Jul 4, 2005

Research Projects

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Transgenic fructan potatoes – stress tolerance and fitness compared to conventional potato strains

(2001 – 2004) Justus-Liebig University Giessen, Institute of Agronomy and Plant Breeding I

Topic

The aim of the project was to analyse the yield formation and fitness of fructan-forming potato strains in comparison with conventional varieties.

The investigations focused on the following questions and hypotheses:

• Does the accumulation of fructans and easily soluble sugars in the tubers lead to increased cold tolerance and therefore better winter survival of the potatoes outdoors?
• Are ecological consequences to be expected from this, in that the potato tuber that now survives the winter may serve as a reservoir for harmful organisms (viruses)?
• Is an increase in potato volunteers to be expected from the cultivation of fructan-forming potatoes?
• Do fructan-forming tubers develop in a different way, are they more stress-tolerant or more stress-prone?
• Are fructan-forming potatoes capable of propagation by seed under field conditions?
• Does the altered carbohydrate metabolism in the fructan-forming potatoes lead to a different composition of substances in the leaves? If so, does this have consequences for insect pests like the potato beetle that colonize the leaf area?

Summary

The investigations into the fitness of genetically modified fructan-forming potatoes showed that vegetative propagation of potatoes and survival over several years is possible under field conditions. There are, however, no differences between conventional and genetically modified strains. Potato seeds also have a high germination ability. The plants that develop from them are, however, very susceptible to frost and are not capable of surviving the winter months. A shortage of water and plant nutrients (N, P, K) leads to morphological, physiological and substance alterations in the plants. These do not, however, lead to increased environmental risks from genetically modified fructan-forming potatoes.

Fructan potatoes have a shorter main shoot and delayed juvenile development, which may make it necessary to increase weed control.

Experiment description

The questions listed above were investigated by means of experiments under both outdoor and greenhouse conditions.

The following questions were researched at the Gießen (university) site using a large number of phenotypic and yield-relevant inspection criteria:

• Germination and survival behaviour of tubers and seeds (pot and field experiments)
• Reaction to dryness (pot experiments)
• Reaction to nutrient shortage (nitrogen (N), phosphorus (P) and potassium (K)) (pot experiments).

The investigations were carried out on selected transgenic strains and compared with the wild type and conventional varieties.

Results

Survival behaviour

Potato tubers can survive the winter under local conditions if frost does not penetrate far into the soil. These potato plants then represent weeds and may act as a reservoir for pathogens in the succeeding crop. There was no difference in the survival behaviour of the tubers in the soil between conventional potatoes and genetically modified fructan potatoes.

Seed propagation

Under both greenhouse and outdoor conditions potatoes are capable of forming generative reproductive organs (seed-containing berries), which are also germinable. There are differences between the varieties/genotypes regarding the size and germinative power of the seeds. Under greenhouse conditions 80 to 100 per cent of the set seeds were germinable after a germination time of about two weeks. The seeds of conventional varieties had the same germinability as the seeds of genetically modified potato strains.

Frost impact (-17 °C) on dry seeds did not affect the seeds’ germination ability. Frost impact on soaked seeds, however, led to a drastic loss of germinability.

T2 plants – the second generation of progeny from the transformed plants propagated via seeds – can survive moderately cold temperatures (-2 °C) following prior acclimatisation and can regenerate. There was no difference in the impact of frost on conventional and genetically modified strains.

Reaction to dryness

Drought stress in potatoes leads to morphological and physiological changes in the plants. Water deficiency results in a drastic reduction in the tuber and leaf mass and an increase in roots and stolons. Water deficiency caused the starch and fructan content in the tubers to increase. The reactions of the genetically modified potatoes to drought stress did not deviate from those of conventional varieties.

Reaction to nutrient deficiency (N, P, K)

A deficiency of the nutrients nitrogen, phosphorus and potassium led to characteristic deficiency symptoms even in the transgenic potato plants. The tuber yields were considerably reduced by the nutrient deficiency. The shortage of nitrogen led to an acceleration of the plant ripening process and to a comparatively high starch content in the tubers. Potassium deficiency led to a higher soluble sugar content in the leaves and to increased starch content in the tubers.

The transformants displayed conflicting reactions to K deficiency. Singe transformants (one gene transferred) reduce the fructan content in the leaves, while double transformants (two genes transferred) increase the fructan content.

Competitive behaviour of the plants

Genetically modified fructan potatoes possess a shorter main shoot and demonstrate delayed juvenile development. These characteristics point to a reduced competitiveness of the plants against e.g. weeds in the field crop, which means that these potato plants may have disadvantages compared with wild plants and the need for weed control measures (herbicides, mechanical cultivation) could be higher.

Plant substances

Genetically modified fructan potatoes have a lower starch content in the tubers. The reduction in starch is compensated for by the formation of fructans, which can represent up to five per cent of the dry weight of the tubers.

Compared with the unmodified parent variety, the leaves of genetically modified potato plants contained twice the amount of soluble carbohydrates, of which 0.5 to 0.9 per cent (dry weight) consisted of fructans.