Master Thesis

Uptake and transformation of methylated thioarsenates in rice

Alina Schindele (09/2017-08/2018)

Support: Britta Planer-Friedrich, Carolin Kerl

Rice (Oryza sativa L.) is the most important staple food for half of the world’s population, but also known as an accumulator for the toxic metalloid arsenic (As), found in paddy soils. Even though inorganic As (iAs) species and methylated arsenates are the predominant species in paddy soils, recent investigations proved the occurrence of methylated thioarsenates, which are widely unexplored regarding the uptake and transformation by rice plants. In the present thesis, the uptake, transport and detoxification mechanisms of monomethylmonothioarsenate (MMMTA) and dimethylmonothioarsenate (DMMTA) by rice plants were investigated with monomethylarsenate (MMA(V)) and dimethylarsenate (DMA(V)) as experimental controls. Synthesized MMMTA was stable over 24 h in a pre-experiment with plants without any indications for biotic degradation, which was in accordance with results from a later stability experiment using only differently prepared plant matrix. MMMTA was detected in the xylem sap of MMMTA exposed plants, although mainly MMA(V) was transported by the plants. These plants transported the lower total As concentrations than MMA(V) exposed plants. With the low translocation found for MMMTA exposed plants, the low total As concentration in the xylem sap was substantiated, although these plants took up the highest amount of total As compared to DMMTA, DMA(V) and MMA(V). Synthesized DMMTA was abiotically stable for 24 h. In presence of rice plants, DMMTA was degraded within 24 h, which was substantiated by an experiment investigating the stability in the plant matrix. The As species sum concentration in the efflux was four times higher compared to MMMTA and DMA(V) but was in the same order of magnitude like the efflux of MMA(V) exposed plants, although the xylem sap of DMMTA exposed plants contained the highest total As concentrations, which consisted mainly of DMA(V) and DMMTA. The uptake efficiency of DMMTA was lower than for MMMTA, but DMMTA showed highest total As concentrations in plant shoots. The translocation from roots into shoots was about six times higher than for MMMTA, but lower than for DMA(V).

In summary, DMMTA is assumed to be biotically degraded in plants, although a highly efficient translocation in the plants was observed. Contrary, for MMMTA no indications for a biotic degradation in the plant were observed as well as the transport in low amounts in the xylem. Further investigations regarding the uptake and metabolism of methylated thioarsenates in rice plants are indispensable to reduce the human health concern originating from toxic As species.