Bachelor Thesis

In-planta thiolation of oxyarsenic species in rice plants

Natascha Lukas (04/2022-09/2022)

Support: Carolin Kerl, Britta Planer-Friedrich

Formation of thioarsenates in rice paddy soils as well as their uptake into rice plants and their presence in rice grains and products has been shown before. In addition to uptake from the soil in-plant thiolation of oxyarsenic species might contribute to the contents of thioarsenates in rice plants. In this study we investigated the thiolation of arsenite, monomethylarsenate (MMA) and dimethylarsenate (DMA) in hydroponically grown rice plants of different Italian varieties. For all oxyarsenic species and all rice varieties thiolated species were observed in the rice roots and shoots after 24 h of exposure. In arseniteexposed plants monothioarsenate (MTA; 0,025 ± 0,002 % in roots; 0,167 ± 0,008 in shoots) was found whereas in MMA-exposed rice plants monomethylmonothioarsenate (MMMTA; 0,31 ± 0,08 % in roots; 0,24 ± 0,09 % in shoots) occurred. The highest percentage of thioarsenates resulted from the thiolation of DMA to dimethylmonothioarsenate (DMMTA; 0,4 ± 0,2 % in roots; 1,5 ± 0,4 % in shoots). Only the dimethylated species had a dithiolated product, dimethyldithioarsenate (DMDTA; 0,18 ± 0,05 % in roots; 0,23 ± 0,07 % in shoots). For MTA and MMMTA it remains unclear whether thiolation occurred in rice roots or shoots and in which plant tissues thioarsenates occurred merely because of translocation. However, distribution of DMMTA and DMDTA between roots and shoots and its change with exposure time indicated that thiolation of DMA and DMMTA might occur in both roots and shoots. Formation of DMDTA was delayed behind DMMTA formation in a kinetics experiment. As DMMTA is mobile in plants and phytotoxic, it is discussed as a possible cause of Straighthead Disease. However, we could not observe a relation between the thiolation of DMA in different rice varieties and their sensitivity to Straighthead Disease. In a mesocosm experiment, MTA, dithioarsenate, trithioarsenate and DMMTA were found in rice shoots, but it was unclear whether they were taken up from the porewater or formed by in-plant thiolation.