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Contributions from
the Column
Focus
“Public debate is essential”
Colonising Africa’s agriculture
GM cotton in a dead end street
“The farmers have the choice”
Not yet mature – it is too early to bet on GM crops
02/2006
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Not yet mature
In conventional plant breeding, genes are transferred all the time. However, genetic engineering speeds up the process and makes it possible to transfer genes across species borders. That opens up lots of possibilities. However, our knowledge of potentially harmful impacts is limited. It is therefore still too early to farm GM crops on a large commercial scale.
[ By Cesare Gessler ]
Since time immemorial, advances in agriculture have been closely linked to the selection of the best strains of plants. For breeding, there are two plant types: self-fertilizers and cross-fertilizers. Self-fertilizers, such as wheat, can inseminate themselves or very close relatives. After a first cross with a donor plant that is intended to contribute new characteristics, a plant can be back-crossed with its plant of origin until it becomes almost identical genetically – except for the new characteristic, which was intentionally crossed in. Most self-fertilizers – such as commercial cereals, including rice – are homozygous. That means that both sets of their chromosomes are broadly identical. They therefore produce identical progeny. A farmer can use his harvest for seed as it has the same characteristics as the parent material.
The pollen of cross-fertilizers such as apples, grapes or bananas, however, cannot fertilize the flowers of its plant of origin. In stead, fertilization requires the pollen of a genetically different plant. When that happens, the seed inherits half the genome of the mother plant and half that of the father plant or pollen donor. Genomes of such plants are heterozygous. Planting the seeds of apples or grapes of a particular variety produces a host of genetically different, individually unique plants. Their fruits are all identical because they are defined by the genetic make-up of the mother plant. But their seeds are different.
To produce seed for a pure variety – that is a genetically identical one – of cross-fertilizers, conventional breeders employ vegetative reproduction methods, such as using cuttings from the plant of origin. Therefore, all apples of a single cultivar like Gala or Golden Delicious are basically clones of a single plant. Every Golden Delicious apple tree, for instance, can be traced back to a seedling found by chance in 19th century America.
Some varieties, however, share characteristics of both cross-fertilizers and self-fertilizers. They are, however, subject to what is known as inbreeding depression. They have the capacity to fertilize themselves – but the more often they do so, the more negative traits they display. If different inbred lines are crossed with one another, however, they produce hybrids with better characteristics. The drawback is that the hybrid seed material cannot be reproduced: inbreeding depression reduces the hybrid effect from one generation to the next. Maize is a classic example. Unlike wheat, which yields seed that farmers can harvest and plant again the next year, or grape-vines, which can be grown from cuttings, hybrid maize seed needs to be bought every year.
So in the case of self-fertilizers, foreign genes can be introduced even by conventional breeding (crossing with a donor, then multiple backcrossing) – but only from species with which crossing is possible. Green genetic engineering merely accelerates this process, additionally allowing genes to be transferred across species borders. For self-fertilizing species, therefore, genetic technology is needed only to introduce alien genes, e.g. from the bacterium Bacillus thuringiensis to produce a more insect-resistant plant. With cross-fertilizers, however, it is a different story: it is not possible for the conventional breeder to cross resistance genes from wild apples to a disease-prone variety like Gala – because varietal purity is lost as a result. Crossing produces new varieties, such as Topaz and Santana, with a flavour and appearance all of their own. That is why there are not many varieties of apple on the market which are resistant to the widespread disease apple scab.
Promising genetic engineering
Under these conditions, the introduction of a specific gene, such as a gene giving resistance to a particular disease, into a specific cultivar like Gala or Golden Delicious is an attractive option: all traits will remain identical in the manipulated plant except for the added genes. Recombinant DNA technology (gene technology) promises to do exactly this. The first target was fire blight. In a pioneering work in 1994, H. Aldwinckle demonstrated that incorporating lytic enzymes genes derived from various microorganisms into the cultivar Royal Gala rendered the plants more resistant to artificial inoculation in greenhouse and field trials. Similarly, the incorporation of fungal chitinase genes and a glucanase gene led, in some transgenic lines of the cultivar McIntosh, to high resistance against scab. However, these measures often went along with reduced plant vigour – something not intended by the scientists involved. At present, no reports on other possible side-effects of these gene products are available.
At our inistitute in Zurich, we used the same technique to introduce an apple-own resistance gene derived from the apple tree Malus floribunda 821 into the scab-susceptible cultivar Gala. This gene, called HcrVf2, was fully functional and made the selected transgenic lines resistant to scab. However, they are still susceptible to that genotype of the pathogen that is able to overcome the resistance of Malus floribunda 821. These works show that it is possible to control scab by introgressing, through DNA recombination technology, foreign genes or an apple-own resistance gene.
Scientifically this is certainly a progress. One may be tempted to proclaim that apple scab will be conquered and even that the application of fungicides will soon become obsolete. However, several aspects must be taken account of. For example, biosafety aspects of expressed target proteins (especially if stemming from foreign genes) must be evaluated. Major effects on non-target organisms can be evaluated. On the other hand, subtle effects may be detectable only after long observation and may harbor unpleasant surprises we are not even able to imagine, such as the formation of new allergenic proteins, different behavior toward other diseases deemed unimportant today or reduced biodegradation of leaf material. The question remains: if we do not detect negative effects, does this mean there aren’t any? Using the apple resistance gene Vf, which is used in many scab resistant cultivars, can avoid these risks at least.
However, two more problems remain to be addressed. Is the resistance durable? And what are the epigenetic effects for the insertion site? As we know, a particular race of the scab pathogen can overcome the effect of a single resistance gene. Therefore, the transformed cultivar can soon become susceptible again. This constraint can be overcome by introgressing one or more functionally different scab resistance genes, similarly to the efforts in conventional breeding today.
The risk of epigenetic effects
The second major and usually unaddressed problem concerns the insert site. At present, it is impossible to precisely target where single genes are inserted into genomes. For each transformation event, the insertion site will be different. This can lead to subtle changes in gene expression at the site of insertion that may become visible only in the later stages of plant development.
The problem is that genes interact. Introgressing one gene may suppress the effect of another gene – a process we call silencing. But it may also amplify the effect of others or even trigger effects that, so far, had been silenced. That effect is called expression. Such epigenetic effects can be easily detected if they visibly change the plant, but it is much more difficult to identify changes in expression with an effect on non-target organisms. The probability of unwanted side-effects is minimised if the inserted gene is placed into the natural site in which it is located in the conventional scab resistant cultivars. We know this exact position. However, we lack the tools to insert a gene exactly where we want to. Efforts are underway to develop such tools, but it is difficult even to estimate when they will be available in model systems and later applicable to plants.
To conclude, an apple transformed with only apple-own DNA and a substitution of non-functional resistance genes, which all common races of the pathogen overcome, with two or more functional resistance genes may be acceptable. That is what we call gene therapy. Such apple cultivars will contribute to a safer and environmentally sustainable apple production, as the need for fungicides may be drastically reduced. Additional safety questions such as outcrossing and weediness have to be discussed but may lose their relevance as such cultivars will be principally identical to resistant cultivars obtained through traditional breeding.
However, to reach this goal great advances in the DNA recombinant technology are needed which are still out of reach. The available technology yields products which are, especially because of unknown epigenetic effects, questionable and in my view not ready to be commercialised. However, we should not condemn the technology but advance it so that the delivered products meet safety requirements and are truly useful to the community.
The fact that we are mainly thinking of herbicide tolerance or resistance against some insects through the Bt gene when talking about GM crops shows that we are only at the beginning of exploiting the potential of green biotechnology. Both traits are debatable, they can be seen as positive or negative, with the point of view depending on the observer’s economic and political position. We know for sure, however, that genetic engineering can be useful for solving serious problems. That was the case when transgenic papaya was made resistant against the ringspot virus. Doing so saved the Hawaiian papaya production. In a similar sense, it obviously would make sense to find ways to reduce and not only to substitute pesticides. Nonetheless, researchers and developers must take seriously any suggested risk.
Dr. Cesare Gessler
is a research director for plant pathology at the Swiss Federal Institute for Technology Zurich. He also heads the research center SafeCrop at the Istituto Agrario San Michele all’Adige
near Trento, Italy.
cesare.gessler@ipw.agrl.ethz.ch
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