Keeping brassicas free from caterpillar damage – without the use of synthetic pesticides – is the goal of research being undertaken at Crop & Food Research near Lincoln, Canterbury.
Research leader Dr Mary Christey has produced plants of these species using molecular techniques – genetic modification – so that the natural pesticide produced by the Bacillus thuringiensis bacteria (known as Bt) is produced by the plant.
Crop & Food Research applied to the Environmental Risk Management Authority (ERMA) for approval to undertake garden scale field tests in Canterbury of the pest resistant forage kale, cabbages, cauliflower and broccoli.
ERMA New Zealand has approved the application to field test genetically-modified Brassica in the Lincoln region. ERMA has included strict controls to manage the risk of GM material escaping from the site.
Bt Brassica: questions and answers
What are brassicas?
Cabbage, broccoli, cauliflower and forage kale are all varieties of the plant Brassica oleracea and all belong to the Brassicaceae or Crucifer family. Other members of the Brassicaceae include swedes (Brassica napus) and turnips (Brassica rapa). Together, these plants are commonly known as brassicas.
Are they related to any native New Zealand plants?
The plants to be field-tested (cabbage, broccoli, cauliflower and forage kale), are all introduced plants. They are not native to New Zealand and are not closely related to any native brassicas, therefore inter-crossing is not possible.
What are the main pests of Brassica crops?
The main pests of brassicas in New Zealand are the caterpillars of cabbage white butterfly, diamond-back moth and soybean looper. Numerous chemical control methods are available for these pests, but some (particularly diamond-back moth) have developed resistance to chemicals. This means that the pests are increasingly difficult to control.
There is growing interest in integrated control methods for these pests, and introducing genes that make Brassica plants resistant to insect pests is one potential tool.
What is Bt? Bt is a soil bacterium called Bacillus thuringiensis (Bt). Bt has been used to control insects for more than 30 years, because the bacterium produces large crystalline proteins that have insecticidal activity. When susceptible insects ingest this protein it is converted into an active toxin that interferes with their gut and kills them.
There are many strains of Bacillus thuringiensis, each producing different proteins that kill different insect groups. The particular Bt genes used in this research kill lepidopteran caterpillars such as diamond-back moth, cabbage white butterfly and soybean looper.
What are Bt brassicas? Bt brassicas are normal Brassica plants into which a gene has been inserted from the soil bacterium Bacillus thuringiensis. Producing such a genetically modified (GM) crop usually makes use of the gene-transferring ability of anothernaturally occurring soil bacterium called Agrobacterium tumefaciens.
In nature, Agrobacterium tumefaciens causes crown gall disease on plants such as grapes and roses. It does this by transferring a portion of its own DNA into the plant cells it infects. The bacterium’s DNA is inserted into the plant’s own DNA, and the expression of the bacterium DNA causes tumours or galls to develop on the plant and the formation of proteins to provide a food source for the bacterium. In other words, Agrobacterium tumefaciens is able to naturally genetically modify the plants it infects.
Scientists developed the concept of replacing the gall-causing genes with genes for a wide range of agronomically useful traits. For example, in our research Bt genes were introduced into Agrobacterium for transfer to plants to allow the production of caterpillar-resistant plants. Only a very small amount of DNA is transferred, about 3000 base pairs. This is minuscule compared to the total DNA content of a broccoli cell, which contains 1324 million base pairs.
Standard tissue culture techniques are used to transfer these genes into plant cells and to regenerate GM plants. These plants are then multiplied in tissue culture and grown under contained greenhouse conditions to produce seed following conventional plant breeding practices. See Figure 1 below for an explanation of the process and see Figure 2 to see how effective the Bt plants are.
Where does the DNA you are inserting actually come from?
The DNA sequence we are using is based on the Bacillus thuringiensis insecticidal gene, but the sequence of the DNA has been altered to ensure that it can express in plants.
Is Bt harmful to humans?
All scientific evidence indicates that Bt at the levels expressed by the plants is not harmful to humans, or to birds or other animals. Bt is very specific to particular insect pests.
The bacterium, as a spray, has been used in biological control of insects for over 30 years, and has a long history of safe use. In New Zealand, several commercial products are available that contain this bacterium. These products are routinely used for caterpillar control in orchards and on vegetables, and are also used by organic growers.
What are the potential environmental benefits of this research?
Reduced insecticide use is the main potential environmental benefit.
In 2004 vegetable brassicas in New Zealand were sprayed with 2.81 kg of active insecticide ingredient per hectare, making these vegetables the 4th highest in terms of insecticide use. Some insecticides currently used in New Zealand to control caterpillar pests and aphids are very toxic to bees, birds and aquatic organisms, and persist in the soil.
If our tests are successful it means Bt brassicas could possibly be grown with fewer applications of chemicals. In the US the average number of insecticide applications for cotton has decreased fivefold, largely because of the introduction of Bt cotton.
Won’t the insects develop resistance to Bt produced by the plants?
In research done so far, we have found that if you feed the Bt brassicas to diamond-back moth and cabbage white caterpillars they all die within 48 hours, leaving the plant virtually undamaged. If the caterpillars are killed before they reach maturity and breed, then resistance does not develop.
As a second line of defence against resistance, we also intend to test the effectiveness of putting two Bt genes, rather than just one, into the brassicas. Any insect pest that encounters the two different types of Bt gene would have to evolve resistance to both types simultaneously, and the biological chances of this happening are very small.
Is there potential for horizontal gene transfer – movement of the Bt gene from the brassicas into other organisms?
It is worth remembering, before considering the likelihood of horizontal gene transfer from Bt brassicas, that Bacillus thuringiensis is a soil bacterium and so already occurs naturally in the environment. Therefore, other organisms in the environment are already exposed to Bt genes by other means.
Plants containing Bt will expose the soil to higher levels of Bt than normal, but overseas in 2006 crops containing Bt were grown on over 30 million hectares of land with no reported adverse effects.
Nonetheless, our field test gives us the opportunity to test for horizontal gene transfer under highly controlled New Zealand-specific conditions.
What about transfer of pollen from the field plants to other non-GM Brassica crops?
The Bt brassicas will not be allowed to produce open flowers in the field. Those that start to form flower heads will be removed before mature flowers open. It is easy to tell when these plants are going to flower. Plants will be monitored closely in the field. Scientifically promising plants will be removed to the glasshouse to flower, so seed can be collected.
Could the Bt brassicas damage non-target, useful organisms?
There are concerns about the potential impact of Bt-containing plants on the soil ecosystem and also on non-target organisms, including pests and their natural enemies, feeding on and/or visiting Bt plants.
In particular it is important to determine whether the Bt toxins have any detrimental effect on biological control agents such as parasitic wasps that are important bio-control agents for biological control of cabbage white butterfly, diamond-back moth and aphids. Other non-target beneficial organisms that need to be assessed include ladybirds and aphid parasitoids. These beneficial non-target organisms may come in contact with the Bt toxin from the transgenic plants by feeding directly on the plant themselves, or indirectly by feeding on target or non-target herbivorous insects.
This field test aims to discover, in a controlled way, whether any of these concerns are justified. Several studies elsewhere on the effects of Bt on non-target organisms have shown little or no direct effects, and have shown beneficial effects in comparison with chemical use. Our initial laboratory tests have also shown only indirect effects.
How big will the test site be?
Initial field tests will be less than 0.05 hectares in size. The application asks for a larger test site – up to 0.4 hectares – later in the test period. The site will be secure, not just from people but also from browsing animals, to ensure that plants cannot be moved away from the test site.
Why apply for a 10-year test?
This time is necessary to progress the breeding work and undertake thorough evaluation.
Why do you need to do a field test now?
The Bt brassicas show good control of caterpillar pests in the laboratory and in the glasshouse. We’ve been doing this research in those contained conditions for more than 5 years, and to confirm the extent of caterpillar control we now need to grow them in normal, realistic field conditions.
What consultation has there been with Māori over this field test?
We discussed the field test with the local iwi Te Rūnanga ō Ngai Tahu. As suggested by Ngai Tahu, we also consulted with Te Taumutu Rūnanga, who are the kaitiaki rūnanga for the Lincoln area. We also undertook national Māori consultation by posting information, including an invitation to reply, to the Chief Executive or senior manager of 80 rūnanga throughout New Zealand.
Is this the first GM test you have done?
No. Crop & Food Research has undertaken 34 contained field tests since 1988 on a range of vegetable and flower crops, and we are very experienced in conducting field tests under strict controls.
Will Bt brassicas damage the organic Brassica industry?
During the research the brassicas will not be allowed to flower and so the test will have no effect on the organic Brassica industry.
But consumers don’t want GM food.
Ultimately, consumers will decide whether these plants have a commercial future. But unless we continue to do this research, under strictly controlled conditions, we can’t explore the opportunities for this technology for the future.
Do you plan to commercialise these crops?
No, we do not plan to produce a commercial variety from these tests. Our motivation, as a Crown Research Institute, is to research ways of producing crops in more sustainable ways for New Zealand industry and consumers. We continue to undertake research involving genetic modification, because it ensures we retain skills and it preserves options for New Zealand’s future.
If, in the future, New Zealanders accept GM food crops, and there is a market for brassicas such as these, commercialisation will take place in partnership with experienced commercial companies.
Figure 1: The sequence of events involved in producing a GM plant.
1 Brassica seed germinating in tissue culture. Leaves or stem segments are removed for production of transgenic shoots.
2 A culture of Agrobacterium grown overnight in a flask.
3 Freshly cut explants (an explant is simply a piece of a plant) after dipping briefly in the bacterial culture.
4 Explants several weeks later showing the regeneration of green, potentially transformed shoots, and white non-transformed shoots.
5 The small green shoots are then removed and cultured to enable the shoots to grow larger and to produce roots to enable transfer to the greenhouse.
6 GM cabbage growing in greenhouse.
Figure 2 (below): Time-lapse video clip over 12 days to compare damage done by diamond-back moth caterpillars to non-Bt and Bt brassicas. The non-Bt plant is on the left, and the plant containing Bt is on the right.
If you cannot see the video options above you may need to download Windows media player or Quicktime software to view.
Cabbage cv. Golden
Acre 2.5 weeks after
cabbage white butterfly
larvae have been
released. The plant on
the left has a Bt gene
and the one on the
right has not.
Working
with us
Cabbage cv. Golden
