30 T. Macek et al. Biotechnology Advances 18 2000 23–34
Force Base eastern cottonwood trees were planted to clean up trichloroethylene from a shal- low, thin aerobic aquifer. The organism was chosen, because “they [eastern cottonwoods]
are indigenous to the site, they are members of the same genus as poplar trees and grow at similarly rapid rates” The trees were planted to intercept and reverse the flow gradient and
to determine if cottonwoods can metabolize TCE and its daughter compounds under field conditions [62]. The system decreased the TCE content; however, the reasons leading to the
choice of eastern cottonwood seems to be quite unsatisfactory and lacking in support of re- search results, including the risk of survival problems for plants after long-term exposure to
the contaminants. The above approach cannot generally be considered as optimal. This is of- ten the problem of agencies funding remediation experiments, where time pressure leads
sometimes to support of projects lacking the time-consuming attempts to explain experimen- tally molecular mechanisms involved in the desired activity.
Many of these problems can be addressed by the selection of the proper type of plants. Phytoremediation has a number of inherent technical limitations. The contaminant must be
within, or drawn toward, the root zones of plants usually the top 3–6 feet of soil. This im- plies water, depth, nutrient, atmospheric, and physical and chemical limitations. In addition,
the site must be large enough to make farming techniques appropriate. It must not present an eminent danger to human health or further environmental harm [8]. There may also be a con-
siderable delay in the time needed for obtaining satisfactory cleanup results between phy- toremediation and ‘dig and dump’ techniques. Although plants have upward of 100 million
miles of roots per acre, their root system may not extend deeply enough to eliminate all of the contamination [61].
10. Future prospects
Only the future can tell whether phytoremediation will become a widely accepted technol- ogy. A growing knowledge of the factors important to phytoremediation can provide a basis
for genetic modification of plants for improved performance. Breeders have been modifying agronomically important plant traits for years. However, in these instances yield and aesthet-
ics were usually the criteria for selection of a plant trait. As Cunningham et al. [2] state, ‘phytoremediation requires a new paradigm in which plants are valued based on what they
adsorb, sequester, destroy and tolerate.’ All of these traits can be specifically targeted by tra- ditional breeding as well as molecular biology.
We believe that molecular biology will allow formation of plants tailored specifically for needs of some particular tasks, and major long-term improvements in phytoremediation should
come when scientists isolate genes from various plant, bacterial, and animal sources that can enhance metal accumulation [44,50,58] or degradation of organics, as described by Borovka et
al. [63] and Newman et al. [64]. These changes will include transformation of plants to add specific proteins or peptides for binding and transporting xenobiotics, increasing the quantity
and activity of plant biodegradative enzymes peroxidases, laccases, oxygenases, dehalogen- ases, nitroreductases, nitrilases, including those that are exported into the rhizosphere and sur-
rounding soil to improve the performance of soil bacteria. Introduction of foreign genes might include those responsible for the synthesis of low molecular weight organic molecules to be ex-
creted in exudates, such as some phenolics, flavonoids, or coumarins that induce rhizospheric
T. Macek et al. Biotechnology Advances 18 2000 23–34 31
bacteria to degrade anthropogenic toxins [21]. Plant-fungal interactions would also seem ripe for exploitation in this area, particularly mycorrhizal associations [65]. Alternatively, trans-
genic plants can be transformed to harbor microbial genes for biodegradation. This is already a routine practice in the engineering of many herbicide-resistant plants and their field testing,
product development, and registration are well advanced. The concept could be extended to ad- dress additional xenobiotics. Biodegradative microbial strains are notoriously unreliable in
their ability to compete with native microorganisms when released into the natural environ- ment. In addition, such release faces general public opposition. In contrast to herbicide resis-
tance, the degradation of xenobiotics is rarely performed by one single enzyme. For the degra- dation of organic xenobiotics, the concerted action of many enzymes is needed, and
introduction of all of the necessary genes into a plant is a formidable task.
11. Conclusions
