The toxicity of arsenic is widely known, but perhaps less widely appreciated is that it's the level of toxicity critically depends on the chemical form. The fern Pteris vittata, is one of a small group of plants that actively accumulates to a startling degree - an arsenic hyperaccumuatlor. P. vittata absorbs arsenic from soil, typically present as the relatively benign arsenate, and changes its chemical form to arsenite, which is one of the more toxic kinds of arsenic. The plant thrives on this toxic regimen, and it most likely does this to defends itself from hungry herbivores. The ability of P. vittata to take up arsenic has generated much excitement because of potential applications for environmental cleanup of drinking water and of contaminated sites.

Using x-ray absorption spectroscopy (XAS) imaging at the Stanford Synchrotron Radiation Laboratory's beam line 9-3, researchers from the University of Saskatchewan studied live specimens of the fern to learn what forms or arsenic are stored where in the plant and how it is converted.

The (XAS) images show that the fern transports arsenate, common in the environment, from the roots to the fronds (leaves) through its vascular tissue. Once in the fronds, the plant chemically alters it to arsenite (the hydrated form of arsenic trioxide). This dangerous compound is stored in sealed compartments within the plant's cells, and sent to tiny hairs that guard its reproductive cells (spores) near the edge of the fronds.

The researchers also studied the second part of the fern life cycle: a tiny plant called a gametophyte that mates pollen and ovule to recombine genetic information. Gametophytes are mostly only one-cell thick. The arsenite is kept in the cell's large central storage compartment. The plant keeps itself healthy by excluding arsenic from its cell walls, reproductive areas and nutrient-absorbing roots.

The study has clearly provided interesting and important insights that contribute to the knowledge that will eventually enable environmental cleanup process to be implemented.

To learn more about this research see the full scientific highlight at:
http://www-ssrl.slac.stanford.edu/ research/highlights_archive/as-fern.html

Pickering, I.J., Gumaelius, L., Harris, H.H., Prince, R.C., Hirsch, G., Banks, J.A., Salt, D.E., George, G.N. "Localizing the biochemical transformations of arsenate in a hyperaccumulating fern". Environ. Sci. Technol. , 2006, 40, 5010-5014.