Genetically modified (GM) tomatoes have been around since the mid-1990s, when scientists created transgenic tomatoes with a prolonged ripening period. In the September issue of Nature Biotechnology, Ruf et al. describe the first successful application of a new transformation method — plastid transgenesis — in tomato. This technique, in which foreign genes are targeted to a cytoplasmic genome, has advantages over the more conventional, nuclear transformation; notably, high expression of the transgene in the fruit offers the promise of using tomatoes to produce biopharmaceuticals and to engineer fruit with a higher nutrient content.

Plastids, such as chloroplasts, are cytoplasmic organelles with small, highly polyploid genomes that, unlike nuclear DNA, are free of higher-order chromatin and epigenetic modification — allowing efficient homologous recombination and high levels of gene expression to occur. Importantly for the environmental impact of GM crops, plastids — like all cytoplasmic organelles — are almost always inherited maternally, thereby eliminating the risk that transgenes will spread through pollen to other plants.

As the aim of the experiment was to induce strong transgene expression in edible plant parts, the authors used a ribosomal RNA promoter Prrn — known to be active in fruit. Transgenic constructs were delivered in culture to plant tissues by gold particle bombardment and integration of the transgenic construct into the plastid genome by homologous recombination was ensured by flanking the foreign genes with plastid-derived 'targeting' sequences. Transformants were then selected on the basis of the antibiotic resistance conferred by the construct and by PCR.

The polyploid nature of the plastid genome has its disadvantages. Because a single cell can contain as many as 10,000 genomes, it is difficult to select pure transgenic lines. The success of plastid transgenesis therefore relies on having a robust method to select transformants: Ruf et al. targeted small leaf pieces for transformation, and carried out all their selection procedures under very low light conditions and over a long period of time. Lack of mosaicism in the tissues was then confirmed by restriction fragment length polymorphism analysis, after which the transformed tissues were used to regenerate a whole plant.

Using this protocol, the authors found that the transgene was expressed throughout the plant, including the fruit, where the transgene product made up 0.5% of the total soluble protein content, making it the first successful transgene to be expressed to high levels in the edible parts of a plant.

Although the technique needs to be optimized before it is commercially useful — shorter selection times are desirable, for example. On the basis of expression studies in tobacco, the authors suggest that the levels of transgene expression in the fruit can reach up to 20% of total soluble protein content. These results open up many appetizing opportunities for biotechnologists, who are always eager to find new ways in which to produce and deliver vaccines, drugs and nutrients.