Reactive oxygen species (ROS) have been shown to be potent signaling molecules. Today, oxidation of cysteine residues is a well-recognized posttranslational protein modification, but the signaling processes steered by such oxidations are poorly understood. To gain insight into the cysteine thiol-dependent ROS signaling in Arabidopsis thaliana, we identified the hydrogen peroxide (H 2 O 2 )-dependent sulfenome: that is, proteins with at least one cysteine thiol oxidized to a sulfenic acid. By means of a genetic construct consisting of a fusion between the C-terminal domain of the yeast (Saccharomyces cerevisiae) AP-1-like (YAP1) transcription factor and a tandem affinity purification tag, we detected ∼100 sulfenylated proteins in Arabidopsis cell suspensions exposed to H 2 O 2 stress. The in vivo YAP1-based trapping of sulfenylated proteins was validated by a targeted in vitro analysis of DEHYDROASCORBATE REDUCTASE2 (DHAR2). In DHAR2, the active site nucleophilic cysteine is regulated through a sulfenic acid-dependent switch, leading to S-glutathionylation, a protein modification that protects the protein against oxidative damage.oxidative stress | redox regulation | cysteine oxidation N umerous posttranslational modifications (PTMs) have been discovered within proteomes, creating a complex landscape of protein diversity and function (1). One of the recognized reversible redox-based PTMs is the oxidation of a cysteine thiol group to a sulfenic acid (Cys-SOH) (2) that acts as regulatory switch in several oxidative stress signal transduction pathways (3). Sulfenic acids, unless they are stabilized into the protein environment, can react rapidly with other protein thiols or with low-molecular weight thiols to form intramolecular and intermolecular disulfides. These mechanisms protect the sulfenic acids against overoxidation to sulfinic (SO 2 H) or sulfonic (SO 3 H) acid and allow sulfur oxygen signaling (2).In plants, the best-known redox regulation mechanisms are the light-dependent thiol-disulfide exchange switches in chloroplast proteins (4). Examples of other redox-regulated proteins are the transcription coactivator NONEXPRESSER OF PR GENES 1 (5), the vacuolar H + -ATPase (6), and several transcription factors (TFs), such as the AP2-type RAP2.4a (7), the G-group of basic leucine zipper TFs (8), and the TEOSINTEBRANCHED1/ CYLOIDEA/PROLIFERATING CELL FACTOR class I TFs (9). The redox relay mechanisms that bridge the signal perception to the final oxidative stress response are largely unknown. Some thiol peroxidases have an H 2 O 2 -dependent signaling function and can act as receptor and transducer (10). In yeast (Saccharomyces cerevisiae), the H 2 O 2 sensor oxidant receptor peroxidase1 (ORP1/glutathione peroxidase 3) controls, together with the transcription factor YAP1 (yeast AP-1-like), a redox regulon via a sulfenic acid thiol-disulfide relay mechanism (11). Upon reaction with H 2 O 2 , the peroxidatic cysteine of ORP1 is oxidized to a sulfenic acid that reacts with the YAP1 C-terminal cysteine-rich domain (...

In plants, fluctuation of the redox balance by altered levels of reactive oxygen species (ROS) can affect many aspects of cellular physiology. ROS homeostasis is governed by a diversified set of antioxidant systems. Perturbation of this homeostasis leads to transient or permanent changes in the redox status and is exploited by plants in different stress signalling mechanisms. Understanding how plants sense ROS and transduce these stimuli into downstream biological responses is still a major challenge. ROS can provoke reversible and irreversible modifications to proteins that act in diverse signalling pathways. These oxidative post-translational modifications (Ox-PTMs) lead to oxidative damage and/or trigger structural alterations in these target proteins. Characterization of the effect of individual Ox-PTMs on individual proteins is the key to a better understanding of how cells interpret the oxidative signals that arise from developmental cues and stress conditions. This review focuses on ROS-mediated Ox-PTMs on cysteine (Cys) residues. The Cys side chain, with its high nucleophilic capacity, appears to be the principle target of ROS. Ox-PTMs on Cys residues participate in various signalling cascades initiated by plant stress hormones. We review the mechanistic aspects and functional consequences of Cys Ox-PTMs on specific target proteins in view of stress signalling events.