Cah Marker Gene Aids Plant
Geneticist Troy Weeks examines wheat cells after bombardment in the gene gun
Scientists aiming to genetically engineer crop plants are always on the
lookout for genes they can use as markers in their experiments. Now, a gene
known as cah may prove a useful addition to today's limited array of
choices. That's according to Agricultural
Research Service geneticist J. Troy Weeks at Lincoln, Nebraska.
Weeks and other biotechnologists pair a marker with an experimental gene
intended to give a plant some prized trait, such as increased resistance to its
worst insect or disease enemies.
The marker clearly flags, for researchers, plant cells that have the new,
useful gene. Other plant tissue can then be discarded so scientists can focus
their efforts on the potentially useful tissue. They nurture it into laboratory
plantlets and later into greenhouse plants.
The marker gene that Weeks and colleagues are testing enables plants to
tolerate a chemical called cyanamide. This versatile compound has some
seemingly contradictory uses. It is an environmentally friendly fertilizer
that, in specific situations, can also act as a herbicide or fungicide.
Weeks' laboratory use of cyanamide exploits the first two of these uses. His
cah marker gene enables callus--clumps of plant tissue-- grown in petri
dishes to convert cyanamide into urea fertilizer. Normally, plants cannot do
In petri dishes, cah-equipped cells soon appear as greenish sections
against the yellow-white callus tissue. In later stages, this callus may
develop healthy shoots and roots. Cells not containing the cah gene may
appear brownish. The few short roots that may develop from this tissue usually
won't survive. But the cah-containing cells, nourished by the cyanamide
fertilizer, will thrive.
In addition to these readily apparent differences in callus, scientists can
check for the cah gene with a fast, simple, and inexpensive assay.
Unlike tests used to detect some other marker genes, the assay for cah
does not require hazardous chemicals.
Where It's From, What It Does
The cah gene comes from a fungus found naturally in soils. Weeks saw
the gene's biotech potential because he knew two things: The fungi--because of
this gene--play an essential role in converting cyanamide into a useful
fertilizer for plants; and without the fungi, cyanamide may kill plants instead
of feeding them.
When used as a fertilizer, cyanamide must be applied before plants emerge.
Soil microorganisms break it down into urea that plants can use. The
microorganisms that do this job include the fungus Myrothecium verrucaria.
The cah gene was borrowed from the fungus.
The gene cues the fungus--or in this case, the genetically engineered plant
cells containing the cah gene--to create an enzyme called cyanamide
hydratase. The enzyme enables the fungus or cah-equipped plant cell to
add the water molecule necessary to convert the fertilizer form of cyanamide,
called calcium cyanamide, into urea. Normally, if the calcium cyanamide
fertilizer were mistakenly applied to plants, they might turn yellow and
die--or at least produce lower yields.
In the laboratory, Weeks moved the cah gene into wheat cells using a
bioblaster, or gene gun. The gun shoots microscopic gold particles--coated with
experimental genes--into plant tissue in petri dishes. Then he grew the cells
on a gelatinous bed of nutrients spiked with cyanamide.
Cells with the cah gene working inside could then convert cyanamide
into urea fertilizer and grow shoots and roots that were the start of new
plants. So far, Weeks has produced more than 100 healthy wheat plants with the
cah gene inside. He is seeking a patent for his work.
Weeks collaborated with Olin D. Anderson of the ARS Western Regional
Research Center in Albany, California; Kelly Y. Koshiyama of the University of
California at Berkeley; Tony Schäeffner of Ludwig-Maximilians University
in Munich; and Ursula Maier-Greiner, formerly at the Munich university.
Maier-Greiner was part of a German team that was the first to discover and copy
the cah gene.
The gene is a potential alternative to marker genes based on resistance to
antibiotic drugs or to widely used commercial herbicides.
In lab experiments using an antibiotic-resistance marker, plant cells
exposed to the antibiotic kanamycin--for example--won't thrive unless they take
up this marker. Some critics fear that when people eat foods derived from such
plants, their intestinal microflora might become resistant to the
pharmaceutical. Biotechnologists see that as highly unlikely.
Marker genes that rely on resistance to widely used commercial herbicides
have also drawn criticism, in part because of concern that the
herbicide-resistance trait might escape if crop plants were to breed with weedy
relatives nearby, perhaps creating super-resistant weeds. Because calcium
cyanamide is not widely used as a herbicide, Weeks sees this risk as nil.
A Benign Alternative?
In new experiments, Weeks wants to determine if outfitting plants with the
cah gene would open the door for growers to use calcium cyanamide
fertilizer more conveniently; that is, not just on bare soil, but also when
plants emerge--or perhaps anytime thereafter--without harming the plants.
"Cyanamide fertilizer apparently poses less risk of nitrate pollution
to groundwater than do the popular urea-based or ammonium-nitrate-based
fertilizers," Weeks says. Also known as lime nitrogen, calcium cyanamide
fertilizer dates back to the early 1900s. It's made from heating lime and
coal.--By Marcia Wood,
Agricultural Research Service Information Staff.
For more information on U.S. patent application number 08/873,001,
"Transformation of Wheat with the Cyanamide Hydratase Gene," contact
J. Troy Weeks, USDA-ARS
Wheat, Sorghum, and Forage
Research Unit, 344 Keim Hall, University of Nebraska, Lincoln, NE 68583;
phone (402) 472-9640, fax (402) 472-4020.
"Cah Marker Gene Aids Plant Transformations" was
published in the July 1998 issue of Agricultural Research magazine.
to see this issue's table of contents.