Fayetteville, Arkansas
May 10, 2004
- Farmers pay several times
more for seed of transgenic crop varieties than seed of
varieties developed by conventional crossbreeding. The higher
price is partly due to the high cost of producing the
transgenic parent plants.
 University
of Arkansas Division of Agriculture geneticist Vibha
Srivastava (left on the photo, with program specialist Andrea
Wilson) is developing a method to greatly reduce the cost of
producing a transgenic crop variety in a two-stage research
project. The first part of the project was a success, she
reported in an article in the
Plant
Biotechnology Journal (2004, 2, pp. 169-179), entitled
"Cre-mediated site-specific gene integration for consistent
transgene expression in rice."
Srivastava, an assistant professor of crop, soil, and
environmental sciences at the U of A, said the current method
used to produce transgenic crop varieties involves inserting
DNA (a gene) into a plant's genome without knowing where the
DNA will enter the genome. The random integration of the new
DNA into the genome causes unintended effects. The result is
highly variable and unstable expression of the desired trait
and other unintended effects in the first generations of
progeny plants from crosses of the genetically transformed
parent plants.
Years of testing, cross-breeding and re-testing are required
to identify plants with the stability to use as parents for
producing seed of the transgenic variety, Srivastava said.
In the first phase of her project, Srivastava used an existing
technique called the Cre/lox system to insert a gene at
a specific site in the genome of a rice plant. She reported
that stable transgenic rice plants were produced in 100
percent of the experimental transgenic lines she produced.
Only three other laboratories have reported using this method
successfully in animals and only one other in tobacco plants.
"What's novel (in the Arkansas findings) is that every single
transgenic rice line we made was stable. That compares to only
50 percent stability of gene expression in the previous plant
study using tobacco," Srivastava said. "We are repeating the
study with as many transgenic lines as we can to see if the
high level of stability holds up."
The research in Srivastava's small laboratory in the Plant
Science Building at the U of A involves inserting a marker
gene, for easy detection, into tiny bits of plant tissue.
Plantlets from the transformed cells are generated in test
tubes and grown in the greenhouse for analysis.
The successful demonstration of consistently stable,
site-specific gene integration in rice means the parent lines
for a transgenic variety could be easily and quickly
developed. But the job is only half done, Srivastava said.
The Cre/lox system requires adding genes that serve no
function other than allowing the site-specific integration of
the desired gene, she said. Those unwanted genes must be
removed before a transgenic variety can be marketed.
"The next step is to develop a 'clean' site-specific
placement," she said.
Removing the unwanted DNA can be done by crossbreeding, but
that would take a long time, Srivastava says, which would
defeat the original purpose.
She has only just begun the work to "clean up" the Cre/lox
DNA in the transformed genomes in the test tube, but she
believes she understands the mechanisms involved and will be
able to complete the job.
"It is just a matter of time," she says.
Program specialist Andrea Wilson and graduate student Magnolia
Ariza-Nieto assist Srivastava in this research.
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