The Rockefeller
University

GM Crops Without Antibiotic Resistance?

A new way of selecting foreign genes in plants could eliminate a potential risk of genetically modified (GM) crops - the transfer of antibiotic-resistance genes into the environment.

Antibiotic-resistance genes are used by genetic engineers to track recombinant genes in plants, allowing transgenic seedlings to be differentiated from seedlings that have failed to take up recombinant DNA.

Nam-Hai Chua and his colleagues now report a system in which the transgenes are linked to a gene encoding isopentenyltransferase (IPT) - an enzyme involved in plant growth hormone biosynthesis - rather than an antibiotic-resistance gene. Using this system, they successfully selected transgenic lettuce and tobacco plants based on their rapid formation of shoots in seedlings compared with nontransgenic plants. The system dispenses with the need for antibiotics and antibiotic-resistance genes and could be applicable to many different crops.

Transgenic crops engineered by conventional methods contain one or two copies of an antibiotic-resistance gene in each cell. As Michael Syvanen discusses in a commentary accompanying the Chua paper, although experimental work and data are inconclusive, concerns have been raised that GM crops could transfer these genes into bacteria, creating "superbugs" resistant to many types of antibiotic.

Now Nam-Hai Chua and colleagues at the Rockefeller University and University of Singapore have used the ipt gene from Agrobacterium - the natural bacteria used to transfer genes into plants - in the place of an antibiotic resistance marker to monitor gene transfer. The IPT enzyme catalyzes the first step in biosynthesis of plant hormones called cytokinins.

These hormones stimulate shoot formation from plant cells, so only cells containing the newly introduced DNA form shoots and differentiate into mature plants. Chua and colleagues used an inducible system to carefully control ipt expression so that it is only turned on when needed for shoot formation, and not long enough to cause abnormalities.

Using their method, healthy, normal transgenic lettuce and tobacco plants were generated containing multiple transgenes. Future refinements to the approach could avoid introducing the bacterial ipt gene altogether, instead using controlled expression of shoot-inducing genes from plants as a selectable marker.