Arsenic is a naturally occurring, toxic metalloid that is known to cause cancers if consumed. Arsenic contamination of soils and water is an environmental problem in many regions of the world, including the US. This research focuses on the fern Pteris vittata, which is remarkable because it and its close relatives are the only known multicellular organisms that are able to tolerate and accumulate very high levels of arsenic. The goals of this research are to understand how this plant is able to take up and store this toxin without succumbing to its deadly effects. Two genes isolated from this plant are hypothesized to be involved in this process. One is an arsenate reductase, which reduces the arsenate, which is taken up from the soil by the root, to arsenite. The other is an arsenite effluxer that is hypothesized to move arsenite into the vacuole of the cell where it is safely sequestered. One goal of this research is to express these genes in another arsenic-sensitive plant (Arabidopsis thaliana) to see if they are able to confer arsenic tolerance to other plants. A second goal is to localize the proteins produced by these genes at the tissue, cellular and subcellular levels in P. vittata. The final goal is to develop basic genomic tools for P. vittata that ultimately will allow us to identify other genes that are involved in arsenic tolerance and hyperaccumulation in this plant. This research has broader impacts in that it will identify a mechanism of arsenic tolerance and accumulation that may be of practical value in the remediation of arsenic contaminated soils. This research also will provide undergraduate and graduate students, including underrepresented minorities, interdisciplinary training in biochemistry, microscopy, genetics and molecular biology.
Arsenic is a naturally occurring metalloid that is toxic to most organisms and is a known human carcinogen. It is a major contaminant of soils and ground water in many regions of the world and an increasing agricultural and health concern. The fern Pteris vittata is exceptional among plants and animals in that it is able to tolerate and accumulate very high levels of arsenic. The goal of this project was to understand how this fern is able to do this by discovering genes that are necessary for arsenic tolerance in this species. One of these genes, called ACR3, encodes a protein that likely transports arsenic from the cytoplasm to the cell’s interior vacuole, where it is sequestered. Interestingly, this gene does not exist in flowering plants, including crop plants. Expressing the ACR3 gene in the flowering plant Arabidopsis thaliana confers arsenic tolerance, indicating that ACR3 is also sufficient for arsenic tolerance. To discover additional arsenic related genes in Pteris vittata, genes that are either up or down-regulated by arsenic were sought by deep sequencing genes from treated and non-treated plants. This approach identified 150 such genes, including ACR3 and gene that encodes GST, known to counteract oxidative stress (which is caused by arsenic exposure) and to detoxify harmful metabolites. These genes suggest that Pteris vittata, unlike other plants, requires relatively few genes to be induced to avoid arsenic toxicity. As a result of this project, new technologies for generating crop plants that accumulate less arsenic could be developed. Funds for this project supported three graduate students and many undergraduate students, who performed most of the research described. To inform the public about this remarkable "arsenic-eating plant", the PI has been interviewed by NPR’s Joe Palca and participates in a local science café where this work is described. Each summer the PI also helps to prepare 20 elite high school students, selected from across the US, for the annual International Biology Olympiad. Data from this project is used extensively in training these students.
