Uptake, accumulation, and metabolic fate of thioarsenates in plants

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The metalloid arsenic is ubiquitous in the environment due to both natural and anthropogenically influenced processes. Arsenic is toxic to all types of organisms and 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 arsenic. Different inorganic and organic arsenic 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 arsenic and the respective organic arsenic species are taken up by plants, too.Recently, we discovered the occurrence of inorganic and organic thiolated, i.e. sulfur-containing, arsenic species under the reducing conditions of rice paddy fields. These forms typically escape routine sampling and analysis methods even though they represent a substantial fraction of total arsenic under various field conditions. Our pilot study demonstrated plant uptake of thioarsenates and respective toxicity. Given the relevance of plant arsenic accumulation for food safety, these two observations urgently demand a mechanistic understanding of thioarsenate effects on plants and the contribution of thioarsenates to the As load of plants. The proposed project aims at dissecting the uptake pathways for various thioarsenates in plants, the metabolic conversion of thioarsenates through reduction and complex formation within plant cells, the mobility of thioarsenate-derived arsenic species between plant tissues and finally the overall contribution of thioarsenate exposure to the arsenic load of plants. The two studied model systems are rice, which is also the main target organism with respect to food safety, and Arabidopsis thaliana, the plant that has enabled many of the major breakthroughs in understanding arsenic transport and metabolism. Mechanistic insights into the plant-thioarsenate interaction will enable approaches to substantially reduce the accumulation of ubiquitous arsenic in crop plants and thus the chronic poisoning of humans.