Biological containment for genetically modified maize

Maize without pollen: No more outcrossing?

The maize on the trial field is in full flower. Staff from the Julius Kühn Institute are pushing their way through the plants and recording the appearance of the male flowers. Why? On this field near Braunschweig the scientists are investigating whether their new concept to prevent the spread of genetically modified maize plants is working. The cultivation of cytoplasmic male-sterile maize is intended to make it possible to prevent cross-pollination with neighbouring maize fields. This is called biological containment or biocontainment. This year is the third and final year of the trial, after which the results will be known.

Dr Heidrun Bückmann of the Julius Kühn Institute in Braunschweig is leading the field trials with CMS maize. The three-year trials are being conducted at three sites: Braunschweig, Groß Lüsewitz and Freising.

Staff from the Julius Kühn Institute document the growth of the CMS maize plants and check whether the male inflorescences have formed anthers. This is an indication that the cytoplasmic male sterility trait is not completely stable under certain environmental conditions. (video in German)

A male inflorescence of conventional maize. The many anthers are clearly visible. They contain the pollen.

A male inflorescence of CMS maize. No anthers have formed.

A panicle of CMS maize on which a few anthers can be seen. Here the CMS trait is not entirely stable.

Male sterility in CMS maize can become unstable as a result of climatic conditions. A weather station records climatic data throughout the trial.

On the left is a plot with CMS maize. Opposite, and separated by a distance of 3.5 metres, is a plot with conventional maize.

The conventional maize normally produces only white grains. If it is fertilized by CMS maize pollen, the cobs will contain some yellow grains as well. The outcrossing rate is calculated based on the ratio of white to yellow grains. The picture shows a maize cob that was grown immediately adjacent to non-sterile yellow-grained maize. In this case, the proportion of yellow grains, and therefore the outcrossing rate, is high.

Heidrun Bückmann fights her way between the head-high maize plants. A strong wind blows through the plants, and her voice can only just be heard above the loud rustling of the leaves as she explains the aims of her work. “The potential spread of newly inserted genes via pollen plays an important role in risk assessments of genetically modified plants. Imagine if these plants produced active pharmaceutical ingredients or industrial raw materials. The spread of this trait to neighbouring fields would have to be prevented, or at least severely restricted.”

The plants on this maize field, which are designed to make this possible, have a special trait. As a result of a natural Mutation in their mitochondria, they are no longer able to produce pollen. Since the mitochondria are located in the cytoplasm of the plant cells, this trait is called cytoplasmic male sterility. This type of maize is therefore known as CMS maize for short.

Good in theory, but needs testing in practice

The plants can lose part or all of their cytoplasmic male sterility trait. “That is why we are conducting these trials here,” explains Heidrun Bückmann. “Sterility in maize can be removed e.g. through extreme environmental conditions, like heat, drought or heavy rain.” This would make the plants less suitable for Confinement.

Her staff are wandering through the field inspecting the male inflorescences of the CMS maize, the panicles. The male inflorescences of conventional maize have hundreds of anthers hanging from them containing maize pollen. The inflorescences of CMS maize should have no anthers at all. But here it becomes clear that CMS maize is not always totally male-sterile, because every now and then the scientists find isolated anthers on some of the plants. “We call these fluctuating panicles,” says Heidrun Bückmann. The field trial is also investigating whether the few anthers actually contain fertile pollen. For this, the researchers previously placed paper bags over female CMS maize flowers on some plants. These plants can no longer be fertilized by pollen from other plants. The pollen from the plant itself is then collected and scattered over the isolated female flowers. If grains develop on these plants, the researchers will know that the pollen was fertile.

But the scientists are primarily interested in whether the CMS maize can fertilize conventional maize on neighbouring trial plots under field conditions. The trial plots with CMS maize and conventional maize are separated by a distance of 3.5 metres. Between them is an empty corridor. Any outcrossing events can be measured later in the year once the maize plants have produced cobs.

It will then be very easy to establish the outcrossing rate. The conventional maize used in the trial produces white grains, while the CMS maize has yellow grains. If the CMS maize plants fertilize the neighbouring white maize plants, yellow grains will be found on the cobs. They will then be counted to determine the outcrossing rate.

Preliminary results look optimistic

Last year it was found that the outcrossing rate in the neighbouring field can be reduced by up to 98 per cent, at least with one of the CMS maize hybrids being investigated here. This poses the question of whether this is sufficient as a biological containment method. “In certain cases,” says Heidrun Bückmann, “this would doubtless not be sufficient for biological safety reasons, for instance, when active pharmaceutical ingredients are to be produced in genetically modified plants.” But if severely restricting outcrossing were sufficient, e.g. with genetically modified plants that produce industrial raw materials, it would be a good approach. Especially, since it would be possible to reduce the outcrossing rate still further by respecting isolation distances between the GM maize and the neighbouring field and planting buffer zones of conventional maize.

Heidrun Bückmann points out another aspect: CMS maize can increase yields. Since these maize plants no longer need energy for pollen production, it benefits the growth of the maize cobs. Since CMS maize cannot fertilize itself, conventional maize hybrids would have to be planted on about 20 per cent of the field to act as pollen donors. But even this has an advantage: fertilization by unrelated maize hybrids leads to a further yield increase in the CMS maize. This ‘Plus-Hybrid’ system was developed a few years ago by researchers at the Swiss Federal Institute of Technology (ETH) in Zurich.