Boosting Built-in Immunity
Plant pathologist Chenglin Yao (left) and biological technician
George Brown check apples that have been genetically engineered to produce
larger amounts of a rot-inhibiting protein.
When disturbed, an octopus changes color and emits a dark, inky fluid so
that predators can't discern its location. A porcupine's needle-sharp quills
fend off would-be attackers.
The plant kingdom, too, has its protective mechanisms. A fascinating example
is a tiny apple growing on a tree. It can protect itself from a formidable
"The problem with this protection is that as the apple matures, the
protection decreases," says William S. Conway, "And this decrease
occurs at a time when the fruit still needs protection."
But Conway, a plant pathologist at the ARS' Horticultural Crops Quality
Laboratory in Beltsville, Maryland, is working with colleague Chenglin Yao to
change this. They are genetically altering the apple to increase the protection
at crucial times.
In this case, the protection is a proteinpresent in several
commodities, including pears, tomatoes, and raspberriesthat Yao found in
Golden Delicious apples in storage.
This is the first time that the protein, called polygalacturonase-inhibiting
protein (PGTP), has been purified from apples.
"This protein inhibits the action of polygalacturonase, an enzyme that
causes fruit rot," Yao says. The enzyme is produced by Botrytis
cinerea, a pathogenic fungus that attacks fruit after it has been
Growers control fruit rots primarily with fungicides because they have been
effective. But because of health and environmental concerns, many of the
"old faithful" fungicides are no longer available. Also because of
these concerns, growers are faced with the need to cut down on the amount of
chemical residues left on the surface of fruit during production and storage.
Estimates vary, Conway says, but between 25 and 30 percent of food harvested
in the United States is lost before it reaches consumers. This loss is due, in
large part, to micro-organisms that cause rots and costs millions of dollars
each year. Consumers ultimately help pay for this loss in the form of increased
"Postharvest losses are more devastating than those in the field
because of the harvesting, handling, and storage costs already incurred,"
That is why he and Yao, an ARS molecular biologist, are trying to develop a
superior Golden Delicious apple treeone that can better protect its
fruit. Yao has purified the protein found in the apples and is now cloning the
gene that is responsible for producing it.
"We have cloned a segment of the gene by using information from amino
acid sequences of the protein," he says. "The idea is to manipulate
the gene so that it produces large amounts of the protein as the fruit gains
"Our studies show that young fruits are not very susceptible to rot
pathogens. But as the fruit reaches maturity, PGIP levels get smaller and the
fruit becomes more susceptible.
"It's at this point that we could put in a strong promoter to stimulate
the PGIP gene to produce the maximum amount of the protein," he says.
It will be some time before Yao produces transgenic apple plants with the
new gene. But once the gene has been successfully inserted into tissue-cultured
plants, they will be ready for field testing. By Doris Stanley,
Conway is with the USDA-ARS
Quality and Safety Laboratory, 10300 Baltimore Ave., Beltsville, MD
20705-2350; phone (301) 504-6980 , fax (301) 504-5107.
"Boosting Built-in Immunity" was published in
the December 1995
issue of Agricultural Research magazine.