More information about the genes added to HoneySweet-
Were other new genes added?
In addition to the coat protein from the PPV, two other genes-beta 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.
The GUS gene 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 GUS gene and protein are naturally found in many common bacteria as well as in tissues of many vertebrates, including humans. The gene and protein are found abundantly in the intestines and in soil and water (Luud et al. 1998. Biosafety of E. coli ?-glucuronidase (GUS) in plants. Transgenic Research 7:157-163). The GUS gene and protein have been exempted from regulation by the EPA, meaning that the gene and protein have always been present in human and domestic animal food and are not known to cause any adverse health effects.
The NPTII gene produces an enzyme that allows the plum tissue to grow in the presence of kanamycin as an indicator. This gene also is exempted from EPA regulation (EPA 59 Federal Register 49353, Sept. 28, 1994) because it is commonly found in food and the environment. It is also deemed safe in foods by the European Food Safety Authority.
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 break 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 are currently in Hawaii, the U.S. mainland, Canada, and Japan. Squash has also been genetically engineered with a virus coat protein.
Could a PPV-resistant tree be developed through conventional breeding?
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 with 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. So the hypersensitive response prevents spread of the virus to other parts of the tree and eliminates the virus from the host tree.
Unfortunately, the hypersensitive response in these hybrid plum trees is regulated by several different genes, making it difficult and time-consuming to incorporate the hypersensitive trait into other cultivars by standard plant breeding methods. Also, there has been a report that some strains of the PPV virus may avoid the hypersensitive resistance mechanism and go on to reproduce and cause disease.
Some stone fruits are tolerant to PPV and tolerance may be confused with resistance. Tolerant trees show few or no symptoms but are fully infected. These tolerant trees actually help the virus spread since aphids feeding on these trees carry the virus to non-infected trees.