Rightly or wrongly, concerns over GM crops contaminating their non-GM kin
have led to a stalemate in progress towards acceptance of GM crops and
foods. We look at a new approach using gene silencing and gene imprinting.

There are no genetically modified triffids stalking suburbia, and no
unkillable "superweeds" choking the farm. But for nervous consumers in
Australia and around the world, the spectre of transgene escape from GM
crops looms larger than any real-world threat.

More than a decade of anti-GM scaremongering has persuaded many consumers
that transgenic crops pose unacceptable risks to their long-term health, and
to the environment, scientific and epidemiological evidence to the contrary.

In a study published in Science in 2002, Dr Mary Rieger of the Australian
Weeds Cooperative Research Centre in Adelaide, showed the risks of transgene
introgression into weedy crop relatives creating "superweeds" were very low.

Rieger found that hybridisation between GM herbicide tolerant canola and the
wild radish (Raphanus raphanistrum) in the field is very rare - among 53
million seedlings raised from Roundup-Ready canola, she found only two
herbicide-tolerant hybrids.

The public's concerns are twofold: that pollen from transgenic crops will
contaminate nearby conventional and organic crops, and that GM crops will
hybridise with weedy relatives, creating intractable superweeds.

GM crops have now been grown for more than a decade, with no adverse
episodes for human health, no greater impact on the environment than their
non-GM counterparts, and, in most cases, real environmental benefits.

But the anti-GM movement's concerns have eroded consumer confidence in GM
crops and foods, and there are good commercial reasons to devise an
effective way of preventing transgene escape through wind-blown or bee-borne
pollen.

Physical containment of transgenic crops and their pollinators, or the use
of extended buffer zones to isolate GM crops from conventional and organic
crops, are costly and impractical. Gene technology itself proffers a
low-cost genetic solution.

At CSIRO Plant Industry's Merbein research laboratories in north-western
Victoria, Professor Steve Swain, Dr Davinder Singh and Dr Angelica Jermakow
have devised an ingenious, broadly applicable strategy to prevent transgene
escape from GM crops.

It exploits a combination of post-transcriptional gene silencing - RNA
interference - and "gene imprinting": natural, methylation-induced
suppression of particular genes, according to whether they are inherited
from the female or male parent.

The CSIRO researchers have demonstrated their system in an Arabidopsis
model. A basic difference from the Technology Protection System developed in
the US in the late 1990s is that the transgenic plants still produce viable
seed - provided similarly-engineered cultivars cross-pollinate with each
other.

If GM pollen is transferred to a non-GM crop of the same species, or a weedy
relative, fertilisation will fail and no viable seed will result.

The system would prevent contamination of non-GM crops, prevent transgenic
crops hybridising with weedy relatives, yet still allow farmers to save GM
seed for replanting the following season.

In the case of crops like maize and sunflowers, farmers have not saved seed
since the 1930s because F1 hybrid seed does not "come true" to type. After
initial resistance to the new-fangled hybrids, farmers gave up saving seed
in return for the superior yield, disease resistance and profitability of F1
hybrids.

The Merbein research team's new system addresses two of the anti-GM
movement's major objections to GM crops: there would be no "contamination"
of non-GM crops, and no superweeds.

But because the solution to these objections depends on the very technology
to which anti-GM activists object it seems unlikely that the anti-GM
movement would endorse its use.

There would also be an issue if companies developing GM crops would have to
find a way to levy fees for re-use of their proprietary technology if
farmers saved GM seed from season to season.

Pollen and ovule
The approach involves introducing two transgene constructs into the selected
crop or cultivar, attached to promoter sequences from early-acting
developmental genes expressed in the endosperm of the developing seed, with
an essential role in seed development.

The two genes are from the MEDEA (or MEA) polycomb gene group in
Arabidopsis, which are also known as Fertilisation Independent Seed 1
(FIS1), and Fertilisation Independent Seed 2 (FIS2).

One transgene construct comprises the protein-coding sequence of a
seed-lethal gene - yet to be selected - under the control of the MEA
promoter. For demonstration purposes, Swain's team used the GUS reporter
gene as a proxy.

The second transgene uses the FIS2 promoter to drive expression of a
so-called hairpin gene, designed to silence expression of the seed-lethal
gene via RNA interference. The construct codes for a double-stranded hairpin
RNA molecule that programs the plant's cells to destroy the messenger RNA of
the seed-lethal gene, blocking synthesis of the encoded protein.

The ingenuity of the approach lies in the fact that, in the MEA:GUS
construct, the MEA promoter begins to drive expression of GUS 48 hours after
pollination when it is inherited from the male (pollen) parent.

If it is inherited via the female (ovule parent), it is expressed before
fertilisation occurs, and again after pollination.

In contrast, the FIS2:GUS construct, designed to silence GUS, is expressed
throughout seed development when maternally inherited, but is repressed by
imprinting when inherited from the pollen parent.

On self-pollinated plants or transgenic plants fertilised by pollen from
non-transgenic plants, the seeds develop normally because the FIS2:hairpin
gene prevents the seed-lethal gene being expressed throughout seed
development.

But if transgenic pollen finds its way onto the flowers a non-transgenic
crop, or a weedy relative, seed development aborts because the non-GM seed
parent has no inbuilt RNAi defence against the MEA:seed lethal gene carried
by GM pollen.

One of the intriguing aspects of the system is that the system would not
prevent fertilisation if the GM crop were fertilised by stray pollen from a
non-GM crop, or by weedy relatives. In the first instance, this could make
non-GM farmers, including organic farmers, liable for "contaminating" GM
crops and compromising farmers' ability to save and re-sow GM seed for which
they have paid a premium.

This would turn a key strategy of the anti-GM movement on its head: the
threat of legal action if pollen from GM crops "contaminates" conventional
and organic crops, compromising their GM-free status, which according to
activists, attracts a premium in international markets.

Swain says the CSIRO system could be adapted to prevent non-GM pollen
fertilising GM crops.

But in practice, GM farmers would be unlikely to require this type of
protection against non-GM pollination, because the Rieger study has shown
that cross-pollination between weeds and GM crops is extremely rare, and
would involve no risk to human health.

And in a field crowded with self-pollinating GM plants, the likelihood that
pollen from any weedy relative, or even a nearby non-GM crop, will pollinate
a single GM flower, is very small, and would poses no credible threat to
human health.

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