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S-Glutathionylation (thiolation) is a ubiquitous redox-sensitive and reversible modification of protein cysteinyl residues that can directly regulate their activity. While well established in animals, little is known about the formation and function of these mixed disulfides in plants. After labeling the intracellular glutathione pool with [ 35 S]cysteine, suspension cultures of Arabidopsis (Arabidopsis thaliana ecotype Columbia) were shown to undergo a large increase in protein thiolation following treatment with the oxidant tert-butylhydroperoxide. To identify proteins undergoing thiolation, a combination of in vivo and in vitro labeling methods utilizing biotinylated, oxidized glutathione (GSSG-biotin) was developed to isolate Arabidopsis proteins/protein complexes that can be reversibly glutathionylated. Following two-dimensional polyacrylamide gel electrophoresis and matrixassisted laser desorption/ionization time of flight mass spectrometry proteomics, a total of 79 polypeptides were identified, representing a mixture of proteins that underwent direct thiolation as well as proteins complexed with thiolated polypeptides. The mechanism of thiolation of five proteins, dehydroascorbate reductase (AtDHAR1), zeta-class glutathione transferase (AtGSTZ1), nitrilase (AtNit1), alcohol dehydrogenase (AtADH1), and methionine synthase (AtMetS), was studied using the respective purified recombinant proteins. AtDHAR1, AtGSTZ1, and to a lesser degree AtNit1 underwent spontaneous thiolation with GSSG-biotin through modification of active-site cysteines. The thiolation of AtADH1 and AtMetS required the presence of unidentified Arabidopsis proteins, with this activity being inhibited by S-modifying agents. The potential role of thiolation in regulating metabolism in Arabidopsis is discussed and compared with other known redox regulatory systems operating in plants.The tripeptide glutathione (GSH; g-Glu-Cys-Gly) serves important functions in plants as a reductant, transiently accumulating under stress conditions as its oxidized disulfide (GSSG). As well as forming disulfides with itself, GSH can also form mixed disulfides with proteinaceous Cys. This S-glutathionylation of proteins is commonly termed thiolation and has recently become established as a widespread reversible posttranslational modification of proteins that occurs in animal and fungal cells exposed to oxidative stress (Klatt and Lamas, 2000). Although originally considered to be a protective mechanism to prevent the irreversible oxidation of reactive protein-bound Cys to their sulfinic and sulfonic acid derivatives, it is now recognized that thiolation can act as a redox-driven regulator of signal transduction cascades and metabolic pathways (Fratelli et al., 2004). As such, thiolation has clear parallels with redox regulation of proteins mediated through intramolecular or intermolecular disulfide formation with other proteinaceous Cys (Lee et al., 2004). In plants, proteomic approaches have focused upon the redox regulation of proteins forming protein disulfides...

Discovered 40 years ago, plant glutathione transferases (GSTs) now have a well-established role in determining herbicide metabolism and selectivity in crops and weeds. Within the GST superfamily, the numerous and plant-specific phi (F) and tau (U) classes are largely responsible for catalyzing glutathione-dependent reactions with xenobiotics, notably conjugation leading to detoxification and, more rarely, bioactivating isomerizations. In total, the crystal structures of 10 plant GSTs have been solved and a highly conserved N-terminal glutathione binding domain and structurally diverse C-terminal hydrophobic domain identified, along with key coordinating residues. Unlike drug-detoxifying mammalian GSTs, plant enzymes utlilize a catalytic serine in place of a tyrosine residue. Both GSTFs and GSTUs undergo changes in structure during catalysis indicative of an induced fit mechanism on substrate binding, with an understanding of plant GST structure/function allowing these proteins to be engineered for novel functions in detoxification and ligand recognition. Several major crops produce alternative thiols, with GSTUs shown to use homoglutathione in preference to glutathione, in herbicide detoxification reactions in soybeans. Similarly, hydroxymethylglutathione is used, in addition to glutathione in detoxifying the herbicide fenoxaprop in wheat. Following GST action, plants are able to rapidly process glutathione conjugates by at least two distinct pathways, with the available evidence suggesting these function in an organ- and species-specific manner. Roles for GSTs in endogenous metabolism are less well defined, with the enzymes linked to a diverse range of functions, including signaling, counteracting oxidative stress, and detoxifying and transporting secondary metabolites.

Plants respond to synthetic chemicals by eliciting a xenobiotic response (XR) that enhances the expression of detoxifying enzymes such as glutathione transferases (GSTs). In agrochemistry, the ability of safeners to induce an XR is used to increase herbicide detoxification in cereal crops. Based on the responsiveness of the model plant Arabidopsis thaliana to the rice safener fenclorim (4,6-dichloro-2-phenylpyrimidine), a series of related derivatives was prepared and tested for the ability to induce GSTs in cell suspension cultures. The XR in Arabidopsis could be divided into rapid and slow types depending on subtle variations in the reactivity (electrophilicity) and chemical structure of the derivatives. In a comparative microarray study, Arabidopsis cultures were treated with closely related compounds that elicited rapid (fenclorim) and slow (4-chloro-6-methyl-2-phenylpyrimidine) XRs. Both chemicals induced major changes in gene expression, including a coordinated suppression in cell wall biosynthesis and an up-regulation in detoxification pathways, whereas only fenclorim selectively induced sulfur and phenolic metabolism. These transcriptome studies suggested several linkages between the XR and oxidative and oxylipin signaling. Confirming links with abiotic stress signaling, suppression of glutathione content enhanced GST induction by fenclorim, whereas fatty acid desaturase mutants, which were unable to synthesize oxylipins, showed an attenuated XR. Examining the significance of these studies to agrochemistry, only those fenclorim derivatives that elicited a rapid XR proved effective in increasing herbicide tolerance (safening) in rice.

By learning lessons from weed science we have adopted three approaches to make plants more effective in phytoremediation: 1. The application of functional genomics to identify key components involved in the detoxification of, or tolerance to, xenobiotics for use in subsequent genetic engineering/breeding programmes. 2. The rational metabolic engineering of plants through the use of forced evolution of protective enzymes, or alternatively transgenesis of detoxification pathways. 3. The use of chemical treatments which protect plants from herbicide injury. In this paper we examine the regulation of the xenome by herbicide safeners, which are chemicals widely used in crop protection due to their ability to enhance herbicide selectivity in cereals. We demonstrate that these chemicals act to enhance two major groups of phase 2 detoxification enzymes, notably the glutathione transferases and glucosyltransferases, in both cereals and the model plant Arabidopsis thaliana, with the safeners acting in a chemical-and species-specific manner. Our results demonstrate that by choosing the right combination of safener and plant it should be possible to enhance the tolerance of diverse plants to a wide range of xenobiotics including pollutants.