Risks associated with genetic engineering in woody plants

Woody plants: Preventing spread

Woody plants have one particular characteristic which is very significant for the risk assessment of genetic interventions: they have an extremely long lifespan. Even the comparatively short-lived poplar, a model object favoured by genetic engineers, can live to a hundred years or more. Transgenic modifications can therefore be effective for long periods of time. This is of relevance for all conceivable risks.

Woody plants, most of which are cross-fertilising, like to cross with their wild species and with related species. Pollen and seed from trees can spread over very long distances depending on the weather conditions. If pollen reaches the upper air layers, it can be carried on the wind for a hundred kilometres. Many species of woody plant even have effective mechanisms for vegetative reproduction.

To generally eliminate the risk of a spread of transgenic plants, strategies are being developed worldwide through which genetic transfer through pollen or seeds can be specifically prevented (biological confinement).

Male sterile early-flowering poplar. Due to the sterility no catkins are produced.

Apple blossom

Apple trees have been brought to flower in their first year through the transfer of a gene from birch.

For many years research has been going on at the Institute for Forestry Genetics in Grosshansdorf to produce male and female sterility in poplars.

Biosafety research projects are currently investigating two confinement systems. In one, male sterility has been generated by stopping pollen production. In the second system, although pollen is produced, it no longer contains the transgene.

Whether such confinement systems will be successful also depends on how stable the introduced gene is in the long term. It is possible that the genetically introduced traits during the course of development and aging. In greenhouse trials with transgenic aspen trees it was observed that instabilities could occur within a short period.

At the Institute for Plant Breeding Research, apple research on horticultural cultures and fruit performed in Dresden-Pillnitz in the framework of the ‘Confinement’ concept of biosafety research has focussed on both male sterility and seedless fruits. The Dresden Institute is also examining the question of gene activity in non-transgenic apple plants can be influenced through grafting onto a transgenic apple plant as stock. That would have the advantage that the reproductive parts of the plant would produce non-transgenic pollen, which would prevent an outcrossing of the genetically modified genotype.

Accelerating research and breeding

Breeding of woody plants is a protracted process, requiring many decades. New traits can be introduced using gene technology in a manageable time frame, but many years or decades are needed to determine whether the genetic modification functions and is durable. To accelerate this process, trees are being induced through the introduction of various ‘early-blooming’ genes to blossom earlier. Poplars, which normally first bloom after about 8 years, can come into flower already after a few months to 3 years.

With apple trees, which normally blossom for the first time after 6 – 10 years, successful transfer of a birch gene has led to plants that start to bloom in the first year. It is planned to introduce transgenic early-flowering apple plants into the conventional breeding processes. At the end of the breeding process, after several crossing steps, those seedlings showing the desired traits but that are no longer transgenic will be used further. Molecular markers are being used to further accelerate the breeding process, i.e. apple seedling are examined at an early stage for molecular markers for specific genes.

Transfer of transgenes to fungi or bacteria?

Trees live in very complex ecosystems with a wide range of interrelationships, e.g. they live in intensive symbiosis with fungal communities in the rhizosphere. Together they form a mycorrhiza, which ensures a supply of water and nutrients. If the transgenes were to transfer to the fungi, they might then spread further to other woody plant species via the fungi. It is possible that modified gene information could also be transferred to endophytic bacteria that normally live inside certain plants.

Several biosafety research projects have investigated possible horizontal gene transfer to mycorrhizal fungi and endophytic bacteria. However, no horizontal gene transfer could be demonstrated either in field trials or in the laboratory under optimised conditions.