Dissecting the integrative antioxidant and redox systems in plant mitochondria. Effect of stress and S-nitrosylation

Abstract

Mitochondrial respiration provides the energy needed to drive metabolic and transport processes in cells. Mitochondria are a significant site of reactive oxygen species (ROS) production in plant cells, and redox-system components obey fine regulation mechanisms that are essential in protecting the mitochondrial integrity. In addition to ROS, there are compelling indications that nitric oxide can be generated in this organelle by both reductive and oxidative pathways. ROS and reactive nitrogen species play a key role in signaling but they can also be deleterious via oxidation of macromolecules. The high production of ROS obligates mitochondria to be provided with a set of ROS scavenging mechanisms. The first line of mitochondrial antioxidants is composed of superoxide dismutase and the enzymes of the ascorbate-glutathione cycle, which are not only able to scavenge ROS but also to repair cell damage and possibly serve as redox sensors. The dithiol-disulfide exchanges form independent signaling nodes and act as antioxidant defense mechanisms as well as sensor proteins modulating redox signaling during development and stress adaptation. The presence of thioredoxin (Trx), peroxiredoxin (Prx) and sulfiredoxin (Srx) in the mitochondria has been recently reported. Cumulative results obtained from studies in salt stress models have demonstrated that these redox proteins play a significant role in the establishment of salt tolerance. The Trx/Prx/Srx system may be subjected to a fine regulated mechanism involving post-translational modifications, among which S-glutathionylation and S-nitrosylation seem to exhibit a critical role that is just beginning to be understood. This review summarizes our current knowledge in antioxidative systems in plant mitochondria, their interrelationships, mechanisms of compensation and some unresolved questions, with special focus on their response to abiotic stress.

Keywords: S-nitrosylation; abiotic stress; ascorbate-glutathione cycle; mitochondria; peroxiredoxin; signaling; sulfiredoxin; thioredoxin.

FIGURE 1

Mitochondrial ascorbate-glutathione cycle. The hydrogen peroxide in the mitochondria produced by ETC is reduced by APX at the expense of ASC to produce MDHA (step 1) that is either reduced to ASC (step 2) or disproportionated to DHA and ASC (step 3). DHAR reduces DHA using GSH as electron donor (step 4), which is regenerated by GR and NADPH (step 5).

FIGURE 2

Trx system in mitochondria. Mitochondrial Trxo is reduced by NADPH-dependent TR (steps 1 and 2). Reduced Trxo can reduce in turn mitochondrial target proteins (step 3).

FIGURE 3

Reaction mechanism of mitochondrial PrxIIF. PrxIIF is oxidized to its sulfenic form in the reduction of peroxides (step 1). PrxIIF-SOH forms an intramolecular disulfide bridge (step 2) that is reduced by the mitochondrial Trxo system (step 3).

FIGURE 4

Catalytic cycle of mitochondrial PrxIIF overoxidation and regeneration by Srx. In physiological conditions mitochondrial PrxIIF is oxidized to its sulfenic form in the reduction of peroxides (step 1). At high concentration of H₂O₂ PrxIIF may be overoxidized to the inactive sulfinic form (PrxIIF-SO₂H; step 2) that is phosphorilated, through a reversible step, in the presence of Srx and ATP (step 3). The phosphoril ester (PrxIIF-SO₂-${\mathrm{P}\mathrm{O}}_{\mathrm{3}}^{\mathrm{2}\mathrm{-}}$) is converted into sulfinate (PrxIIF-SO-S-Srx) with Srx and Pi is released (step 4). A reducing agent (mitochondrial Trxo) reduces the heterocomplex to release PrxIIF-SOH and Srx-Trxo (step 5). The complex Srx-Trxo is subsequently reduced to Srx-SH by Trxo (step 6). The sulfenic form of PrxIIF is reduced by Trxo that forms the intermolecular complex PrxIIF-Trxo (step 7) and the active PrxIIF-SH is released by another Trxo (step 8) that forms the dimer Trxo-Trxo. The binary complexes between the three proteins in the cycle (sulfinic PrxIIF, Srx and Trxo) are in bold type. (cif. ref. Iglesias-Baena et al., 2011).

FIGURE 5

Interaction between ascorbate–glutathione cycle and Trx/PrxIIF/Srx system in ROS and RNS signaling in plant mitochondria. Superoxide radicals (${\mathrm{O}}_2^{\cdot -}$) produced by the mitochondrial ETC (step 1) are dismutated to molecular O₂ and H₂O₂ by the activity of Mn-SOD (step 2). H₂O₂ is reduced by two different systems: ascorbate-glutathione cycle (step 3) and Trx/PrxIIF/Srx system (step 4). In the ascorbate-glutathione cycle, H₂O₂ is reduced by APX (step 5) and throughout the cycle as indicated in Figure 1. In the Trx/PrxIIF/Srx system, H₂O₂ is reduced by the peroxidase activity of PrxIIF (step 6) that produces its sulfenic form (PrxIIF-SOH) and is reduced by the Trx system (step 7) as indicated in Figures 2 and 3. Under oxidative stress, PrxIIF-SOH can be overoxidized to the sulfinic form (PrxIIF-SO₂H; step 8) that gains chaperone activity (step 9) losing its peroxidase activity (step 10). This loss of activity increases H₂O₂ concentration in the mitochondria (step 11) allowing the signaling (step 12). PrxIIF-SO₂H can be regenerated to the reduced form by the action of Srx (step 13) and Trx (step 7) as indicated in Figure 4. The generation of NO^• in the mitochondria (step 14) allows signaling (step 15) in addition of forming GSNO by reduction with GSH from ascorbate-glutathione cycle (step 16). NO^• can react with ${\mathrm{O}}_2^{\cdot -}$, produced by the mitochondrial ETC, to form ONOO^- that bursts nitrosative stress (step 17). Under oxidative or nitrosative stress, PrxIIF-SH can be glutationylated or S-nitrosylated (step 18) in order to protect the enzyme against overoxidation and to gain chaperone activity (step 19). PrxIIF-S-SG and PrxIIF-S-SNO lose the peroxidase activity (step 20), allowing the signaling by H₂O₂ (step 11 and 12). These post-translational modifications could be reverted to PrxIIF-SH by the Srx activity (step 21) as it happens with the 2-Cys Prx. On the other hand, salt stress induces an increase of APX and MDHAR (step 22), which produces a decrease in the concentration of H₂O₂ (step 23) allowing acclimation (step 24). During the oxidative mechanism of senescence there are decreases of APX and MDHAR activities (step 25) which produce an increase in H₂O₂ (step 26) allowing signaling (step 12).