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Selenium metabolism has been an area of active research because of the essentiality as well as toxicity of selenium to animals and humans. Biologically based selenium volatilization has been a particular area of interest for its potential in making detoxification of selenium pollution highly effective. Recently, we have isolated a broccoli BoCOQ5-2 methyltransferase gene involved in the ubiquinone biosynthetic pathway and found that it promoted selenium volatilization in both bacteria and plants. The identification of BoCOQ5-2 methyltransferase as a facilitator of selenium volatilization showed that selenium metabolism is regulated by other metabolic processes outside of the selenium/sulfur metabolic pathway. The interplay between ubiquinone and selenium metabolisms is possible through the protective function of ubiquinone against oxidative stresses induced by selenium. This observation could lead to new approaches to enhance selenium phytoremediation.
Selenium (Se) as an essential micronutrient is an environmentally hazardous element to animals and humans at high levels. Plants and microorganisms adapted to high-Secontaminated environment develop unique mechanisms to metabolize Se into less toxic forms. Therefore, they are not only able to take up and accumulate high levels of Se, but also convert the toxic forms of Se into volatile compounds that are 500 to 700 times less toxic.1 Biologically based Se volatilization offers an environmentally friendly and low-cost approach for cleanup of Se from soil and water.2,3
As an analog of sulfur, Se uptake, assimilation and volatilization are believed to share the same metabolic pathway of sulfur.1,4 Significant progress has been made in our understanding of Se metabolic process. Although alteration of Se metabolism has been achieved as a result of overexpressing some key genes in the pathway,5 little is known of the factors outside of the sulfur/selenium metabolic pathway that may regulate or affect Se metabolism.
The recent discovery of a ubiquinone biosynthetic pathway gene BoCOQ5-2 from broccoli that promotes Se volatilization in both transgenic bacteria and Arabidopsis has added an interesting new perspective to Se metabolism.6 BoCOQ5-2 functionally complemented a yeast coq5 mutant and caused an increase in the cellular ubiquinone level in transgenic bacteria. Characterization of bacteria and Arabidopsis ectopically expressing BoCOQ5-2 revealed that both Se volatilization and tolerance to selenate and selenite were significantly enhanced in the transgenic organisms.6
What is the connection between these two seemingly unrelated processes? Although BoCOQ5-2 is a methyltransferase, it does not seem likely that BoCOQ5-2 is directly involved in the methylation of Se compounds as BoCOQ5-2 is a C-methyltransferase in mitochondria and secluded from the subcellular locations where Se metabolism is believed to takes place. We speculate that BoCOQ5-2 may help bacteria and Arabidopsis deal with the oxidative stresses caused by Se toxicity through enhancing the capacity of ubiquinone biosynthesis to facilitate Se metabolism.
Apart from the disruption of protein function caused by replacing sulfur moiety of proteins with Se, another important mechanism of Se toxicity involves the formation of CH3Se− which enters a redox cycle and generates superoxide and oxidative stress.7 Indeed, exposing Arabidopsis plants to selenite has been shown to induce the production of hydrogen peroxide and superoxide in leaves.8 As a universal antioxidant, ubiquinone exists in all living things.9,10 A wide range of research demonstrates that ubiquinone is an important protectant against oxidative stresses. While ubiquinone deficient mutants are hypersensitive to those elicitors which are known to induce the production of reactive oxygen species, the ubiquinone overproducers are resistance to them.11–14 Overexpression of BoCOQ5-2 in transgenic Arabidopsis helped plants detoxify reactive oxygen species induced by selenite as the transgenic plants exhibited less staining of reactive oxygen species in comparison with control plants.6 These studies clearly show that ubiquinone contents in cells are important for cell tolerance to oxidative stresses. This may also explain the observation that the increase in Se volatilization in transgenic bacteria is much remarkable than that of transgenic Arabidopsis as the increase in cellular ubiquinone level in transgenic bacteria is much more marked than that in transgenic Arabidopsis.6
Transgenic Arabidopsis provides a nice model system showing that expression of BoCOQ5-2 promotes Se volatilization most likely through enhanced oxidative stress tolerance. However, as a Se non-accumulator, Arabidopsis has a limited ability in metabolizing Se. In contrast, Se accumulators possess high capacity to assimilate and volatilize Se. It is likely that manipulation of genes altering cellular ubiquinone level in Se accumulators would dramatically increase their capacity to assimilated and volatilize Se. The finding that expression of BoCOQ5-2 stimulates Se volatilization in bacteria and plants also broadens the ken of research in selenium metabolism to include players other than the enzymes that directly catalyze the reactions in the selenium/sulfur metabolic pathway. In addition to ubiquinone, a number of natural antioxidants such as vitamins C and E are also known to provide protection against oxidative stresses.15,16 Furthermore, elevated glutathione biosynthesis has been shown to help protect against Ni-induced oxidative damage in Thlaspi Ni hyperaccumulators17 and against Pb-induced oxidative damage in maize callus culture.18 Thus, it would be of interest to examine if alteration of the levels of these compounds affecting oxidative stress tolerance in microorganisms and plants also influences Se assimilation and volatilization.
Given the interacting nature of plant metabolic processes, it is not surprising that Se metabolism could be impacted by other pathways and factors. The successful demonstration of alteration of ubiquinone biosynthesis in stimulating Se volatilization through enhanced oxidative stress tolerance has opened a new perspective for study of Se metabolism as well as genetic engineering of microorganisms and/or Se accumulators for the remediation of Se-contaminated environment.
Previously published online: www.landesbioscience.com/journals/psb/article/10235