CONICET | Buscador de Institutos y Recursos Humanos

1. Oxidative reactive species paradigm: redox signal transduction vs. antioxidants stress response It is believed that over 2.2 billion years ago, the presence of O2 in the Earth´s atmosphere was originated due to the evolution of cyanobacteria photosynthetic activity, which used the sun energy and water to produce sugars with the release of O2. Since then, several organisms began to evolve an antioxidant defense that gives them the capacity to tolerate O2 and use it for metabolic transformation and biosynthesis (Halliwell 2006; Slesak, Libik, Karpinska et al. 2007). Considering the relationship between aerobic organisms and O2, it is remarkable that this molecule can be both, beneficial and damaging. Beneficial for the reason that O2 is essential as electron exchanger for an efficient respiration and photosynthetic activity and it is largely used for signal transduction; while damaging because O2 can cause dysfunction on cell components by irreversible modifications on DNA, proteins, sugars and lipids. Hence, there is a balance/imbalance between both sides which is critical for the cell functionality and survival (Slesak, Libik, Karpinska et al. 2007). During evolution in an aerobic atmosphere, which unavoidably exposes organisms to an oxidative environment, plants and all other aerobic life forms evolved different antioxidative systems to protect cell components. Consequently, a delicate homeostasis between oxidative and reductive reactions is maintained inside cells, which is critical in the use of O2 in metabolic pathways. The presence of intracellular oxidative and reducing species determine a transitory redox environment, defined as the summation of the reduction potential and reducing capacity of the linked redox couples occurring inside the cell (Schafer and Buettner 2001). When this redox environment suffers an imbalance and reduced or oxidized species are favored, cells are exposed to a redox stress. Both, oxidative and reductive stress can trigger redox cascades that bring changes in the oxidized/reduced status of biomolecules. Changes in the cellular redox environment can alter signal transduction, DNA and RNA synthesis, protein synthesis, enzyme activation, and even regulation of the cell cycle. Thus, the redox environment might determine if a cell will proliferate, differentiate, or perish (Moller, Jensen and Hansson 2007; Neill, Desikan, Clarke et al. 2002). Figure 1 schematizes how different agents can create stress conditions and also molecular damages, the whole situation challenging the cell to trigger defense mechanisms. On the other hand, Figure 2 shows a simplified scenario of different metabolic routes related with the flux of redox equivalents operating under physiological conditions as well as to coping situations of oxidative stress. The use of O2 in metabolic reactions through electron transport chains such as in photosynthesis and respiration, produces NADPH and energy (ATP), but also generates different ?Reactive Oxygen Species? (ROS) as by-products (Apel and Hirt 2004; Buchanan and Balmer 2005) (Figure 1). ROS are different oxygen derived species; for example, free radicals containing one or more unpaired electrons such as superoxide anion radical (O2.-), hydroxyl radical (·OH), per‑hydroxyl radical (HO2∙), singlet oxygen (1O2); and non radical derivatives, such as hydrogen peroxide (H2O2) (Foyer, Noctor, Buchanan et al. 2009). In addition, plants may also be exposed to ?Reactive Nitrogen Species? (RNS) and ?Reactive Sulfur Species? (RSS) (Figure 1). The major RNS in the cell is nitric oxide (·NO) that can reacts with O2, O2.-, oxidized glutathione (GSSG) and transition metals generating peroxinitrite (ONOO−), nitrosoglutathione (GSNO), and metal-NO adducts, respectively (Neill, Bright, Desikan et al. 2008). Other components like ozone (O3), nitric dioxide (NO2), and sulfur dioxide (SO2), which are major air pollutants, can cause oxidative modifications as well (Moller, Jensen and Hansson 2007). To struggle with this different oxidative species, aerobic organism has developed an antioxidant network composed by many antioxidant systems (Figure 1). The term antioxidant can be considered to describe any compound capable of quenching ROS without itself undergoing conversion to a destructive radical (Noctor and Foyer 1998). The antioxidant network is composed by numerous proteins, enzymes and metabolites that act in synchrony to ameliorate oxidative stress situations. Some of the proteins/enzymes acting in plant antioxidative networks are catalase (CAT), peroxiredoxin (PRX), thioredoxin (TRX), glutaredoxin (GRX), and sulfiredoxin (SRX); whereas main metabolites involved include ascorbate, glutathione (GSH), tocopherol, and NAD(P)H. The enzymatic and non‑enzymatic antioxidant components work together to form different groups of antioxidant systems. Plants, as photosynthetic aerobic organisms, are highly exposed to oxidative conditions. Moreover, due to their sessile lifestyle, plants are unconditionally exposed to a range of biotic and abiotic deleterious stresses that can trigger an oxidative burst. Besides, these oxidative species can function initially as signal transducers (Figure 1). The situation of oxidative stress depends on the sort of ROS that is produced, the concentration and the site where molecules are generated, their interaction whit other molecules present in the organism, as well as the developmental stage and the past of the cell (Moller, Jensen and Hansson 2007). Accumulation of H2O2, for example, is perceived by the plant as a signal of environmental change. As a diffusible signal-transducer molecule, H2O2 alerts metabolism to the presence of both biotic and abiotic threats (Noctor and Foyer 1998). If the plant response is not enough to overcome this threat, the oxidative stress is established. Comprehension of occurrence and function of antioxidant metabolic routes is of high interest to understand: (i) how plants detoxify oxidative species, (ii) how they rearrange the whole metabolism under oxidative conditions, (iii) how signal transduction pathways operate in response to oxidative stress, and (iv) how cells repair macromolecules after oxidative damage.