutated with Cys152 or Cys251. Cys71 is located in a flexible N-terminal arm of ALAD, which could allow intramolecular disulfide bridges with Cys residues at the surface of the remaining globule ALAD structure. As a result, we propose different roles of Cys residues for redox control, catalytic activity and Mg2+-dependent assembly.

Redox regulation in chloroplast thylakoid lumen: The pmf changes everything, again

Photosynthesis is the foundation of life on Earth. However, if not well regulated, it can also generate excessive reactive oxygen species (ROS), which can cause photodamage. Regulation of photosynthesis is highly dynamic, responding to both environmental and metabolic cues, and occurs at many levels, from light capture to energy storage and metabolic processes. One general mechanism of regulation involves the reversible oxidation and reduction of protein thiol groups, which can affect the activity of enzymes and the stability of proteins. Such redox regulation has been well studied in stromal enzymes, but more recently, evidence has emerged of redox control of thylakoid lumenal enzymes. This review/hypothesis paper summarizes the latest research and discusses several open questions and challenges to achieving effective redox control in the lumen, focusing on the distinct environments and regulatory components of the thylakoid lumen, including the need to transport electrons across the thylakoid membrane, the effects of pH changes by the proton motive force (pmf) in the stromal and lumenal compartments, and the observed differences in redox states. These constraints suggest that activated oxygen species are likely to be major regulatory contributors to lumenal thiol redox regulation, with key components and processes yet to be discovered.

CBSX2 is required for the efficient oxidation of chloroplast redox-regulated enzymes in darkness

Thiol/disulfide-based redox regulation in plant chloroplasts is essential for controlling the activity of target proteins in response to light signals. One of the examples of such a role in chloroplasts is the activity of the chloroplast ATP synthase (CFoCF1), which is regulated by the redox state of the CF1γ subunit and involves two cysteines in its central domain. To investigate the mechanism underlying the oxidation of CF1γ and other chloroplast redox-regulated enzymes in the dark, we characterized the Arabidopsis cbsx2 mutant, which was isolated based on its altered NPQ (non-photochemical quenching) induction upon illumination. Whereas in dark-adapted WT plants CF1γ was completely oxidized, a small amount of CF1γ remained in the reduced state in cbsx2 under the same conditions. In this mutant, reduction of CF1γ was not affected in the light, but its oxidation was less efficient during a transition from light to darkness. The redox states of the Calvin cycle enzymes FBPase and SBPase in cbsx2 were similar to those of CF1γ during light/dark transitions. Affinity purification and subsequent analysis by mass spectrometry showed that the components of the ferredoxin-thioredoxin reductase/thioredoxin (FTR-Trx) and NADPH-dependent thioredoxin reductase (NTRC) systems as well as several 2-Cys peroxiredoxins (Prxs) can be co-purified with CBSX2. In addition to the thioredoxins, yeast two-hybrid analysis showed that CBSX2 also interacts with NTRC. Taken together, our results suggest that CBSX2 participates in the oxidation of the chloroplast redox-regulated enzymes in darkness, probably through regulation of the activity of chloroplast redox systems in vivo.

Breakdown of Arabidopsis thaliana thioredoxins and glutaredoxins based on electrostatic similarity-Leads to common and unique interaction partners and functions

The reversible reduction and oxidation of protein thiols was first described as mechanism to control light/dark-dependent metabolic regulation in photosynthetic organisms. Today, it is recognized as an essential mechanism of regulation and signal transduction in all kingdoms of life. Proteins of the thioredoxin (Trx) family, Trxs and glutaredoxins (Grxs) in particular, catalyze thiol-disulfide exchange reactions and are vital players in the operation of thiol switches. Various Trx and Grx isoforms are present in all compartments of the cell. These proteins have a rather broad but at the same time distinct substrate specificity. Understanding the molecular basis of their target specificity is central to the understanding of physiological and pathological redox signaling. Electrostatic complementarity of the redoxins with their target proteins has been proposed as a major reason. Here, we analyzed the electrostatic similarity of all Arabidopsis thaliana Trxs, Grxs, and proteins containing such domains. Clustering of the redoxins based on this comparison suggests overlapping and also distant target specificities and thus functions of the different sub-classes including all Trx isoforms as well as the three classes of Grxs, i.e. CxxC-, CGFS-, and CC-type Grxs. Our analysis also provides a rationale for the tuned substrate specificities of both the ferredoxin- and NADPH-dependent Trx reductases.

The tomato chloroplast stromal proteome compendium elucidated by leveraging a plastid protein-localization prediction Atlas

Tomato (Solanum lycopersicum) is a model species for studying fruit development, wounding, herbivory, and pathogen attack. Despite tomato's world-wide economic importance and the role of chloroplasts as metabolic hubs and integrators of environmental cues, little is known about the stromal proteome of tomato. Using a high-yielding protocol for chloroplast and stromal protein isolation, MudPIT nano-LC-MS/MS analyses, a robust in-house protein database (the Atlas) for predicting the plastid localization of tomato proteins, and rigorous selection criteria for inclusion/exclusion in the stromal proteome, we identified 1,278 proteins of the tomato stromal proteome. We provide one of the most robust stromal proteomes available to date with empirical evidence for 545 and 92 proteins not previously described for tomato plastids and the Arabidopsis stroma, respectively. The relative abundance of tomato stromal proteins was determined using the exponentially modified protein abundance index (emPAI). Comparison of the abundance of tomato and Arabidopsis stromal proteomes provided evidence for the species-specific nature of stromal protein homeostasis. The manual curation of the tomato stromal proteome classified proteins into ten functional categories resulting in an accessible compendium of tomato chloroplast proteins. After curation, only 91 proteins remained as unknown, uncharacterized or as enzymes with unknown functions. The curation of the tomato stromal proteins also indicated that tomato has a number of paralogous proteins, not present in Arabidopsis, which accumulated to different levels in chloroplasts. As some of these proteins function in key metabolic pathways or in perceiving or transmitting signals critical for plant adaptation to biotic and abiotic stress, these data suggest that tomato may modulate the bidirectional communication between chloroplasts and nuclei in a novel manner. The stromal proteome provides a fertile ground for future mechanistic studies in the field of tomato chloroplast-nuclear signaling and are foundational for our goal of elucidating the dynamics of the stromal proteome controlled by the solanaceous-specific, stromal, and wound-inducible leucine aminopeptidase A of tomato.

NTRC and TRX-f Coordinately Affect the Levels of Enzymes of Chlorophyll Biosynthesis in a Light-Dependent Manner

Redox regulation of plastid gene expression and different metabolic pathways promotes many activities of redox-sensitive proteins. We address the question of how the plastid redox state and the contributing reducing enzymes control the enzymes of tetrapyrrole biosynthesis (TBS). In higher plants, this metabolic pathway serves to produce chlorophyll and heme, among other essential end products. Because of the strictly light-dependent synthesis of chlorophyll, tight control of TBS requires a diurnal balanced supply of the precursor 5-aminolevulinic acid (ALA) to prevent the accumulation of photoreactive metabolic intermediates in darkness. We report on some TBS enzymes that accumulate in a light intensity-dependent manner, and their contents decrease under oxidizing conditions of darkness, low light conditions, or in the absence of NADPH-dependent thioredoxin reductase (NTRC) and thioredoxin f1 (TRX-f1). Analysis of single and double trxf1 and ntrc mutants revealed a decreased content of the early TBS enzymes glutamyl-tRNA reductase (GluTR) and 5-aminolevulinic acid dehydratase (ALAD) instead of an exclusive decrease in enzyme activity. This effect was dependent on light conditions and strongly attenuated after transfer to high light intensities. Thus, it is suggested that a deficiency of plastid-localized thiol-redox transmitters leads to enhanced degradation of TBS enzymes rather than being directly caused by lower catalytic activity. The effects of the proteolytic activity of the Clp protease on TBS enzymes were studied by using Clp subunit-deficient mutants. The simultaneous lack of TRX and Clp activities in double mutants confirms the Clp-induced degradation of some TBS proteins in the absence of reductive activity of TRXs. In addition, we verified previous observations that decreased chlorophyll and heme levels in ntrc could be reverted to WT levels in the ntrc/Δ2cp triple mutant. The decreased synthesis of 5-aminolevulinic acid and porphobilinogen in ntrc was completely restored in ntrc/Δ2cp and correlated with WT-like levels of GluTR, ALAD, and other TBS proteins.

Plastid 2-Cys peroxiredoxins are essential for embryogenesis in Arabidopsis

The redox couple formed by NADPH-dependent thioredoxin reductase C (NTRC) and 2-Cys peroxiredoxins (Prxs) allows fine-tuning chloroplast performance in response to light intensity changes. Accordingly, the Arabidopsis 2cpab mutant lacking 2-Cys Prxs shows growth inhibition and sensitivity to light stress. However, this mutant also shows defective post-germinative growth, suggesting a relevant role of plastid redox systems in seed development, which is so far unknown. To address this issue, we first analyzed the pattern of expression of NTRC and 2-Cys Prxs in developing seeds. Transgenic lines expressing GFP fusions of these proteins showed their expression in developing embryos, which was low at the globular stage and increased at heart and torpedo stages, coincident with embryo chloroplast differentiation, and confirmed the plastid localization of these enzymes. The 2cpab mutant produced white and abortive seeds, which contained lower and altered composition of fatty acids, thus showing the relevance of 2-Cys Prxs in embryogenesis. Most embryos of white and abortive seeds of the 2cpab mutant were arrested at heart and torpedo stages of embryogenesis suggesting an essential function of 2-Cys Prxs in embryo chloroplast differentiation. This phenotype was not recovered by a mutant version of 2-Cys Prx A replacing the peroxidatic Cys by Ser. Neither the lack nor the overexpression of NTRC had any effect on seed development indicating that the function of 2-Cys Prxs at these early stages of development is independent of NTRC, in clear contrast with the operation of these regulatory redox systems in leaves chloroplasts.

Structural and functional insights into the chloroplast division site regulators PARC6 and PDV1 in the intermembrane space

Chloroplast division involves the coordination of protein complexes from the stroma to the cytosol. The Min system of chloroplasts includes multiple stromal proteins that regulate the positioning of the division site. The outer envelope protein PLASTID DIVISION1 (PDV1) was previously reported to recruit the cytosolic chloroplast division protein ACCUMULATION AND REPLICATION OF CHLOROPLAST5 (ARC5). However, we show here that PDV1 is also important for the stability of the inner envelope chloroplast division protein PARALOG OF ARC6 (PARC6), a component of the Min system. We solved the structure of both the C-terminal domain of PARC6 and its complex with the C terminus of PDV1. The formation of an intramolecular disulfide bond within PARC6 under oxidized conditions prevents its interaction with PDV1. Interestingly, this disulfide bond can be reduced by light in planta, thus promoting PDV1-PARC6 interaction and chloroplast division. Interaction with PDV1 can induce the dimerization of PARC6, which is important for chloroplast division. Magnesium ions, whose concentration in chloroplasts increases upon light exposure, also promote the PARC6 dimerization. This study highlights the multilayer regulation of the PDV1-PARC6 interaction as well as chloroplast division.

Mechanisms of cerium-induced stress in plants: A meta-analysis

A comprehensive evaluation of the effects of cerium on plants is lacking even though cerium is extensively applied to the environment. Here, the effects of cerium on plants were meta-analyzed using a newly developed database consisting of approximately 8500 entries of published data. Cerium affects plants by acting as oxidative stressor causing hormesis, with positive effects at low concentrations and adverse effects at high doses. Production of reactive oxygen species and its linked induction of antioxidant enzymes (e.g. catalase and superoxide dismutase) and non-enzymatic antioxidants (e.g. glutathione) are major mechanisms driving plant response mechanisms. Cerium also affects redox signaling, as indicated by altered GSH/GSSG redox pair, and electrolyte leakage, Ca²⁺, K⁺, and K⁺/Na⁺, indicating an important role of K⁺ and Na⁺ homeostasis in cerium-induced stress and altered mineral (ion) balance. The responses of the plants to cerium are further extended to photosynthesis rate (A), stomatal conductance (g_s), photosynthetic efficiency of PSII, electron transport rate, and quantum yield of PSII. However, photosynthesis response is regulated not only by physiological controls (e.g. g_s), but also by biochemical controls, such as via changed Hill reaction and RuBisCO carboxylation. Cerium concentrations <0.1-25 mg L^-1 commonly enhance chlorophyll a and b, g_s, A, and plant biomass, whereas concentrations >50 mg L^-1 suppress such fitness-critical traits at trait-specific concentrations. There was no evidence that cerium enhances yields. Observations were lacking for yield response to low concentrations of cerium, whereas concentrations >50 mg Kg^-1 suppress yields, in line with the response of chlorophyll a and b. Cerium affects the uptake and tissue concentrations of several micro- and macro-nutrients, including heavy metals. This study enlightens the understanding of some mechanisms underlying plant responses to cerium and provides critical information that can pave the way to reducing the cerium load in the environment and its associated ecological and human health risks.

The ferredoxin/thioredoxin pathway constitutes an indispensable redox-signaling cascade for light-dependent reduction of chloroplast stromal proteins

To ensure efficient photosynthesis, chloroplast proteins need to be flexibly regulated under fluctuating light conditions. Thiol-based redox regulation plays a key role in reductively activating several chloroplast proteins in a light-dependent manner. The ferredoxin (Fd)/thioredoxin (Trx) pathway has long been recognized as the machinery that transfers reducing power generated by photosynthetic electron transport reactions to redox-sensitive target proteins; however, its biological importance remains unclear, because the complete disruption of the Fd/Trx pathway in plants has been unsuccessful to date. Especially, recent identifications of multiple redox-related factors in chloroplasts, as represented by the NADPH-Trx reductase C, have raised a controversial proposal that other redox pathways work redundantly with the Fd/Trx pathway. To address these issues directly, we used CRISPR/Cas9 gene editing to create Arabidopsis mutant plants in which the activity of the Fd/Trx pathway was completely defective. The mutants generated showed severe growth inhibition. Importantly, these mutants almost entirely lost the ability to reduce several redox-sensitive proteins in chloroplast stroma, including four Calvin-Benson cycle enzymes, NADP-malate dehydrogenase, and Rubisco activase, under light conditions. These striking phenotypes were further accompanied by abnormally developed chloroplasts and a drastic decline in photosynthetic efficiency. These results indicate that the Fd/Trx pathway is indispensable for the light-responsive activation of diverse stromal proteins and photoautotrophic growth of plants. Our data also suggest that the ATP synthase is exceptionally reduced by other pathways in a redundant manner. This study provides an important insight into how the chloroplast redox-regulatory system operates in vivo.

Back to the future: Transplanting the chloroplast TrxF-FBPase-SBPase redox system to cyanobacteria

Fructose-1,6-bisphosphatase (FBPase) and sedoheptulose-1,7-bisphosphatase (SBPase) are two essential activities in the Calvin-Benson-Bassham cycle that catalyze two irreversible reactions and are key for proper regulation and functioning of the cycle. These two activities are codified by a single gene in all cyanobacteria, although some cyanobacteria contain an additional gene coding for a FBPase. Mutants lacking the gene coding for SBP/FBPase protein are not able to grow photoautotrophically and require glucose to survive. As this protein presents both activities, we have tried to elucidate which of the two are required for photoautrophic growth in Synechocystis sp PCC 6803. For this, the genes coding for plant FBPase and SBPase were introduced in a SBP/FBPase mutant strain, and the strains were tested for growth in the absence of glucose. Ectopic expression of only a plant SBPase gene did not allow growth in the absence of glucose although allowed mutation of both Synechocystis' FBPase genes. When both plant FBPase and SBPase genes were expressed, photoautrophic growth of the SBP/FBPase mutants was restored. This complementation was partial as the strain only grew in low light, but growth was impaired at higher light intensities. Redox regulation of the Calvin-Benson-Bassham cycle is essential to properly coordinate light reactions to carbon fixation in the chloroplast. Two of the best characterized proteins that are redox-regulated in the cycle are FBPase and SBPase. These two proteins are targets of the FTR-Trx redox system with Trx f being the main reductant in vivo. Introduction of the TrxF gene improves growth of the complemented strain, suggesting that the redox state of the proteins may be the cause of this phenotype. The redox state of the plant proteins was also checked in these strains, and it shows that the cyanobacterial redox system is able to reduce all of them (SBPase, FBPase, and TrxF) in a light-dependent manner. Thus, the TrxF-FBPase-SBPase plant chloroplast system is active in cyanobacteria despite that these organisms do not contain proteins related to them. Furthermore, our system opens the possibility to study specificity of the Trx system in vivo without the complication of the different isoforms present in plants.

The circadian night depression of photosynthesis analyzed in a herb, Pulmonaria vallarsae. Day/night quantitative relationships

Although many photosynthesis related processes are known to be controlled by the circadian system, consequent changes in photosynthetic activities are poorly understood. Photosynthesis was investigated during the daily cycle by chlorophyll fluorescence using a PAM fluorometer in Pulmonaria vallarsae subsp. apennina, an understory herb. A standard test consists of a light induction pretreatment followed by light response curve (LRC). Comparison of the major diagnostic parameters collected during day and night showed a nocturnal drop of photosynthetic responses, more evident in water-limited plants and consisting of: (i) strong reduction of flash-induced fluorescence peaks (FIP), maximum linear electron transport rate (Jmax, ETR_EM) and effective PSII quantum yield (Φ_PSII); (ii) strong enhancement of nonphotochemical quenching (NPQ) and (iii) little or no change in photochemical quenching qP, maximum quantum yield of linear electron transport (Φ), and shape of LRC (θ). A remarkable feature of day/night LRCs at moderate to high irradiance was their linear-parallel course in double-reciprocal plots. Photosynthesis was also monitored in plants subjected to 2-3 days of continuous darkness ("long night"). In such conditions, plants exhibited high but declining peaks of photosynthetic activity during subjective days and a low, constant value with elevated NPQ during subjective night tests. The photosynthetic parameters recorded in subjective days in artificial darkness resembled those under natural day conditions. On the basis of the evidence, we suggest a circadian component and a biochemical feedback inhibition to explain the night depression of photosynthesis in P. vallarsae.

Molecular Regulation and Evolution of Redox Homeostasis in Photosynthetic Machinery

The recent advances in plant biology have significantly improved our understanding of reactive oxygen species (ROS) as signaling molecules in the redox regulation of complex cellular processes. In plants, free radicals and non-radicals are prevalent intra- and inter-cellular ROS, catalyzing complex metabolic processes such as photosynthesis. Photosynthesis homeostasis is maintained by thiol-based systems and antioxidative enzymes, which belong to some of the evolutionarily conserved protein families. The molecular and biological functions of redox regulation in photosynthesis are usually to balance the electron transport chain, photosystem II, photosystem I, mesophyll and bundle sheath signaling, and photo-protection regulating plant growth and productivity. Here, we review the recent progress of ROS signaling in photosynthesis. We present a comprehensive comparative bioinformatic analysis of redox regulation in evolutionary distinct photosynthetic cells. Gene expression, phylogenies, sequence alignments, and 3D protein structures in representative algal and plant species revealed conserved key features including functional domains catalyzing oxidation and reduction reactions. We then discuss the antioxidant-related ROS signaling and important pathways for achieving homeostasis of photosynthesis. Finally, we highlight the importance of plant responses to stress cues and genetic manipulation of disturbed redox status for balanced and enhanced photosynthetic efficiency and plant productivity.

Insight into the regulatory networks underlying the high lipid perennial ryegrass growth under different irradiances

Under favourable conditions, perennial ryegrass (Lolium perenne) engineered to accumulated high lipid (HL) carbon sink in their leaves was previously shown to also enhance photosynthesis and growth. The greater aboveground biomass was found to be diminished in a dense canopy compared to spaced pots. Besides, the underlying genetic regulatory network linking between leaf lipid sinks and these physiological changes remains unknown. In this study, we demonstrated that the growth advantage was not displayed in HL Lolium grown in spaced pots under low lights. Under standard lights, analysis of differentiating transcripts in HL Lolium reveals that the plants had elevated transcripts involved in lipid metabolism, light capturing, photosynthesis, and sugar signalling while reduced expression of genes participating in sugar biosynthesis and transportation. The plants also had altered several transcripts involved in mitochondrial oxidative respiration and redox potential. Many of the above upregulated or downregulated transcript levels were found to be complemented by growing the plants under low light. Overall, this study emphasizes the importance of carbon and energy homeostatic regulatory mechanisms to overall productivity of the HL Lolium through photosynthesis, most of which are significantly impacted by low irradiances.

Reduction in Phosphoribulokinase Amount and Re-Routing Metabolism in Chlamydomonas reinhardtii CP12 Mutants

The chloroplast protein CP12 is involved in the dark/light regulation of the Calvin–Benson–Bassham cycle, in particular, in the dark inhibition of two enzymes: glyceraldehyde−3-phosphate dehydrogenase (GAPDH) and phosphoribulokinase (PRK), but other functions related to stress have been proposed. We knocked out the unique CP12 gene to prevent its expression in Chlamydomonas reinhardtii (ΔCP12). The growth rates of both wild-type and ΔCP12 cells were nearly identical, as was the GAPDH protein abundance and activity in both cell lines. On the contrary, the abundance of PRK and its specific activity were significantly reduced in ΔCP12, as revealed by relative quantitative proteomics. Isolated PRK lost irreversibly its activity over-time in vitro, which was prevented in the presence of recombinant CP12 in a redox-independent manner. We have identified amino acid residues in the CP12 protein that are required for this new function preserving PRK activity. Numerous proteins involved in redox homeostasis and stress responses were more abundant and the expressions of various metabolic pathways were also increased or decreased in the absence of CP12. These results highlight CP12 as a moonlighting protein with additional functions beyond its well-known regulatory role in carbon metabolism.

Low light-regulated intramolecular disulfide fine-tunes the role of PTOX in Arabidopsis

Disulfide-based regulation links the activity of numerous chloroplast proteins with photosynthesis-derived redox signals. The plastid terminal oxidase (PTOX) is a thylakoid-bound plastoquinol oxidase that has been implicated in multiple roles in the light and in the dark, which could require different levels of PTOX activity. Here we show that Arabidopsis PTOX contains a conserved C-terminus domain (CTD) with cysteines that evolved progressively following the colonization of the land by plants. Furthermore, the CTD contains a regulatory disulfide that is in the oxidized state in the dark and is rapidly reduced, within 5 min, in low light intensity (1-5 µE m^-2  sec^-1 ). The reduced PTOX form in the light was reoxidized within 15 min after transition to the dark. Mutation of the cysteines in the CTD prevented the formation of the oxidized form. This resulted in higher levels of reduced plastoquinone when measured at transition to the onset of low light. This is consistent with the reduced state of PTOX exhibiting diminished PTOX oxidase activity under conditions of limiting PQH₂ substrate. Our findings suggest that AtPTOX-CTD evolved to provide light-dependent regulation of PTOX activity for the adaptation of plants to terrestrial conditions.

Structural analysis revealed a novel conformation of the NTRC reductase domain from Chlamydomonas reinhardtii

In plant chloroplasts, thiol regulation is driven by two systems. One relies on the activity of thioredoxins through their light dependent reduction by ferredoxin via a ferredoxin-thioredoxin reductase (FTR). In the other system, a NADPH-dependent redox regulation is driven by a NADPH-thioredoxin reductase C (NTRC). While the thioredoxin system has been deeply studied, a more thorough understanding of the function of this plant specific NTRC is desirable. NTRC is a single polypeptide harbouring a thioredoxin domain (Trx) at the C-terminus of a NADPH-dependent Thioredoxin reductase (TrxR). To provide functional and structural insights, we studied the crystal structure of the TrxR domain of the NTRC from Chlamydomonas reinhardtii (CrNTRC, Cre01.g054150.t1.2) and its Cys136Ser (C136S) mutant, which is characterized by the mutation of the resolving cysteine in the active site of the TrxR domain. Furthermore, we confirmed the role of NTRC as electron donor for 2-Cys peroxiredoxin (PRX) also in C. reinhardtii. The structural data of TrxR were employed to develop a scheme of action which addresses electron transfer between TrxR and Trx of NTRC and between NTRC and its substrates.

Oxidative regulation of chloroplast enzymes by thioredoxin and thioredoxin-like proteins in Arabidopsis thaliana

Plants modulate photosynthesis activity in response to the surrounding environment. It is well known that the redox-responsive protein thioredoxin (Trx) activates photosynthesis-related enzymes in the light. However, the factors involved in deactivating them are not well understood. Recent in vitro experiments suggest that several Trx and Trx-like proteins serve as oxidation factors for Trx-targeted proteins; thus, we examined their functions in vivo. Consequently, we found that f-type Trx and two types of Trx-like proteins, Trx-like 2 and atypical Cys His-rich Trx, were involved in oxidative deactivation of photosynthesis-related enzymes (e.g., fructose-1,6-bisphosphatase, Rubisco activase, and the ATP synthase γ-subunit). Thus, this study reveals the functions of oxidation factors in vivo and elucidates the regulation system for photosynthesis in the dark.

Thioredoxin (Trx) is a protein that mediates the reducing power transfer from the photosynthetic electron transport system to target enzymes in chloroplasts and regulates their activities. Redox regulation governed by Trx is a system that is central to the adaptation of various chloroplast functions to the ever-changing light environment. However, the factors involved in the opposite reaction (i.e., the oxidation of various enzymes) have yet to be revealed. Recently, it has been suggested that Trx and Trx-like proteins could oxidize Trx-targeted proteins in vitro. To elucidate the in vivo function of these proteins as oxidation factors, we generated mutant plant lines deficient in Trx or Trx-like proteins and studied how the proteins are involved in oxidative regulation in chloroplasts. We found that f-type Trx and two types of Trx-like proteins, Trx-like 2 and atypical Cys His-rich Trx (ACHT), seemed to serve as oxidation factors for Trx-targeted proteins, such as fructose-1,6-bisphosphatase, Rubisco activase, and the γ-subunit of ATP synthase. In addition, ACHT was found to be involved in regulating nonphotochemical quenching, which is the mechanism underlying the thermal dissipation of excess light energy. Overall, these results indicate that Trx and Trx-like proteins regulate chloroplast functions in concert by controlling the redox state of various photosynthesis-related proteins in vivo.

Light potentials of photosynthetic energy storage in the field: what limits the ability to use or dissipate rapidly increased light energy?

The responses of plant photosynthesis to rapid fluctuations in environmental conditions are critical for efficient conversion of light energy. These responses are not well-seen laboratory conditions and are difficult to probe in field environments. We demonstrate an open science approach to this problem that combines multifaceted measurements of photosynthesis and environmental conditions, and an unsupervised statistical clustering approach. In a selected set of data on mint (Mentha sp.), we show that 'light potentials' for linear electron flow and non-photochemical quenching (NPQ) upon rapid light increases are strongly suppressed in leaves previously exposed to low ambient photosynthetically active radiation (PAR) or low leaf temperatures, factors that can act both independently and cooperatively. Further analyses allowed us to test specific mechanisms. With decreasing leaf temperature or PAR, limitations to photosynthesis during high light fluctuations shifted from rapidly induced NPQ to photosynthetic control of electron flow at the cytochrome b6f complex. At low temperatures, high light induced lumen acidification, but did not induce NPQ, leading to accumulation of reduced electron transfer intermediates, probably inducing photodamage, revealing a potential target for improving the efficiency and robustness of photosynthesis. We discuss the implications of the approach for open science efforts to understand and improve crop productivity.

Autophagy Is Involved in the Viability of Overexpressing Thioredoxin o1 Tobacco BY-2 Cells under Oxidative Conditions

Autophagy is an essential process for the degradation of non-useful components, although the mechanism involved in its regulation is less known in plants than in animal systems. Redox regulation of autophagy components is emerging as a possible key mechanism with thioredoxins (TRXs) proposed as involved candidates. In this work, using overexpressing PsTRXo1 tobacco cells (OEX), which present higher viability than non-overexpressing cells after H₂O₂ treatment, we examine the functional interaction of autophagy and PsTRXo1 in a collaborative response. OEX cells present higher gene expression of the ATG (Autophagy related) marker ATG4 and higher protein content of ATG4, ATG8, and lipidated ATG8 as well as higher ATG4 activity than control cells, supporting the involvement of autophagy in their response to H₂O₂. In this oxidative situation, autophagy occurs in OEX cells as is evident from an accumulation of autolysosomes and ATG8 immunolocalization when the E-64d autophagy inhibitor is used. Interestingly, cell viability decreases in the presence of the inhibitor, pointing to autophagy as being involved in cell survival. The in vitro interaction of ATG4 and PsTRXo1 proteins is confirmed by dot-blot and co-immunoprecipitation assays as well as the redox regulation of ATG4 activity by PsTRXo1. These findings extend the role of TRXs in mediating the redox regulation of the autophagy process in plant cells.

Monitoring cellular redox dynamics using newly developed BRET-based redox sensor proteins

Reactive oxygen species are key factors that strongly affect the cellular redox state and regulate various physiological and cellular phenomena. To monitor changes in the redox state, we previously developed fluorescent redox sensors named Re-Q, the emissions of which are quenched under reduced conditions. However, such fluorescent probes are unsuitable for use in the cells of photosynthetic organisms because they require photoexcitation that may change intracellular conditions and induce autofluorescence, primarily in chlorophylls. In addition, the presence of various chromophore pigments may interfere with fluorescence-based measurements because of their strong absorbance. To overcome these problems, we adopted the bioluminescence resonance energy transfer (BRET) mechanism for the sensor and developed two BRET-based redox sensors by fusing cyan fluorescent protein–based or yellow fluorescent protein–based Re-Q with the luminescent protein Nluc. We named the resulting redox-sensitive BRET-based indicator probes “ROBINc” and “ROBINy.” ROBINc is pH insensitive, which is especially vital for observation in photosynthetic organisms. By using these sensors, we successfully observed dynamic redox changes caused by an anticancer agent in HeLa cells and light/dark-dependent redox changes in the cells of photosynthetic cyanobacterium Synechocystis sp. PCC 6803. Since the newly developed sensors do not require excitation light, they should be especially useful for visualizing intracellular phenomena caused by redox changes in cells containing colored pigments.

A chloroplast redox relay adapts plastid metabolism to light and affects cytosolic protein quality control

In chloroplasts, thiol-dependent redox regulation is linked to light since the disulfide reductase activity of thioredoxins (Trxs) relies on photo-reduced ferredoxin (Fdx). Furthermore, chloroplasts harbor an NADPH-dependent Trx reductase (NTR) with a joint Trx domain, termed NTRC. The activity of these two redox systems is integrated by the redox balance of 2-Cys peroxiredoxin (Prx), which is controlled by NTRC. However, NTRC was proposed to participate in redox regulation of additional targets, prompting inquiry into whether the function of NTRC depends on its capacity to maintain the redox balance of 2-Cys Prxs or by direct redox interaction with chloroplast enzymes. To answer this, we studied the functional relationship of NTRC and 2-Cys Prxs by a comparative analysis of the triple Arabidopsis (Arabidopsis thaliana) mutant, ntrc-2cpab, which lacks NTRC and 2-Cys Prxs, and the double mutant 2cpab, which lacks 2-Cys Prxs. These mutants exhibit almost indistinguishable phenotypes: in growth rate, photosynthesis performance, and redox regulation of chloroplast enzymes in response to light and darkness. These results suggest that the most relevant function of NTRC is in controlling the redox balance of 2-Cys Prxs. A comparative transcriptomics analysis confirmed the phenotypic similarity of the two mutants and suggested that the NTRC-2-Cys Prxs system participates in cytosolic protein quality control. We propose that NTRC and 2-Cys Prxs constitute a redox relay, exclusive to photosynthetic organisms that fine-tunes the redox state of chloroplast enzymes in response to light and affects transduction pathways towards the cytosol.

Barley stripe mosaic virus γb protein disrupts chloroplast antioxidant defenses to optimize viral replication

The plant antioxidant system plays important roles in response to diverse abiotic and biotic stresses. However, the effects of virus infection on host redox homeostasis and how antioxidant defense pathway is manipulated by viruses remain poorly understood. We previously demonstrated that the Barley stripe mosaic virus (BSMV) γb protein is recruited to the chloroplast by the viral αa replicase to enhance viral replication. Here, we show that BSMV infection induces chloroplast oxidative stress. The versatile γb protein interacts directly with NADPH-dependent thioredoxin reductase C (NTRC), a core component of chloroplast antioxidant systems. Overexpression of NbNTRC significantly impairs BSMV replication in Nicotiana benthamiana plants, whereas disruption of NbNTRC expression leads to increased viral accumulation and infection severity. To counter NTRC-mediated defenses, BSMV employs the γb protein to competitively interfere with NbNTRC binding to 2-Cys Prx. Altogether, this study indicates that beyond acting as a helicase enhancer, γb also subverts NTRC-mediated chloroplast antioxidant defenses to create an oxidative microenvironment conducive to viral replication.

Chloroplast dismantling in leaf senescence

In photosynthetic plant cells, chloroplasts act as factories of metabolic intermediates that support plant growth. Chloroplast performance is highly influenced by environmental cues. Thus, these organelles have the additional function of sensing ever changing environmental conditions, thereby playing a key role in harmonizing the growth and development of different organs and in plant acclimation to the environment. Moreover, chloroplasts constitute an excellent source of metabolic intermediates that are remobilized to sink tissues during senescence so that chloroplast dismantling is a tightly regulated process that plays a key role in plant development. Stressful environmental conditions enhance the generation of reactive oxygen species (ROS) by chloroplasts, which may lead to oxidative stress causing damage to the organelle. These environmental conditions trigger mechanisms that allow the rapid dismantling of damaged chloroplasts, which is crucial to avoid deleterious effects of toxic by-products of the degradative process. In this review, we discuss the effect of redox homeostasis and ROS generation in the process of chloroplast dismantling. Furthermore, we summarize the structural and biochemical events, both intra- and extraplastid, that characterize the process of chloroplast dismantling in senescence and in response to environmental stresses.

Chloroplast Lipids Metabolism and Function. A Redox Perspective

Plant productivity is determined by the conversion of solar energy into biomass through oxygenic photosynthesis, a process performed by protein-cofactor complexes including photosystems (PS) II and I, and ATP synthase. These complexes are embedded in chloroplast thylakoid membrane lipids, which thus function as structural support of the photosynthetic machinery and provide the lipid matrix to avoid free ion diffusion. The lipid and fatty acid composition of thylakoid membranes are unique in chloroplasts and cyanobacteria, which implies that these molecules are specifically required in oxygenic photosynthesis. Indeed, there is extensive evidence supporting a relevant function of glycerolipids in chloroplast biogenesis and photosynthetic efficiency in response to environmental stimuli, such as light and temperature. The rapid acclimation of higher plants to environmental changes is largely based on thiol-based redox regulation and the disulphide reductase activity thioredoxins (Trxs), which are reduced by ferredoxin (Fdx) via an Fdx-dependent Trx reductase. In addition, chloroplasts harbour an NADPH-dependent Trx reductase C, which allows the use of NADPH to maintain the redox homeostasis of the organelle. Here, we summarise the current knowledge of chloroplast lipid metabolism and the function of these molecules as structural basis of the complex membrane network of the organelle. Furthermore, we discuss evidence supporting the relevant role of lipids in chloroplast biogenesis and photosynthetic performance in response to environmental cues in which the redox state of the organelle plays a relevant role.

Biochemical Basis for Redox Regulation of Chloroplast-Localized Phosphofructokinase from Arabidopsis thaliana

Various proteins in plant chloroplasts are subject to thiol-based redox regulation, allowing light-responsive control of chloroplast functions. Most redox-regulated proteins are known to be reductively activated in the light in a thioredoxin (Trx)-dependent manner, but its regulatory network remains incompletely understood. Using a biochemical procedure, we here show that a specific form of phosphofructokinase (PFK) is a novel redox-regulated protein whose activity is suppressed upon reduction. PFK is a key enzyme in the glycolytic pathway. In Arabidopsis thaliana, PFK5 is targeted to chloroplasts and uniquely contains an insertion sequence harboring two Cys residues (Cys152 and Cys157) in the N-terminal region. Redox shift assays using a thiol-modifying reagent indicated that PFK5 is efficiently reduced by a specific type of Trx, namely, Trx-f. PFK5 enzyme activity was lowered with the Trx-f-dependent reduction. PFK5 redox regulation was bidirectional; PFK5 was also oxidized and activated by the recently identified Trx-like2/2-Cys peroxiredoxin pathway. Mass spectrometry-based peptide mapping analysis revealed that Cys152 and Cys157 are critical for the intramolecular disulfide bond formation in PFK5. The involvement of Cys152 and Cys157 in PFK5 redox regulation was further supported by a site-directed mutagenesis study. PFK5 catalyzes the reverse reaction of fructose 1,6-bisphosphatase (FBPase), which is reduced and activated specifically by Trx-f. Our data suggest that PFK5 redox regulation, together with that of FBPase, constitutes a checkpoint for switching light/dark metabolism in chloroplasts.

Crystal structure of chloroplastic thioredoxin z defines a type-specific target recognition

Thioredoxins (TRXs) are ubiquitous disulfide oxidoreductases structured according to a highly conserved fold. TRXs are involved in a myriad of different processes through a common chemical mechanism. Plant TRXs evolved into seven types with diverse subcellular localization and distinct protein target selectivity. Five TRX types coexist in the chloroplast, with yet scarcely described specificities. We solved the crystal structure of a chloroplastic z-type TRX, revealing a conserved TRX fold with an original electrostatic surface potential surrounding the redox site. This recognition surface is distinct from all other known TRX types from plant and non-plant sources and is exclusively conserved in plant z-type TRXs. We show that this electronegative surface endows thioredoxin z (TRXz) with a capacity to activate the photosynthetic Calvin-Benson cycle enzyme phosphoribulokinase. The distinct electronegative surface of TRXz thereby extends the repertoire of TRX-target recognitions.

NTRC Effects on Non-Photochemical Quenching Depends on PGR5

Non-photochemical quenching (NPQ) protects plants from the detrimental effects of excess light. NPQ is rapidly induced by the trans-thylakoid proton gradient during photosynthesis, which in turn requires PGR5/PGRL1-dependent cyclic electron flow (CEF). Thus, Arabidopsis thaliana plants lacking either protein cannot induce transient NPQ and die under fluctuating light conditions. Conversely, the NADPH-dependent thioredoxin reductase C (NTRC) is required for efficient energy utilization and plant growth, and in its absence, transient and steady-state NPQ is drastically increased. How NTRC influences NPQ and functionally interacts with CEF is unclear. Therefore, we generated the A. thaliana line pgr5 ntrc, and found that the inactivation of PGR5 suppresses the high transient and steady-state NPQ and impaired growth phenotypes observed in the ntrc mutant under short-day conditions. This implies that NTRC negatively influences PGR5 activity and, accordingly, the lack of NTRC is associated with decreased levels of PGR5, possibly pointing to a mechanism to restrict upregulation of PGR5 activity in the absence of NTRC. When exposed to high light intensities, pgr5 ntrc plants display extremely impaired photosynthesis and growth, indicating additive effects of lack of both proteins. Taken together, these findings suggest that the interplay between NTRC and PGR5 is relevant for photoprotection and that NTRC might regulate PGR5 activity.

Redox regulation of chloroplast metabolism

Regulation of enzyme activity based on thiol-disulfide exchange is a regulatory mechanism in which the protein disulfide reductase activity of thioredoxins (TRXs) plays a central role. Plant chloroplasts are equipped with a complex set of up to 20 TRXs and TRX-like proteins, the activity of which is supported by reducing power provided by photosynthetically reduced ferredoxin (FDX) with the participation of a FDX-dependent TRX reductase (FTR). Therefore, the FDX-FTR-TRXs pathway allows the regulation of redox-sensitive chloroplast enzymes in response to light. In addition, chloroplasts contain an NADPH-dependent redox system, termed NTRC, which allows the use of NADPH in the redox network of these organelles. Genetic approaches using mutants of Arabidopsis (Arabidopsis thaliana) in combination with biochemical and physiological studies have shown that both redox systems, NTRC and FDX-FTR-TRXs, participate in fine-tuning chloroplast performance in response to changes in light intensity. Moreover, these studies revealed the participation of 2-Cys peroxiredoxin (2-Cys PRX), a thiol-dependent peroxidase, in the control of the reducing activity of chloroplast TRXs as well as in the rapid oxidation of stromal enzymes upon darkness. In this review, we provide an update on recent findings regarding the redox regulatory network of plant chloroplasts, focusing on the functional relationship of 2-Cys PRXs with NTRC and the FDX-FTR-TRXs redox systems for fine-tuning chloroplast performance in response to changes in light intensity and darkness. Finally, we consider redox regulation as an additional layer of control of the signaling function of the chloroplast.

Chloroplast-derived photo-oxidative stress causes changes in H2O2 and EGSH in other subcellular compartments

Metabolic fluctuations in chloroplasts and mitochondria can trigger retrograde signals to modify nuclear gene expression. Mobile signals likely to be involved are reactive oxygen species (ROS), which can operate protein redox switches by oxidation of specific cysteine residues. Redox buffers, such as the highly reduced glutathione pool, serve as reservoirs of reducing power for several ROS-scavenging and ROS-induced damage repair pathways. Formation of glutathione disulfide and a shift of the glutathione redox potential (EGSH) toward less negative values is considered as hallmark of several stress conditions. Here we used the herbicide methyl viologen (MV) to generate ROS locally in chloroplasts of intact Arabidopsis (Arabidopsis thaliana) seedlings and recorded dynamic changes in EGSH and H2O2 levels with the genetically encoded biosensors Grx1-roGFP2 (for EGSH) and roGFP2-Orp1 (for H2O2) targeted to chloroplasts, the cytosol, or mitochondria. Treatment of seedlings with MV caused rapid oxidation in chloroplasts and, subsequently, in the cytosol and mitochondria. MV-induced oxidation was significantly boosted by illumination with actinic light, and largely abolished by inhibitors of photosynthetic electron transport. MV also induced autonomous oxidation in the mitochondrial matrix in an electron transport chain activity-dependent manner that was milder than the oxidation triggered in chloroplasts by the combination of MV and light. In vivo redox biosensing resolves the spatiotemporal dynamics of compartmental responses to local ROS generation and provides a basis for understanding how compartment-specific redox dynamics might operate in retrograde signaling and stress acclimation in plants.

Flexibility of Oxidized and Reduced States of the Chloroplast Regulatory Protein CP12 in Isolation and in Cell Extracts

In the chloroplast, Calvin–Benson–Bassham enzymes are active in the reducing environment created in the light by electrons from the photosystems. In the dark, these enzymes are inhibited, mainly caused by oxidation of key regulatory cysteine residues. CP12 is a small protein that plays a role in this regulation with four cysteine residues that undergo a redox transition. Using amide-proton exchange with solvent, measured by nuclear magnetic resonance (NMR) and mass-spectrometry, we confirmed that reduced CP12 is intrinsically disordered. Using real-time NMR, we showed that the oxidation of the two disulfide bridges is simultaneous. In oxidized CP12, the C₂₃–C₃₁ pair is in a region that undergoes a conformational exchange in the NMR-intermediate timescale. The C₆₆–C₇₅ pair is in the C-terminus that folds into a stable helical turn. We confirmed that these structural states exist in a physiologically relevant environment: a cell extract from Chlamydomonas reinhardtii. Consistent with these structural equilibria, the reduction is slower for the C₆₆–C₇₅ pair than for the C₂₃–C₃₁ pair. The redox mid-potentials for the two cysteine pairs differ and are similar to those found for glyceraldehyde 3-phosphate dehydrogenase and phosphoribulokinase, consistent with the regulatory role of CP12.

Evaluation of CBSX Proteins as Regulators of the Chloroplast Thioredoxin System

The chloroplast-localized cystathionine β-synthase X (CBSX) proteins CBSX1 and CBSX2 have been proposed as modulators of thioredoxins (Trxs). In this study, the contribution of CBSX proteins to the redox regulation of thiol enzymes in the chloroplast Trx system was evaluated both in vitro and in vivo. The in vitro biochemical studies evaluated whether CBSX proteins alter the specificities of classical chloroplastic Trx f and Trx m for their target proteins. However, addition of CBSX proteins did not alter the specificities of Trx f and Trx m for disulfide bond reduction of the photosynthesis-related major thiol enzymes, FBPase, SBPase, and NADP-MDH. In vivo analysis showed that CBSX-deficient mutants grew similarly to wild type plants under continuous normal light conditions and that CBSX deficiency did not affect photo-reduction of photosynthesis-related thiol enzymes by Trx system at several light intensities. Although CBSX proteins have been suggested as modulators in the chloroplast Trx system, our results did not support this model, at least in the cases of FBPase, SBPase, and NADP-MDH in leaves. However, fresh weights of the cbsx2 mutants were decreased under short day. Since Trxs regulate many proteins participating in various metabolic reactions in the chloroplast, CBSX proteins may function to regulate other chloroplast Trx target proteins, or serve as modulators in non-photosynthetic plastids of flowers. As a next stage, further investigations are required to understand the modulation of Trx-dependent redox regulation by plastidal CBSX proteins.

Decreased Levels of Thioredoxin o1 Influences Stomatal Development and Aperture but Not Photosynthesis under Non-Stress and Saline Conditions

Salinity has a negative impact on plant growth, with photosynthesis being downregulated partially due to osmotic effect and enhanced cellular oxidation. Redox signaling contributes to the plant response playing thioredoxins (TRXs) a central role. In this work we explore the potential contribution of Arabidopsis TRXo1 to the photosynthetic response under salinity analyzing Arabidopsis wild-type (WT) and two Attrxo1 mutant lines in their growth under short photoperiod and higher light intensity than previous reported works. Stomatal development and apertures and the antioxidant, hormonal and metabolic acclimation are also analyzed. In control conditions mutant plants displayed less and larger developed stomata and higher pore size which could underlie their higher stomatal conductance, without being affected in other photosynthetic parameters. Under salinity, all genotypes displayed a general decrease in photosynthesis and the oxidative status in the Attrxo1 mutant lines was altered, with higher levels of H₂O₂ and NO but also higher ascorbate/glutathione (ASC/GSH) redox states than WT plants. Finally, sugar changes and increases in abscisic acid (ABA) and NO may be involved in the observed higher stomatal response of the TRXo1-altered plants. Therefore, the lack of AtTRXo1 affected stomata development and opening and the mutants modulate their antioxidant, metabolic and hormonal responses to optimize their adaptation to salinity.

M-Type Thioredoxins Regulate the PGR5/PGRL1-Dependent Pathway by Forming a Disulfide-Linked Complex with PGRL1

In addition to linear electron transport, photosystem I cyclic electron transport (PSI-CET) contributes to photosynthesis and photoprotection. In Arabidopsis (Arabidopsis thaliana), PSI-CET consists of two partially redundant pathways, one of which is the PROTON GRADIENT REGULATION5 (PGR5)/PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE1 (PGRL1)-dependent pathway. Although the physiological significance of PSI-CET is widely recognized, the regulatory mechanism behind these pathways remains largely unknown. Here, we report on the regulation of the PGR5/PGRL1-dependent pathway by the m-type thioredoxins (Trx m). Genetic and phenotypic characterizations of multiple mutants indicated the physiological interaction between Trx m and the PGR5/PGRL1-dependent pathway in vivo. Using purified Trx proteins and ruptured chloroplasts, in vitro, we showed that the reduced form of Trx m specifically decreased the PGR5/PGRL1-dependent plastoquinone reduction. In planta, Trx m4 directly interacted with PGRL1 via disulfide complex formation. Analysis of the transgenic plants expressing PGRL1 Cys variants demonstrated that Cys-123 of PGRL1 is required for Trx m4-PGRL1 complex formation. Furthermore, the Trx m4-PGRL1 complex was transiently dissociated during the induction of photosynthesis. We propose that Trx m directly regulates the PGR5/PGRL1-dependent pathway by complex formation with PGRL1.

Comparative physiological and transcriptomic analysis of pear leaves under distinct training systems

Canopy architecture is critical in determining the light interception and distribution, and subsequently the photosynthetic efficiency and productivity. However, the physiological responses and molecular mechanisms by which pear canopy architectural traits impact on photosynthesis remain poorly understood. Here, physiological investigations coupled with comparative transcriptomic analyses were performed in pear leaves under distinct training systems. Compared with traditional freestanding system, flat-type trellis system (DP) showed higher net photosynthetic rate (P_N) levels at the most time points throughout the entire monitored period, especially for the interior of the canopy in sunny side. Gene ontology analysis revealed that photosynthesis, carbohydrate derivative catabolic process and fatty acid metabolic process were over-represented in leaves of DP system with open-canopy characteristics. Weighted gene co-expression network analysis uncovered a significant network module positive correlated with P_N value. The hub genes (PpFKF1 and PpPRR5) of the module were enriched in circadian rhythm pathway, suggesting a functional role for circadian clock genes in mediating photosynthetic performance under distinct training systems. These results draw a link between pear photosynthetic response and specific canopy architectural traits, and highlight light harvesting and circadian clock network as potential targets for the input signals from the fluctuating light availability under distinct training systems.

Two chloroplast thioredoxin systems differentially modulate photosynthesis in Arabidopsis depending on light intensity and leaf age

Various regulatory mechanisms have evolved in plants to optimize photosynthetic activity under fluctuating light. Thioredoxins (TRX) are members of the regulatory network balancing activities of light and carbon fixation reactions in chloroplasts. We have studied the impact of two chloroplast TRX systems, the ferredoxin-dependent TRX reductase (FTR) and the NADPH-dependent TRX reductase C (NTRC) on regulation of photosynthesis by mutants lacking or overexpressing a component of either system. Plants were subjected to image-based phenotyping and chlorophyll fluorescence measurements that allow long-term monitoring of the development and photosynthetic activity of the rosettes, respectively. Our experiments demonstrate that NTRC and FTR systems respond differently to variation of light intensity. NTRC was an indispensable regulator of photosynthesis in young leaves, at light-intensity transitions and under low light intensities limiting photosynthesis, whereas steady-state exposure of plants to growth or higher light intensities diminished the need of NTRC in regulation of photosynthesis. In fluctuating light, overexpression of NTRC increased the quantum yield of Photosystem II (YII) at low light and stimulated the relaxation of non-photochemical quenching (NPQ) after high light exposure, indicating that overexpression of NTRC improves leaf capacity to convert light energy to chemical energy under these conditions. Overexpression of chimeric protein (NTR-TRXf) containing both the thioredoxin reductase and TRXf activity on an ntrc mutant background, did not completely recover either growth or steady-state photosynthetic activity, whereas OE-NTR-TRXf plants exposed to fluctuating light regained the wild-type level of Y(II) and NPQ.

Exploring the Functional Relationship between y-Type Thioredoxins and 2-Cys Peroxiredoxins in Arabidopsis Chloroplasts

Thioredoxins (Trxs) are small, ubiquitous enzymes that catalyze disulphide–dithiol interchange in target enzymes. The large set of chloroplast Trxs, including f, m, x and y subtypes, use reducing equivalents fueled by photoreduced ferredoxin (Fdx) for fine-tuning photosynthetic performance and metabolism through the control of the activity of redox-sensitive proteins. Although biochemical analyses suggested functional diversity of chloroplast Trxs, genetic studies have established that deficiency in a particular Trx subtype has subtle phenotypic effects, leading to the proposal that the Trx isoforms are functionally redundant. In addition, chloroplasts contain an NADPH-dependent Trx reductase with a joint Trx domain, termed NTRC. Interestingly, Arabidopsis mutants combining the deficiencies of x- or f-type Trxs and NTRC display very severe growth inhibition phenotypes, which are partially rescued by decreased levels of 2-Cys peroxiredoxins (Prxs). These findings indicate that the reducing capacity of Trxs f and x is modulated by the redox balance of 2-Cys Prxs, which is controlled by NTRC. In this study, we explored whether NTRC acts as a master regulator of the pool of chloroplast Trxs by analyzing its functional relationship with Trxs y. While Trx y interacts with 2-Cys Prxs in vitro and in planta, the analysis of Arabidopsis mutants devoid of NTRC and Trxs y suggests that Trxs y have only a minor effect, if any, on the redox state of 2-Cys Prxs.

Biochemical insight into redox regulation of plastidial 3-phosphoglycerate dehydrogenase from Arabidopsis thaliana

Thiol-based redox regulation is a post-translational protein modification for controlling enzyme activity by switching oxidation/reduction states of Cys residues. In plant cells, numerous proteins involved in a wide range of biological systems have been suggested as the target of redox regulation; however, our knowledge on this issue is still incomplete. Here we report that 3-phosphoglycerate dehydrogenase (PGDH) is a novel redox-regulated protein. PGDH catalyzes the first committed step of Ser biosynthetic pathway in plastids. Using an affinity chromatography-based method, we found that PGDH physically interacts with thioredoxin (Trx), a key factor of redox regulation. The in vitro studies using recombinant proteins from Arabidopsis thaliana showed that a specific PGDH isoform, PGDH1, forms the intramolecular disulfide bond under nonreducing conditions, which lowers PGDH enzyme activity. MS and site-directed mutagenesis analyses allowed us to identify the redox-active Cys pair that is mainly involved in disulfide bond formation in PGDH1; this Cys pair is uniquely found in land plant PGDH. Furthermore, we revealed that some plastidial Trx subtypes support the reductive activation of PGDH1. The present data show previously uncharacterized regulatory mechanisms of PGDH and expand our understanding of the Trx-mediated redox-regulatory network in plants.

Thioredoxin-dependent control balances the metabolic activities of tetrapyrrole biosynthesis

Plastids are specialized organelles found in plants, which are endowed with their own genomes, and differ in many respects from the intracellular compartments of organisms belonging to other kingdoms of life. They differentiate into diverse, plant organ-specific variants, and are perhaps the most versatile organelles known. Chloroplasts are the green plastids in the leaves and stems of plants, whose primary function is photosynthesis. In response to environmental changes, chloroplasts use several mechanisms to coordinate their photosynthetic activities with nuclear gene expression and other metabolic pathways. Here, we focus on a redox-based regulatory network composed of thioredoxins (TRX) and TRX-like proteins. Among multiple redox-controlled metabolic activities in chloroplasts, tetrapyrrole biosynthesis is particularly rich in TRX-dependent enzymes. This review summarizes the effects of plastid-localized reductants on several enzymes of this pathway, which have been shown to undergo dithiol-disulfide transitions. We describe the impact of TRX-dependent control on the activity, stability and interactions of these enzymes, and assess its contribution to the provision of adequate supplies of metabolic intermediates in the face of diurnal and more rapid and transient changes in light levels and other environmental factors.

Efficient photosynthesis in dynamic light environments: a chloroplast's perspective

In nature, light availability for photosynthesis can undergo massive changes on a very short timescale. Photosynthesis in such dynamic light environments requires that plants can respond swiftly. Expanding our knowledge of the rapid responses that underlie dynamic photosynthesis is an important endeavor: it provides insights into nature's design of a highly dynamic energy conversion system and hereby can open up new strategies for improving photosynthesis in the field. The present review focuses on three processes that have previously been identified as promising engineering targets for enhancing crop yield by accelerating dynamic photosynthesis, all three of them involving or being linked to processes in the chloroplast, i.e. relaxation of non-photochemical quenching, Calvin–Benson–Bassham cycle enzyme activation/deactivation and dynamics of stomatal conductance. We dissect these three processes on the functional and molecular level to reveal gaps in our understanding and critically discuss current strategies to improve photosynthesis in the field.

Redox regulation of PGRL1 at the onset of low light intensity

PGR5-LIKE PHOTOSYNTHETIC PHENOTYPE1 (PGRL1) regulates photosystem I cyclic electron flow which transiently activates non-photochemical quenching at the onset of light. Here, we show that a disulfide-based mechanism of PGRL1 regulated this process in vivo at the onset of low light levels. We found that PGRL1 regulation depended on active formation of key regulatory disulfides in the dark, and that PGR5 was required for this activity. The disulfide state of PGRL1 was modulated in plants by counteracting reductive and oxidative components and reached a balanced state that depended on the light level. We propose that the redox regulation of PGRL1 fine-tunes a timely activation of photosynthesis at the onset of low light.

Effects of Light and Daytime on the Regulation of Chitosan-Induced Stomatal Responses and Defence in Tomato Plants

Closure of stomata upon pathogenesis is among the earliest plant immune responses. However, our knowledge is very limited about the dependency of plant defence responses to chitosan (CHT) on external factors (e.g., time of the day, presence, or absence of light) in intact plants. CHT induced stomatal closure before dark/light transition in leaves treated at 17:00 hrs and stomata were closed at 09:00 hrs in plants treated at dawn and in the morning. CHT was able to induce generation of reactive oxygen species (ROS) in guard cells in the first part of the light phase, but significant nitric oxide production was observable only at 15:00 hrs. The actual quantum yield of PSII electron transport (ΦPSII) decreased upon CHT treatments at 09:00 hrs in guard cells but it declined only at dawn in mesophyll cells after the treatment at 17:00 hrs. Expression of Pathogenesis-related 1 (PR1) and Ethylene Response Factor 1 were already increased at dawn in the CHT-treated leaves but PR1 expression was inhibited in the dark. CHT-induced systemic response was also observed in the distal leaves of CHT-treated ones. Our results suggest a delayed and daytime-dependent defence response of tomato plants after CHT treatment at night and under darkness.

Light Regulates the Cytokinin-Dependent Cold Stress Responses in Arabidopsis

To elucidate the effect of light intensity on the cold response (5°C; 7 days) in Arabidopsis thaliana, we compared the following parameters under standard light (150 μmol m^-2 s^-1), low light (20 μmol m^-2 s^-1), and dark conditions: membrane damage, photosynthetic parameters, cytokinin oxidase/dehydrogenase (CKX) activity, phytohormone levels, and transcription of selected stress- and hormone-related genes and proteome. The impact of cytokinins (CKs), hormones directly interacting with the light signaling pathway, on cold responses was evaluated using transformants overexpressing CK biosynthetic gene isopentenyl transferase (DEX:IPT) or CK degradation gene HvCKX2 (DEX:CKX) under a dexamethasone-inducible promoter. In wild-type plants, cold treatment under light conditions caused down-regulation of CKs (in shoots) and auxin, while abscisic acid (ABA), jasmonates, and salicylic acid (SA) were up-regulated, especially under low light. Cold treatment in the dark strongly suppressed all phytohormones, except ABA. DEX:IPT plants showed enhanced stress tolerance associated with elevated CK and SA levels in shoots and auxin in apices. Contrarily, DEX:CKX plants had weaker stress tolerance accompanied by lowered levels of CKs and auxins. Nevertheless, cold substantially diminished the impact from the inserted genes. Cold stress in dark minimized differences among the genotypes. Cold treatments in light strongly up-regulated stress marker genes RD29A, especially in roots, and CBF1-3 in shoots. Under control conditions, their levels were higher in DEX:CKX plants, but after 7-day stress, DEX:IPT plants exhibited the highest transcription. Transcription of genes related to CK metabolism and signaling showed a tendency to re-establish, at least partially, CK homeostasis in both transformants. Up-regulation of strigolactone-related genes in apices and leaves indicated their role in suppressing shoot growth. The analysis of leaf proteome revealed over 20,000 peptides, representing 3,800 proteins and 2,212 protein families (data available via ProteomeXchange, identifier PXD020480). Cold stress induced proteins involved in ABA and jasmonate metabolism, antioxidant enzymes, and enzymes of flavonoid and glucosinolate biosynthesis. DEX:IPT plants up-regulated phospholipase D and MAP-kinase 4. Cold stress response at the proteome level was similar in all genotypes under optimal light intensity, differing significantly under low light. The data characterized the decisive effect of light-CK cross-talk in the regulation of cold stress responses.

New Light on Chloroplast Redox Regulation: Molecular Mechanism of Protein Thiol Oxidation

Thiol-based redox regulation is a posttranslational protein modification that plays a key role in adjusting chloroplast functions in response to changing light conditions. Redox-sensitive target proteins are reduced upon illumination, which turns on (or off in a certain case) their enzyme activities. A redox cascade via ferredoxin, ferredoxin-thioredoxin reductase, and thioredoxin has been classically recognized as the key system for transmitting the light-induced reductive signal to target proteins. By contrast, the molecular mechanism underlying target protein oxidation, which is observed during light to dark transitions, remains undetermined over the past several decades. Recently, the factors and pathways for protein thiol oxidation in chloroplasts have been reported, finally shedding light on this long-standing issue. We identified thioredoxin-like2 as one of the protein-oxidation factors in chloroplasts. This protein is characterized by its higher redox potential than that of canonical thioredoxin, that is more favorable for target protein oxidation. Furthermore, 2-Cys peroxiredoxin and hydrogen peroxide are also involved in the overall protein-oxidation machinery. Here we summarize the newly uncovered "dark side" of chloroplast redox regulation, giving an insight into how plants rest their photosynthetic activity at night.

Thioredoxin-like2/2-Cys peroxiredoxin redox cascade acts as oxidative activator of glucose-6-phosphate dehydrogenase in chloroplasts

Thiol-based redox regulation is crucial for adjusting chloroplast functions under fluctuating light environments. We recently discovered that the thioredoxin-like2 (TrxL2)/2-Cys peroxiredoxin (2CP) redox cascade supports oxidative thiol modulation by using hydrogen peroxide (H2O2) as an oxidizing force. This system plays a key role in switching chloroplast metabolism (e.g. Calvin–Benson cycle) during light to dark transitions; however, information on its function is still limited. In this study, we report a novel protein-activation mechanism based on the TrxL2/2CP redox cascade. Glucose-6-phosphate dehydrogenase (G6PDH) catalyzes the first step of the oxidative pentose phosphate pathway (OPPP). Biochemical studies, including redox state determination and measurement of enzyme activity, suggested that the TrxL2/2CP pathway is involved in the oxidative activation of G6PDH. It is thus likely that the TrxL2/2CP redox cascade shifts chloroplast metabolism to night mode by playing a dual role, namely, down-regulation of the Calvin–Benson cycle and up-regulation of OPPP. G6PDH was also directly oxidized and activated by H2O2, particularly when H2O2 concentration was elevated. Therefore, G6PDH is thought to be finely tuned by H2O2 levels in both direct and indirect manners.