Top Crop Manager

Features Herbicides Seed & Chemical
New herbicide tolerance system unveiled

Uses new glyphosate tolerance trait.

November 14, 2007
By Top Crop Manager


Corn and soybean producers will soon have a new herbicide tolerance production
system available for their toolbox. Optimum GAT, a new trait developed by Pioneer
Hi-Bred International, a subsidiary of DuPont, will take herbicide tolerance
to a new level. It will be the first system on the market to combine glyphosate
tolerance with tolerance to ALS (Group 2) herbicides.

In addition to excellent glyphosate tolerance, this new trait will allow growers
to complement their glyphosate with Group 2 herbicides for additional contact
and residual control on grassy and broadleaf weeds. Ultimately, the system will
provide growers with the flexibility to tailor weed control programs to local

Soybeans with the Optimum GAT trait are on-track to be on the market in North
America as early as 2009. Corn hybrids are still in development and are expected
to be available soon after soybeans with the Optimum GAT trait are launched.


The ALS family of Group 2 herbicides includes sulfonylureas (SU), imidazolinones
(IMI), triazolopyrimidines (TP) and pyrimidinylthiobenzoates (PTB).

ALS tolerance has long been used by DuPont as a marker trait for research.
Meanwhile, Pioneer has been working to develop a completely new glyphosate resistance
trait. The next big step was to stack the two traits together to create the
Optimum GAT system. The name Optimum GAT is derived from 'Glyphosate ALS Tolerant'.

"The Optimum GAT glyphosate tolerance trait is not the same as Monsanto's
Roundup Ready trait," says Linda Castle, research co-ordinator for Pioneer
in Redwood City, California. "Current glyphosate resistance technology
changes the protein that glyphosate inhibits to make it tolerant to the herbicide.
Optimum GAT works by detoxifying the glyphosate within the plant so that the
herbicide doesn't inhibit the native target protein."

Herbicide resistance tool
The combination of Group 9 and Group 2 tolerance in one system gives growers
two different modes of action to control tough weeds. Wild buckwheat, lamb's
quarters, ragweed and horsetail are just a few of the weeds that can be hard
to control with glyphosate alone. More importantly, the combination of a Group
9 (glyphosate) with a Group 2 (ALS) will reduce the risk of developing herbicide
resistant weeds.

First system developed through gene shuffling
The Optimum GAT system originated when a new bacterial enzyme was discovered
that could modify glyphosate so it would no longer be toxic to plants. However,
when the new gene was inserted into corn and soybeans, it was not strong enough
by itself to give true herbicide tolerance.

"A lot of times, the native genes we find in bacteria or fungi are just
not good enough, so we use gene shuffling technology to make them better,"
Castle says. "In the case of Optimum GAT, we used it to improve the enzyme's
activity on glyphosate to make the plants tolerant. It took three years of research
to do it but when we were done, we had plants that expressed very robust glyphosate
detoxification and tolerance."

Gene shuffling is like super plant breeding at the genetic level. It works
to improve a trait's potential for success by improving the functionality of
the gene. With basic plant breeding, researchers select two parents with desired
traits and then check the offspring for improvements. Gene shuffling works the
same way, but at the genetic level.

"We have a parent gene, in this case the glyphosate acetyltransferase
gene. From this we created a number of progeny genes, which are sequence variants
of the original," Castle says. To grasp the basic concept of gene shuffling,
try thinking of a gene as a linear alphabet: A to Z. Many variants
of the same alphabet can be made by substituting one or more letters in bold,
(A to Z). Each of the progeny is then checked for improved activity. The genes
that demonstrate improvements become the next generation of parents and the
process is repeated several times.

"Traditional breeding lets you screen a few variations at a time,"
Castle says. "Shuffling technology allows us to screen millions of variations
at one time. Imagine taking millions of slight variations of a corn plant and
screening them all at once. This technology allows us compress hundreds of generations
of research into a very short time frame. Additionally, many of these traits,
like glyphosate detoxification or insect tolerance, are traits you could never
get from traditional plant breeding."

Optimum GAT is the first-ever agricultural trait developed through gene shuffling.
DuPont and Pioneer hold the exclusive right to use gene shuffling for agricultural

Major shift for Pioneer
In the past, Pioneer has worked with other companies to co-develop traits. Optimum
GAT marks the first time that Pioneer has developed its own trait. It will not
be the last. Prior to this, the company also has licensed traits, like Roundup
Ready and Bt, from other companies and inserted them into its own germplasm.

"Now that we've integrated the gene shuffling technology that we used
to develop Optimum GAT into our breeding program, we plan to use it to rapidly
bring new hybrids and varieties to market," Castle says. "Pioneer
already has the most extensive, elite germplasm base in the industry and has
80 years of experience developing new hybrids and in-breds. We are currently
using gene shuffling to improve insect and disease resistance, herbicide resistance,
nutritional improvements and of course yield enhancements. It's a truly remarkable
tool." -30-