As part of a $9 million research contract from the U.S. Department of Agriculture, UA plant sciences professor Judith Brown studies the molecular interactions between a tiny insect and a bacterium that causes citrus greening disease, a threat to commercial and residential citrus growing efforts.
Commercial and residential citrus growers dread this bacteria-caused disease because it kills every affected citrus tree. In heavily infested plants, only the leaf veins remain green while the remainder of the leaf turns yellow and eventually dies, earning the condition its alternative name, Huanglongbing disease – Chinese for "yellow dragon."
In the U.S., major citrus growing areas are in Arizona, Florida, California and Texas, with oranges, grapefruit, tangerines, tangelos and lemons accounting for the bulk of cultivated citrus fruit. The disease-causing pathogen is transmitted by the Asian Citrus Psyllid, Diaphorina citri, a tiny insect distantly related to aphids and whiteflies, all of which use a syringe-like mouth apparatus to pierce plant tissue and suck out the sugar-laden sap.
"Citrus greening disease affects the whole plant and interferes dramatically with growth and ability to produce quality fruit," said Judith Brown, a professor in the School of Plant Sciences in the UA’s College of Agriculture and Life Sciences, who serves as the principal investigator on the UA portion of the grant.
Brown's research group has been studying the molecular mechanisms used by the bacterium to get into the insect, multiply and then enter the host plant when the psyllid feeds on it, with the goal of finding ways to block the transmission process. It is the first project conceived and supported by the citrus growing industry as a novel approach to managing the disease.
Fruit from an infected tree typically is not marketable, Brown explained. For example, an orange might be green or splotchy, with some of its segments fused together or only half developed, and it is pithy in texture, rather than sweet and juicy.
"If you cut open an infected fruit, you'd think, 'They didn't leave this on the tree long enough to ripen,'" Brown said.
The story of citrus greening disease begins somewhere in China, India or Pakistan; the exact home range of the Asian Citrus Psyllid is still unclear. Carried by international commerce and travel, the insects have since spread across the world, taking with them the disease-causing bacterial pathogen, Candidatus Liberibacter asiaticus.
"Citrus greening disease has become a national problem," Brown said. "We have been aware of this disease for quite some time. The pathogen itself has been found in almost all citrus growing regions of the world."
Brown explained that while it took the psyllid insect pests 30 to 40 years to spread from the original introduction area in the southerly Americas, the pathogen was introduced multiple times through citrus tree cuttings grafted onto existing trees, a common practice commercial growers and gardening enthusiasts apply to improve the quality or quantity of fruit. In the U.S., the Asian citrus psyllid first appeared in Florida in 1998.
Before growers and home owners realized that the pathogen spreads by the psyllid insects, it was common to simply take out an infected tree and destroy it without too much economic consequence, Brown explained. This changed in August 2005, when Liberibacter was found in Florida for the first time.
"From that point, they monitored the disease and discovered that it was moving westward from Florida and possibly also northward from Mexico toward Texas, Arizona and California," Brown said. "Right now, Arizona is only state in which the psyllid has been found but not the bacterium."
Once the psyllid was found carrying the bacterium in Florida, the risk of infection leading to transmission and incidence of disease skyrocketed, she added.
According to Brown, psyllids often get into groves unseen and go unnoticed while building up their population. By the time they're detected, they have adapted well, populations have exploded and transmission of the bacteria to the trees has already occurred.
"Eradication is not an option at this point," she said. "We have to learn how to live with this disease and the psyllid that transmits the pathogen. In Arizona, citrus is an important crop commercially, but also to many homeowners and urban public-use areas. Many of us love to have our citrus trees in the yard."
"Our research aims at disarming the psyllid to disable it to transmit the bacterium," Brown said. "If we can eliminate its ability to transmit the pathogen, it becomes just a pest, and no longer a carrier of a deadly plant disease."
"Specifically, we identify and study the proteins involved in the bacteria's various life cycle processes," explained Brown, who has been working for three years studying a potato psyllid as a model organism. "From the time they set up shop inside the insect's gut until they multiply and finally relocate to the mouth parts, from where they enter a citrus plant."
Success so far
For example, the bacteria are thought to rely on adhesive proteins to be able to stick to the gut wall, while others are likely to suppress the insects' defenses.
"We surmise the bacterium break through the gut wall and escape into the blood and that's how they reach the oral region," she said. "But we don't know how that happens. Do they use an enzyme?"
"If we could figure out what enzyme was employed by the bacterium to leave the gut, we could find ways to prevent that from happening, so the bacteria would remain trapped and couldn't get into the plant. Similarly, if we know what it uses to adhere to the gut wall, we could interfere with that process so the bacteria could never grow to large numbers. Or, if we could figure out what they use to get into the salivary region before they get into the plant, we could arrest the pathway in that step."
According to John Caravetta, associate director of the Arizona Department of Agriculture, the state's citrus industry contributes some $37 million, not counting the economic impact of citrus nursery stock produced for ornamental use by homeowners.
"So far, we have been successful in fighting the incursion of the Asian citrus psyllid, which has been detected in 53 sites since October 2009," Caravetta said. "Given that we are under constant pressure from surrounding areas like California and Texas, the big issue right now is to prevent the spread of the insect and the bacterium from entering the state."
He said that his department has been working closely and successfully with the federal agency, the commercial growers, UA's Agricultural Cooperative Extension Service and the public to keep the pests at bay. Through public service announcements and outreach geared toward the public and the master gardeners in the extension offices, the goal is to communicate the risks of moving fruit or trees.
"Our success relies heavily on the participation of the public," Caravetta explained. "We ask the public to either purchase and consume fruit from commercial sources, especially when travelling, or fruit locally produced and sold at Farmer's markets, and to purchase citrus plants from outlets such as local nurseries and garden centers."
"The response from the community has been outstanding," he added. "Don't move citrus, and you won't move the problem."