Why do we need marker genes?
New genes are introduced into plant cells to give them novel properties. Some new genes might give the plant the ability to produce its own insecticide, or some might enable it to make a particular vitamin.

But inserting new genes into a plant cells is a hit and miss process. Only a fraction of the target plant cells will take up the new gene successfully.

Even then, there is often no simple way of telling whether a plant cell has taken up the new gene. That's why marker genes are used.

Marker genes are introduced together with the new gene and enable scientists to tell when the new gene is present, like a form of labelling. If the marker gene is there, so is the new gene.

How do marker genes 'label' plant cells?
Some marker genes give cells the ability to withstand treatment with antibiotics or herbicides (weedkillers). To identify cells that contain these genes, scientists simply treat all the cells with that particular antibiotic or herbicide and only the cells that have the marker gene will grow.

Sometimes, marker genes are not active in the final plant. For example, Syngenta used an antibiotic resistance marker gene in the development of its Bt-176 maize, which can protect itself from attack by a devastating insect pest, the European corn borer. However, this marker gene is not active in the plant.

Other marker genes may make cells turn a distinct colour when treated with chemicals or glow under a certain kind of light.

How does the PMI marker gene system work?
Plants need many kinds of nutrients to grow, including sugars. There are many different kinds of sugars and not all plants can utilise every kind of sugar.

Many kinds of plant cannot use sugars like mannose, for example.

The PMI marker system gives plant cells the ability to digest mannose, whereas other plant cells may not be able to. If plant cells are grown on a food source that contains mannose as the only sugar, then only the plant cells containing the active PMI gene will be able to digest mannose and grow - all the others will starve.

Therefore, cells that contain the PMI gene and the new gene can be easily identified and grown into plants. The following diagram illustrates how the system works.

The PMI gene allows scientists to insert new genes into plant cells without using negative selection marker systems such as antibiotic or herbicide resistance. The PMI gene is transformed into plant cells together with a gene of interest. Not all cells will take up and express the new DNA so the cells are grown in the presence of mannose, which most plants cant utilise as food. Only those cells with the PMI gene grow. These cells can be regenerated into plants which carry the new trait.