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Faculty for Biology, Chemistry, and Earth Sciences

Environmental Geochemistry Group - Prof. Dr. Britta Planer-Friedrich

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Master Thesis

Transportation and re-translocation of methylated arsenic species in rice shoots

Jannis Claßnitz (10/2020-11/2021)

Support: Britta Planer-Friedrich, Andrea Colina Blanco

Methylated thioarsenates have been detected in paddy fields and were recently discovered in commercially available rice products. These arsenic species, especially dimethyl monothioarsenate (DMMTA), are very toxic for humans, yet the exact mechanisms of their accumulation in the rice grains remain largely unknown. Here, three experiments were conducted to better understand the behavior of DMMTA in the rice shoot in comparison with its oxyarsenate counterpart, dimethylarsenate (DMA). A stem girdling experiment showed that both species are very dependent on the phloem transport in the shoot to reach the grain. In a second experiment, arsenic was remobilized from the plant’s flag leaf to a larger degree when the leaf was fed DMA rather than DMMTA. The arsenic speciation changed in planta in both experiments to form very similar fractions of DMA and DMMTA, regardless of the As species the plant was exposed to. In the grains, As was present as 85.9–87.9% DMA and 6.9–10.0% DMMTA in the experiments. The flag leaf feeding experiment showed an increasing fraction of DMMTA along the pathway of the nutrients from the leaf to the grain, suggesting the possibility of thiolation during transport. Therefore, a third experiment was conducted to test the effect of thiol-containing glutathione on the transformation observed but the results remained inconclusive. Nonetheless, the conducted experiments showed that thiolation and de-thiolation of methylated arsenates can occur in planta.

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