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

June 10, 2010

WEST LAFAYETTE, Ind. - Isolating a gene that allows a type of fern to tolerate high levels of arsenic, Purdue University researchers hope to use the finding to create plants that can clean up soils and waters contaminated by the toxic metal.

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."

Without a genome sequenced for Pteris vittata, Banks and Salt used a method of gene identification called yeast functional complementation. They combined thousands of different Pteris vittata genes into thousands of yeast cells that were missing a gene that makes them tolerant to arsenic.

The yeast was exposed to arsenic, with most of it dying. The yeast strains that lived had picked up the genes from Pteris vittata that convey arsenic resistance.

To confirm that this was the correct gene, its function was knocked down and the plant was exposed to arsenic. Without the gene functioning properly, the plant could not tolerate arsenic.

"It tells us that this gene is necessary for the plant to function on arsenic," said Banks, whose findings were published in the early online version of the journal Plant Cell. "We looked for a similar gene in the plant Arabidopsis. We couldn't find it. It can't be found in any flowering plant."

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.

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

"Potentially you could take these genes and put them in any organism that could suck the arsenic out of the soil," Banks said.

Salt said rice plants could be modified with the gene to store arsenic in the roots of plants - instead of rice grains - in contaminated paddies.

Banks and Salt found another gene in Pteris vittata that looks almost exactly the same as the one that controls arsenic tolerance. When the fern was exposed to arsenic, the confirmed arsenic-tolerance gene increased its expression while the similar gene did not.

Salt said the gene that regulates arsenic tolerance could be a duplicate of the other that has changed slightly to give itself a new function.

"The fact that it has these two genes could be a sign of evolution," Salt said. "One of the thoughts of gene evolution is that one copy could continue to do what it has always done, while the duplicate can develop another function."

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

Banks hopes findings such as this will lead to more research emphasis on non-flowering plants. She said there are characteristics in plants such as Pteris vittata that cannot be found in other organisms.

The next step in their research is to put the arsenic-tolerance gene from Pteris vittata into Arabidopsis to see whether it gives the new plant the same characteristics.

The National Science Foundation funded the research.

Writer: Brian Wallheimer, 765-496-2050, bwallhei@purdue.edu

Sources:   Jody Banks, 765-494-5895, banksj@purdue.edu

                    David Salt, 765-496-2112, dsalt@purdue.edu

Ag Communications: (765) 494-2722;
Keith Robinson, robins89@purdue.edu
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A Vacuolar Arsenite Transporter Necessary for Arsenic Tolerance in the Arsenic Hyperaccumulating Fern Pteris vittata
is Missing in Flowering Plants

Emily Indriolo, GunNam Na, Danielle Ellis, David E. Salt, and Jo Ann Banks

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.