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An obscure infection may deliver life-saving treatments to diseased citrus trees

by | Jun 10, 2021


A recent ruling by the United States Court of Appeals for the District of Columbia Circuit left Florida citrus growers with one less tool in their fight against a disease that has decimated Florida’s citrus industry.

However, there may be another treatment in the works.

On Monday, an order was issued vacating the U.S. Environmental Protection Agency’s (EPA) recent registration of a pesticide, AgLogic Aldicarb, for use on oranges and grapefruit in Florida, following a court challenge claiming, among other things, that the pesticide threatens the health of workers and wildlife.

Aldicarb, according to AgLogic Chemical, LLC, had been used routinely for several decades to fight Huanglongbing, or citrus greening disease, with no observed adverse effects reported on endangered species in citrus groves in the past.

In a statement today, the company said it was extremely disappointed in the ruling and said it is considering its next steps.

Meanwhile, a study was published this week in the journal Frontiers in Microbiology which may bring hope for a different kind of treatment for this devastating disease.

In the past decade, Huanglongbing has caused a 72 percent decline nationally in oranges used for juice, and a 21 percent decrease in the American fresh citrus fruit market. Growers in other parts of the world are similarly affected, and it continues to spread unabated.

Scientists are hoping the RNA of an obscure infection can one day be used like a Trojan horse to deliver life-saving treatments to diseased citrus trees.

The infection, citrus yellow vein disease, was discovered 64 years ago in Riverside, California and has never been seen elsewhere in the world. Decades later, University of California Riverside researchers have finally unraveled the associated pathogen’s genetic codes, opening the door to testing whether this apparently benign infection could be used as a vehicle to transport antibacterial and antiviral agents into citrus plants’ vascular systems, where infections usually take place.

Cells use RNA to convert the information stored in DNA into proteins that carry out different functions. Yellow vein disease is associated with small, independently mobile RNA, also called iRNA, which spread through a plant’s vascular system. This spreading mechanism could be a new way to send treatments for Huanglongbing or other diseases into plants.

Researchers learned that the basically harmless citrus yellow vein disease spread between cells in infected plants by the iRNA disguising itself with plant proteins that lets it pass through cellular connective tissue

Yellow vein disease iRNA is also surprisingly small, even for a microscopic organism, with only one functional gene.

“The iRNA is amazing because it’s able to manipulate plant cells to help it replicate, despite having only one functional gene,” explained Kiran Gadhave, a UCR microbiologist, corresponding author of the paper, and lead researcher of the iRNA project. “In addition to its potential therapeutic value, it’s just a scientific curiosity. This is as small as it gets.”

Though they believe the pathogen to be benign, the research team is doing additional testing to make sure it won’t affect fruit quality or quantity, tree height, or any other markers of health. Symptoms in greenhouse-grown trees were mild. Now this is being tested in a field trial in the living laboratory at the UCR Agricultural Experiment Station AgOps.

According to a release on the study, the challenges with iRNA is managing unintended effects, costs and efficient delivery.

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