Fronds of ladder brake ferns hyperaccumulate toxic arsenic using a special transporter protein that spatially isolates the chemical in vacuoles.

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"The fern Pteris vittata can tolerate 100 to 1,000 times more arsenic than other plants. Jody Banks, a professor of botany and plant pathology, and David Salt, a professor of horticulture, uncovered what may have been an evolutionary genetic event that creates an arsenic pump of sorts in the fern.

"'It actually sucks the arsenic out of the soil and puts it in the fronds,' Banks said. 'It's the only multi-cellular organism that can do this.'

"Banks and Salt found that the protein encoded by [an isolated] 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.

"Banks said understanding how the Pteris vittata functions with arsenic could lead to ways to clean up arsenic-contaminated land

"The plant might have evolved to accumulate arsenic, Banks and Salt theorized, as a defense against animals or insects eating them." (ScienceDaily 2010)

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"The fern Pteris vittata tolerates and hyperaccumulates exceptionally high levels of the toxic metalloid arsenic, and this trait appears unique to the Pteridaceae. Once taken up by the root, arsenate is reduced to arsenite as it is transported to the lamina of the frond, where it is stored in cells as free arsenite. Here, we describe the isolation and characterization of two P. vittata genes, ACR3 and ACR3;1, which encode proteins similar to the ACR3 arsenite effluxer of yeast. Pv ACR3 is able to rescue the arsenic-sensitive phenotypes of yeast deficient for ACR3. ACR3 transcripts are upregulated by arsenic in sporophyte roots and gametophytes, tissues that directly contact soil, whereas ACR3;1 expression is unaffected by arsenic. Knocking down the expression of ACR3, but not ACR3;1, in the gametophyte results in an arsenite-sensitive phenotype, indicating that ACR3 plays a necessary role in arsenic tolerance in the gametophyte. We show that ACR3 localizes to the vacuolar membrane in gametophytes, indicating that it likely effluxes arsenite into the vacuole for sequestration. Whereas single-copy ACR3 genes are present in moss, lycophytes, other ferns, and gymnosperms, none are present in angiosperms. The duplication of ACR3 in P. vittata and the loss of ACR3 in angiosperms may explain arsenic tolerance in this unusual group of ferns while precluding the same trait in angiosperms." (Indriolo et al. 2010)

Fern's evolution gives arsenic tolerance that may clean toxic land

Journal article
A Vacuolar Arsenite Transporter Necessary for Arsenic Tolerance in the Arsenic Hyperaccumulating Fern Pteris vittata Is Missing in Flowering PlantsThe Plant CellJune 9, 2010
Emily Indriolo, GunNam Na, Danielle Ellis, David E. Salt, Jo Ann Banks

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