Oct 20, 2003

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Conflict over transgenic apple trees

Apples: Deliberate releases planned

Genetically modified apple trees are to be tested in the field for the first time in Germany. The aim is to produce apple trees with increased resistance to the dreaded apple diseases, fire blight, apple scab and powdery mildew. This has sparked violent protests. At the Dresden-Pillnitz site a coalition comprising local action groups, organic farmers and greens are attempting to prevent the release. Critics fear that the transgenes will be able to spread despite safety precautions, with incalculable consequences for the ecosystem.

The cultivated apple (Malus domestica) belongs to the Rosaceae family. It can be successfully crossed with other cultivars and related wild species. Cross-pollination by insects is the principal method of fertilisation. The pollen is too heavy to be carried by the wind.

Every apple tree grown from a pip is unique. To obtain more trees with exactly the same apples, the breeder must attach a branch from this tree onto a rootstock. This is known as grafting.

Transgenic apple plantlets in a jar. First the foreign genes are transferred to leaf sections. These so-called primary transformants are then grown on in the jar and vegetatively propagated, i.e. new plants are produced from plant parts.

Fire blight is caused by a bacterium, which first appeared in Germany in 1971. Infected plant parts look as though they have been burnt. The bacteria get into the plant through open wounds and cuts and in particular via blossom. They overwinter in the bark and multiply in the spring. Under warm, humid conditions bacterial ooze is produced. The infection is spread mainly by insects but also by rain, wind, birds and pruning tools.

Apple scab , the most significant apple disease, is a fungus that overwinters in fallen leaves. In spring the winter spores are released, triggering primary infections if the leaves are sufficiently damp. Summer spores are produced in the infected areas (all parts of the tree above ground), leading to further infection. Pale olive green patches appear initially on leaves and fruit, turning brownish black later.

Apple powdery mildew is a fungus which, unlike apple scab, only feeds on living tissue and overwinters in the leaf and flower buds. It thrives on hot, humid weather and affects all plant parts above ground. Infected leaves have a powdery coating, curl upwards and become hard and brittle. Infected fruits are covered in a web-like structure.

The plan

After grapevines and poplars, the apple is now the third genetically modified woody plant to be field tested in Germany. The Federal Centre for Breeding Research on Cultivated Plants (BAZ) plans to deliberately release transgenic apple trees at two sites over a twenty-year period. Up to 5000 trees are to be planted during this twenty-year period on land belonging to the BAZ in Quedlinburg, Saxony-Anhalt, whilst on land belonging to the Institute of Fruit Breeding in Dresden-Pillnitz, Saxony, there are plans to plant as many as 1000 trees.

At the Quedlinburg site several transgenic strains are to be tested for their resistance to bacteria and fungi on the 0.2-hectare plot. The plants here will be “own-rooted”, i.e. without grafted rootstocks, since fructification is unnecessary. After just four years, before the plants have matured and produce flowers, they are to be pulled up and destroyed. Any flower buds found are to be removed immediately by hand.

At the Pillnitz site (one hectare) the plan is to plant only grafted trees and to study them with regard to their fruiting qualities. For this reason it is both necessary and desirable to allow these trees to flower and produce fruit.

To prevent insects spreading the transgenic pollen, the inflorescences will be covered with polyester pollination bags. Bags like this are also used in the normal breeding process. When the trees mature and flowers form, a polytunnel will be stretched over the rows of trees.

Risk to the ecosystem?

Opponents of the deliberate release believe that such measures are inadequate. They fear that native apple varieties could be “contaminated”, with long-term repercussions for the ecosystem. In addition the good reputation of Pillnitz as a centre for traditional fruit growing is at stake. With its many meadows with scattered fruit trees, nature conservation sites and organic crop areas, they believe it is totally unsuitable for the deliberate release of “genetically engineered” apple trees.

Institute director Viola Hanke assesses the risk of the transgenes being spread by pollen transfer as low, even if, despite isolating the inflorescences, individual grains of pollen were to escape into the environment.

Dreaded apple diseases

In the last thirty years the bacterial disease fire blight has spread to many European countries and is causing significant damage. Fire blight is highly contagious and affects pomaceous trees and shrubs in the Rosaceae family, for example apples, pears and quinces. Once established, it spreads like wildfire. There are no effective means of controlling it which are neither harmful to health nor the environment. In Germany an antibiotic may be used only in exceptional circumstances.

Apple scab and powdery mildew are fungal diseases which can also lead to heavy yield losses. Both diseases can be checked using targeted preventative measures. In the case of powdery mildew, this involves in particular careful pruning and the complete removal of all shoots with infected buds. With apple scab it is important to ensure that fallen leaves are composted thoroughly and quickly to prevent fungal spores overwintering in them. Both diseases can also be controlled with fungicides if the infestation is particularly severe.

In recent decades conventional methods have been used to breed highly resistant apple varieties, including at the Institute of Fruit Breeding in Pillnitz, where for some years now researchers have also been looking to improve the resistance of plants using genetic engineering techniques.

The genetic approach

The Institute of Fruit Breeding has been working on the development of transgenic apple trees since 1997. Different gene constructs have been inserted into apple leaf sections, which have then been grown into plants and tested for resistance to bacteria and fungi. To test the stability of the foreign genes, the transgenic plantlets have been rooted, transferred to the greenhouse and then grafted in various combinations

The apple plants, which are now to be released, each contain one of eight different gene constructs which should increase the plant’s resistance to pathogens. The genes originate from different organisms including bacteriophages, one fungus and the silkworm moth. They all control the production of proteins which are effective against bacteria and fungi. One gene from the T4 bacteriophage, for instance, enables the apple plants to produce lysozyme, a protein which attacks the bacterial cell wall. This gene has also been tested in potatoes.

Another gene from the fungus Trichoderma harzianum is responsible for the production of the enzyme chitinase, which destroys the fungal cell walls. A similar approach is already being tested on grape vines.

Count-down

Opponents of the release have announced any number of objections, but the deadline for these has now expired. The Robert Koch Institute plans to make a decision on the release application from the Federal Centre for Breeding Research on Cultivated Plants (BAZ) by the end of October.

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