Translocation and transformation of selenium in hyperaccumulator plant Cardamine enshiensis from Enshi, Hubei, China

Aims Cardamine enshiensis (C. enshiensis) is a selenium (Se) hyperaccumulator plant found in Enshi, a typical seleniferous area in southwestern Hubei Province, central China. The aim of this work was to investigate the translocation and transformation of Se in C. enshiensis, which may help to elucidate the mechanism on the tolerance and hyperaccumulation of Se in C. enshiensis. Methods The contents of Se in roots, shoots and leaves of C. enshiensis were measured by ICP-MS. The spatial distribution and chemical forms of Se in roots, shoots and leaves of C. enshiensis were studied using synchrotron radiation microfocused X-ray fluorescence analysis (mu-SRXRF) and in situ Se K-edge X-ray Absorption Near Edge Spectroscopy (mu-XANES). The Se species in roots, shoots and leaves were further studied by RP-HPLC-ICPMS. Results Roots, shoots and leaves of C. enshiensis contained an average of 2985, 3329 and 2491 mg/kg Se DW, respectively. mu-SRXRF found that Se was primarily located in the cortex, endodermis, and vascular cylinder in roots, while it was in the epidermis, cortex and vascular bundle of shoots and concentrated in the leaf veins and the peripheral parts. In situ mu-XANES analysis showed that the root vascular tissue contained 16% SeO42-, 19% C-Se- (using selenocystine, SeCys as model compound) and 65% C-Se-C (using methylselenocystine, MeSeCys as model compound), and shoot vascular bundle contained 10% SeO42-, 28% C-Se- and 62% C-Se-C while leaves had 84% C-Se- and 16% SeO32-. RP-HPLC-ICPMS confirmed that the major Se species were MeSeCys and SeCys. Conclusions C. enshiensis is a hyperaccumulator plant with the majority of Se in it as organic Se species like MeSeCys and SeCys. Se was translocated from roots via shoots to leaves through vascular tissues, and was deposited in the cortex and endodermis of roots, epidermis and cortex of shoots and periphery of leaves. The high concentration of organic Se species like MeSeCys suggests that C. enshiensis may be used for health promotion in people suffered from cancer and other diseases. Besides, the hyperaccumulation of Se in C. enshiensis may also identify itself to be applied for phytoremediation in Se-contaminated areas.