GM virus could save Florida's orange industry
A genetically modified virus could save Florida’s citrus growing industry, which has seen 90% of groves infected with the fatal citrus greening disease at an annual cost of $1bn. Southern Gardens Citrus, one of the largest orange juice manufacturers in the world, has now established a process through which antibacterial proteins from the spinach plant are inserted into trees via their genome. A treated plant is then grafted onto an infected tree, enabling its new proteins to attack the disease. If the Environmental Protection Agency grants approval, growers could be using the system from 2019. 

Florida’s citrus growers are running out of time. Since 2005, when a deadly disease called citrus greening first showed up in the state, they’ve been fighting a losing battle to slow the spread of the sugar-sucking bacterium behind the scourge. Today, it has infected 90 percent of Florida’s citrus groves. Fifth-generation farmers are abandoning their acres, packing factories are shuttering their operations, and the state is hemorrhaging a billion dollars every year.

One way to ensure the survival of Florida’s citrus industry---and most of the country’s orange juice---is to produce resistant trees, either with traditional breeding or genetic engineering. Both approaches are proving problematic. Despite exhaustive searches, no one has found a naturally immune tree. And it will take 10 to 20 years to engineer and approve an artificially resistant tree, even with new gene-editing tools like Crispr. Growers can’t wait that long. So to buy them time, one local citrus company is developing something more like an arboreal vaccine, using a genetically modified virus to deliver bacteria-killing spinach proteins.

The treatment starts with a harmless virus that lives in nearly all of Florida’s citrus trees. Years ago, University of Florida plant pathologist Bill Dawson modified a local strain of the citrus tristeza virus so that anyone could insert new bits of DNA into its genome, turning it into a protein factory---otherwise known as a viral vector. Meanwhile, Southern Gardens Citrus, one of the world's largest orange juice manufacturers, was getting on the biotech bandwagon. Initially, the company planned to rid Florida of the citrus greening disease by breeding genetically modified trees. But each one takes at least three years just to mature. “What Florida needs is something quick,” says Dawson, “because whatever you’ve heard, it’s worse than you can imagine.” Something quick, Southern Gardens realized, would be to instead inoculate trees with the vector, which they then licensed from Dawson's lab.

With the vector as its syringe, Southern Gardens just needed a drug to inject. Luckily, it had already screened dozens of genes in its search for a resistant tree---from vegetables, viruses, and even a pig. One particularly promising set of genes came from the spinach plant, coding for a group of antibacterial proteins called defensins. (You have defensins too, fighting off microbes in your saliva.) These spinach defensins worked especially well to chop up C. liberibacter, the bacterium that kills citrus trees by choking off their food supplies.

Southern Gardens inserted the spinach genes into the viral vector; all that was left to do was take it to where the bacteria live.

Trees don’t have veins, but they do have a circulatory system of sorts. Called phloem, these pipe-like tissues transport water, sugar, and nutrients from the roots to the leaves and fruit. That’s where C. liberibacter make their home, living off the trees’ food, preventing the fruits from accumulating sugars (that’s why they stay small and green, hence “greening disease”), and eventually starving the tree to death. To access that network, Southern Gardens grafts a vector-treated plant onto the infected tree. As the virus replicates inside the tree, it becomes a spinach defensin factory, and those proteins travel through the phloem attacking every C. literibacter they meet.