HoneySweet Plum Trees:
A Transgenic Answer to the Plum Pox Problem

Frequently Asked Questions

What is plum pox?
Plum pox is a plant disease that infects stone fruit trees including peach, nectarine, plum, apricot and cherries. The disease, which is also called Sharka (the Slavic name for plum pox) disease, is caused by the plum pox virus (PPV). It is considered the most serious virus disease of stone fruit trees. Plum pox is spread from tree to tree by aphids and through infected budwood used for grafting, which is the normal method of propagating stone fruit trees. Symptoms of plum pox infection include leaf and fruit yellowing, fruit deformation, and premature fruit drop. A tree suffering from plum pox can go into serious decline, especially if the tree also becomes infected with other viruses.

What does plum pox mean to me as a consumer?
Presently, plum pox is not an issue for consumers. In the U.S., PPV has only been found in a few places in Pennsylvania and is being eradicated by destroying the infected and surrounding trees. Pennsylvania is not a major plum-producing area, so the consumer market for fresh and dried plums in the United States has not really been affected at this time.

The concern is that if plum pox becomes widespread in the U.S., the disease could cause a major disruption in the availability of plums, prunes and other stone fruits if no solution to the plum pox problem is available. So to ensure that the U.S. consumer continues to have access to the widest variety of fruit, the Agricultural Research Service began a research program to develop plum trees with resistance to PPV. More information about plum pox outbreaks in the U.S.

What is being done to eradicate the PPV infection in the U.S.?
In the U.S., eradication of PPV is done by eliminating infected trees plus all potentially susceptible trees within a buffer zone around infected areas. Each buffer zone extends from 500 meters in a circle beyond the perimeter of an infected area. In a larger zone that extends from 500 to 1,000 meters beyond the perimeters of the first zone is area, 100 percent of the stone fruit trees also are checked for the presence of the virus.

What is the U.S. Department of Agriculture (USDA) doing about plum pox?
The Animal and Plant Health Inspection Service (APHIS), an agency of USDA, is responsible for preventing the introduction of plant pathogens into the U.S. All fruit nursery stock for importation is tested for a range of known fruit tree pathogens, especially those that are not known to occur in the United States (exotic pathogens). APHIS plum pox program

What is the Agricultural Research Service (ARS) doing about plum pox?
ARS has an active research program to develop a better understanding of the virus and how mechanisms of resistance may work in trees. The primary goal of the research program is to produce PPV-resistant trees.

Because developing a PPV-resistant plum tree is not a simple or quick project, ARS has not waited until plum pox has a major presence in the U.S. to begin research. Rather, ARS has taken the proactive step of developing a PPV-resistant tree and doing the testing required to allow genetically engineered trees to become available, before plum pox precipitates a crisis in this country. (Link: Researchers involved in developing and testing HoneySweet)

How was the PPV-resistant plum developed?
ARS researchers have developed a PPV-resistant tree through genetic engineering. The gene for the PPV coat protein was separated from the PPV and inserted into a piece of carrier DNA. This new piece of DNA was inserted into a bacterium—Agrobacterium tumefaciens—that was used to infect cells extracted from plum seeds. Seed cells that were found to have incorporated the new gene into the plum DNA were then regenerated and grown into complete plum trees. These trees have the new gene in their DNA and are resistant to PPV.

The new PPV-resistant variety is named HoneySweet.

Do you know exactly what new genetic material is in HoneySweet?
Yes, we know all of the genes that have been added to HoneySweet.

Are the added genes in the fruit? Will I be eating foreign genes?
The new DNA is in the fruit. But genes are broken by digestive enzymes in the stomach. We have analyzed the fruit from HoneySweet and compared it with fruit from other plum trees, and there is no significant difference in composition in terms of nutrients usually measured in plums (sugars, acids, vitamins, fiber, etc.).

Can the virus DNA harm people?
Plant viruses do not infect animals or people. Thousands of infected trees grow in Europe and the fruit from these trees is eaten by people. There has been no instance of harm from eating the fruit. Also, a portion of all produce, whether from commercial growers or the home garden, is infected by various viruses. Such fruit contains coat protein and coat protein genes from these viruses, and people have always eaten them. In that sense, HoneySweet is no different than many other fruits that contain coat protein genes.

Is HoneySweet safe to eat? How do I know that?
HoneySweet will only be released for sale or consumption when its safety is determined by all three agencies that regulate genetic engineering of crops in the United States—APHIS, the U.S. Food and Drug Adminstration (FDA) and the U.S. Environmental Protection Agency (EPA). (More information about the genes added to HoneySweet)

How did you show that the HoneySweet trees are resistant to plum pox?
Trees were tested in a greenhouse for five years. Plant tissue infected with PPV was grafted onto the new trees, but none of the trees developed plum pox disease. Trees were also grown in Spain, Poland and Romania, where PPV is indigenous, and none of the engineered trees have ever been infected by transmission through aphids, which naturally carry the PPV virus from tree to tree.

Have HoneySweet trees been grown in orchards in the United States?
Only one experimental planting has been grown in the U.S. Permits for this planting were obtained from APHIS, which put very strict restrictions on how many trees could be grown, where they could be grown, how far the trees had to be from other susceptible trees, what had to done when the trees were in blossom, and what had to done with cuttings from the tree—to ensure that test trees were kept under very tight control.

After seven years under strict control, APHIS allowed the trees to blossom and be pollinated so fruit could be produced. This permission was granted with the proviso that there were no other stone fruit or other Prunus species in the area that could be pollinated by HoneySweet.

In addition to the ARS research location where HoneySweet was developed, the trees have been grown in Spain, Poland and Romania under restrictions specific to each country.

Could HoneySweet pollinate other trees?
HoneySweet will cross with other domestic plum trees.

To find out how far HoneySweet plum pollen will travel to other plum trees, almost 3,000 seeds were evaluated over six years at distances from 230 to more than 3,500 feet from the HoneySweet planting. Over the six years of testing, only two seeds were found to have received HoneySweet pollen. These were at a distance of about 1,700 feet from the HoneySweet trees. (More information about pollination and crossbreeding )

Have APHIS, EPA and FDA approved the release of HoneySweet?

APHIS has "deregulated" HoneySweet, which means APHIS has made a determination that the organism is not a plant pest and found that it will have no significant impact on other plants. Deregulated products have an established history of safe use in U.S. agriculture, and APHIS has the authority to bring any deregulated item back under regulation if new information becomes available that demonstrates unanticipated effects or risks to plant health. APHIS has been safely regulating GE organisms since 1986 and has overseen the deregulation of over 70 GE plants, including corn, cotton, rapeseed (canola), soybean, flax, sugar beet, squash, and papaya. Soybeans, corn, cotton, and canola that are herbicide tolerant or insect resistant are the most frequently planted deregulated crops.

With deregulated status, HoneySweet and its progeny can be freely moved and planted without the need for permits or other regulatory oversight by APHIS. HoneySweet is the second tree to be deregulated by APHIS. In September 1996, a genetically engineered papaya became the first tree to be deregulated.

FDA has reviewed HoneySweet and had no questions about the fruit's safety and wholesomeness.

An application for registration has been filed with EPA for HoneySweet.

Am I likely to see HoneySweet plums in my grocery store soon?
People are not likely to see these plums and prunes in stores anytime in the near future. Plum pox is still being contained and eradicated at this time. But as with most foreign diseases, there is the potential that plum pox will become established in the U.S., and all of our current plum tree varieties are susceptible. Since it takes years to develop and multiply new varieties, we can't wait until the problem becomes endemic to do the research to have resistant trees. USDA needs to be ready before the problem becomes a crisis. (More information about how HoneySweet is expected to be used)

Is ARS consulting with plum growers and the nursery industry about the planned release of a genetically engineered (GE) plum?
Research horticulturalist Ralph Scorza, who has been heading ARS's research program, spoke about his research on this plum and the planned deregulation at the International Fruit Tree Fruit Association (IFTA) meeting in February 2006 in Hershey, Pa.. He will be consulting with grower and industry groups in the near future as HoneySweet nears release. More information for the plum orchard industry

Where can I get more information about HoneySweet development and ARS plum pox research?
Contact Kim Kaplan, ARS Information Staff, 301, 504-1637, Kim.Kaplan@ars.usda.gov.

Links to information about plum pox from—

• APHIS
   
• Penn State University
   
• West Virginia University
   
• Purdue University

Links to information about agricultural genetic engineering from—

• U.S. Department of Agriculture
   
• Colorado State University
   
• EuropaBio
   
• University of British Columbia