Analysis of glutathione S-transferase (GST) gene expression in transgenic plants and its effects on environmental stress
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Plants cannot escape from biotic and abiotic stress factors such as extreme temperatures, high light intensity, drought, UV radiation, heavy metals, and pathogen attack. Anthropogenous stress factors, such as herbicides, also affect or block metabolism, growth, and development, and is being used every year. Plants have versatile detoxification systems to counter the phytotoxicity of a wide range of natural and synthetic compounds, which are present in the environment. Glutathione S-transferases (GSTs) are the enzymes that detoxify natural and exogenous toxic compounds by conjugation with glutathione. Glutathione, an endogenous tripeptide, is important as a reducing agent, nucleophilic scavenger, and alleviate the chemical toxicity in the plants by the reaction of GST. GSTs play an important role in the phase Ii of conjugation with xenobiotics, and have another role as glutathione peroxidase. Glutathione conjugates are can be transported to the vacuoles or apoplast and are generally much less toxic than the parent compounds. Oxygen radicals are also highly harmful to the cell components. Those toxic reactive oxygen species damage DNA, lipid layer, and proteins. Many GSTs can also act as glutathione peroxidases to sacvenge toxic peroxides from cells. A cDNA named Gh-5 was isolated from cotton fiber cDNA library, using PCR-based cloning methods. Analysis of this cDNA revealed high sequence similarity with other plant GSTs. Expression of Gh-5 in E. coli resulted in protein extracts with high GST activity. Overexpression of Gh-5 in tobacco has led to about a two-fold increase in GST specific activity compared to the nonexpressing control plants. These results indicate that Gh-5 encodes an active GST both bacterial and in plant cells. Antisense GST constructs were also developed to deactivate native cotton GST and to evaluate the effect of the decreased enzyme in cotton plants. In order to determine whether GST overexpression could confer protection to plants against oxidative stresses, transgenic plants were exposed to herbicides, salinity, low temperature, heavy metals and photooxidation. The GST overexpressing plants showed enhanced seedling tolerance to low temperature and salinaty. These results indicate that increased level of GST can protect plants from certain types of environmental stresses.