The sporophyte of the fern Pteris vittata, which tolerates and accumulates very high levels of the deadly toxin arsenic. Researchers from Purdue University have identified a gene (ACR3) from P. vittata that is necessary for the plant's tolerance to arsenic.
Jody Banks, professor of botany and plant pathology, and David Salt, professor of horticulture--both from Purdue University, discovered that the fern sucks the arsenic out of the soil and into its fronds. "It's the only multicellular organism that can do this," says Banks. It encodes a vacuolar protein that likely pumps arsenic from the cytoplasm into the vacuole of the cell, where it is sequestered. This gene is present in many organisms, including yeast, mosses, lycophytes, ferns and gymnosperms, but not flowering plants.
Banks and Salt did not have a gene sequence for P. vittata, so they used yeast functional complementation, a method of gene identification. The researchers took thousands of different P. vittata genes and combined them with thousands of yeast cells that were missing a gene, thus allowing them to tolerate arsenic. When exposed to arsenic, most of the yeast cells died; however, those strains that had picked up the genes from P. vittata that convey arsenic resistance lived.
The researchers wanted to confirm that this was the correct gene, so they "knocked down" its function and exposed the plant to arsenic. The results were conclusive--the plant could not tolerate arsenic without the gene functioning properly--confirming that the gene is necessary for the plant to function on arsenic said Banks.
Banks and Salt found that the protein encoded by this gene ends up in the membrane of the plant cell's vacuole. Salt said the protein acts as a pump, moving arsenic into the cell's equivalent of a trashcan. "It stores it away from the cytoplasm so that it can't have an effect on the plant," Salt said.
The gene--which Banks says cannot be found in any flowering plant--allows P. vittata to tolerate 100 to 1,000 times more arsenic than other plants. Banks sees the gene's potential in cleaning up environmental hazards, where plants could be created that can clean up soils and waters contaminated by arsenic. "Potentially you could take these genes and put them in any organism that could suck the arsenic out of rice grains in contaminated paddies. [This research was supported by a grant from the National Science Foundation.] (Date of Image: 2010)
Credit: Jo Ann Banks, Purdue University