Uptake, accumulation, and metabolic fate of thioarsenates in plants

由于自然和人类活动的影响，类金属砷在环境中普遍存在。砷对所有类型的生物都是有毒的，对人类来说，它被归类为1类致癌物质。砷被植物不经意地通过转运蛋白吸收，以获得必要或有益的营养。因此，从饮食中摄取植物性食物，特别是大米谷物，是人类接触慢性有毒砷的主要途径。在环境中发现了不同的无机和有机砷形态。通常认为，在植物中积累砷的两种主要无机砷形态是氧化条件下的砷酸盐和还原条件下的亚砷酸盐。此外，根际细菌可以甲基化砷，相应的有机砷形态也被植物吸收。最近，我们发现在稻田还原条件下，存在无机和有机硫醇化的砷形态，即含硫的砷形态。这些形态通常避开常规的采样和分析方法，即使它们在各种现场条件下代表了总砷的相当大一部分。我们的初步研究证明了植物对硫代砷酸盐的吸收和各自的毒性。鉴于植物砷积累与食品安全的相关性，这两个观察迫切需要从机理上了解硫代砷酸盐对植物的影响以及硫代砷酸盐对植物砷负荷的贡献。该项目旨在剖析各种硫代砷酸盐在植物中的吸收途径，硫代砷酸盐在植物细胞内通过还原和络合物形成的代谢转化，硫代砷酸盐衍生的砷物种在植物组织之间的迁移，以及硫代砷酸盐暴露对植物砷负荷的总体贡献。这两个被研究的模式系统是水稻和拟南芥，前者也是食品安全的主要目标生物，后者使人们在理解砷的运输和代谢方面取得了许多重大突破。对植物-硫代砷酸盐相互作用的机械性见解将使能够大幅减少农作物中普遍存在的砷的积累，从而减少对人类的慢性中毒。

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.