|
A New Glyphosate Tolerance Gene
Chad
Lee, University of Kentucky
Crops
with the popular brand name of Roundup Ready® may have
some competition in a few years. Scientists have developed a new set
of genes that allows plants to tolerate glyphosate herbicide.
A team
of researchers associated with three private companies reported
their success story in the May 21, 2004 edition of Science
(vol. 304, pp. 1151-1154). The newly discovered gene strategy makes
it possible for plants to produce an enzyme which deactivates
glyphosate. Crops that can deactivate glyphosate could be sprayed
with this herbicide at much later growth stages, such as during
reproductive development. Current Roundup Ready crops tolerate
glyphosate because they have been inserted with a gene that encodes
for a key enzyme called EPSPS that inhibits glyphosate. Although the
EPSPS enzyme in Roundup Ready crops is not inhibited by glyphosate,
it does not deactivate the herbicide. Since Roundup Ready crops do
not deactivate glyphosate, they must be sprayed at earlier growth
stages.
The
researchers found this gene in strains of the bacterium Bacillus
licheniformis. Although the original or native bacterial gene
they discovered encoded for an enzyme that deactivated glyphosate,
the enzyme did not work fast enough to be effective against field
doses of glyphosate. The scientists used methods known as “DNA
shuffling” and “functional selection” to basically build a gene.
They built this gene by recombining different parts of the native
gene from different bacterial strains.
Scientists use DNA shuffling to build new genes based on the
rearranging and recombining of the original gene. Scientists use
functional selection to screen the products of these new gene
combinations for activity against glyphosate. The most promising
native genes are “selected” and then used to build new genes. This
process of building new genes and screening the products of genes is
advanced by repeating cycles of shuffling and selection until the
scientists generate and identify the genes they want.
In the
case of the new glyphosate tolerant gene, scientists completed the
gene shuffling and functional selection processes 11 times. By the
11th cycle, scientists were able to develop a gene that
coded for an enzyme with almost a 10,000-fold increase in activity
against glyphosate compared with the original B. licheniformis
gene.
As the
shuffling and selection cycles were repeated, they resulted in corn
plants with increasing degrees of tolerance to glyphosate. For
example, corn plants with genes from the 5th cycle
tolerated 104 fl oz/acre of Roundup UltraMAX but showed some injury
symptoms. Plants with genes from the 7th cycle showed
very little injury to glyphosate. Corn plants with genes from the 10th
and 11th cycles survived 156 fl oz / acre of Roundup
UltraMAX and showed no injury symptoms.
The
scientists are currently testing corn plants with genes from several
of the shuffling cycles to determine if the genes will be
commercially viable. The scientists made no predictions about when
this new source of glyphosate tolerance could hit the market.
However, some of the members of the scientific team are employed by
Pioneer Hi-Bred. Based on the fact that these scientists are
associated with Pioneer and that they are reporting this story now
indicates to me that we could see some new competition in the
glyphosate tolerance market in five or six years.
Back to Grain Crops Main Page |
Report Archives | 
