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Poplars as a renewable raw material

“From a genetic perspective the potential is huge.”

Poplars are fast-growing trees. They can grow up to four metres in just one year. This makes them attractive as a source of renewable raw materials. Poplar wood is becoming increasingly important as a raw material for paper manufacture and as a climate-friendly, low-cost fuel. The higher demand presents new challenges for science and industry.

Can the yield per hectare and forms of wood use be extended using breeding methods? What characteristics are relevant and what potential do genetic methods offer? GMO Safety spoke to Dr Matthias Fladung about the significance of genetic engineering in poplar breeding and the associated requirements in the area of biological safety research.

PD Dr. Matthias Fladung, Federal Research Centre for Forestry and Forest Products, Institute for Forest Genetics and Forest Tree Breeding, Großhansdorf near Hamburg.

GMO Safety: How important are poplars as a renewable raw material in Germany? What is the current research status?

Matthias Fladung: The poplar has been enjoying a kind of renaissance in recent years. It grows fast, is easy to plant and the wood is of good quality. Poplars are often used with spruce for paper manufacture. For this reason, efforts have been made in recent years to push poplar cultivation. What is lacking is the genetic engineering, the breeding. Hardly any work has been done in the area of poplar breeding in Germany since the end of the 80s because the demand for wood, particularly poplar wood, was no longer very great. Today we still have a lot of breeding material that was selected 20 or 30 years ago, as well as a few varieties, although these are ones that met the requirements of the time. No varieties have been developed that meet today’s requirements.

GMO Safety: Which traits play a role when it comes to optimising poplars as a renewable raw material and making poplar cultivation more economical? Is there any room for improvement here at all?

Matthias Fladung : With classic breeding, the main aim was, and still is, volume production. The breeders try to optimise growth speeds and wood formation and to generate natural resistance, primarily against fungi. Compared with agricultural crops, however, we are still at the beginning of the breeding process for poplars. Agricultural crops, whether maize, oilseed rape, wheat or cabbage varieties, were optimised over decades by means of breeding – but poplars were not. This is purely a consequence of the much longer generation times for trees.

GMO Safety: Genetic methods open up a broad range of new possibilities for plant breeding. What initiatives are there in this area?

Matthias Fladung: At the moment researchers are working intensively on transferring genes that confer e.g. resistance to chewing insects or to fungi. They are also looking at altering the lignin content for paper manufacture, improving growth or modifying the flowering of trees to prevent outcrossing. Then of course there are the various types of tolerance – to temperature fluctuations, high salt content or drought. Phytoremediation, cleaning up soil that is polluted with heavy metals using plants such as poplars, is also of relevance. Finally, I can imagine that poplar wood will also become attractive for the production of modern biomass-to-liquid fuels, if researchers succeed in optimising the fuel conversion process by developing poplars that produce the necessary chemical substances themselves.

GMO Safety: Would it not be possible to produce some of these properties using conventional breeding methods? What is the advantage of genetic engineering?

Matthias Fladung: I don’t want to say that it would not be possible at all. After all, we can see what conventional breeding has achieved in herbaceous plants, maize, cabbage, wheat etc. The problem with trees is simply the long generation times. It takes at least eight to ten years for a tree to flower for the first time once it has germinated, a bit less for poplars, and a spruce tree does not flower for 20 to 30 years. Then you have to make the appropriate optimisations, conduct planting trials and finally back-crosses. With these timescales, a scientist can carry out maybe two or three crosses in his lifetime and that’s it. With the help of genetic engineering you can reduce the long generation times considerably, by transferring early-flowering genes or genes that code for particular traits into the plant.

GMO Safety: Before you can produce the desired characteristics in poplars, the underlying genes must first be identified and isolated in the genome. What is the status of the research in this area?

Matthias Fladung: In 2004 the genome of the Western balsam poplar, Populus trichocarpa, was fully sequenced. It should be noted that this makes the poplar the third plant in the world (after thale cress and rice) and the first tree species in the world for which the genome has been fully sequenced. So we have the sequence of the poplar genome. We can read the book, but understanding it is still a bit problematical. At the moment a lot of effort is being invested worldwide into understanding the letters and putting them into words. There will be lots of new findings that can be made use of, I am convinced of that.

GMO Safety: Within the area of biosafety research you have dealt with the possible environmental impacts of genetically modified woody plants. What specific safety issues need to be considered when growing genetically modified poplars?

Matthias Fladung: Right at the top of the list is preventing vertical gene transfer, i.e. the transfer of the inserted genes to the next generation. The plants must either not flower at all or at least not produce any fertile flowers. Another factor is low (or ideally no) capacity for vegetative reproduction. Poplars have a tendency to reproduce and spread via root suckers. And finally, there is horizontal gene transfer, i.e. the potential spread of transgenes across species boundaries. We have investigated this aspect in two projects, but not found any indications that we should be worried about it in practice. We cannot rule it out entirely, but our research shows that it is at least not one of the central safety issues. In connection with the longevity of woody plants, we also of course have to consider the stability of the gene expression. If you are suppressing flower formation using a genetic approach, you have to be certain that it will work long-term.

GMO Safety: Genetically modified trees are not yet used in forestry in Germany. Are you assuming that this will change in the near future?

Matthias Fladung: Classic forest managers will not plant genetically modified trees in their forests – the resistance is simply to great. But fast-growing trees like poplars could be an alternative for farmers if the set-aside premium is scrapped as planned. They can plant huge areas, leave the trees to grow for a few years and make an investment from which they will benefit within a manageable timeframe. The classically-bred varieties at least will represent an alternative for farmers in the foreseeable future. Whether genetically modified trees will also enter this market, however, is debatable.

GMO Safety: The environmental safety of genetically modified woody plants is in the cross-fire of public debate. Can you envisage acceptance for them in the long term? Matthias Fladung: I think that if genetically modified plants are produced that cannot enter the food chain then a certain degree of acceptance is possible. But it must be ensured satisfactorily that the transgenes do not contaminate the gene pool of natural ecosystems where woody plants, unlike crop plants, come into contact with natural relatives. The results from the BMBF joint biosafety research project that we coordinated on the environmental impacts of transgenic woody plants showed that sterility can in fact be induced. The next thing is to test the reliability of these sterility systems. There is certainly a need for further research work at this level.

GMO Safety: Do genetically modified poplars have other advantages, apart from the economic aspects?

Matthias Fladung: Yes, especially thinking about lignin. Have you ever visited a pulp factory? The first thing you notice when you go in is an unpleasant smell. Aggressive chemicals are added to the wood to separate the lignin from the cellulose. Wastewater and acids pollute the environment, and the surrounding streams are too warm. These ecological problems may have been reduced by a few improved methods of cellulose extraction, but pulp factories are nevertheless still a crime against nature. The need for paper is, however, greater than ever. So if it is possible to prepare the wood in such a way that the lignin can be removed more easily, without such aggressive chemicals and using less energy, that would be a contribution towards environmental protection.

GMO Safety: In summary: great potential and a great need for research?

Matthias Fladung: Yes, there is certainly a need for research – that will never end. The potential is huge, and there aren’t that many research teams working on poplars, so progress is not that great. In future, though there will be some action – if not in Germany, then in other countries. Experiments are currently being conducted in North America on the reduction of lignin. In Europe we had a release trial in France. The results showed very clearly that low-lignin poplars do not present any disadvantages in the environment and were not more susceptible to wind breakage or to fungi. In five to ten years I am sure we will obtain further results so that we will be able to say whether it makes sense to plant low-lignin trees or not. This is simply a matter of time. In Germany too we will not be able to shut ourselves off from this in the long term.

GMO Safety: Thank you for talking to us.