Effects of Thiosulfate as a Sulfur Source on Plant Growth, Metabolites Accumulation and Gene Expression in Arabidopsis and Rice

Plants are considered to absorb sulfur from their roots in the form of sulfate. In bacteria like Escherichia coli, thiosulfate is a preferred sulfur source. It is converted into cysteine (Cys). This transformation consumes less NADPH and ATP than sulfate assimilation into Cys. In Saccharomyces cerevisiae, thiosulfate promoted growth more than sulfate. In the present study, the availability of thiosulfate, the metabolite transformations and gene expressions it induces were investigated in Arabidopsis and rice as model dicots and monocots, respectively. In Arabidopsis, the thiosulfate-amended plants had lower biomass than those receiving sulfate when sulfur concentrations in the hydroponic medium were above 300 μM. In contrast, rice biomass was similar for plants raised on thiosulfate and sulfate at 300 μM sulfur. Therefore, both plants can use thiosulfate but it is a better sulfur source for rice. In both plants, thiosulfate levels significantly increased in roots following thiosulfate application, indicating that the plants absorbed thiosulfate into their root cells. Thiosulfate is metabolized in plants by a different pathway from that used for sulfate metabolism. Thiosulfate increases plant sulfide and cysteine persulfide levels which means that plants are in a more reduced state with thiosulfate than with sulfate. The microarray analysis of Arabidopsis roots revealed that 13 genes encoding Cys-rich proteins were upregulated more with thiosulfate than with sulfate. These results together with those of the widely targeted metabolomics analysis were used to proposes a thiosulfate assimilation pathway in plants.