Increasing evidences have indicated that much of the injury to plants due to various environmental stresses is associated with oxidative damage through direct or indirect formation of reactive oxygen species (ROS) (Apel and Hirt 2004). Production and scavenging of ROS is tightly linked with the presence of micronutrients such as Zn, Fe, Mn, and Cu in plant tissues. These micronutrients are components of antioxidant defense enzymes and modulation in the activity of these enzymes in nutrient-deficient plants is well documented. In addition, deficiency of micronutrients may affect other physiological attributes of plants, such as electron transport, water relations, and gas exchange that could directly or indirectly influence ROS metabolism in plants.
Table 16.1 Physiological functions of micronutrients in higher plants
Zinc Catalytic, cocatalytic, and structural role in more than 300 enzymes. Constituent of carbonic anhydrase, carboxypeptidase, alcohol dehydrogenase, CuZn superoxide dismutase, Zn-finger motif class of transcription factors, alkaline phosphatase (microorganisms), phospholipase, RNA polymerase, Zn-PPiase (tonoplast), fructose 1,6 bisphosphatase, aldolase. Role in the integrity of ribosomes. Role in protein, RNA, DNA, and carbohydrates metabolism. Role in membrane integrity and metabolism of reactive oxygen species.
Iron Role in biological redox systems. Constituent of prosthetic group of heme proteins: cytochromes, leghemoglobin, catalase, peroxidases, ascorbate peroxidase, nitrogenase (microorganisms). Constituent of nonheme groups: ferredoxin, aconitase, Fe-superoxide dismutase, xanthine oxidase, ACC oxidase (ethylene forming enzyme), lipooxygenase. Role in chlorophyll synthesis: synthesis of 8-aminolevulinic acid, protoporphyrinogen, and protochlorophyllide. Role in chloroplast development, photosynthesis, and lignin biosynthesis.
Manganese Role in biological redox systems. Constituent of enzymes: water splitting (oxygen evolving)
complex of PSII and Mn-superoxide dismutase. Role in TCA cycle in oxidative and nonoxidative carboxylations. Role in the activity of NADPH-dependent malic enzyme, malate dehydrogenase, and isocitrate dehydrogenase. Role in the activity of PEP carboxykinase, phenylalanine ammonia lyase, peroxidases, IAA oxidase, phytoene synthase, allantoate amidohydrolases, arginase, RNA polymerase. Role in development of thylakoid membranes, lipids and carotenoids synthesis.
Copper Role in biological redox systems. Constituent of oxidases: cytochrome oxidase, diamine oxidase, phenol oxidase, DOPA oxidase, tyrosinase, phenolase, polyphenol oxidase, laccase, plastocyanin, CuZn superoxide dismutase. Role in pollen formation and viability, pollination, desaturation of lipids, role in biosynthesis of lignin, quinones, carotenoids
Nickel Constituent of urease, hydrogenases (microorganisms). Role in nitrogen metabolism.
Molybdenum Role in biological redox systems. Constituent of nitrogeanse (microorganism), nitrate reductase, xanthine dehydrogenase (oxidase), sulfite oxidase. Role in metabolism of nitrogeneous compounds (purine, ureides, protein)
Boron Constituent of cell wall in cross-links of rhamnogalacturonan II. Role in cell wall synthesis and cell extension, integrity of membranes, sugar transport, pollen germination and pollen tube elongation. Effective on the metabolism of phenolics, IAA, carbohydrates, proteins, RNA.
Chlorine Role in water splitting (oxygen evolving) complex of PSII. Role in osmosis and water relations.
Regulation of tonoplast proton pumps. Role in plant organs and stomata movements.
The photosynthetic electron transport system is the major source of active oxygen in plant tissues, having potential to generate singlet oxygen (1O2) and superoxide (O2"). The major oxygen-consuming processes associated with photosynthesis are the oxygenase reaction of ribulose-1, 5-bisphosphate carboxylase (Rubisco), which is the initiating reaction of the photorespiratory pathway, and direct reduction of molecular oxygen by the photosystem I (PSI) electron transport chain:
In addition, certain photosystem II (PSII) components are also capable of converting molecular O2 to high-energy singlet oxygen. A cyanide-insensitive respiratory pathway in chloroplasts that competes for electrons with photosynthetic electron transport may also reduce oxygen (Asada 1999). Induced production of O2" is also catalyzed by NAD (P) H-oxidizing enzyme systems localized in different cell compartments, such as cell walls, plasmamembranes, cytosol and microsomes, peroxisomes, and mitochondria (Murphy and Auh 1996) . High levels of Fe accumulation in plant cells are also responsible for the initiation of severe oxidative stress because they produce ROS by various cellular reactions (Becanne et al. 1998).
The reduced ferrous (Fe II) compounds can be oxidized causing production of H2O2 or O2":
2Fe (II)+ O2 + 2H+ ® 2Fe (III)+ H2O2 Fe (II )+ O2 ® Fe (III)+ O/-
The powerful oxidant hydroxyl radical ('OH) is produced by the oxidation of Fe (II) by H2O2 to 'OH, which is known as the Fenton reaction:
Increased Fe accumulation has been reported for plants subjected to various stress conditions such as Zn deficiency, root anoxia, drought and light (Becanne et al. 1998). Under these stress conditions, accumulation of Fe is associated with enhanced lipid peroxidation and chlorophyll (Chl) damage. In the case of drought stress, chlo-roplast membranes enhance their production of O2" in response to Fe accumulation, and Fe-catalyzed formation of O2 radicals has been considered as a major factor contributing to drought damage in plant cells (Price and Hendry 1991). Plants grown under flooded conditions accumulate high levels of Fe (Sahrawat et al. 1996) and oxidation of Fe (II) leads to production of O2% which is suggested as a cause of flooding damage and post-anoxic injury to plants (Neue et al. 1998).
3.2 Antioxidant Defense System
By catalyzing detoxification of O2" to O2 and H2O2 and blocking O2" driven cell damage, SODs are a major component of the antioxidative defense system of plant cells (Bowler et al. 1994).
In many cases, the protective effect of SOD is enhanced by the presence of H2 O2 scavenging enzymes (see below), which prevent the production of hydroxyl radicals that may otherwise be formed in the presence of O/- as reductant and an appropriate catalyst such as metal ions, qui-nones, and Ferredoxin via the Haber-Weiss reaction:
According to their metal cofactor, SODs are classified into three types containing either Mn (Mn-SOD), Fe (Fe-SOD), or Cu and Zn (CuZn-SOD). In general, Mn-SOD is located in mitochondria, Fe-SOD in chloroplasts, and CuZn-SOD in chloroplasts and the cytosol (Bowler et al.
1994) . Accordingly, all three isozymes of SOD have a metal cofactor, and under deficiency conditions decline in the activity of related isozymes is expected to occur. Activity of isoforms of SOD is useful for determining the micronutrient status of plants, for example Zn (Cakmak et al. 1997; Hajiboland 2000), Fe (Iturbe-Ormaetxe et al.
1995), and Mn (Yu et al. 1998) nutritional status.
Oxidative stress is essentially a regulated process; the equilibrium between the oxidative and antioxidative capacities determines the fate of the plant. Plants possess very efficient scavenging systems for ROS that protect them from destructive oxidative reactions. Under nonstressful conditions the antioxidant defense system provides adequate protection against active oxygen and free radicals. Under stress conditions, however, an imbalance between production and scavenging of ROS causes oxidative damage (Apel and Hirt 2004) .
Enzymes involved in the metabolism of H2O2, namely, catalase, peroxidases, and ascorbate peroxidase are heme proteins. Catalase (CAT) catalyzes the conversion of hydrogen peroxide into water and O2:
Peroxidases (POD) catalyze the conversion of hydrogen peroxide to water. For example, ascorbate peroxidase (APX) is involved in the
Fig. 16.1 Production and scavenging of reactive oxygen species (ROS) in plants. Light energy absorbed by chlorophyll (Chl) is converted to electrochemical potential, which oxidizes H2O to O2 and generates the electrons for CO2 reduction and production of photoassimilates. Singlet oxygen (:O2) is formed in chloroplasts when photoexcited Chl in the triplet state ( 3 Chl) reacts with dioxygen (O2 ). Carotenoids react with :O2 and dissipate excess light energy as heat. Production of 2 O2 and O2^- is inevitable under light conditions. Impairment in utilization of electrons and absorbed light energy for CO2 fixation in nutrient-deficient plants or under environmental stresses increases possibility of ROS generation. O^- is converted to hydrogen peroxide (H2O2) by superoxide dismutase (SOD) which contains Zn, Cu, Mn or Fe as metal component. In the case of reduction in the SOD activity under deficiency conditions, H2O2 produces hydroxyl radical OOH), a highly reactive and potent ROS. Hydrogen peroxide is scavenged by Fe-containing ascorbate peroxidase (APX), peroxidases (POD) and catalase (CAT). Under Fe deficiency, scavenging reaction is impaired and H2 O2 is accumulated. Reaction oxygen species reacts with membranes, DNA, and proteins and causes cell damage sand death
Fig. 16.1 Production and scavenging of reactive oxygen species (ROS) in plants. Light energy absorbed by chlorophyll (Chl) is converted to electrochemical potential, which oxidizes H2O to O2 and generates the electrons for CO2 reduction and production of photoassimilates. Singlet oxygen (:O2) is formed in chloroplasts when photoexcited Chl in the triplet state ( 3 Chl) reacts with dioxygen (O2 ). Carotenoids react with :O2 and dissipate excess light energy as heat. Production of 2 O2 and O2^- is inevitable under light conditions. Impairment in utilization of electrons and absorbed light energy for CO2 fixation in nutrient-deficient plants or under environmental stresses increases possibility of ROS generation. O^- is converted to hydrogen peroxide (H2O2) by superoxide dismutase (SOD) which contains Zn, Cu, Mn or Fe as metal component. In the case of reduction in the SOD activity under deficiency conditions, H2O2 produces hydroxyl radical OOH), a highly reactive and potent ROS. Hydrogen peroxide is scavenged by Fe-containing ascorbate peroxidase (APX), peroxidases (POD) and catalase (CAT). Under Fe deficiency, scavenging reaction is impaired and H2 O2 is accumulated. Reaction oxygen species reacts with membranes, DNA, and proteins and causes cell damage sand death detoxification of H2O2 through conversion of ascorbate to dehydroascorbate:
Catalase is involved in the protection of chloroplasts from free radicals produced during the water-splitting reaction of photosynthesis. Activity of APX has mainly been reported from chloroplast and cytosol. In the chloroplasts, SOD and APX enzymes exist in both soluble and thylakoid-bound forms (Arora et al. 2002).
Antioxidant metabolites such as glutathione (GSH), a-tocopherol, carotenoids, and proline are also involved in scavenging ROS. A relationship between lipid peroxidation and proline accumulation has been reported in plants subjected to diverse kinds of stress (Molinari et al. 2007; Ashraf and Foolad 2007) . Proline acts as a free radical scavenger to protect plants away from damage by oxidative stress (Wang et al. 2009) and is an important compatible osmosolute serves as a protectant for enzymes and cellular structures under osmotic and drought stress (Hasegawa et al. 2000; Banu et al. 2009). There are evidences to demonstrate that accumulation of proline under Zn (Hajiboland and Amirazad 2010b) and B (Hajiboland, unpublished data) deficiency conditions may play a role in the tolerance of deficient plants to the other environmental stresses such as drought.
Factors involved in the production of ROS, their reactions, and scavenging are presented schematically in Fig. 16.1.
Was this article helpful?
An Open Letter To Anyone Who Wants To Lose Up To 20 Pounds In 30 Days The 'Low Carb' Way. 30-Day Low Carb Diet 'Ketosis Plan' has already helped scores of people lose their excess pounds and inches faster and easier than they ever thought possible. Why not find out what 30-Day Low Carb Diet 'Ketosis Plan' can do for you by trying it out for yourself.