Biological Approaches to Preventing Gene Flow

Farmers will sometimes need to take measures to manage the spread of pollen from transgenic crops. These could include observing required separation distances or surrounding transgenic plantings with buffer strips of conventional crops. Although such physical measures can be useful, there are also biological ways to prevent pollen traffic. Such approaches could prevent the release of transgenic pollen in the first place. No single strategy, however, will be applicable to all crops. That’s why researchers are investigating several different possibilities that could reduce pollen flow for several of Europe’s most important crops.

Keeping flowers from opening in the first place

Cleistogamous rapeseed flowers opened to varying degrees
Cleistogamous rapeseed flowers opened to varying degrees
One simple way to keep transgenic plants from releasing their pollen is to ensure that their flowers do not open. Some naturally occurring plants have flowers that never open, but are still able to self-pollinate and set seed. These are known as cleistogamous plants. Different lines of cleistogamous rapeseed were developed by induced mutation at a lab of the national institute for agronomic research (INRA) in France. Cleistogamous rapeseed could be useful for reducing the spread of transgenes. One project is looking to find out how reliably cleistogamy prevents gene flow in rapeseed and to see if cleistogamous, transgenic rapeseed could be a good way of securing biocontainment.

The prospect of pollen-free maize

Normal maize tassels are loaded with pollen (left). CMS maize, however, is pollen free (right).
Normal maize tassels are loaded with pollen (left). CMS maize, however, is pollen free (right).
Some naturally occurring plants are male sterile. This means they are unable to produce functioning male flowers and cannot release viable pollen. If GM crops were made male sterile, out-crossing could be effectively nipped in the bud.

Male sterile maize lines are already used by seed companies to produce hybrid seed. Hybrids are plants derived from distantly related parents, which for many species results in more robust offspring. This phenomenon is known as hybrid vigour or the heterosis effect. To take advantage of this, hybrid cultivars are produced by crossing two distantly related lines and collecting the resulting seed. Hybrid seed producers must find ways to prevent the parental lines from self-pollinating. For maize, this is usually done by manually removing the male flowers from one of the parents, a process known as detasseling. Sometimes, seed producers save themselves the trouble by growing male sterile maize instead.

Male sterile maize seems like a great way of preventing the spread of GMOs. It has even been known for decades that male sterile maize plants can have higher yield. The strategy, however, has one major setback. It has to do with the fact that most crops need pollination to produce seed and fruit. An entire field of male sterile maize, for instance, would never be practical. Without pollen, the ears wouldn’t be able to develop kernels.

One way to overcome this could be to mix male sterile maize together with conventional maize that can still produce pollen. The non-transgenic maize would act as pollen donors to fertilise the transgenic plants. If pollen drifts to neighbouring fields, it wouldn’t be a problem for co-existence.

Particularly in the case of maize, planting conventional pollen donors offers some unique advantages:

Securing GM sunflowers

Male sterility could also be useful for preventing the spread of pollen from transgenic sunflower. Unlike maize, no genetically modified sunflowers are commercially grown in Europe today. This could change, however, even in the not-so-distant future. Transgenic sunflower with resistance to important fungal diseases has already been developed. In addition, sunflower is a good candidate for genetic engineering to produce high-value compounds in its oil. Co-Extra researchers will be studying male sterility in sunflower to see if it might be a good way of preventing the spread of transgenes.

Biocontainment for transgenic tomatoes

Male sterility is also a biocontainment option for tomato. Besides transgenic tomatoes with delayed ripening, which were first commercialised in 1996, GM tomatoes have also been developed with resistance to viral diseases, enhanced nutritional value, resistance to insect pests, and better ability to thrive in salty soils. In fact, 75 field trials with transgenic tomatoes have taken place in the EU since 1992. Co-Extra researchers will be testing male sterility to see if it can keep transgenes from spreading to other tomato plants.

Locking foreign genes out of pollen

A new breakthrough approach has enabled a third technique for reducing gene flow. Although technically more challenging, genetically transforming plastids offers several advantages over conventional plant genetic engineering.

Most of the DNA in a plant cell is found in the central cellular compartment known as the nucleus. Every plant cell, however, has more locations for genetic information. This information is found in organelles known as plastids and mitochondria. Plastids occur in several subtypes, each carrying out specialised functions. The most important type of plastid is the chloroplast, the site of photosynthesis. Every green plant cell contains dozens of chloroplasts. Although it’s easier to genetically engineer nuclear DNA, foreign genes can also be added to the DNA contained in plastids. Plants with genetically engineered plastids are called “transplastomic”.

Transplastomic plants are different than traditional transgenic plants in several ways. For one, foreign genes can be expressed stably at much higher levels in transplastomic plants.  Another important difference enables a new possibility for containing foreign genes. Pollen from some plant species is free of plastids. The fertilised seed obtains all of its plastids from the female parent. If transgenes are only contained in chloroplasts, pollen is typically transgene-free.

Genetically engineering plastids remains a technical challenge. To date, researchers have only succeeded at transforming plastids in a few species such as tobacco, potato, and tomato. Another drawback is that plastids can occasionally make their way into pollen, which is called “paternal leakage”. Researchers are currently assessing transplastomic plants to see how reliably foreign genes are actually contained.