Ubi7New Tool for Potato
Plant physiologist William Belknap uses a DNA synthesizer to make rot-fighting
genes for experimental potatoes.
Tomorrow's potatoes might easily fend off rot-causing microbes, thanks in
part to a friendly gene known as ubiquitin7. ARS researchers snipped
part of the gene, hooked it onto a soft-rot-fighting gene, and inserted this
new combination into experimental potatoes.
In preliminary laboratory and greenhouse tests, the tubers resisted attack
by the microorganism that causes soft rot, according to William R. Belknap of
the Agricultural Research Service's
Western Regional Research Center in Albany, California.
The piece of the ubiquitin7, or ubi7, gene that scientists
prize is called a promoter. Its job: to turn a gene on. To a genetic engineer,
a strong promoter is an invaluable tool.
For more than three decades, scientists have known that promoters borrowed
from one gene can be fitted onto another to boost the second gene's
effectiveness. Ideally, the borrowed promoter also dictates when and where
within the plant the gene should turn on.
Ubi7 captured the attention of Belknap's team because wounding turned
on that gene in tubers analyzed in their laboratory.
Joan Garbarino, a research scientist with Demeter BioTechnologies, Ltd.,
examines greenhouse potato plants containing Demeter's microbe-resistance
"When wounding occurs," says Joan E. Garbarino, "rot may
follow. We reasoned that hooking rot-fighting genes to the ubi7 promoter
would make the anti-rot gene more effective when tubers were damagedthe
time they need protection the most."
That is what happened in Garbarino's laboratory tests with
quarter-inch-thick slices of greenhouse-grown potatoes. The tubers had the
ubi7 promoter, hitched to the anti-rot genes, working inside.
To simulate the injury that might occur as potatoes make their way from
field to table, Garbarino stabbed the slices with a toothpick, then exposed
them to the rot-causing organism, Erwinia carotovora. After 5 days,
tubers with the ubi7-driven anti-rot genes had 85 to 96 percent less
decay than those without this gene-promoter combination. Slices from
conventional potatoes turned watery with decay.
Garbarino works in Belknap's laboratory under a cooperative research and
development agreement initiated in 1994 between Demeter BioTechnologies, Ltd.,
of Durham, North Carolina, and ARS. The company developed the rot-resistance
genes used in the experiments. ARS and Demeter are seeking a patent for the
Not Just a Problem in Potatoes
The E. carotovora bacterium that causes soft rot gets its name from
carrots, the crop in which it was first isolated and named. A soil-dwelling
microbe, it invades potatoes and other crops either in the field or in storage.
The rot causes afflicted tissue to become soft and watery and then turn slimy
For their lab tests, ARS plant physiologist William Belknap and Demeter's Joan
Garbarino will harvest tubers from these genetically engineered potato
E. carotovora also ruins other vegetables and fruits including
cucumbers, onions, tomatoes, lettuce, and some ornamental plants like iris.
"We refer to Erwinia carotovora as the 'flesh-eating
bacteria' of the plant world,'" says Richard D. Ekstrom, Demeter's
president. Though ARS tests have been limited to potatoes, the scientists
anticipate that these other crops might also benefit from new protection
offered by Erwinia-resistance genes linked to the ubi7 gene's
Potatoes need antimicrobial protection because damage from machinery and
routine jostling and tumbling during harvest and handling "is unavoidable,
despite the concerted effort by growers and shippers to handle tubers
gently," says Belknap.
In their laboratory experiments, Belknap and Garbarino worked with David R.
Rockhold of the Albany center and former Albany colleagues Timothy M. Rickey
and Teruko Oosumi.
Belknap provided dozens of the genetically engineered tubers to ARS
scientists at Aberdeen, Idaho, for the 1997 growing season. They're completing
the first-ever outdoor tests of the anti-rot genes hitched to the ubi7
ARS plant pathologist Dennis L. Corsini and geneticist Joseph J. Pavek lead
those experiments. They're scrutinizing the high-tech tubers not only for soft
rot resistance, but also for size and other traits important to growers and
Previously at Albany, the team had discovered another ubiquitin gene, which
they named ubi3. In early tests, it seemed the ubi3 promoter
should turn on genes in wound tissue when injury occurred. "But we
couldn't get that to happen," Garbarino says. "We saw ubi3 go
to work in wounded and nonwounded tissue alike, even in leaves, for
Tests of potato genetic material, or DNA, loaded onto an agarose gel, reveal
the presence of an experimental gene.
However, an increasing needworldwidefor new promoters has meant
that plant genetic engineering researchers have been anxious to try out
ubi3. So far, Belknap has shipped the promoter to more than three dozen
labs in the United States and abroad.
"We make ubi3 available free of charge to any molecular biology
lab that needs it," says Belknap. "It seems to be a good all-purpose
promoter for switching a gene on and leaving it on."
To home in on the ubi3 and ubi7 genes, Belknap used a
ubiquitin-seeking molecular probe from Peter H. Quail of the University of
California at Berkeley and ARS Plant Gene Expression Center at Albany. The
probe derives from the ubiquitin1 gene that was discovered by Quail,
former Albany colleague Alan H. Christensen, and others.
Potatoes are America's most popular vegetable. U.S. potato growers produced
about 25 million tons of the tubers in l996, worth about $2 billion. Though
some are raised for fresh-market sale, most are processed into fries, chips,
dehydrated flakes, or other top-selling products.
By Marcia Wood,
Research Service Information Staff, 800 Buchanan St., Albany, CA 94710; phone
William R. Belknap is in the
Improvement and Utilization Research Unit, Western Regional Research
Center, 800 Buchanan St., Albany, CA 94710; phone (510) 559-6072, fax (510)
559-5775. Dennis L. Corsini and
Joseph J. Pavek are in the USDA-ARS
Small Grains and Potato
Germplasm Research Unit, 1691 South 2700 West, Aberdeen, ID 83210; phone
(208) 397-4162, fax (208) 397-4165.
Peter H. Quail is with the
USDA-ARS and University of California at Berkeley Plant Gene Expression Center,
800 Buchanan St., Albany, CA 94710; phone (510) 559-5900, fax (510)
"Ubi7New Tool for Potato Breeders" was published in
the January 1998 issue of Agricultural Research magazine. Click
here to see this issue's table of