Wednesday, 07 June 2017 12:50

Researchers Identify Mechanism to Stop Citrus Tree Disease

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Published in mBiosphere

Drive down the center of Florida south of Orlando on U.S. Route 27 where once stood plentiful citrus tree groves on either side of the road and today you’re more likely to see skeletonized trees, laments Professor Dean Gabriel, a plant pathologist at the University of Florida.


The cause? Huanglongbing, or citrus greening, a disease caused by Candidatus Liberibacter asiaticus and introduced by insects called Asian citrus psyllids that has been decimating citrus trees in the state over the last 10 years or so. Citrus greening causes a severe decline in the trees, Gabriel says — leaves turn a blotchy, mottled yellow color years after the initial infection, the fruits produced are smaller and have an off-taste, and fruit yield is much reduced. As a result, the state’s overall citrus production has declined by about 60 percent over the last six years.

mSphere: A Small Wolbachia protein Directly Represses Phage Lytic Cycle Genes in 'Candidatus Liberibacter asiaticus within Psyllids

Grapefruits are more sensitive than oranges, says Gabriel: “The grapefruits are going to disappear first and, in a few years, if we don’t find a cure, more juicing plants and packing houses will close; all are now operating way below their normal capacity. At some point they become uneconomical to run and they close. Once they’re closed, nobody’s going to reopen them because it would take years to get the production back again. There’s a huge need for a cure.”


But finding a cure has been tricky. When psyllids carrying but not affected by Candidatus Liberibacter feed on healthy trees, they inject the bacterium into the trees’ phloem, a tubular system normally used to transport sugars produced during photosynthesis from the leaves of a plant to the rest. The bacterium suppresses the plants’ defenses as it moves, Gabriel says: “It’s like a little cunning burglar sneaking in under the radar.” It impacts the tree from its roots to its shoots, he says, and has a long incubation period: “By the time disease is detected in one tree, the entire grove is thoroughly infested and much more difficult to treat.”


Scientists could treat the trees by injecting them with penicillin, but no one would want to eat the fruit, Gabriel says. They also could produce GMO trees but many are opposed to that as well. And attempts to isolate Candidatus Liberibacter to grow in culture also have proved fruitless, as the disease, once removed from its host, historically has destroyed itself.


Now Gabriel’s group may have come a step closer to finding a treatment. In work published in mSphere this week, they describe identifying a small protein from the Wolbachia bacterium in psyllids that can “cross-talk,” moving to Candidatus Liberibacter within the insects to silence its prophage genes, thereby helping prevent an insect immune reaction that would likely be detrimental to both bacteria.


The protein could serve as a potential target to develop spray treatments to protect trees against the psyllids, and could potentially help the trees themselves fight off bacterial invasion, Gabriel says. Wolbachia is a natural bacterium present in up to 60 percent of all insect species.


“In this case, one bacterium is doing a favor to the whole bacterial community living within the psyllid by shutting down a potential threat to survival of insect host,” he says.


In a series of laboratory experiments, the team discovered that expression of proteins that help drive the spread of Candidatus were suppressed when they were treated with extracts from the psyllids. Further studying the process, they identified a fragment of the protein doing part of suppression as encoded by the Wolbachia strain and secreted into the insect. This protein could move within the insect into Candidatus Liberibacter, bind itself to a genetic region that would normally promote prophage activity, and repress these genes.


Now that a protein target has been identified, says Gabriel, it could be commercially synthesized and added to culture media, where Candidatus Liberibacter may be more likely to grow. The group has a grant from the U.S. Department of Agriculture to pursue that work.

Last modified on Wednesday, 07 June 2017 14:53
Karen Blum

Karen Blum, a writer in the Baltimore area, has been covering health and science for 20+ years. Her work has been published in daily newspapers including The Baltimore Sun and The Palm Beach Post; national news websites like msn.com and WebMD.com; and magazines and news services for health professionals including Anesthesiology News, Pharmacy Practice News, Internal Medicine News, and MedPageToday.com.

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