Cellular sequestration of cadmium in the hyperaccumulator plant species Sedum alfredii

Spatial imaging of cadmium (Cd) in the hyperaccumulator Sedum alfredii was investigated in vivo by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and X-ray microfluorescence (µ-XRF) imaging. Preferential Cd accumulation in the pith and cortex was observed in stems of the Cd hyperaccumulating ecotype (HE), whereas Cd was restricted to the vascular bundles in its contrasting nonhyperaccumulating ecotype (NHE). Cadmium concentrations of up to 15,000 µg g-1 were measured in the pith cells which was many fold higher than concentrations in the stem epidermis and vascular bundles in the HE plants. In the leaves of HE, Cd was mainly localized to the mesophyll and vascular cells rather than epidermis. The distribution patterns of Cd in both stems and leaves of HE was very similar to Ca, but not Zn, irrespective of Cd exposure levels. Extended X-ray absorption fine structure spectroscopy analysis (EXAFS) showed that Cd in the stems and leaves of HE was mainly associated with oxygen ligands, and a larger proportion (about 70% in leaves and 47% in stems) of Cd was bound with malic acid, which was the major organic acid in the shoots of the plants. These results indicate that a majority of Cd in HE accumulates in the parenchyma cells, especially in stems, and is likely associated with Ca pathways and bound with organic acid (malate), which is indicative of critical role of vacuolar sequestration of Cd in the HE S. alfredii.