Marker Genes Alternatives to Antibiotic Resistance Markers

New marker systems have been developed, but whether these new systems truly offer a safety advantage remains to be investigated.

Possible alternative marker gene systems:

• Herbicide resistance genes: After treatment with an herbicide, only the plants harbouring the herbicide resistance gene and gene of interest survive.

• Marker genes that enable the plant cell to use a particular food source: If the plant cells are fed only this one energy source, only the plants that have successfully incorporated the new genes will be able to grow. An example of this approach is the PMI gene. This marker gene enables the plant cell to use mannose, a type of sugar, as a source of energy. This marker gene was used to develop 3272 maize, a GM maize line recently submitted to European Commission for authorisation.

Visible markers: A fluorescent protein marks transformed plant cells

Genes that enable the plant to produce proteins that bind to toxic heavy metals, thereby allowing transformed cells to survive heavy metal treatments (e.g. cadmium). Visible markers: These marker genes make transgenic plants visually recognisable. The gene for the green fluorescent protein (GFP) makes genetically modified plant cells appear green when exposed to UV light. A major disadvantage to this technique is that modified and unmodified cells must later be visually separated, which can be very tedious.

Marker gene removal

After genetically modified cells have been selected, marker genes no longer serve a function. It is difficult, however, to remove marker genes from GM plants. Some strategies have been devised to make this happen.

A number of techniques use genetic tools that act as "molecular scissors" that can actually cut certain genes out of the genome. The gene encoding these scissors is introduced along with the marker gene and the gene of interest. Once transformed cells have been selected, scientists activate the gene encoding the scissors by an external stimulus. The scissors then cut out the marker gene and the gene for the scissors themselves, making the resulting plant completely free of marker genes. The "cre/lox" marker gene removal system works in this way.

Future GM plants are less likely to contain antibiotic resistance genes. In fact, LY 038, which was recently submitted for authorisation, is the first application for a marker gene free GMO in the EU. Its marker gene was removed using the cre/lox system schematised above. It is hoped that marker gene removal will simplify safety assessments. Nontheless, currently used GM cultviars possessing antibiotic resistance genes are still considered safe.