Poplar Tree

Trees with rabbit genes accelerate cleaning of soil

In a recent study, Sharon Doty, of the University of Washington, Seattle have developed transgenic poplars with an enhanced uptake and metabolism of toxic volatile pollutants. In doing so, they have delivered a technology that is likely to lead to the wider application of phytoremediation in the field.

Poplars use an enzyme called cytochrome P450 to break down contaminants. Mammalian cytochrome P450 has been used in the past to create GM plants that can detoxify herbicide-treated fields.

Although poplars already naturally remove contaminants from the environment, the rabbit liver enzyme speeds up the process.

Tests on six-inch tall GM poplar cuttings which had a gene from a rabbit inserted into them showed that they could remove up to 91% of a chemical called trichloroethylene(TCE) from the water used in their feed. This chemical, used as an industrial degreaser and one of the most common contaminants of ground water, was broken down by the plants into harmless byproducts more than 100 times faster than by unaltered plants. Trichloroethylene is turned into a harmless salt, water and carbon dioxide.

After Dr Doty's team inserted the gene into the tree from a rabbit they also produced P450, but at a much faster rate. Ultimately, the scientists would like to manipulate the plant's own genes to achieve the same goal.

The engineered trees were capable of the enhanced metabolism of five volatile toxic compounds: TCE, vinyl chloride, carbon tetrachloride, chloroform and benzene.

• trichloroethylene(TCE)- commonly used as an industrial solvent & it is one of the most common contaminants of ground water
• Vinyle chloride- used to make plastic. It is highly toxic, flammable and carcinogenic.
• Carbon tetrachloride- Exposure to high concentrations of carbon tetrachloride (including vapor) can affect the central nervous system, degenerate the liver and kidneys and may result (after prolonged exposure) in coma and even death. Chronic exposure to carbon tetrachloride could result in cancer.
• Chloroform – a byproduct of disinfection of drinking water. chloroform vapors depress the central nervous system.
• Benzene- carcinogenic, may cause death in high level breathing.

ADVANTAGES/DISADVANTAGES

In comparison with other clean-up technologies, phytoremediation has potentially many advantages, including low installation and maintenance costs, less disruption of the environment and other beneficial side effects such as carbon sequestration and biofuel production. However, phytoremediation also suffers from several limitations, among which the most commonly evoked are the slow rate of removal, incomplete metabolism and potential increase in bioavailability of toxic contaminants. Indeed, in the absence of significant detoxification, parent compounds and toxic metabolites can accumulate inside plant tissues and eventually return to the soil or volatilize into the atmosphere.

RESEARCH
Among the different transgenic clones tested, the most efficient one, line 78, expressed CYP2E1 at a 3.7- to 4.6-fold higher level and exhibited the highest level of TCE metabolism (>100-fold higher than in non-transgenic controls). When cultivated in hydroponic solution spiked with toxic compounds, line 78 was capable of extracting 90% of TCE (compared with 3% extracted by non-transgenic controls), 99% of chloroform (compared with 20% by controls) and 92–94% of carbon tetrachloride (compared with 20% by controls). Enhanced metabolism of organic pollutants in transgenic plants is associated with a faster uptake, which can be explained by a steeper concentration gradient inside plant tissues. Transgenic plants were also shown to remove volatile compounds from contaminated air at a higher rate than non-transgenic controls: 79% of TCE (none removed by controls), 49% of vinyl chloride (compared with 29% by controls) and 40% of benzene (compared with 13% by controls).

Figure 1. Phytoremediation involves several processes:

pollutants in soil and groundwater can be taken up inside plant tissues (phytoextraction) or adsorbed to the roots
(rhizofiltration); pollutants inside plant tissues can be transformed by plant enzymes (phytotransformation) or can volatilize into the atmosphere

(phytovolatilization);pollutants in soil can be degraded by microbes in the root zone (rhizosphere bioremediation) or incorporated in soil material (phytostabilization)

EVALUATION

Overall, the technology is very efficient for treating any contaminated sites. Transgenic poplars are much better than normal poplars since they enhance the uptake and metabolism of organic pollutants. Despite being a slow process, the transgenic poplars are fast growing thus increasing efficiency. Trangenic poplars have low installation and maintenance cost. Moreover, it is a good source for paper/ wood manufacturing and also good biomass. Eventhough gene pollution may occur, poplar trees can grow several years without flowering. As such, the tress could be harvested before their seeds germinates and spread their genes.

Resources:
http://news.nationalgeographic.com/news/2007/10/071015-plants-toxic.html http://www.guardian.co.uk/environment/2007/oct/16/gmcrops http://www.iasvn.org/uploads/files/phyto-transgenic_0626083555.pdf