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Heavy metals inhibit diverse sites of several metabolic pathways and thus
are toxic for plants when occurring at high concentrations. Plant species
that tolerate greater concentrations of pollutants in soil can be used to
clean the environment, a characteristic referred to as phytoremediation.
Poplars are now proposed as efficient cleaners of soil containing high
concentrations of zinc, August 2005 by Katharina Schoebi, Checkbiotech.

High concentrations of heavy metals, such as zinc, are toxic for plants.
However, a plant with the right characteristics can extract zinc out of the
soil, thereby cleaning the environment. The efficiency of phytoremediation
depends largely on the plant?s ability to escape deleterious concentrations
of the toxic form of the pollutant and the active oxygen species that might
be generated in the stressed tissue.

Poplar plants are good candidates for phytoremediation purposes because they
have fast growth rates and produce a large biomass. Their extensive root
system ensures an efficient uptake of the soil water containing the
pollutants. Once the poplars have taken up the zinc, the loss via
transpiration is improbable. However, the zinc may be lost due to falling
leaves, especially in autumn. By collecting and afterwards incinerating
them, the contaminated biomass is abolished and the trees can be used for
several cycles of decontamination.

When the poplars finally are cut down, their wood may be used for paper,
matches or other products, which allows for a maximum utilization of the
cleansing trees. A further advantage is, that the genome of poplars has
recently been sequenced, which increases how fast breeding and selection of
the right clones can occur.

Several inorganic pollutants ? including heavy metals, such as cadmium,
mercury, and zinc ? can be taken up by poplars. Yet, the heavy metal
tolerance of the trees is restricted, because they can only remediate a
given amount. Genetic engineering of poplars, however, can significantly
increase their remediative capacity.

Poplar plants with an increased amount of an enzyme called
?-glutamylcysteine-synthetase (?-ECS), which is pivotal for the biosynthesis
of glutathione, take up more cadmium than naturally growing poplars.
Glutathione protects the cell against oxidative stress caused by
environmental factors and also plays a role in heavy metal detoxification in
plants.

An additional enzyme known as glutathione S-transferase isoenzyme (GST) is
responsible for the reactions between glutathione and several substances not
belonging to the organism. Thus, GST plays a crucial role in the degradation
of toxic substances.

With regards to the poplar?s reaction to toxic concentrations of the metal
zinc, however, only little is known. Dr. Tamas Komives from the Plant
Protection Institute at the Hungarian Academy of Sciences in Budapest and
his colleagues have now examined the stress reactions of different poplar
lines to zinc. They published their results in the journal Environment
International.

In their experiment, the researchers used two transgenic poplar lines
(Populus canescens) that significantly overproduced the amount of ?-ECS. The
researchers cut little discs from the poplar leaves and put them into
different concentrations of zinc sulfate. To compare the reaction of the
transgenic cells to zinc with that of normal cells, Dr. Komives and his team
additionally put cells of two non-transformed poplars (P. nigra and P.
canescens) in the zinc sulfate. After 21 days, the contents of cadmium,
chromium, copper and zinc were determined.

Upon examining their results, Dr. Komives and his team noticed that only
high concentrations of zinc resulted in a significant reduction in foliage
and total dry mass in all poplar lines. A somewhat lower zinc concentration
led to bleaching of the leaves and retained growing activity. Still lower
concentrations had no toxic effects on the poplar trees, irrespective if
they were genetically engineered or non-transformed.

However, the researchers observed a difference in the reaction to zinc
sulfate between P. nigra and P. canescens. P. nigra was more sensitive to
zinc than P. canescens and genetically engineered lines of P. canescens .The
researchers also observed that the zinc concentrations in the leaves
augmented gradually with the amount of zinc sulfate supplied. However, there
was no difference in zinc uptake between the transgenic and the
non-transgenic trees.

Dr. Komives and his team additionally found out, that all plants showed an
increase in cadmium, chromium and copper contents with the application of
zinc sulfate, but the genetically engineered plants accumulated more of the
metals than the others.

Since GST detoxifies active oxygen species that are produced in stress
situations, its activity corresponds to the amount of stress the plants
experience. Hence, the GST can be regarded as a stress indicator. The
researchers therefore measured the enzyme activity of GST in the plants.

Upon examining the data, Dr. Komives? research showed, that GST was more
active in the leaves of non-transgenic plants. This indicated that
genetically engineered poplars are more tolerant to zinc exposure and thus
also more suitable for phytoremediation than their non-transgenic poplar
varieties.

Currently, poplars are already being used for phytoremediation projects ?
most notably in the US, but evermore so in Europe, too. ?There is a huge
potential in phytoremediation, unfortunately mostly unexploited,? Dr.
Komives told Checkbiotech.

In a preliminary study, he and his team have planted 1,100 young
non-transgenic trees at an industrial waste site. However, there the
pollutants are mostly organic compounds. ?The trees are clearly suffering
because of the phytotoxicity of the pollutants,? Dr. Koemives said. This
prompted the researchers to ask for permission to plant the stress-tolerant
transgenic poplars in this area. The request is currently being evaluated.

Although the researchers have done their experiments with poplar trees,
there are also some non-woody plants known as zinc-hyperaccumulators. In Dr.
Komives? opinion, trees are arguably the best choice for phytoremediation
projects ? willows would be his second choice.

Dr. Komives and his team cooperate with several laboratories in the EU. They
would like to learn more about using phytoremediation in EU-candidate
countries, such as Bulgaria and Romania. ?I think it is sad that the EU does
not support more phytoremediation projects,? Dr. Kömives said.

[www.nki.hu]

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