August 13, 2001
Genetic secrets of metal-eating plants uncovered
WEST LAFAYETTE, Ind. Genes thought to allow plants to accumulate large amounts of metal in their tissues have been identified and cloned by a Purdue University scientist.
The finding is expected to lead to new crop plants that can clean up industrial contamination, new foods that fight disease and reduced work for some farmers.
David E. Salt, associate professor of plant molecular physiology and principal investigator on the project, says that this discovery opens up new avenues for plant breeders.
"This is really one of the first tools that we've got to manipulate this process of metal hyperaccumulation," he says. "So what we're going to do now is to start expressing these genes in nonaccumulating plants to see if we can turn them into metal-accumulating plants."
The genes were identified from the tiny wild mustard Thlaspi goesingense, a plant that lives in the Austrian Alps, where it hyperaccumulates nickel. The plant is similar to the nonmetal-accumulating plant Arabidopsis thaliana, which is commonly used in scientific research.
The research is published in the Tuesday (8/14) issue of the Proceedings of the National Academy of Science.
Salt says more than 350 species of plants are known to accumulate metal such as nickel, zinc, copper, cadmium, selenium or manganese in high levels.
"The plant species that we're interested in can accumulate 1 percent of their dry biomass as nickel. In a normal plant you might expect to find 10 to 100 parts per million of nickel in their tissue, and these plants can accumulate 10,000 parts per million," he says. "So they obviously have this extraordinary capacity to accumulate metals, and they do this in the wild without any interference from man. They just do this for a living."
Hyperaccumulating plants store the metal in microscopic structures in their cells called vacuoles. The vacuoles are membrane-lined structures that protect the rest of the cell from the toxic effects of the metal. Interestingly, the protective membranes that surround the vacuoles closely resemble cell membranes in the human liver that serve a similar function.
Scientists aren't completely sure why some rare plants try to grab as much metal as they can, but studies indicate that they do this to stop insects and other creatures from eating them.
Just as people hate to bite down on a piece of aluminum foil, insects tend to avoid eating plants that taste like metal.
"You can imagine if you're a bug and you bite down on a plant and it's got 10,000 parts per million of nickel in its leaf, it's not going to taste too good," Salt says, laughing.
Scientists are interested in using metal hyperaccumulating plants as a means to clean up contaminated brownfield sites. Researchers believe that soil polluted with heavy metal or radioactive materials could be cleaned up by using crop plants that could absorb the material. This process is called bioremediation, or more specifically when using plants, phytoremediation.
"Imagine if you have a site contaminated with cadmium. Right now your options are to put a fence around it and put a sign up telling people to stay out, build a parking lot over it, or dig up all of the soil and truck it to a landfill, which is very expensive," Salt says. "The idea would be that you could take plants that accumulate metal you could essentially farm the metal out of the ground. Over five or 10 years, by growing crop rotations there, you could remove the metal from the site. The nice thing is that it's cheap, and you're left with a soil at the end of it which could be used for other things."
Salt says the metal hyperaccumulating plants found in nature would not be used for phytoremediation because they are all small and slow growing. Instead, scientists could move the genes Salt and his colleagues have identified into fast growing, large plants, such as grasses.
Another benefit of Salt's work could be functional foods foods that contain micronutrients missing from diets in certain areas. Metals are essential nutrients in small doses, but some regions of the world lack foods that contain sufficient levels of these micronutrients, which causes severe health problems. Using the genetic tools Salt and his colleagues have identified, scientists could begin to bioengineer foods that contain these essential micronutrients.
A third application of the research would be for improved crop nutrition. "Instead of adding zinc to the soil because you live in a zinc-deficient region, why not have the wheat plant itself be more zinc-efficient so that you can reduce agricultural inputs?" Salt asks.
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Functional activity and role of cation-efflux family members in Ni hyperaccumulation in Thlaspi goesingense
Michael W. Persans, Ken Nieman, Dept. of Chemistry, Northern Arizona University; David E. Salt, Purdue University
The ability of Thlaspi goesingense to hyperaccumulate Ni appears to be governed in part by enhanced accumulation of Ni within leaf vacuoles. We have characterized genes from T. goesingense encoding putative vacuolar metal ion transport proteins, termed Metal Tolerance Proteins (TgMTPs). These proteins contain all the features of Cation Efflux (CE) family members, and evidence indicates they are derived from a single genomic sequence (TgMTP1) that gives rise to an unspliced (TgMTP1t1) and a spliced (TgMTP1t2) transcript. Heterologous expression of these transcripts in yeast lacking the TgMTP1 orthologues COT1 and ZRC1 complement the metal sensitivity of these yeast strains; suggesting that TgMTP1s are able to transport metal ions into the yeast vacuole in a manner similar to COT1 and ZRC1. The unspliced and spliced TgMTP1 variants differ within a histidine rich, putative metal binding domain, and these sequence differences are reflected as alterations in the metal specificities of these metal ion transporters. When expressed in yeast, TgMTP1t1 confers the highest level of tolerance to Cd, Co and Zn whereas TgMTP1t2 confers the highest tolerance to Ni. TgMTP1 transcripts are highly expressed in T. goesingense compared to orthologues in the non-accumulators Arabidopsis thaliana, Thlaspi arvense, and Brassica juncea. We propose that the high level expression of TgMTP1 in T. goesingense accounts for the enhanced ability of this hyperaccumulator to accumulate metal ions within shoot vacuoles.