Genetically modified cyanophycin potatoes

Will new plant substance lead to changes in potato characteristics?

It is a stormy, overcast afternoon in mid-July. Genetically modified potatoes which provide the raw material for a biodegradable plastic are growing in neat rows on a fenced-in release site belonging to the University of Rostock. Overwintering trials will be conducted on the potato tubers this winter and their rotting behaviour will be observed.

Science live: Will new plant substance lead to changes in potato characteristics? (Video clip in German)

Various strains of genetically modified potatoes are growing on the release site.

Prof. Inge Broer, Chair of Agrobiotechnology at the University of Rostock

At various different times, potato tubers will be removed from the sacks buried on the trial field.

For the rotting experiments the potato tubers are buried in the trial field

and assessed using a special grading system.

The genetically modified potatoes (left) have been graded 4.28, because a large proportion of tubers (over 30%) have already turned to brown slime, i.e. the cellular tissue has broken down. By contrast, the near-isogenic strain (right) displays virtually no symptoms and was therefore graded 0.3.

Dr Christoph Unger is currently investigating potato samples from last winter in the laboratory. The cyanophycin content is determined using a specific method (Lockau, Humboldt-Univeristät Berlin).

Finely chopped and freeze-dried potato samples.

Ceramic beads are used to pulverise the potato powder even more finely.

Cyanophycin is detected using a special protein test (the Bradford assay). The colour intensity indicates the amount of cyanophycin present in the sample.

Inge Broer, Professor of Agrobiotechnology at the University of Rostock, has to make herself known to the security guard who has hurried up to us before we are allowed to enter the trial field. Since the latest destruction of a trial field in early July, the release site, where a variety of genetically modified plants are growing, is guarded around the clock. Several rows of ‘bioplastic potatoes’, cultivated here for special biosafety research studies, are growing undisturbed; some are already in flower. These potatoes contain a gene from a cyanobacterium (blue-green algae). The polypeptide cyanophycin, which the bacteria use to store nitrogen, among other things, will now be produced in the potatoes, providing the raw material for a biodegradable plastic. Some of the genetically modified potatoes produce the peptide in all plant parts, whilst others produce it only in the tubers.

“We are currently propagating the potatoes here so that we will have enough tuber material available in winter for our overwintering trials,” says Inge Broer. She points to one of the rows of potatoes: “This is the near-isogenic parent line with no genetic modification which is being grown and studied by way of comparison.” There is also a transgenic control; these are potatoes which contain only the marker gene required for the genetic transformation, which is present in all transgenic potatoes.

Overwintering trials

In the winter, tubers from the different type of potato will be buried on the trial field at two different sites and at two different depths. On six different occasions throughout the winter, 10 tubers of each variety will be dug up from each site and examined in the laboratory.

“We want to examine whether the genetically modified potatoes are perhaps better able to survive the winter to re-emerge the following year, in other words giving rise to new potato plants.” According to Inge Broer, the production of cyanophycin in the potatoes can disrupt the carbohydrate metabolism, which in turn can affect the freezing point. Increased frost resistance in genetically modified potatoes is undesirable because it could increase the risk of the potatoes growing wild along the field margins.

During the first trial winter of 2008/2009, tubers from the various different potato varieties were studied in detail. Initially the focus was on developing suitable methods. For example, a grading system was devised to provide a standardised means of assessing frost damage in the different trials by visual examination. The grades range from 0 (no symptoms) to 5 (more than 50% brown slime). The results from the first winter indicate that no increased frost resistance has been observed in the cyanophycin potatoes compared with the control lines.

Renewable raw material without the need for additional fields

The potatoes being tested here are a prototype for genetically modified plants which could supply specific substances for use as renewable raw materials. The biodegradable plastic which can be obtained from cyanophycin could replace petroleum-based plastics, like certain specific substances that are added to detergents as water softeners. Inge Broer and her colleagues still need to do a great deal of pioneering work. Suitable techniques must be developed to extract the substance from the plants. The plants themselves must be optimised to produce as much of the desired substance as possible. Methods will be developed to assess the biological safety of these plants.

To compete with petroleum-based products, products made from renewable resources must be at least as good and provide equal value for money. Scientists developing the cyanophycin potatoes have already succeeded in increasing the yield from an initial 1% to 6% of the dry mass of the potato. “The idea,” explains Inge Broer, “is that we won’t need any additional field space to grow these potatoes because we could supply cyanophycin as a by-product of potatoes that are actually grown for starch production.”

Research into the production of cyanophycin is not restricted to potatoes. The gene in question has also been inserted into tobacco plants, for example.

Elaborate process: how much cyanophycin do the tubers contain?

Christoph Unger is currently working in the laboratory, measuring the cyanophycin content of potato samples from last winter. As soon as the potato samples have been collected in the winter, they are pulverised and freeze-dried at minus 70 degrees. This extracts all the water from them, leaving much reduced, light, pale potato crumbs. In a series of different stages the crumbs are then further pulverised to a fine powder, all proteins except cyanophycin are dissolved out and finally the cyanophycin concentration is measured using a special protein detection method. “You can immediately tell how much cyanophycin is present by the intensity of the colour reaction,” explains Christoph Unger as he pushes the protein test into a reader which accurately measures the protein content. “At the moment it looks as if the cyanophycin content remains very stable throughout the entire period of overwintering, regardless of what condition the potatoes are in.” Whether further investigations will confirm this trend and what this signifies for the degradation process of cyanophycin and the associated micro-organisms are the type of questions that Christoph Unger will be investigating together with soil analysis experts from the University of Trier.

Potatoes on trial

When the potato tubers arrive in the laboratory from the field in the winter, pieces are removed from them with a cork borer, cut into slices and separated into “physiological compartments” in an elaborate series of steps; specific substances, such as enzymes, are studied both intercellularly, i.e. between the cells, in the cell walls and in the intercellular fluid, and in the cytoplasm, the cell’s interior. The presence of sugar in the intercellular fluid and in the cytoplasm, for example, is an indication of frost resistance. Plants protect themselves from frost by splitting long chain sugar polymers (polysaccharides) into simple sugars, such as glucose, because this lowers the freezing point. The pH value of the intercellular fluid also provides clues about the plant’s susceptibility to frost. The pH value of an intact potato ranges from 6.2 to 6.4. If the pH value is more alkaline (over 7), this may be an indication of frozen cells; if it is more acidic (below 6) it may indicate a fungal infection, for example.

However, frost resistance is not the only subject being investigated in this complex series of analyses. Christoph Unger is also investigating whether the rotting behaviour of the transgenic potatoes is different to that of conventional potatoes and whether this could potentially affect the quality of the farmland. Peroxidase is the enzyme of interest here. If the activity of this enzyme is high in the plant, it serves as a type of stress indicator, since the plant uses it to ward off pathogens, for example. In preliminary investigations the genetically modified potatoes exhibited higher levels of peroxidase following a period of storage. Reason enough for Christoph Unger to take a closer look. Increased levels of peroxidase could increase the stability of the cell walls, thereby reducing the speed of rotting. One of the tasks over the next two years will be to investigate this correlation.