Field trial: Oilseed rape with closed flowers

“After this year we will know whether cleistogamic oilseed rape is a suitable method for the biological containment of foreign genes.”

A flowering oilseed rape field near Braunschweig: While one half of the field glows bright yellow, the other half looks somewhat paler. This half is being used to grow cleistogamic oilseed rape, i.e. plants with flowers that do not open. The idea is to prevent the plants from releasing pollen into the environment. This trait could be used in future for the biological containment of foreign genes in genetically modified oilseed rape plants. This field trial is designed to show whether the method works in practice.

Staff from the Julius Kühn Institute are out in the oilseed rape field carrying out controlled pollination.

Project director, Alexandra Hüsken: “Cleistogamic oilseed rape could be a way of preventing, or at least reducing, outcrossing in GM plants.”

cleistogamic and conventional oilseed rape

Cleistogamic oilseed rape (right) with closed flowers. Left: a conventional oilseed rape plant with open flowers.

artificial pollination 1
artificial pollination 2
artificial pollination 3

Hand pollination:
(1) The petals and filaments are removed from the oilseed rape flowers, leaving only the stigma. (2) The exposed stigma is pollinated with a cleistogamic oilseed rape flower. (3) The inflorescence is covered with a bag to prevent pollen from other plants reaching the stigma.

“This is not our only experiment,” explains the project director, Alexandra Hüsken from the Julius Kühn Institute. “We have conducted these outcrossing experiments over several years at various other sites.”

Her six colleagues are busy working in different parts of the field. On a windy, clammy day they are trying to carry out controlled pollination of individual conventional oilseed rape plants either with the plants’ own pollen or with pollen from cleistogamic oilseed rape. When DNA analyses are carried out later in the year to see whether the cleistogamic oilseed rape has released pollen and fertilized plants on the half of the field with conventional rape, the seeds from the plants pollinated by hand will serve as control samples.

Chemical mutagenesis was used to induce cleistogamy in the oilseed rape plants used in this trial. The DNA analysis looks for a certain point mutation in the genome that is only found in the cleistogamic rape. If the cleistogamic rape outcrosses with conventional oilseed rape, this point mutation will be found in the resulting seeds.

Hand pollination requires skill and perseverance. Individual conventional oilseed rape plants are selected and any flowers that have already opened are removed with scissors. Only the young, still unopened buds are retained because these are the ones that have not yet been pollinated. The buds, which are not much more than 2 cm in length, are carefully prepared using tweezers. The petals and filaments, which would produce pollen later, are removed with care, leaving only the stigma, the female part of the flower. The flowers are then pollinated by hand. A cut mature flower from a cleistogamic rape plant is opened slightly and the stigma is inserted into it several times, so that the pollen from the cleistogamic flower attaches to the stigma. Then the pollinated inflorescences are covered with a bag to prevent them being fertilized by pollen from other plants. A white stick is placed in the ground to mark the location of the hand-pollinated plants on the field.

Is pollen release prevented completely?

“To be sure that cleistogamic oilseed rape really is a safe method of biological containment, these trials must be conducted under field conditions and a range of different environmental conditions,” says Alexandra Hüsken. The cleistogamy trait is not necessarily stable. It could be that individual flowers of cleistogamic rape plants open slightly under certain site or weather conditions, releasing pollen into the environment.

“This is the last year of the trial, so we will soon know,” she says. So will the cleistogamic oilseed rape be worthless if the cleistogamy trait does not prove to be totally stable? “No,” says Ms Hüsken, “because the trait can still have a practical use.” The plant would still be of interest for restricting the outcrossing of GM rape, e.g. plants with a modified fatty acid pattern. In this case, cleistogamy would not be used to prevent outcrossing altogether, but to facilitate coexistence. “If cultivation of cleistogamic GM rape makes it easier to comply with the threshold values for GMO levels in neighbouring fields, that is already worth something.” These oilseed rape plants would not, however, be suitable for guaranteeing biological containment of foreign genes that produce active pharmaceutical substances.

But Ms Hüsken sees potential even in this area: “If the cleistogamic oilseed rape plants do not yet contain the pollen entirely, it is possible that the trait could be improved and stabilised.” Cleistogamy is, she explains, dependent on a number of genetic factors and cold be optimised through further breeding stages.