Research articleExcess copper induced oxidative stress and response of antioxidants in rice
Highlights
► Excess Cu negatively affects growth of the rice plant. ► Damage was more severe in root than shoot. ► Excess Cu led to oxidative stress in rice plant by inducing ROS generation. ► The plant tolerated the oxidative stress by increasing antioxidant defense system. ► Increased level of proline also reflects its association in tolerating the stress.
Introduction
Heavy metal toxicity is one of the major environmental problems to the present world because of its increasing level caused by both natural sources (e.g., wind-blown dust, decaying vegetation, forest fires and sea spray) and human activities (e.g., mining, metal production; wood production and phosphate fertilizer). Even in trace concentration, they caused serious problem to all the organisms. Among the heavy metals, copper (Cu) is an essential micronutrient for plant growth and various biochemical processes. It is required in much important biological function since they are constituents of many enzymes and proteins. It is a co-factor of enzymes like plastocyanin, cytochrome c, and Cu/Zn− superoxide dismutase (Cu/Zn–SO). However, excessive Cu adversely affects plant growth and metabolism. Even slightly higher concentration of Cu from the optimal level induces toxicity to the plant [1]. Being a redox active metal, Cu catalyzes the production of reactive oxygen species (ROS), such as superoxide (O2•−), hydrogen peroxide (H2O2) and hydroxyl radicals (OH•), via Haber–Weiss and Fenton reactions [2]. ROS are damaging to essential cellular components such as DNA, proteins and lipids therefore induction of ROS production led to oxidative stress affecting plant growth and alteration of antioxidant system [3].
Plants have evolved certain mechanisms to tolerate heavy metal stress such as metal exclusion, metal accumulation and binding of heavy metal by strong ligands like cysteine-rich proteins, metallothioneins (MTs) and thiol-rich peptides, phytochelatins (PCs) [4], [5]. Moreover, plants have stimulated antioxidant system to combat the oxidative injury induced by the heavy metals [6]. These include several ROS-removing enzymes such as superoxide dismutase (SOD), catalases (CAT), guaiacol peroxidase (GPX), ascorbate peroxidase (APX), glutathione reductase (GR), and low molecular mass antioxidants scavengers such as ascorbate (ASC) and glutathione (GSH). It has been suggested that Cu induced antioxidative reactions in the roots of Brassica juncea and in Arabidopsis [4]. These induced cellular antioxidants scavenged ROS thereby preventing the damage caused by the overproduction of ROS. Generation of proline is also one of the vital responses of plant under Cu toxicity which is possibly associated with the protection of plant cells against oxidative damage and with signal transduction [7]. Proline might protect plants from metal toxicity by chelating heavy metals in the cytoplasm or as a hydroxyl radical scavenger [8]. It can be considered as a non-enzymic antioxidant which is involved in counteracting the ROS damage. However different plants, plant parts and metals evolved different strategies to resist the toxicity.
Rice is an important staple food of the world population containing high nutritional quality among cereal crops. Half of the world population depends on rice for survival but due to certain heavy metal toxicity including Cu caused reduction of the crop yield. The present investigation was undertaken to test the effects of Cu on growth, induction of oxidative stress and antioxidative responses in rice (Oryza sativa L.) variety MSE-9.
Section snippets
Changes of growth and copper uptake
The effect of different Cu concentrations on the growth of rice plant was given in Table 1. A gradual decrease in shoot and root elongation was observed with the increase in Cu concentration and length of treatment. Maximum inhibition was occurred at 100 μM Cu after 5th day in shoot (18.84%) and root (27.59%) with respect to control. At the same concentration, fresh mass of shoot is decreased by 35.31% while in root 44.44% is found to be decreased (Table 1) suggesting higher impact of Cu
Changes of growth and copper uptake
The decrease in growth of rice plant when raised under increasing level of Cu suggested Cu induced toxicity at elevated concentration and the impact was found more in root than shoot. The greater sensitivity of root to Cu toxicity might be due to the higher Cu content in root as maximum accumulation of Cu in rice root was observed. Similar observation was reported in Elsholtzia splendens [9] under Cu treatment. On the basis of this result, Cu toxicity is directly correlated with the
Plant cultivation and treatments
Viable rice seed (MSE-9) variety was procured from Regional Agricultural Research Station (AAU), Akbarpur, Karimganj, Silchar. The seeds were surface sterilized with 0.1% mercuric chloride (HgCl2) solution, rinsed with distilled water and set for germination in clean petri dishes with wetted filter papers at 30 °C in dark. Uniformly germinated seeds were selected and were transferred to the plastic pots containing Hoagland nutrient medium (pH = 6.2). The seedlings were grown in a growth chamber
Acknowledgments
The authors thank Regional Agricultural Research Station (AAU), Akbarpur, Karimganj, Silchar for providing Rice seedlings throughout the experimental work and the financial support by UGC (UGC fellowship) is also highly acknowledged.
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