Submitted to: Planta
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/26/2004
Publication Date: 8/1/2004
Citation: Poynton, C.Y., Huang, J.W., Blaylock, M.J., Kochian, L.V., Elless, M.P. 2004. Mechanisms of arsenic hyperaccumulation in pteris species: root as influx and translocation. Planta. 219:1415-1423. Interpretive Summary: Arsenic (As) contamination of soils and groundwater is becoming a serious environmental problem. Arsenic occurs naturally in the environment, but has also been anthropogenically elevated from the widespread usage of arsenic as a basis for pesticides used on agricultural land in the USA prior to 1968. Arsenic, predominantly from natural sources, has also been found to be contaminating groundwater in parts of the Indian sub-continent and South-East Asia and in the USA, and the US Environmental Protection Agency has recently lowered the Maximum Contaminant Level for drinking water from 50 to 10 ug As L-1. Phytoremediation is a potentially attractive alternative to energy-intensive, high-cost traditional methods for remediation of contaminated soils and groundwater. This new technology employs the use of higher plants capable of accumulating high levels of contaminants in shoots. Following harvesting, the shoot biomass can be disposed of in a final repository after volume reduction (e.g., ashing). There is considerable current interest in As phytoremediation, due to the recent discovery of As-hyperaccumulating ferns in the genus, Pteris. The mechanism of As hyperaccumulation in these ferns is poorly understood. In this study, radioactive As was used to quantify root As uptake and translocation in both hyperaccumulating and non-accumulating ferns. It was found that the As hyperaccumulator absorbed As more efficiently from the soil or groundwater, and more rapidly translocated it to the shoot, where it appears to have been converted to a non-toxic form of As for storage in the leaf. A better understanding of plant As hyperaccumulation is essential if we are going to design high biomass plant species better suited for phytoremediation of As contaminated sites.
Technical Abstract: Several species of fern from the Pteris genus are able to accumulate extremely high concentrations of arsenic in the fronds. We have conducted unidirectional As influx and translocation experiments with 73As radiolabeled arsenate, and found that the concentration-dependent influx of arsenate into roots was significantly larger in two of these As hyperaccumulating species, Pteris vittata and Pteris cretica cv Mayii, than in Nephrolepis exaltata, a non-accumulating fern. The arsenate influx could be described by Michaelis-Menten kinetics and the kinetic parameter Km was found to be lower in the Pteris species, indicating higher affinity of the transport protein for arsenate. Quantitative analysis of kinetic parameters showed that phosphate inhibited arsenate influx in a directly competitive manner, consistent with the hypothesis that arsenate enters plant roots on a phosphate transport protein. The significantly augmented translocation of arsenic to the shoots that was seen in these As hyperaccumulator species is proposed to be due to decreased sequestration of As in the roots, as a larger fraction of arsenic could be extracted from roots of the Pteris species than from roots of N. exaltata. This leaves a larger pool of mobile arsenic available for translocation to the shoot, probably predominantly as arsenite.