May 5, 2000

After years of lab and greenhouse studies, a researcher at Texas A&M Agricultural Research and Extension Center in the Lower Rio Grande Valley has developed grapefruit trees with built-in resistance to a devastating virus and the insect that transmits the disease.

Twenty transgenic Rio Red trees, with resistance to the citrus tristeza virus (CTV) and its vector, the brown citrus aphid, have been planted in various field research plots in Weslaco after receiving USDA permission to do so. They are the first transgenic grapefruit trees to be planted in the nation.

"The virus resistance in these transgenic trees is pathogen-derived," said Dr. Erik Mirkov, the project leader and a molecular biologist at the Texas A&M Center. "The gene is actually from the virus, much like vaccinations that are given to humans to prevent disease. The insect resistance is from a naturally occurring protein from the snow drop lily, a plant commonly found in the northeast United States. This protein has toxicity to insects, but not to birds and mammals."

The transgenic trees will be allowed to mature to fruit-bearing age, Mirkov said, then evaluated on their agronomic traits to make sure they still produce the fruit preferred by growers and consumers.

Tristeza, which means sadness in Spanish, is a term used to describe the effects of the devastating virus that has destroyed millions of citrus trees worldwide. The virus is quickly transmitted from tree to tree and region to region by the brown citrus aphid, a small but highly efficient transmitter.

Although CTV has been detected in Valley citrus as far back as the mid-1950s, there appear to be no severe strains of the disease here since symptoms have never been detected. Constant monitoring has failed to detect any signs of the brown citrus aphid either. But the threat to the industry is that, in time, severe strains of the disease and/or insect will find its way to Valley citrus trees.

Both CTV and the brown citrus aphid have been reported in Florida and Mexico. Severe strains of CTV have also been detected in dooryard citrus trees in East Texas near Louisiana, which also has a high incidence of the virus.

"Fortunately, the aphids found in Texas are not effective vectors of CTV,"
said Mirkov, "but the threat of the arrival here of the brown citrus aphid from Florida, which is highly efficient, is the reason we’re trying to prepare for CTV with the program and several others."

The agricultural research center in Weslaco is one of only six labs worldwide (four in the United States and one each in Spain and Mexico) doing transgenic citrus research. Mirkov began his transgenic citrus research in Weslaco in 1996 and cleared a difficult hurdle when he became the first to develop methods of transferring genes to Rio Red grapefruit, a plant that does not readily lend itself to genetic transfer.