ngin - Norfolk Genetic Information Network

6 February 2003


Secreting genes in cell subcompartment no guarantee against environmental escape.
Nature, 6 February 2003

A new study could raise concerns about next-generation genetically modified (GM) crops. It suggests that genes can move from a plant cell's chloroplast - its energy factory - into its pollen grains and therefore into the environment at large.

Bioengineers had hoped that modifying plants by secreting genes in the self-contained chloroplast might eliminate the risk of those genes leaking into their wild relatives or other organisms. This is a major concern surrounding existing GM crops, which contain genes in their cell nuclei (the compartment that contains most plant DNA).

But the study finds that there's a 1-in-16,000 chance of genes moving between the subcellular compartments. This probability was thought to be tens or hundreds of times smaller. "We knew this could happen but I am surprised the frequency is so high," says Anil Day, who works on chloroplasts at the University of Manchester, UK.

Study leader Jeremy Timmis of the University of Adelaide in Australia is keen to allay possible fears. The chance of genes transferring from chloroplast to pollen and then actually working is vanishingly small, he stresses. "I don't want to add fuel to the fire of ignorance surrounding GM crops."

But knowing how often genes could jump will be important for assessing new GM technologies. "We can use this number for risk evaluation," says Day, who has advised the British government on the risks of GM crops.

Chloroplasts evolved from single-celled organisms that are thought to have been captured by plant cells early in their evolution. They still contain their own DNA, but it is not passed on during plant sex. So biotechnology companies are experimenting with inserting into the chloroplast genes for herbicide tolerance, insect resistance, vaccines and the like.

Inside story

Timmis' team put two marker genes into tobacco plants' chloroplasts and looked for them in about 250,000 of their offspring. Eighteen of the seedlings bore the genes in the nuclei of their cells1.

One of the markers was designed to function in the nucleus, and did so. The other was suited to the chloroplast; it jumped into nucleus but could not function there. This, Timmis explains, illustrates just how unlikely it is that a chloroplast gene would both escape and function elsewhere.

It also hints at why only a few thousand genes have successfully moved from plant chloroplast into their nuclear DNA during hundreds of millions of years of evolution.

"It's an exceedingly rare event," says Timmis. In other words, genes in chloroplasts of GM plants are still overwhelmingly likely to stay put.  Nonetheless, biotechnologists should pay heed to the findings. "It's a low frequency, but not when you consider the global acreage of a crop," warns Day. It might be wise to tweak altered genes to make it even harder for them to get turned on in the nucleus, he adds.


1.Huang, C. Y., Ayliffe, M. A. & Timmis, J. N. Direct measurement of the transfer rate of chloroplast DNA into the  nucleus. Nature, published online, doi:10.1038/nature01435 (2003). |Article

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