Uptake, accumulation, and metabolic fate of thioarsenates in plants
Final Report Abstract
The metalloid arsenic (As) is ubiquitous in the environment due to both natural and anthropogenically influenced processes. Arsenic is toxic to all types of organisms. For humans it is categorized as a class 1 carcinogen. Arsenic is inadvertently taken up by plants via transporters for essential or beneficial nutrients. Therefore, dietary intake of plant-derived food and especially of rice grains is a major route for human exposure to chronically toxic As. Different inorganic and organic As species are found in the environment. The two main inorganic As species generally considered to account for most of the As accumulation in plants are arsenate under oxidizing conditions and arsenite under reducing conditions. Also, rhizosphere bacteria can methylate As and the respective organic As species are taken up by plants, too. Motivation for the present project had been our previous discovery of inorganic and methylated As-sulfur species, so-called thioarsenates, in paddy soil pore waters. Thioarsenates typically escape routine sampling and analysis methods, even though they represent a substantial fraction of total As under various field conditions and our pilot study had shown uptake of monothioarsenate (MTA) in rice and ensuing toxicity. Given that rice is the most important staple food worldwide, these two observations urgently demand a more thorough mechanistic understanding of uptake pathways for various thioarsenates, their metabolic conversion through reduction and complex formation within plant cells, their mobility, and finally, the overall contribution of thioarsenate exposure to the As load of plants. Besides rice, Arabidopsis thaliana was studied, which is the plant system that has enabled many of the major breakthroughs in understanding As transport and metabolism. Our project yielded a number of crucial insights into the interaction of thioarsenates with plants. Most importantly, we found efficient uptake and in planta mobility of dimethylmonothioarsenate (DMMTA), a highly toxic thioarsenate according to studies with human cells and our own experiments on plants. Furthermore, DMMTA can be formed in plants from other, much less harmful methylated As species such as dimethylarsenate (DMA), most likely via reactions with reduced sulfur. DMMTA mobility in plants is at least partly explained by the fact that it escapes the main As detoxification pathway in plants, namely the formation of phytochelatins. Substantial amounts of DMMTA can even reach rice grains. Market sampling showed that DMMTA is in fact ubiquitous in commercial rice, regardless of region of origin. This poses a potentially serious food safety threat which we communicated in publications and directly to regulatory authorities.
Publications
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Iron Plaque at Rice Roots: No Barrier for Methylated Thioarsenates. Environmental Science & Technology, 53(23), 13666-13674.
Kerl, Carolin F.; Ballaran, Tiziana Boffa & Planer-Friedrich, Britta
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Methylated Thioarsenates and Monothioarsenate Differ in Uptake, Transformation, and Contribution to Total Arsenic Translocation in Rice Plants. Environmental Science & Technology, 53(10), 5787-5796.
Kerl, Carolin F.; Schindele, Ruth Alina; Brüggenwirth, Lena; Colina, Blanco Andrea E.; Rafferty, Colleen; Clemens, Stephan & Planer-Friedrich, Britta
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Safer food through plant science: reducing toxic element accumulation in crops. Journal of Experimental Botany, 70(20), 5537-5557.
Clemens, Stephan
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Detection of Thioarsenates in Rice Grains and Rice Products. Journal of Agricultural and Food Chemistry, 69(7), 2287-2294.
Colina, Blanco Andrea E.; Kerl, Carolin F. & Planer-Friedrich, Britta
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Dimethylated Thioarsenates: A Potentially Dangerous Blind Spot in Current Worldwide Regulatory Limits for Arsenic in Rice. Journal of Agricultural and Food Chemistry, 70(31), 9610-9618.
Planer-Friedrich, Britta; Kerl, Carolin F.; Colina, Blanco Andrea E. & Clemens, Stephan
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Dimethylmonothioarsenate Is Highly Toxic for Plants and Readily Translocated to Shoots. Environmental Science & Technology, 56(14), 10072-10083.
Pischke, Erik; Barozzi, Fabrizio; Colina, Blanco Andrea E.; Kerl, Carolin F.; Planer-Friedrich, Britta & Clemens, Stephan
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In Planta Arsenic Thiolation in Rice and Arabidopsis thaliana. Environmental Science & Technology, 57(51), 21846-21854.
Colina, Blanco Andrea E.; Pischke, Erik; Higa, Mori Alejandra; Kerl, Carolin F.; Clemens, Stephan & Planer-Friedrich, Britta
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Widespread occurrence of dimethylmonothioarsenate (DMMTA) in rice cakes: Effects of puffing and storage. Food Chemistry, 436, 137723.
Colina, Blanco Andrea E.; Higa, Mori Alejandra & Planer-Friedrich, Britta
