Accumulation and uptake of light rare earth elements in a hyperaccumulator Dicropteris dichotoma

Abstract

Dicropteris dichotoma, a natural perennial fern, grows in acidic soil in southern China. It hyperaccumulates several light rare earth elements La, Ce, Pr and Nd (LREEs) up to about 0.7% of its dry leaf biomass. Through electron microscopic and X-ray microanalyses, LREEs deposits were observed in the cell wall, intercellular space, plasmalemma, vesicles and vacuoles of the root endodermis and stele cells but not in the Casparian band of the fern adventitious root. In addition, LREE deposits were observed in the phloem and xylem of the fern rhizome. These results indicate that at least a portion of the LREEs can be transported symplastically. Histidine and organic acids appear to play a role in stimulating the accumulation of LREEs. This was evident in ferns cultivated in soil amended by histidine, malic and citric acids over a 60-day period. Concentrations of LREEs in D. dichotoma leaves increased by 21–78% as opposed to control, which only increased by 6–10%. While this suggests that histidine and organic acids promote the uptake and sequestration of LREEs in D. dichotoma, the enhancement mechanism of each acid appears to differ. Histidine promoted sequestration of LREEs by forming complexes with LREEs in cells. Organic acids, however, increased the release of LREE from soil, and the uptake of LREEs by fern roots. Most of the LREEs (81–89%) in the cell wall are the result of one of these mechanisms involved in LREE hyperaccumulation. Mass analysis indicated that the molecular weight of LREE-binding peptides was approximately 2000.