An enhancing effect of exogenous brassinolide on the growth and antioxidant activity in Chlorella vulgaris cultures under heavy metals stress
Introduction
Biochemical adaptations to pathogens and abiotic stresses in plants involve various changes in the biochemistry of the cell. These changes include the evolution of new metabolic pathways, the accumulation of low molecular weight metabolites, the synthesis of special proteins, detoxification mechanisms and changes in phytohormone levels. One of the biochemical changes that occurs when plants are subjected to heavy metals stress is the production of reactive oxygen species (ROS) such as the superoxide radical (O2−), hydrogen peroxide (H2O2), and the hydroxyl radical (OH). Although, under normal growth conditions, the production of ROS in cells is very low, there are many stresses which can enhance the production of ROS. These stresses include drought stress and desiccation, salt stress, chilling, heat shock, heavy metals, ultraviolet radiation, ozone, mechanical stress, nutrient deprivation, pathogen attack and high light stress. ROS can have a detrimental effect on normal metabolism through oxidative damage to lipids, proteins, and nucleic acids. However, many plants that live in heavy metals pollutions have evolved several adaptations. Recent studies have demonstrated that the activities of these antioxidant enzymes and the levels of antioxidant molecules can be increased under various environmental stresses. To control the level of ROS and to protect the cells, plants possess low molecular weight antioxidants (ascorbic acid, reduced glutathione, carotenoids, tocopherols) and antioxidant enzymes such as superoxide dismutase (SOD; E.C. 1.15.1.1), ascorbate peroxidase (APX; E.C. 1.11.1.11) and catalase (CAT; E.C. 1.11.1.6) that scavenge ROS. Superoxide dismutase is the major superoxide radical scavenger and its enzymatic action results in H2O2 and O2 formation. The product of SOD activity (H2O2) is still toxic and must be eliminated by conversion to H2O in subsequent reactions. CAT and several classes of peroxidases like APX then scavenge the H2O2 produced. The ascorbate–glutathione cycle seems to be a mechanism of great importance in controlling the cellular redox status, especially after application of heavy metals. Ascorbic acid is a primary as well as a secondary antioxidant. As secondary antioxidant it plays an important role in the regeneration of α-tocopherol. Also, non-protein thiol groups, especially glutathione, exert several important roles in protection of plants from environmental stress factors, especially in the case of cadmium and lead toxicity (Mittler, 2002, Sharma and Dietz, 2009, Triantaphylidès and Havaux, 2009).
The object of the present study was to determine the interaction of brassinolide (BL) with heavy metals (cadmium, copper and lead) on the growth and level of antioxidants in Chlorella vulgaris cultures. Obtained results could be important for better understanding of the role of brassinosteroids in biochemical adaptation of C. vulgaris to stress conditions present in polluted aquatic ecosystem.
Section snippets
Plant material and growth conditions
C. vulgaris Beijerinck (Trebouxiophyceae) was grown under controlled conditions at 25 ± 0.5 °C. Illumination was supplied during the 16-h photoperiod (8-h dark period) by a bank of fluorescent lights yielding a photon flux of 50 μmol m−2 s−1 at the surface of the tubes. Permanent synchronous growth was established according to the method of Pirson and Lorenzen (1966) under conditions developed by Sayegh and Greppin (1973). The culture medium used was modified Knop's medium consisting of the following
Results
On the basis of previously published results (Bajguz and Czerpak, 1998), a concentration of 10−8 M BL was mixed with selected heavy metals (10−6 to 10−4 M). BL at this concentration had the greatest effect on the growth and cellular chemical composition of light-grown C. vulgaris. Data were determined for each of the parameters (number of cells and antioxidants level) on 24 h after the initiation of BL and/or heavy metals treatment.
The effects of BL mixed with heavy metals on the growth (expressed
Discussion
Algal cultures have a remarkable ability to take up and accumulate heavy metals from their environment. Many studies on heavy metals polluted waters have revealed that metal pollution decreases algal diversity, productivity and alters algal species composition (Harding and Whitton, 1976, Fahmi et al., 1982, Foster, 1982, Takamura et al., 1989, Gupta and Chandra, 1994, Bajguz, 2000, Mallick, 2004). Heavy metals are nondegradable and can accumulate and concentrate as they move up the food chain.
Conclusion
Metals are important pollutants in the aquatic environment. The growth of C. vulgaris has been inhibited in the cultures treated with heavy metals. The present study revealed that the BL-treated cultures of C. vulgaris grown in the presence of heavy metals had significantly enhanced level of antioxidative enzymes (catalase, peroxidase and superoxide dismutase), the level of ascorbate and reduced glutathione, compared with untreated plants (receiving Cu, Pb and Cd alone) as well as to the
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