Pollen and ovule

The approach involves introducing two transgene constructs into the selected crop or cultivar, attached to promoter sequences from early-acting developmental genes expressed in the endosperm of the developing seed, with an essential role in seed development.

The two genes are from the MEDEA (or MEA) polycomb gene group in Arabidopsis, which are also known as Fertilisation Independent Seed 1 (FIS1), and Fertilisation Independent Seed 2 (FIS2).

One transgene construct comprises the protein-coding sequence of a seed-lethal gene - yet to be selected - under the control of the MEA promoter. For demonstration purposes, Swain's team used the GUS reporter gene as a proxy.

The second transgene uses the FIS2 promoter to drive expression of a so-called hairpin gene, designed to silence expression of the seed-lethal gene via RNA interference. The construct codes for a double-stranded hairpin RNA molecule that programs the plant's cells to destroy the messenger RNA of the seed-lethal gene, blocking synthesis of the encoded protein.

The ingenuity of the approach lies in the fact that, in the MEA:GUS construct, the MEA promoter begins to drive expression of GUS 48 hours after pollination when it is inherited from the male (pollen) parent.

If it is inherited via the female (ovule parent), it is expressed before fertilisation occurs, and again after pollination.

In contrast, the FIS2:GUS construct, designed to silence GUS, is expressed throughout seed development when maternally inherited, but is repressed by imprinting when inherited from the pollen parent.

On self-pollinated plants or transgenic plants fertilised by pollen from non-transgenic plants, the seeds develop normally because the FIS2:hairpin gene prevents the seed-lethal gene being expressed throughout seed development.

But if transgenic pollen finds its way onto the flowers a non-transgenic crop, or a weedy relative, seed development aborts because the non-GM seed parent has no inbuilt RNAi defence against the MEA:seed lethal gene carried by GM pollen.

One of the intriguing aspects of the system is that the system would not prevent fertilisation if the GM crop were fertilised by stray pollen from a non-GM crop, or by weedy relatives. In the first instance, this could make non-GM farmers, including organic farmers, liable for "contaminating" GM crops and compromising farmers' ability to save and re-sow GM seed for which they have paid a premium.

This would turn a key strategy of the anti-GM movement on its head: the threat of legal action if pollen from GM crops "contaminates" conventional and organic crops, compromising their GM-free status, which according to activists, attracts a premium in international markets.

Swain says the CSIRO system could be adapted to prevent non-GM pollen fertilising GM crops.

But in practice, GM farmers would be unlikely to require this type of protection against non-GM pollination, because the Rieger study has shown that cross-pollination between weeds and GM crops is extremely rare, and would involve no risk to human health.

And in a field crowded with self-pollinating GM plants, the likelihood that pollen from any weedy relative, or even a nearby non-GM crop, will pollinate a single GM flower, is very small, and would poses no credible threat to human health.