Transformation and Subcellular Distribution of Arsenic and Mechanism of its Sub-lethal Toxicity in Plants

Speciation of arsenic (As) in rice, the most As affected crop, and Ceratophyllum demersum, a good laboratorymodel for shoot, through chromatography and localization in the later through synchrotron based techniqueswas performed to explore the mechanism of As toxicity. In rice plant, exposed to inorganic arsenate (AsV) andmethylated As, most of the AsV and methylarsonate (MAV) were efficiently reduced to arsenite (AsIII) and MAIIIrespectively, but dimethylarsinate (DMAV) did not transformed. A large proportion of AsIII and MAIII were complexedwith thiols showing up to 20 and 16 As species respectively in the roots. Many of them were identifiedas new As-thiol species. Despite high complexation in root, more MA was translocated to shoot, with shoot/roottransfer factor being in order DMA>MA>inorganic As in rice.C. demersum, also displayed up to 60% accumulated As in the form of thiol complexed-AsIII. Most of which wasin epidermis of mature leaves as revealed by tissue resolved X-ray absorption near-edge spectroscopy (μ-XANES) of intact hydrated leaves. At sublethal concentration, As predominantly accumulated in the nucleus ofthe epidermal cells, as revealed by μ-X-ray fluorescence (μ-XRF), indicating replacement of P by As in DNA molecules,providing in vivo evidence for the proposed toxicity mechanism of AsV. While at lethal concentration,vacuole was the main storage site of As, yet a significant increase of unbound AsIII in mesophylls of young matureleaves occurred. This small amount of As reaching chloroplasts already caused a strong and specific inhibitionof tetrapyrrole biosynthesis and severe growth retardation.Taken together these results establish the mechanism of As toxicity and reported yield loss in paddy rice growingin As contaminated areas. Further, significant translocation of MAIII (more cytotoxic than AsIII) to rice shootscould also be an important factor inducing straighthead (spikelet sterility disorder) in rice