Jul 15, 2003

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Field Trial

Soil detoxification using genetically modified poplars:

June 2003. The industrial landscape of Mansfelder Land, Saxony-Anhalt; row upon row of waste dumps, some pyramid-shaped, derelict and abandoned manufacturing plants. Here, in this former copper mining area, genetically modified poplars are undergoing a three-year field trial.

In laboratory trials scientists at the University of Freiburg had discovered that wild type poplars have an unusual capacity for accumulating cadmium. This ability can be further strengthened by a genetic modification. In the greenhouse the transgenic poplars absorbed up to 15% of the available cadmium within one month.

“If this also happens in the field,” explains project leader Andreas Peuke, “then a piece of land contaminated with heavy metals could be remediated within about 25 years.”

Poplars are perfectly equipped to do this. They grow throughout the year and they grow quickly, so they produce a lot of biomass. They also have a spreading root system, which enables them to penetrate a long way into the soil. Approximately every three to five years the trees would be harvested and the plant material incinerated in a controlled manner.

But at the moment this is still only an idea. The initial aim is to study genetically modified poplars at release sites in Germany and Russia to find out whether they also absorb heavy metals in field conditions, as anticipated, and whether this ability is permanent, in other words whether the genetic modification remains stable. Trees are long-lived and the increased ability to absorb heavy metals is of little use if it only lasts a short while.

The control area

Andreas Peuke leads the way to the first of three release sites in Mansfelder Land. The soil in the three areas is contaminated with varying amounts of copper; there is one heavily contaminated area, one moderately contaminated area and also a control area with only slight contamination.

Genetically modified poplars in a field-like trial. Here too there are strict safety procedures: the transgenic plants cannot be planted directly in the soil; they must be planted in pots standing on a protective film. A net is stretched over them to prevent them being eaten by birds and they are surrounded by a rabbit-proof fence.

We walk through waist-high grass and undergrowth to the control area where the soil is only slightly contaminated. A fence to keep out wild animals surrounds the plot of land. “It’s of little use with the wild boar” comments Andreas Peuke knowingly “if they can’t get around it, they just crash straight through.” With a manageable plot like this - the trial plot measures 50 metres square - animals can normally find their way around it. For larger areas the fencing is fitted with gates so that the wild boar can charge in on one side and out again on the other.

The poplars were planted here the previous year. They show very variable growth rates, some are only half a metre high, whilst others are already towering above our heads. “The plants develop quite differently outside” Andreas Peuke remarks, “they look more robust than the plants in the greenhouse and they have smaller, tougher leaves.” Three or four of the young trees have withered. Transgenic and wild type poplars cannot be distinguished from one another. They have been randomly planted amongst one another. Wild type poplars grow around the margins.

Strict safety procedures

Much has to be done to ensure the safe containment of the genetically modified trees - the transgenic plant material must be prevented from spreading beyond the borders of the trial plots.This means that the trees must be prevented from flowering.

Poplars do not normally flower in the first three years (the duration of the trial), but they still need constant checking and any flowering buds found have to be removed. All this despite the fact that the plants in question are female clones, which produce no pollen that could spread. At the end of the trial the plants and their roots have to be dug up.

The leaves and woody material will be shredded and burnt in an incinerator with flue gas purification. Since poplars can form suckers, the land must be monitored for a further two years after completion of the trial to ensure that any emerging shoots are destroyed. A 15-meter wide strip around the release site is also included in this monitoring.

In the autumn a net must be stretched over the entire area to prevent any leaves from blowing away and then rotting in the soil - this will also prevent the re-contamination of the soil with heavy metals.

Laboratory, greenhouse and field-like conditions

The foreign gene inserted in the poplars comes from the E.coli bacterium. This gene causes the plants to produce more glutathione. Increased glutathione content makes the plants more tolerant of heavy metals. Under heavy-metal stress they are then able to produce phytochelatins. These are small heavy metal-binding and detoxifying peptides, which are produced from the basic building block glutathione and are transported into the vacuoles, the cavities in the plant cells filled with cellular sap.

The scientists from Freiburg did not perform the genetic transformation themselves. They are propagating plants which have already been transformed. “Usually poplars can be propagated by simply putting a stem cutting in the ground”, explains Andreas Peuke, “but that doesn’t work with these poplars, because their genetic material contains aspen genes and aspen doesn’t do that.”

The plantlets develop leaves with buds, which are then individually picked and each leaf with a bud is placed in a test-tube on moist agar. When they are large enough, the young plants are planted in proper soil in the greenhouse, but to begin with they still remain covered with a glass jar. The conditions in the greenhouse are mild, UV light is filtered by the glass and there is no wind.

In the next stage the young plants are exposed to wind and weather in field-like conditions.

First harvest

The poplars were only planted out this year on the two remaining plots in Mansfelder Land. So in Autumn 2002 only the plants from the control plot were harvested. Ten of each type were removed from the field and the wood and leaves were examined to see whether the transgene was still present. It was evident that the poplars remained stable throughout the first growing period in terms of the genetic modification. As expected, there was no difference between the wild type and the transgenic poplars in terms of heavy metal content, since the soil on the control plot is only slightly contaminated with copper.

At the end of September/beginning of October the newly planted poplars will then be harvested for the first time.

Then next year, in cooperation with the University of Tübingen, researchers will address the question of whether the genetic modification has an impact on soil fungi and whether the close symbiotic relationship between the trees and the fungi is affected. The ability to absorb heavy metals is also dependent on this symbiosis remaining unaffected. And finally the researchers aim to examine whether the foreign gene is passed on to fungi or soil bacteria, in other words whether horizontal gene transfer takes place.