Plum Pox FAQ - More information about the genes added to
More information about the genes added to
Were other new genes added?
In addition to the coat protein
from the PPV, two other genesbeta glucuronidase (GUS) and neomycin
phosphotransferase (NPTII)were added in the same piece of DNA that carried
the coat protein to provide markers that allow researchers to easily track the
presence or absence of the new DNA. GUS is a gene that produces an enzyme that
will react with an indicator chemical (X-Glu) by turning it blue, demonstrating
that the tissue has the new genes in it. The NPTII gene produces an enzyme that
makes the tissue resistant to the antibiotic kanamycin. If tissue grows in the
presence of kanamycin, we know the new genes are actually in the
cellincluding the PPV coat protein gene since it is located next to the NPTII
gene in the carrier DNA.
Does the tree or fruit contain antibiotics? Are these harmful to
people or animals eating the fruit or leaves?
The fruit and leaves do
not contain any antibiotics, only the antibiotic resistance gene. In addition,
it is possible that the leaves and fruit may contain a small amount of the
enzyme produced by the NPTII gene that produces kanamycin resistance. This gene
and the enzyme it produces have been approved by the FDA in 1994 as safe to be
present in food.
Does the resistant tree make the virus coat protein?
If it does not make protein, what makes the tree resistant?
The resistance is due to RNA silencing, which is a natural process in
plants that gives them some adaptive protection against viruses. In the
silencing process, the introduced PPV coat protein gene induces the plant to
make an enzyme that breaks down the coat protein, which prevents virus
infection. This is a natural mechanism that plants use not only for virus
resistance, but also to regulate many normal cell processes.
Is there other transgenic fruit that has been made by adding virus
coat protein DNA?
Yes. Papaya has been genetically engineered to resist
ringspot virus by adding a virus coat protein. These papayas are being grown in
Hawaii and sold there and in the U.S. mainland and Canada. Squash has also been
genetically engineered with a virus coat protein.
Could a PPV-resistant tree be developed through conventional
A few highly resistant stone fruit or ornamental stone fruit
varieties have been developed through conventional breeding. Hybrid plum
cultivars have been identified that respond to PPV by a hypersensitive
response. This means that once virus infection occurs, the plant tissue
surrounding the infection site quickly dies. PPV, like all other viruses, can
survive and multiply only in living host cells. Therefore, the hypersensitive
response prevents spread of the virus to other parts of the tree and eliminates
the virus from the host.
Unfortunately, the hypersensitive response in these plum hybrids is
regulated by several different genes in the tree, making it difficult and
time-consuming to incorporate the hypersensitive trait into other cultivars by
standard plant breeding methods.