October 2004
An End to Citrus Sadness?
After years of laboratory work, a scientist in Texas is inching closer to identifying a gene in the DNA of a citrus relative that may change the way citrus is produced. For at least a
century, scientists have known that the citrus relative, Poncirus, carries resistance to citrus tristeza virus, which has wiped out millions of acres of citrus trees worldwide. The disease is commonly called tristeza, the Spanish word for sadness. Until now, tapping into the tristeza resistance of Poncirus was futile because crossing the plant with commercial citrus varieties inevitably resulted in fruit so sour as to make it inedible. One hundred years of crosses and back-crosses failed to eliminate the powerful Poncirus pungency.
Currently, growers can only protect their trees from tristeza by not using sour orange rootstocks, which are highly susceptible to the virus. Before the threat of tristeza, sour orange rootstocks had been widely used because of their ability to resist many soil-borne diseases. Unfortunately, switching from the tristeza-susceptible sour orange rootstocks not only reduced the production and quality of citrus, but the alternative rootstocks growers began using brought other diseases that were not a problem with sour orange rootstocks.
Dr. Erik Mirkov, a virologist and molecular biologist at the Texas A&M University System Agricultural Research and Extension Center at Weslaco, is coming very close to identifying and isolating the Poncirus gene responsible for tristeza resistance. "Through a variety of molecular procedures, including genetic linkage mapping and physical genetic mapping, we've narrowed down the location of the resistance gene to a tiny fraction of the entire Poncirus genome," Mirkov said. "But because of the limitations of those mapping tools, they cannot help us make the region smaller, so we now have 10 candidate genes. We know one of them is the gene that gives resistance, but we don't know which one it is yet."
The gene search has taken Mirkov's work from the laboratory to the greenhouse. "We're taking each of those 10 genes and one at a time transforming grapefruit trees, the varieties grown here in the Rio Grande Valley," he said. "Once we confirm that we have the gene in the grapefruit plant, we challenge the tree by inoculating it with the tristeza virus. If the plant becomes infected, we know that's not the resistant gene. Obviously, when a tree with a gene remains uninfected, that means we have identified it and confirmed that the candidate gene is the one we want." Three of the 10 candidate genes have already been eliminated. Plants with two of the remaining seven genes have been inoculated and results will be known in about a month. Transgenic plants for the other five genes are still in growth chambers and not yet large enough to be inoculated. Once the gene is cloned, it will be transferred to commercial citrus varieties, allowing growers to avoid the devastation of citrus tristeza virus and return to the far more productive and profitable sour orange rootstocks. Depending on the pace of the government regulatory approval process, Mirkov hopes growers can begin planting the resistant varieties in about five years. (AgNews, 9/17/04).
