Transgenic apple varieties – Approaches for the prevention of outcrossing and dispersal

Topic

Cultivated apples can successfully outcross with numerous wild species and cultivated relatives. The production of the first genetically modified apple strains and the impending commercialisation of transgenic apple trees have focused attention on possible consequences of any outcrossing of the transgenes .

• Male sterility. It is possible to prevent pollen transfer of the transgenes to other woody plants by using male-sterile apple breeding lines. With genetic engineering, it is possible to insert the male sterility trait directly into apple breeding lines. The aim of this research project is to produce transgenic apple plants characterised by male sterility (no pollen formation).

• Seedless fruit. In addition, the project is following genetic engineering approaches to parthenocarpic fruit development, i.e. the formation of seedless fruit. Parthenocarpy is known to occur naturally in apples. The research project hopes in this way to help prevent outcrossing of transgenes and the uncontrolled spread of genetically modified seeds in the environment.

The project work is of fundamental importance for future genetic engineering approaches in apples: the combination of useful genes, e.g. disease resistance, with genes for pollen sterility or parthenocarpy would make it much easier for transgenic woody plants to be used.

Summary

A number of transgenic strains were created for the production of male-sterile apple plants and apple plants with parthenocarpic fruit. For the transfer, genes were used that have already been successfully used in other crop plants.

Most of these strains are in the greenhouse, with the first blossom expected next spring. In follow-up projects, these flowers will then be investigated to see whether the transfer of the foreign genes achieved the desired objective.

In addition, a birch gene was transferred which is designed to stimulate blossom formation in apple plants. Early successes have been achieved with the transfer of this gene.

Experiment description

Production of transgenic apple strains

An important focus of the project is the production of transgenic apple strains. Various genetic constructs were used that impart male sterility and seedlessness. Both transgenic and non-transgenic apple plants were used as parent material. The transgenic apple strains already contained resistance to fire-blight, a major pathogen among fruit species.

Stability of the genetic modification

In the coming years, the successfully modified strains will be subjected to various molecular genetic and morphological tests in the laboratory and greenhouse, and later in the field. Of particular interest is whether strains can be selected that stably develop the characteristics of male sterility and/or seedlessness under natural environmental conditions in the open.

Results

Production of transgenic apple strains

Transgenic apple plantlets in a jar and in pots

So far, numerous transformations have been carried out with various apple strains and constructs to achieve male sterility. This work produced several transgenic strains, which were rooted and transferred to the greenhouse. Here they were grafted onto rootstock commonly used in fruit growing. The first blossom is expected to appear on these trees in the spring of 2006. The flowers will then be tested for male sterility. To produce transgenic seedless apple strains, gene constructs were used that have already resulted in seedlessness in tomatoes and aubergines. These transgenic strains are currently being rooted and transferred to the greenhouse. In the spring of 2006 some of these plants will be grafted onto rootstock. This means that the first blossom will be expected in 2007. These flowers can then be tested to see whether the genetic modification has been successful.

Stability of the genetic modification

Since it takes several years for apple trees to form blossom for the first time, a birch gene was transferred with the aim of stimulating blossom formation. This should make it possible to shorten long and costly breeding programmes significantly. This gene was used to produce a large number of transgenic strains. Some of the plants blossomed after only a few weeks. These are currently being tested to see whether the flowers are functional.

Over the next few years, the transgenic plants will be cultivated to the flowering phase and characterised using molecular genetic techniques. Morphological testing can then begin on the floral organs. At this stage, individual flowers will be tested for pollen formation, vigour and germinability. In order to test the stability of the new traits (male sterility), these plants must be studied and analysed over several years.