By Jennifer Dorsett
Field Editor

Scientists at the University of California Riverside have made big strides this summer in the fight against huanglongbing, citrus greening disease, one of the most serious citrus plant diseases in the world.

Citrus greening disease is spread by the invasive Asian citrus psyllid. Infected trees produce fruits that are green, misshapen and bitter and are unsuitable for sale as fresh fruit or for juice, according to the U.S. Department of Agriculture (USDA).

Although the disease is not harmful to humans or animals, it is deadly to trees, most of which die within a few years of infection, costing growers millions of dollars in losses.

But a UC Riverside researcher recently announced she found the first substance capable of controlling the devastating disease, a peptide spray containing a naturally occurring molecule found in wild citrus relatives.

“This peptide is found in the fruit of greening-tolerant Australian finger limes, which has been consumed for hundreds of years,” UC Riverside Geneticist Hailing Jin said.

The peptide offers advantages over other antibiotics currently used to treat the disease, according to Jin, who has been researching the issue for five years. She explained that unlike antibiotic sprays, the peptide is stable even when used outdoors in high heat, easy to manufacture and safe for humans.

“Most antibiotics are temperature sensitive, so their effects are largely reduced when applied in the hot weather,” she said. “By contrast, this peptide is stable even when used in 130-degree heat.”

The peptide was discovered after Jin and her team examined numerous citrus plants like the Australian finger lime, which is known to possess natural tolerance for citrus greening disease.

She was able to isolate the genes which contribute to this innate immunity, and one of those genes produces the peptide that she tested extensively over the course of two years.

Jin noted trees treated with the peptide spray showed a marked improvement early on.

“You can see the bacteria drastically reduced, and the leaves appear healthy again only a few months after treatment,” she said.

The peptide only needs to be applied a few times each year, so it’s cost-effective for citrus growers. It can be applied by injection or foliage spray. It moves systemically through plants and remains stable, creating a stronger treatment effect.

In addition to Jin’s breakthrough, UC Riverside scientists have also developed new, extensive models of the huanglongbing bacterium, which may lead to other discoveries in controlling citrus greening.

“They show you all the biological processes, and how they work together,” James Borneman, UC Riverside microbiology professor, said. “They also show you which molecular pathways, if blocked, will kill the organism.”

The research team made models for six different strains of the bacterium, which enabled them to identify as many as 94 enzymes essential for its survival.

These enzymes are now considered targets for creating new antibacterial treatments.

The team also identified metabolites the bacteria need to grow.

“Just like when humans break down the food they eat into small components, called metabolites, which feed our cells, bacterial cells also require metabolites for their growth,” Borneman said.

Knowing the metabolites needed for the bacterium’s growth may enable scientists to cultivate it in a laboratory setting, which is not currently possible.

This project involved a collaboration between UC Riverside, UC San Diego, Texas A&M University and USDA, Borneman noted.

He cautioned that while the peptide spray and metabolite models are exciting, any one solution to citrus greening may be short-lived, as microbes tend to mutate and acquire resistance in response to drugs and other mitigation efforts.

“Microbes almost always adapt to control measures, perpetuating the ‘arms race’ between pathogens and hosts,” Borneman said. “There won’t be one thing that will fix this disease. We likely will need to address all three components associated with the disease—the bacterium, the insect that transmits it, and the citrus plants—to find a long-lasting solution.”