Sterile plants: The key to hybrid varieties
Hybrid varieties of maize have been widely used for many years. Plants raised from this type of seed are larger, more productive and more vigorous than those from conventional varieties. To obtain hybrid seed, two inbred lines must be crossed with one another. For breeders, this is an expensive, time-consuming procedure which is greatly simplified if they are able to use sterile plants.
Hybrid forms of some crop varieties were discovered in the natural gene pool some time ago and used for hybrid breeding . Today it is also possible to produce breeding lines without viable pollen using genetic engineering. This concept could also be used to prevent the spread of foreign genes.
For decades plant breeders have used conventionally bred plants which produce no pollen, or no viable pollen for controlled pollination in seed production. These types of "male-sterile" sugar beet, maize, sunflower and oilseed rape plants are unable to fertilise themselves or other plants.
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However, to ensure that these sterile plants still produce seed, they must be fertilised with pollen from another, non-sterile plant line. Homozygous mother and father lines, each with specific characteristics, are brought together, enabling breeders to obtain the particularly productive hybrid varieties. Hybrids have a broader spectrum of genetic information than purebred lines, because the different alleles (gene variants) of the parents are combined. The offspring of two homozygous lines are characterised by greater productivity and vigour. This phenomenon, which can be observed throughout the plant and animal kingdom, is known as the heterosis effect . However, so far it has not been possible to produce sterile lines for all crop varieties using conventional breeding techniques. Producing hybrid seed from these crops is therefore impossible or extremely time-consuming and expensive. Genetic engineering can be used to overcome this limitation. |
Barnase-barstar system: Hybrid varieties of oilseed rape
In the 1990s the Belgian company PGS (Plant Genetic Systems) developed a genetic engineering system to engineer pollen sterility. This was used predominantly with GM oilseed rape varieties. These have been grown extensively in Canada for years. In 1997 the EU also approved two oilseed rape lines and their offspring under the designation MS1xRF1, which contain the PGS sterility system in addition to herbicide resistance to the active substance glufosinate.
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MS (male sterile) is the breeding line which produces no viable pollen.
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RF (restore fertility) is the second breeding line which cancels out the sterility of the first line.
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Hybrids derived from the MS and RF lines produce fertile pollen.
The sterility of the MS line can be attributed to a newly introduced gene derived from the bacterium Bacillus amyloliquefaciens, whose activity leads to cell death. This toxic effect is caused by a special enzyme (barnase), which cleaves the RNA molecule in the cells. To ensure that only the male flower parts are affected, the toxin gene is linked to a genetic switch, or promoter . This switch activates the gene only in specific cells that are needed for the development of the male inflorescence. As a result, no pollen is produced on the filaments (anthers) (see diagram below).
The RF lines contain a gene for an active substance (barstar), which neutralises the RNA-cleaving barnase. The gene for this inhibitor was also isolated from B. amyloliquefaciens.
In the offspring that result from crossbreeding the MS and RF lines, although the barnase and barstar genes are both active, they cancel each other out: Breeders obtain "normal", viable seed.
The barnase-barstar system is also used with other crop plants. With potatoes it is to be used to confer resistance to fungal diseases. It is also used in seed sterility systems.
No pollen production with the barnase gene: In addition to the pistil with the stigma (the female inflorescence), a hermaphrodite flower also produces filaments (anthers). This male inflorescence is responsible for pollen production.
Sterile plants in agriculture
The aim of the PGS system was to simplify the breeding of high-yield hybrid varieties. In principle it can also be used to biologically contain genetically modified plants and so prevent the spread of foreign genes. However, only the barnase gene is needed for this.
Male-sterile maize, oilseed rape, tobacco, sugar beet, sunflowers, potatoes, tomatoes, wheat and cauliflower have already been produced as a result of introducing this gene. However, in agricultural practice problems are associated with the cultivation of these plants. Plants grown for their seed or fruits must first be pollinated, but where does the pollen come from, since its development has been so successfully prevented?
One option is to grow the sterile GM plants together with normal non-sterile plants in one field. The conventional plants then serve as pollen donors and pollinate the GM plants. However, heavy yield losses can be expected if the flowering periods of the two plant varieties do not coincide.
Crops that are grown for their vegetative parts (leaves, beets, tubers, wood), on the other hand, do not need to be pollinated. In these cases, male-sterile varieties can be used for agriculture without any additional measures.
More from GMO Safety
- Pollen sterility: Plants without pollen
- Potatoes: Resistance to the Phytophtora fungus. The barnase-barstar system
- Seed sterility: Seeds that don't germinate
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