Glutathione, a tripeptide with the sequence γ-Glu-Cys-Gly, exists either in a reduced form with a free thiol group or in an oxidized form with a disulfide between two identical molecules. We describe here briefly the pathways involved in the synthesis, reduction, polymerization, and degradation of glutathione, as well as its distribution throughout the plant and its redox buffering capacities. The function of glutathione in xenobiotic and heavy metal detoxification, plant development, and plant-pathogen interactions is also briefly discussed. Several lines of evidence indicate that glutathione and glutaredoxins (GRXs) are implicated in the response to oxidative stress through the regeneration of enzymes involved in peroxide and methionine sulfoxide reduction. Finally, emerging functions for plant GRXs and glutathione concern the regulation of protein activity via glutathionylation and the capacity of some GRXs to bind iron sulfur centers and for some of them to transfer FeS clusters into apoproteins.

Summary: The cellular glutathione redox buffer is assumed to be part of signal transduction pathways transmitting environmental signals during biotic and abiotic stress, and thus is essential for regulation of metabolism and development. Ratiometric redox‐sensitive GFP (roGFP) expressed in Arabidopsis thaliana reversibly responds to redox changes induced by incubation with H₂O₂ or DTT. Kinetic analysis of these redox changes, combined with detailed characterization of roGFP2 in vitro, shows that roGFP2 expressed in the cytosol senses the redox potential of the cellular glutathione buffer via glutaredoxin (GRX) as a mediator of reversible electron flow between glutathione and roGFP2. The sensitivity of roGFP2 toward the glutathione redox potential was tested in vivo through manipulating the glutathione (GSH) content of wild‐type plants, through expression of roGFP2 in the cytosol of low‐GSH mutants and the endoplasmic reticulum (ER) of wild‐type plants, as well as through wounding as an example for stress‐induced redox changes. Provided the GSH concentration is known, roGFP2 facilitates the determination of the degree of oxidation of the GSH solution. Assuming sufficient glutathione reductase activity and non‐limiting NADPH supply, the observed almost full reduction of roGFP2 in vivo suggests that a 2.5 mm cytosolic glutathione buffer would contain only 25 nm oxidized glutathione disulfide (GSSG). The high sensitivity of roGFP2 toward GSSG via GRX enables the use of roGFP2 for monitoring stress‐induced redox changes in vivo in real time. The results with roGFP2 as an artificial GRX target further suggest that redox‐triggered changes of biologic processes might be linked directly to the glutathione redox potential via GRX as the mediator.

We provide here an exhaustive overview of the glutathione (GSH) peroxidase (Gpx) family of poplar (Populus trichocarpa). Although these proteins were initially defined as GSH dependent, in fact they use only reduced thioredoxin (Trx) for their regeneration and do not react with GSH or glutaredoxin, constituting a fifth class of peroxiredoxins. The two chloroplastic Gpxs display a marked selectivity toward their electron donors, being exclusively specific for Trxs of the y type for their reduction. In contrast, poplar Gpxs are much less specific with regard to their electron-accepting substrates, reducing hydrogen peroxide and more complex hydroperoxides equally well. Site-directed mutagenesis indicates that the catalytic mechanism and the Trx-mediated recycling process involve only two (cysteine [Cys]-107 and Cys-155) of the three conserved Cys, which form a disulfide bridge with an oxidation-redox midpoint potential of −295 mV. The reduction/formation of this disulfide is detected both by a shift on sodium dodecyl sulfate-polyacrylamide gel electrophoresis or by measuring the intrinsic tryptophan fluorescence of the protein. The six genes identified coding for Gpxs are expressed in various poplar organs, and two of them are localized in the chloroplast, with one colocalizing in mitochondria, suggesting a broad distribution of Gpxs in plant cells. The abundance of some Gpxs is modified in plants subjected to environmental constraints, generally increasing during fungal infection, water deficit, and metal stress, and decreasing during photooxidative stress, showing that Gpx proteins are involved in the response to both biotic and abiotic stress conditions.

▪ Abstract Thioredoxins, the ubiquitous small proteins with a redox active disulfide bridge, are important regulatory elements in plant metabolism. Initially recognized as regulatory proteins in the reversible light activation of key photosynthetic enzymes, they have subsequently been found in the cytoplasm and in mitochondria. The various plant thioredoxins are different in structure and function. Depending on their intracellular location they are reduced enzymatically by an NADP-dependent or by a ferredoxin (light)-dependent reductase and transmit the regulatory signal to selected target enzymes through disulfide/dithiol interchange reactions. In this review we summarize recent developments that have provided new insights into the structures of several components and into the mechanism of action of the thioredoxin systems in plants.

Mitochondria contain thioredoxin (Trx), a regulatory disulfide protein, and an associated flavoenzyme, NADP/Trx reductase, which provide a link to NADPH in the organelle. Unlike animal and yeast counterparts, the function of Trx in plant mitochondria is largely unknown. Accordingly, we have applied recently devised proteomic approaches to identify soluble Trx-linked proteins in mitochondria isolated from photosynthetic (pea and spinach leaves) and heterotrophic (potato tubers) sources. Application of the mitochondrial extracts to mutant Trx affinity columns in conjunction with proteomics led to the identification of 50 potential Trx-linked proteins functional in 12 processes: photorespiration, citric acid cycle and associated reactions, lipid metabolism, electron transport, ATP synthesis/transformation, membrane transport, translation, protein assembly/folding, nitrogen metabolism, sulfur metabolism, hormone synthesis, and stress-related reactions. Almost all of these targets were also identified by a fluorescent gel electrophoresis procedure in which reduction by Trx can be observed directly. In some cases, the processes targeted by Trx depended on the source of the mitochondria. The results support the view that Trx acts as a sensor and enables mitochondria to adjust key reactions in accord with prevailing redox state. These and earlier findings further suggest that, by sensing redox in chloroplasts and mitochondria, Trx enables the two organelles of photosynthetic tissues to communicate by means of a network of transportable metabolites such as dihydroxyacetone phosphate, malate, and glycolate. In this way, light absorbed and processed by means of chlorophyll can be perceived and function in regulating fundamental mitochondrial processes akin to its mode of action in chloroplasts.