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Ecological research into possible environmental risks of genetically modified virus-resistant sugar beet (Main focus: Environmental behaviour of transgenic sugar beet)

(1992 – 2000) RWTH Aachen University; Chair of Biology V

Topic

Three successive projects investigated whether genetically modified sugar beet with resistance to rizomania, a disease triggered by viruses, behaves differently in the environment to conventional beet.

The project focussed on three key topics.

Key topic (1): Environmental behaviour of transgenic sugar beet

Research topics:

  • Can transgenic sugar beet overcome weeds? Does the virus resistance give it a higher competitive strength in the case of rizomania infection? Is there any effect on germinability?
  • How do beet overwinter and persist? Are there differences for transgenic beet?
  • Does outcrossing (gene transfer) take place to relatives like the mangold, beetroot and wild beet? How much gene transfer occurs between wild, cultivated and volunteer beet?
  • Which plants and arthropods are found in the vicinity of the release plots? Are there differences compared with similar areas?

Summary

Environmental behaviour of transgenic sugar beet

There are no differences between the transgenic and isogenic non-transgenic sugar beet, either in their germinability or their overwintering and persistence. When infected with the virus, the virus-resistant beet were able to use their ecological advantage for increased biomass production only in the first stage of life. With regard to progeny (seed production), however, once again no differences were found compared with the isogenic controls.

Ecological basis data on sugar beet

  • Overwintering: Sugar beet can survive mild winters. Long periods of frost greatly reduce the overwintering capability.
  • Persistence: Conventional sugar beet was unable to survive two years in undisturbed fallow ground.
  • Gene transfer between cultivated and wild beet does occur. This means that a transfer of the virus-resistance gene is possible. Since the rizomania virus has not yet been found in wild beet populations, the wild beet does not gain any selection advantage through the virus resistance gene.

Experiment description

Various experiments and studies were carried out at two release locations – with and without virus infection. They included:

  • Transgenic sugar beet with resistance to the rizomania virus
  • Non-transgenic control variety (isogenic strain)
  • Conventional virus-resistant sugar beet variety

Long-term observation sites were selected in the vicinity of individual release plots. At these sites observations were carried out over several years, focussing in particular on certain arthropods, in order to detect changes in the species composition.

Results

Survival and persistence ability

No indications of altered naturalization and dispersal behaviour were found in the transgenic virus-resistant sugar beet (see table).

Characteristic Experiment method Result
Competitive strength White goose-foot (Chenopodium album), a typical weed, was planted in various densities on fields with conventional or transgenic sugar beet. Serious virus infection: the transgenic plants fared better in competition with the weed than the non-transgenic ones. They form more biomass. Weaker or no virus infection: transgenic plants fared worse than reference plants.
Germinability Seeds of transgenic and conventional sugar beet were buried. Their germination rate was assessed after one and two years. No differences between transgenic beet and conventional isogenic control lines.
Overwintering and survival ability Transgenic sugar beet and reference plants were left unharvested on the field in the autumn under virus infection and infection-free conditions. The surviving plants were examined the following spring. No differences between transgenic and conventional plants. Sugar beet can survive mild winters. They do not usually survive frost periods below -4° C.
Conventional sugar beet was sown and observed over two years with no further cultivation (fallow). No persistence found
Survival ability through seeds - bolting Study of bolting (more detailed description below) Seed weight Germinability: Seeds of transgenic and conventional sugar beet were buried. Their germination rate was assessed after one and two years. No noticeable differences between transgenic beet and conventional isogenic reference lines.

Persistence ability in seed form (bolting, seed weight, seed dormancy)

Sugar beet can also survive cold and unfavourable weather conditions in the form of seeds. For this to occur, the beet must flower early (bolting) and produce seeds (annual habit).

To investigate the seed dormancy (pause in development) of transgenic and conventional sugar beet seeds, experiments were set up in 1997 in which seed bags were buried at a depth of 30 cm. A first random sample was taken in 1998. The seeds remaining in the field were dug out in 1999 and the germination rate was assessed. The number of transgenic seedlings was also determined using the herbicide resistance trait.

  • Bolting: No significant differences were found between transgenic and isogenic control plants. All the hybrids investigated showed increased bolting when attacked by the virus. This is attributed to location factors and the milder winder climate at the infection site.
  • There were no obvious differences in the total weight of seeds produced . Neither the plant types investigated nor the infection conditions had any effect.
  • Seed dormancy: The experiment results indicate that the genetic modification does not lead to an increase in seed dormancy. The longer the seeds remain in the ground, the smaller the proportion of transgenic plants (In the sample taken in 1998, the number of seedlings was so small that it was not possible to determine the proportion of transgenic plants.).

Gene transfer to other plants (vertical gene transfer)

Unlike sugar beet cultivation, seed production involves flowering of the sugar beet plants. As part of the experiment, large quantities of beet and mangold seeds were produced.

Receptor plants (fertile beet) were used to assess the outcrossing in seed production at distances of one metre and 1100 metres (Various protection measures against uncontrolled pollen dispersal were implemented.).

Honey from hives adjacent to the sugar beet flowers was examined for beet pollen.

Wild relatives from the vicinity of seed production were examined in the greenhouse and in the field for evidence of gene transfer (e.g. acquisition of traits like pollen sterility, red or hairy leaves). The plants were tested for the rizomania virus (ELISA).

  • At a distance of one metre from the flowering pollen donors, the receptor plants had two per cent transgenic progeny. At a distance of 1100 metres east and west there was zero per cent. The transgenic pollen donor flowered very poorly in 1995, but in the following year (1996) no gene transfer was found over 1100 metres despite relatively good pollen production by the transgenic plants.
  • In a similar experiment in 1998 with pollen-sterile receptor plants, up to 75 per cent transgenic progeny were found at a distance of one metre, and up to 40 per cent transgenic progeny at a distance of 200 metres.
  • Three 10-gram samples of honey contained neither beet pollen nor hemp pollen (from the ring of hemp used as a protective strip). There was no beet pollen in the honey, despite the fact that honey bees were observed making regular trips to beet flowers.
  • The virus resistance is transferred to mangold, beetroot and wild beet through cross-pollination. This means that outcrossing must be expected with large-scale use of sugar beet. Incrossing of genes from cultivated forms to wild beet has already occurred (e.g. red colouring from beetroot). However, since the virus was not detected in the places where wild beet was found, this resistance does not offer any selective advantage there.

Long-term observation sites (long-term monitoring)

Normal cultivated areas in the vicinity of the release fields were selected for long-term observation. The plant species on these areas were recorded each spring and summer. The arthropods collected in pit traps were assessed according to number and species.

For comparison, release fields with transgenic sugar beet and conventional fields were included in the monitoring experiment.

Since ground beetles (carabids) were the most common, from 1994 onwards only these were trapped and assessed. The assessment took account of sites with and without virus infection.

  • No beet or beet relatives were found on the observation sites.
  • Mites and hymenopterans and spiders represent significant proportions of the arthropods.
  • Ground beetle species typical of arable fields were found everywhere; including species typical for beet fields and on the wheat field a species that prefers living on grain crops.
  • Overall, no significant differences in species diversity or the abundance of the individual species were found. Because of the small area, the results should be interpreted with care.