Plant chloroplasts may leak GM transgenes

An Adelaide University study has shown that confining transgenes to nature's own handy bottle, to prevent them escaping from genetically modified crops into non-GM crops or their weedy relatives, will not necessarily prevent transgenes escaping.

Dr Jeremy Timmis and his colleagues at Adelaide University have confirmed that the bottle -- the plant chloroplast -- has a tendency to leak, albeit at a very low rate.

Chloroplasts are the tiny, chlorophyll-packed organelles in plant and algal cells that house the machinery of photosynthesis. Each chloroplast has its own small complement of genes, independent of the main genetic library in the plant cell's nucleus.

Nuclear genes are packaged into pollen -- along with any transgenes that might be inserted into the nucleus, such as the Bt insecticide genes and herbicide-tolerance genes used in many of today's GM crops.

One way of blocking transgene escape via pollen would be to insert transgenes into the chloroplast genome, because pollen lacks chloroplasts.

But the Adelaide University team's finding that transgenes inserted into chloroplasts occasionally migrate into the nucleus is not unexpected.

Over evolutionary timescales, many native chloroplast genes have been transferred to the nucleus -- as reported this week by Australian Biotechnology News, in its article on the plant ancestry of the malaria parasite Plasmodium, 466 ex-chloroplast genes now reside in the parasite's nucleus

Timmis and his colleagues inserted a marker gene, conferring resistance to the antibiotic kanamycin, into the chloroplasts of tobacco plants.

They then fertilised wild-type plants with pollen from the transgenic plants, and grew 250,000 resulting hybrid seeds on kanamycin-laced media.

In about one in 16,000 of the seedlings, the gene had jumped from chloroplast to nucleus to become heritable, via pollen transfer to other plants.

Transgenes must be specially modified with special DNA 'switches' to function properly in the chloroplast, but many such genes would still be viable in the nucleus, according to Nature.

Timmis' team suggests such integration events would have the same potential to change the course of the plant's evolution as normal mutation events involving nuclear genes.

They say further experiments are now necessary to determine whether this low, background rate of transfer, presents a realistic risk of transgenes jumping into pollen, and from there, into non-GM crops, through wind-blown or insect-carried pollen grains.

Some media have reported the Adelaide University group's findings as supporting claims by anti-GM activists that transgene transfer to non-GM crops or weeds will create "superweeds".

But according to GM crops expert Dr Rick Roush, of the CRC for Weed Management, in Adelaide, the actual risk of that happening is extremely low -- as Timmis was at pains to point out in his paper.

The chloroplast is derived from an ancient bacterial symbiont, and retains its original bacterial promoters -- gene switches -- that work very inefficiently in the nucleus.

To obtain detectable expression of the antibiotic-resistance transgene, the Timmis group had to equip it with nuclear-gene promoters -- something that was extremely unlikely to happen spontaneously under field conditions.

Even then, the Adelaide University researchers had hand-pollinated the wild-type tobacco plants; in the field, any vagrant transgenic pollen from a crop like GM canola would have to compete with vast quantities of non-GM pollen from nearby flowers to successfully fertilise a flower and produce seeds with the GM trait.

Roush said suggestions that the chloroplast had been regarded as the great hope for plant transgene containment in future were inaccurate.

Molecular geneticists' chief interest in the chloroplast was in using it to obtain higher levels of transgene expression, through the great abundance of chloroplasts in plant cells.

Nor did chloroplast containment solve the problem of accidental transgene transfer, because it provided only a one-way barrier. It did not prevent pollen from weeds fertilising transgenic crops to produce hybrids carrying transgenic traits, said Roush.