Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/3/2005
Publication Date: 5/15/2005
Citation: Li, L., Lyi, S.M., Heller, L., Rutzke, M., Welch, R.M., Kochian, L.V. 2005. Molecular and biochemical characterization of the selenocysteine se-methyltransferase gene and se-methylselenocysteine synthesis in broccoli (brassica oleracea var. italica). Plant Physiology. 138:409-420. Interpretive Summary: Cancer is the second leading cause of death in the USA with over 500,000 deaths annually. Diet plays an important role in the onset of cancer. Plant foods altered with higher levels of anticarcinogens could reduce cancer risk. A large body of convincing evidence suggests that selenium (Se) acts as a cancer preventive agent when given in pharmacological amounts. Increasing Se in widely consumed plant foods could lower the risk of cancer to the general public, reaching all sectors of society in sustainable ways. Some Brassica species such as broccoli accumulate a Se compound (e.g. Se-methylselenocysteine) that shows superior anticarcinogen properties. To ultimately develop crops with the ability to accumulate high level of Se-methylselenocysteine, we have isolated and characterized a selenocysteine methyltransferase responsible for the formation of the functional form of Se from broccoli. We have showed that when the plant was fertilized with different forms of Se and sulfate, the gene expression and Se-methylselenocysteine accumulation were highest with selenate treatment and dramatically reduced with selenite and high level of sulfate supply. The isolation of this gene provides molecular tool to enhance the functional form of Se production in crops with increased health-promoting properties. Also our results provide important information to maximize the selenium production in the popular vegetable.
Technical Abstract: Selenium plays an indispensable role in human nutrition and has been implicated to have important health benefits, including being a cancer preventive agent. While different forms of selenium vary in their anticarcinogenic efficacy, Se-methylselenocysteine has been demonstrated to be one of the most effective chemopreventive compounds. Broccoli is known for its ability to accumulate high levels of selenium, mainly in the form of Se-methylselenocysteine. Thus, it serves as a good model to study the regulation of Se-methylselenocysteine accumulation in plants. A cDNA encoding selenocysteine Se-methyltransferase, the key enzyme responsible for Se-methylselenocysteine formation, was cloned from broccoli using an Arabidopsis homocysteine S-methyltransferase gene probe. This clone, designated as BoSMT, was functionally expressed in E. coli and its identity was confirmed by its substrate specificity in the methylation of selenocysteine. The BoSMT gene represents a single copy gene in the broccoli genome. Examination of BoSMT gene expression and Se-methylselenocysteine accumulation in response to selenate, selenite and sulfate treatments showed that the BoSMT transcript and Se-methylselenocysteine synthesis were significantly up-regulated in plants exposed to selenate but was low in plants supplied with selenite. High levels of sulfate suppressed selenate uptake and resulted in a dramatic reduction of BoSMT mRNA level and Se-methylselenocysteine accumulation. Our results revealed that Se-methylselenocysteine accumulation closely correlates with the BoSMT gene expression as well as the total Se status in tissues, and provide important information for maximizing the Se-methylselenocysteine production in this beneficial vegetable plant.