An Asian citrus psyllid feeds on a citrus tree, leaving the citrus greening bacteria. The bacteria will starve the tree of nutrients and eventually kill it.Photo courtesy of the University of FloridaDean Gabriel, a plant bacteriology specialist with UF/IFAS, helped sequence and map the genome of the most prevalent form of the disease in Florida, and now he and colleagues have done the same for a new strain of the disease discovered in Brazil.

There is no cure for either strain, although researchers believe that knowing the genetic makeup of the disease is critical to finding one. Gabriel said by having that “roadmap” of the bacteria genome, they will be certain there are no surprises in the Brazilian species, which has now been found in Texas.  In addition, the mapping should help guide them to improvements in control methods and toward more usable genes and treatments.

“What the genome does, it lets you know everything that the organism has and doesn’t have in its artillery for offense and defense — and it lets you design a strategy to control it,” Gabriel said.

Researchers often liken having the genetic sequence for an organism to having its list of parts.

“Having all the genetic information is like having a detailed roadmap of the organism,” said Jackie Burns, director of the UF/IFAS Citrus Research and Education Center in Lake Alfred.

Citrus greening was first discovered by farmers in China in 1911 and made its first appearance in Florida in 2005. It is spread by a tiny insect called the Asian citrus psyllid that feeds on the trees, leaving bacteria that starve the tree of nutrients. Infected trees produce fruits that are unsuitable for sale as fresh fruit or for juice and most die within a few years. The disease has already affected millions of citrus trees in North America.

Gabriel’s team’s work will be outlined in a research paper that will be published in February in the journal Molecular Plant-Microbe Interaction. The genome map is already available online, at GenBank.

To map the bacteria’s DNA genome, Gabriel’s Brazilian colleagues first diced up and crushed tissue from the veins of infected citrus trees where the organism was most highly concentrated.  They used chemicals to extract DNA and purified it. The team had to separate the tree DNA from that of the bacterium.

DNA comprises four nucleotides, which fall into an order to encode genes specific to an organism. Gabriel likened it to examining beads on a necklace: The beads come in only four colors, and the color sequence determines each gene – in this case, the DNA greening “necklace” held 1,195,201 beads, or 1,044 genes.

To obtain the nucleotide sequence from the purified bacterial DNA, they used state-of-the-art sequencing machines at the UF Interdisciplinary Center for Biotechnology Research.

The research was funded by the Citrus Research and Development Foundation Inc., an affiliate of UF’s Institute of Food and Agricultural Sciences.

In the battle against greening, UF/IFAS researchers have tried everything from working on ways to eradicate the psyllid to grafting trees that show better resistance to greening.

Frederick Gmitter, a citrus breeder and faculty member at IFAS’ Citrus REC, said his research team has found new experimental rootstocks that seem to be supporting healthier trees – even ones with citrus greening.  In addition, his team is studying “escape trees,” which are trees that remain unscathed, even when surrounded by thousands of infected trees.