SiFo project: Effects of transgenic zeaxanthin potatoes on soil life and soil quality

Varietal effect

July 2006. The sky is clear and the sun has been shining all morning over the farm. On a potato field behind the farm buildings ten young people are at work. A few others are standing at the edge of the field talking. It could be a farm holiday, were it not for the laboratory gloves on the hands holding the pitchforks.

Today, Michael Schloter of the Technische Universität München (TUM) is at the field to explain things to journalists and farmers from the region, because this field is being used to grow genetically modified potatoes. “We are investigating the effects of these potatoes on the microflora in the root area, i.e. on soil bacteria and soil fungi”, explains Schloter. “The microflora is affected by a number of factors, like the time of year, type of crop and method of cultivation. But excretions from the plant roots also cause changes.”

Dr. Michael Schloter, project leader at the Technical University of Munich

Trial field with security fence

The trial field, one of two release sites. Intense opposition at the other site meant that the approval was delayed until it was too late for sowing. The fence is designed to prevent animals carrying transgenic tubers out of the field. The crop is surrounded by a cordon of mustard.

Michael Schloter (2nd from right) in talks with an organic farmer, a conventional farmer and the field owner

The trial field with five conventional varieties and transgenic zeaxanthin potatoes with two different constructs

Four rows of the same variety are planted next to each other. In total there are five conventional varieties and transgenic zeaxanthin potatoes with two different constructs.


The sticks mark sampling points The mobile “laboratory” – this is where the plants are cleaned, the roots removed and the samples frozen.

Individual plants are dug up…

The roots are largely cleaned of soil.

…the roots are largely cleared of soil…

Roots are frozen.

…and are then frozen

The excretions – technically called exudates – from the potatoes on the trial field could differ from those from conventional varieties. These are so-called zeaxanthin potatoes – a genetically engineered variety that accumulates zeaxanthin in its tubers. Zeaxanthin is a substance that can prevent age-related blindness. “However, we are not only investigating differences between the transgenic potatoes and their parent variety”, stresses Schloter, pointing to the cultivation plan. The potato rows are highlighted in seven colours, with blocks of four identical rows. “This trial is also recording, for the first time, what effect varietal differences in general have on soil microflora. In addition to the two zeaxanthin potato lines, which have been produced in different ways, the field also contains the parent line Baltica and four other conventional varieties: Ditta, Désirée, Sibu and Selma.

The activity of the soil organisms

Today the members of the joint research project are taking root samples of all seven potato varieties. To do this they dig up individual potato plants, quickly remove most of the earth and cut off the roots with sterile shears. The reason for the hurry is not so much the heat, but rather the fact that the researchers are also interested in seeing what bacterial mRNA is present in the roots and on the root surface. Since the mRNA usually degrades within just a few minutes, the samples are immediately frozen in dry ice. “The mRNA is interesting because it shows us which genes are actually active”, explains Schloter. “When we analyse the DNA we find out only what genetic potential is present, but not which parts are really active. The genetic potential in the soil is, incidentally, huge”, stresses the soil ecologist and holds up a handful of soil: “According to a conservative estimate, there are ten thousand species living in each gram of soil.”

However, the five research groups from Germany and Austria working on the project are not investigating the potato plants and their root areas (the rhizosphere) just to focus on the species diversity. They are also interested in the nutrient dynamics, which are inextricably linked to soil life and plant growth, and in plant pests and their antagonists. “If a plant can be modified in such a way that it favours the antagonists in its root zone, some chemical treatments could become superfluous”, says Schloter, explaining the reasoning behind the analyses. But adverse effects cannot be ruled out.

His own attention is focused on the nitrogen cycle. “Most of the bacterial groups involved have been identified. Now we want to know which of their genes are involved and how they are regulated.”

For instance, he believes it is conceivable that plants could be modified through breeding so that their root exudates affect soil bacteria in a way that improves their digestion of nutrients bound in the soil, thereby making them available to the plants. But that is all in the future. First of all, the trial is being conducted for safety research purposes – to investigate whether the genetic modification has a greater effect on soil life than would normally be expected as the result of varietal differences.

Safety questions

It is also questions of safety that have brought one conventional and one organic farmer from the region to the trial field today. They want to know how the potatoes are being prevented from spreading outside the field, and whether the transgenic plants can harm animals. “In order to prevent any animals eating the tubers or carrying them out of the field, we have fenced in the whole trial”, informs Schloter. The organic farmer insists: “And the pollen?” Schloter explains that pollen does not play a role in the spread of potatoes. They are propagated only via the tubers. Although pollen can pollinate other potato plants in a radius of up to 20 metres, the seeds do not produce potato plants that are robust enough to survive. In addition, the zeaxanthin is produced specifically in the tuber. “No new gene has been added to the potato plants”, explains Schloter. “The conversion of zeaxanthin into a different substance in the tuber has simply been interrupted.” Nevertheless: if the zeaxanthin potatoes later bear berries, some plants will be covered by cages to protect them against birds, while the other transgenic plants will have their berries removed.

Schloter stresses the fact that regardless of whether the zeaxanthin potato will ever be grown commercially, this project will supply important findings about the conversion of substances in agricultural soil, and will also provide the basis for further safety research in the area of transgenic potatoes.

Wheat as follower crop

While Michael Schloter has been talking to visitors at the edge of the field, the sampling has progressed. The sun is now right overhead, but only a short lunch break is permitted – after all, the samples need to be taken within a short timeframe, since altered weather conditions also affect the microflora in the soil, and the samples would no longer be comparable. But it doesn’t look like rain today.

Before more potatoes are planted on the field, it is to be sown with winter wheat, because another important aspect of the project is to investigate possible impacts on the follower crop – winter wheat. Preliminary results have shown that a large number of fungi harmful to cereals, so-called fusaria, live in the root area of some varieties of potato. The new research will show whether the frequency of fusaria in the root zone of the potato plants is increased by genetic modifications.