Phytoremediation: a connection of chemistry and biology

In our anthropocentric culture, plants are thought of as a source of food, fiber, medicine, wood, slowing the rate of global warming, waste and water treatment, and a beautiful prop for our abode.  Secondarily, some may entertain ecocentric thoughts and appreciate the natural aesthetics of plants, as well as their ability to provide habitat for other species and link the large, biochemical cycles of the biosphere.  We can now add to the anthrocentric list the ability of plants to serve us in cleaning up soil contaminated with organic and inorganic wastes, the ultimate goal being either to alter the chemical and physical nature of the contaminant in the soil so that it is no longer a risk to humans or the environment, or removing the pollutant altogether.

Plant biology and physiology determine the efficiency of contaminant extraction.  The contaminant must be in a biologically accessible form, where absorption of the contaminant by the plant root can occur.  The contaminant must be transferred from the root through the stem to the harvestable portion of the plant.  The rate of contaminant removal is therefore dependent on the total biomass gathered during harvesting, the number of harvests per year, and the metal concentration that accumulated in the harvestable portion of the plant.  Harvested plants can be processed to reclaim valuable metals (Ni, Zn, Cu), or can be reduced in terms of biomass by drying, eating of microbes, or further chemical or physical means.  Disposal would still be necessary, although valuable metals would be reclaimed and the total amount of contamination reduced.  As plants can assimilate and naturally degrade organic contaminants, only inorganic contaminants (that can not be broken down at the elemental level) are extracted in this manner.  The only other alternative to date is the costly process of extraction of contaminated soils and reburial; the cost savings of phytoremediation is huge and continues to be a major factor driving current phytoremediation research.  Additionally, carving out contaminated soils decimates the local landscape and the habitats it supports.

As efficiency of the phytoremediation process is dependent on the total biomass of contaminants a plant can accumulate, “hyperaccumulators” are used for the job.  These include poplar trees (included in the image of a poplar test plot above) and sunflowers, among others.  Even though they are hyperaccumulators, they are still limited (by what humans would like) by the biomass they can accumulate in a given time.  Genetic modification of natural hyperaccumulators can introduce biochemical traits that enhance hyperaccumulation.  These can include genes that control the synthesis of proteins or peptides that sequester metals, genes that synthesis transport proteins for root uptake of metals and transport through the plant, or genes that change the oxidation state of heavy metals.  Likewise, genes that take slow-growing hyperaccumulators with few and shallow roots and make them fast-growing hyperaccumulators with deep and dense roots would be beneficial.  Where previous planned cross-breeding regimes or even iRNA have been modifying plant traits for years (iRNA more recently) primarily for appearance or increased plant production, genetically modified plants for phytoremediation is really a new paradigm, where plants are valued on the pollutants they absorb, sequester, safely destroy or alter, and tolerate (Cunningham and Ow, 1996).

Regulation of genetically modified plants for phytoremediation initially fell under the Federal Plant Pest Act.   Permit applications had to include a description of the modifications being made to the plant, scientific data regarding the stability of the changes, a description of the proposed field test with procedures detailing how the plant will be confined to the test plot, and environmental effects.  Key concerns were listed by Glass (1997) as falling under four main categories: (1) Introduced DNA, (2) Environmental Impacts, (3) Environmental Impacts, and (4) Test Conditions.  Is the introduced DNA stable?  Are infectious, pathogenic, toxic, or deleterious functions also encoded in the introduced DNA?  Will the host plant be able to cross with related species (particularly wild relatives)?  Is the host plant considered a weed or have weed characteristics or have acquired weed characteristics that would enhance its competitiveness versus natives?  Are there unintended effects on wildlife?  Is there the possibility of gene transfer to other plants?

This permit process first posed in 1987 was eased in 1993 to where a notification with statements of whether the plant contained harmful DNA sequences, and annual test result submissions is all that is needed.  Field testing can begin within 30 days of notification.  After several years of safe testing, notifications of further testing is unecessary leading to “delisting” of the genetically modified plant.  Delisted genetically modified plants can then be commercially used, where only applicable regulation in the case of their use for phytoremediation falls under hazardous waste laws.  Is this the current state of regulation of transgenic plants?

The phytoremediation lab we are conducting is designed to achieve four goals: (1) to gain an appreciation for the chemistry behind biology by using a biological system to remediate zinc-contaminated water, (2) to understand the problem posed by contaminated soils and examine phytoremediation as a potential solution, (3) to examine current regulation with regard to transgenic or genetically modified plants, and (4) to further engage in our discussion of anthropocentric versus biocentric viewpoints.  Crops are grown and modified for our consumption, should specific plants be cultivated and modified to clean up our mess as well?

Cunningham, Scott D. and David W. Ow. 1996. Promises and Prospects of Phytoremediation. Plant Physiol. 110:715-719.

Glass, David J. 1997. Prospects For Use and Regulation of Transgenic Plants in Phytoremediation. Battelle Press.

Mrs. Bauchiero

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