Reactive Oxygen Species
Ever since the introduction of oxygen by the oxygen releasing photosynthetic organisms Reactive oxygen species (ROS) have been the unwelcomed guests of aerobic life (Halliwell, 2006). Being a free radical molecular oxygen contains two unpaired electrons sharing same spin quantum number that makes it preferable to accept electrons, generating the so called Reactive oxygen species. Most dominating ROS include Hydrogen peroxide (H2O2), Hydroxyl radical (OH*), Singlet oxygen (1O2), Superoxide radical (O2*-) etc., (Choudhary et al., 2014).Vast number of metabolic pathways operating in various cellular cubicles are continuously producing ROS as their byproducts. Chloroplast, Mitochondria and peroxisomes are the dominant cellular organelles releasing ROS because of their higher oxidizing metabolic activities and fast rates of electron flow (Suzuki et al., 2011). ROS levels need to be strictly controlled to guarantee the signaling functions of these molecules and to prevent toxicity. Reduced ROS scavenging efficiency would therefore be expected to increase oxidative damage and cell death (Miller et al. 2008, de Pinto et al 2012).
ROS production in different organelles
Oxygenic conditions and profusion of photosensitizers and PUFA in chloroplast predispose photosynthetic organisms to oxidative damage. Reactive oxygen species are being released both in light as well as in dark conditions. Chloroplast and Mitochondria appear to be the most prominent sources of ROS production in light (Foyer and Noctor, 2003). While in darkness mitochondria seem to play their part in ROS production (Moller, 2001)
ROS Generation in Mitochondria:
Mitochondria popularly known as the energy factories are the main targets of ROS as well as the primary producers of toxic metabolic by products mainly in the form of H2O2, differing from their animal matching part bearing a specific set of ETC components and functional processes such as photorespiration. Distinctive cellular environment of this plant cell organelle creates O2 and energy rich carbohydrates because of photosynthesis (Rasmusson., 2004; Noctor and Foyer, 2006).Mitochondrial electron transport chain is able to harbour electrons with adequate amount of available free energy that leads to direct reduction of 02 molecules, considered to be the inevitable source of mitochondrial ROS Produce associated with aerobic respiration (Rhoads et al., 2006).Conversely during normal respiratory conditions ROS production occurs in mitochondria. However, ROS production is triggered during abiotic and biotic stress conditions. Popular among the ROS producing sites in mitochondria seem to be Complex I and III of ETC. Being moderately reactive in aqueous solutions further reduction of O2 to H2O2 occurs through SOD Dismutation ( Raha and Robinson, 2000; Moller, 2001; Sweetlove and Foyer, 2004). From the studies it has been assessed that 15% of the O2 consumed in mitochondria leads to the production of H2O2. Upon reaction with Fe2+ and Cu+ H2O2 produces highly lethal OH which in turn can traverse membranes while leaving mitochondria Moller, 2001; Rhodes et al., 2006; Grene, 2002. Mitochondrial membrane peroxidation by OH results in the formation of several cytotoxic compounds that may lead to cellular damage upon reaction with protein, lipids and nucleic acids. Mitochondrial behavior, localization and production of ROS during programmed cell death (PCD) induced by UV-C exposure (Gao et al., 2008).It was observed that in response to UV-C exposure protoplast accumulate ROS rapidly, restricted to mitochondria and chloroplast. Plant bioenergetics is strongly affected during abiotic stress conditions. ROS production in mitochondria is under the control of energy dissipating system. From this it is presumed that during stressful conditions mitochondria play a vital role in cell adaptation. It has been demonstrated that in durum wheat energy dissipating system reduce production of mitochondrial ROS. It has been elucidated that in control as well as in hyper osmotic conditions mitochondrial potential diminishes that in turn hampers large scale production of ROS (Pastore et al., 2007)
Chloroplasts and ROS Generation:
Chloroplasts are the main seats of photosynthesis in almost all plant species including algae and other higher plants. Chloroplasts are equipped with a complex network of organized thylakoids harbouring all light capturing gadgets making them capable for harvesting light energy. During photosynthesis oxygen generated in the chloroplasts accepts electrons while travelling through photosystems that in turn results in the form of O2.Therefore triplet chlorophyll, electron transport chain are among the ROS centers present in photosystems( PSI and PSII) mark chloroplasts the major sources of ROS production in the form of O2, 1O2 and H2O2. Elevated levels of ROS production in chloroplast occurs during abiotic stress conditions. During normal conditions from excited photosystems electron flow is directed towards NAPH that gets reduced to NADPH which reduces the final electron acceptor CO2 in Calvin cycle. During overloading circumstances of Electron transport chain a chunk of electron flow is abstracted from ferredoxin to O2, reducing it to O2 via Mehler reaction (Wise and Naylor, 1987; Elstner, 1991). During low light conditions, 1O2 is formed at PSII which is considered as a natural byproduct of photosynthesis. O2 is instinctively dismutated to H2O2 by the action of Cu Zn- SOD on the external membrane surface of stroma (Takahashi et al., 1988). It is because of these conditions chloroplasts are considered to be tha major sources of ROS production. Several studies have linked ROS production in chloroplast with that of hypersensitive response (Mur et al., 2008). It has been demonstrated that chloroplast generated Reactive oxygen species are able to transmit the spread wound induced programmed cell death through maize tissues (gray et al., 2002). Studies carried out on transgenic tobacco expressing Bcl-2 an animal apoptotic gene have clearly elucidated the engrossment of chloroplast in PCD induced through oxidative stress (Chen and Dickman, 2004). It has been validated that the chloroplast generated ROS are the main players involved in the PCD the studies were confirmed on the Arabidopsis thaliana suspension cultures were the plants grown in the light possessed functional and well developed chloroplasts as compared with the plants grown in the dark. An increase in apoptotic like-PCD induction was also observed in plants treated with antioxidants and exposed to light, confirming the role of ROS in apoptotic like-PCD regulation (Doyle et al., 2010). Elevated levels of ROS has been reported to diminish cytochrome respiratory pathway and net photosynthetic rate in cucumber in response to chilling, however in mitochondria and chloroplasts chilling induced ROS accumulation resulted in alleviation of protective mechanism by way of ROS scavenging mechanisms, thermal dissipationand alternative respiratory pathways (Hu et al., 2008)
ROS Generation in peroxisomes:
Peroxisomes are single lipid bilayer, small spherical microbodies with oxidative type of metabolism, undoubtedly the key players among the producers of intracellular ROS. During the normal course of their metabolism peroxisomes harvest O2 radicals like chloroplast and mitochondria. It has been demonstrated that there are two sites of O2 generation in peroxisomes (del Rio et al., 2002) Oxidation of xanthine and hypoxanthine to uric acid occurs via xanthine oxidase the whole process occurs in organelle matrix (Corpas et al., 2001) the second and important site is peroxisome membranes were O2 is released by the peroxisomes electron transport chain composed of a flavoprotein NADH and cytochrome b (del Rio et al., 2002). MDHAR is actively involved in O2 generation by peroxisome membranes. Photorespiratory glycolate oxidase reactions, disproportionation of O2 radicals, b-oxidation of fatty acids, enzymatic reactions of flavinoxidases are among the important metabolic processes accountable for H2O2 generation in peroxisomes (Huang et al., 1983; del Rio et al., 2002). Several studies have demonstrated that peroxisomes are also the sources of NO radicals. Peroxisomes actively participate in various metabolic processes like Jasmonic acid and auxin biosynthesis, degradation of brached chain amino acids, photomorphogenasis and glycine betaine production (Hu 2007). It has been also suggested that peroxisomes are also the store house of some regulatory proteins like phosphatases, kinases and heat shock proteins (Hayashi and Nishimura, 2003; Reumann et al., 2004). Overproduction of H2O2 and O2 in peroxisomes promote oxidative damage but moderate levels act as signaling molecules mediating pathogen induce PCD in plants (McDowell and Dangl, 2000, Grant and Loake, 2000). On the basis of above observations it has been advocated peroxisomes are in the capacity to release vital signaling molecules in the form of O2, H2O2 and NO playing a pivotal role in incorporated cell communication system. Apart from chloroplast, mitochondria and peroxisomes Cell wall, apoplast, plasma membrane and endoplasmic reticulam are among the important sources of ROS generation (Gill and tuteja, 2010; Sharma et al., 2012; Voothulru and Sharp, 2013)
Oxidative stress intimates any disparity between reactive oxygen species and systems capacity to detoxify these byproducts of metabolism. Failure of normal rodox state leads to toxic effects through production of free radicals and peroxides damaging cell components like lipids, proteins and DNA.
Abiotic stresses like salinity, drought, and extremes of temperature, UV radiations, chemical toxicity, etc. are among the strong candidates that hamper plant growth and result in agricultural productivity globally. All these kind of stresses are accompanied by the overproduction of ROS (Suzuki et al., 2012). Pathogen invasion is among one of the causes that paves the way for the generation of highly oxidizing ROS (O Brien et al., 2012). Overproduction of ROS and its byproducts being noxious result in the oxidative stress, it is therefore the necessary to keep the harmony between generation and metabolism of ROS and its byproducts so that plant can perform its vital cell metabolic functions smoothly (Gechev et al., 2006; Gill and Tuteja, 2010; Anjum et al., 2012; Gill et al., 2012).
During the normal life cycle plants are continuously releasing Reactive oxygen species from various cellular compartments with intense rate of electron flow (Suzuki et al., 2011). It is because of this fact that plants are fully equipped with different enzymatic and non- enzymatic antioxidant machinery for scavenging ROS and to keep them under control during favorable conditions of growth. On the other hand during stressful conditions the imbalance between ROS production and their scavenging may lead to oxidative stress (Smirnoff, 1993; Mullineax & Karpinski 2002; Miller et al ., 2010).
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