Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/30/2009
Publication Date: 8/5/2009
Citation: Li, L., Zhou, X., Yuan, Y., Yang, Y., Rutzke, M., Thannhauser, T.W., Kochian, L.V. 2009. Involvement of a broccoli COQ5 methyltransferase in the production of volatile selenium compounds. Plant Physiology. 151:528-540. Interpretive Summary: Selenium is an essential micronutrient for animal and human nutrition, but becomes toxic at higher concentrations. Therefore, reducing the levels of selenium salts in selenium contaminated soils is of great important. Plants provide a promising solution for cleanup of selenium polluted environments by converting toxic selenium salts into less toxic volatile selenium compounds. To identify plant genes whose products promote the production of volatile selenium compounds, we used a genomics-based approach and isolated a broccoli COQ5 methyltransferase gene. When it was expressed in bacteria and transgenic plants, it specifically promotes selenium volatilization. This enzyme represents the first plant enzyme that is not involved in sulfur/selenium metabolism yet mediates selenium volatilization. The cloning and characterization of it from this economically important vegetable crop extends our understanding of selenium metabolism in plants. Such information may lead to ways to facilitate more efficient removal of selenium contaminated soils through genetic engineering of the process in plants and also possibly may be used to enhance the accumulation of health-promoting bioactive forms of selenium by suppression of their conversion into volatile selenium compounds.
Technical Abstract: Selenium plays an indispensable role in human nutrition and health, but becomes toxic at higher concentrations. Therefore, selenium volatilization is of great interest because it provides important means for cleanup of selenium polluted environments. To identify novel genes whose products are involved in Se volatilization from plants, a broccoli cDNA encoding COQ5 methyltransferase (BoCOQ5-2) in ubiquinone biosynthetic pathway was isolated, and authenticated by functionally complementing a yeast coq5 mutant and by detecting increased cellular ubiquinone level in the Bo-COQ5-2 transformed bacteria. Proteomics analysis of the differentially expressed proteins between the bacteria cells carrying BoCOQ5-2 and those containing empty vector further supports its functional role in ubiquinone biosynthesis. BoCOQ5-2 was found to specifically promote selenium volatilization but not sulfur emission in both bacteria and transgenic Arabidopsis plants. When exposed to selenate or selenite, bacteria expressing BoCOQ5-2 produced an over 160-fold or 11-fold increase, respectively, in selenium volatilization. Consequently, the BoCOQ5-2 transformed bacteria exhibited dramatically enhanced tolerance to selenate and selenite, and contained reduced levels of total selenium in the cells. Transgenic Arabidopsis expressing BoCOQ5-2 also showed a three-fold increase in Se volatilization when treated with selenite and exhibited significant tolerance to selenium. BoCOQ5-2 represents the first plant enzyme that is not directly involved in sulfur/selenium metabolism yet mediates selenium volatilization. This discovery opens up new aspects in understanding the complete metabolism of selenium and may lead to ways for more efficient phytoremediation of selenium contaminated environments.