Greening Breakthrough

Working with several research colleagues, Drs. David Hall and Yong-Ping Duan of the USDA Agricultural Research Service have sequenced the genome of the bacteria that causes citrus greening. The two scientists from the USDA’s Agricultural Research Service Horticultural Research Lab in Fort Pierce believe this work could lead to better understanding of greening and hopefully even more significant breakthroughs.


The bacterium, Candidatus Liberibacter asiaticus, causes greening by plugging the phloem tissues in the plant, which reduces the passage of nutrients and can lead to death.

“We have sequenced more than 95% of the bacterium’s genome, analyzed some of the genetic information contained in the genome, and identified some targets for disease control,” says Duan. “We began working on the genome about 14 months ago, and expect to close the gaps in a few weeks. After that, research will focus on identifying genes and gene products produced at different regions of the genome in order to better understand the biology of the bacterium and why it causes the disease.”

The Secret Is In The Psyllid

Both scientists stress that sequencing the genome of the agent that causes greening has been a global, collaborative effort. While others in the search have tried to extract bacterial DNA from infected plants, Hall and Duan focused on pulling the DNA from infected psyllids. The approach worked and opened the doors for greater research findings.

Benefits Ahead

We anticipate that genetic information from the genome will lead to a method of culturing the bacterium,” says Duan. “Also, once the functional genomics of the genome have been investigated, we anticipate that scientists will better understand why the bacterium is pathogenic in citrus, which should lead to methods of disrupting pathogenesis. Some obvious targets (genes) have already been identified for disease-control research.”

Hall adds, if their work helps to lead to culturing the greening bacteria, it would be a major step in fighting the disease.

“HLB bacteria have never been successfully cultured in vitro, although many attempts have been made over the last 25 years,” says Hall. “Sequencing the genome allows the scientific community to better understand the bacterium, to identify the bacterium’s enzymatic and metabolic pathways, to understand the bacterium’s nutritional requirements, and to determine what makes it pathogenic not only in citrus but in the Asian citrus psyllid. 

“We stand a much better chance of culturing the bacterium now that we have sequenced its genome. If we could culture the bacterium, many avenues of research would be opened.”