In Battle to Save Vineyards, Scientists Try Tricking Bacteria

Every grapevine in the 28-acre Bonny Doon Vineyard had to be ripped from the earth and torched in 1994. New vines might have faced the same fate the following year. Instead, owner Randall Grahm, numb from years of battling an incurable plague, sold his whole vineyard of dead and dying Syrah, Viognier, and Marsanne grapes.

But that was just the beginning of a statewide killing spree by a new duo behind Pierce’s disease: the sap-sucking insect known as the glassy-winged sharpshooter and the vine-choking bacteria Xylella fastidiosa. Together they drained more than $30 million out of Northern California’s $3 billion-a-year grape industry in the late 1990s. The wine industry retaliated with millions of dollars of pest-management and protection measures — in a battle it’s still fighting.

Now scientists have come up with a new and cheaper tactic: confuse the germs as soon as the sharpshooter delivers them into a healthy vine. Steve Lindow, a plant pathologist from the Department of Plant and Microbial Biology (PMB), is using something similar to a Jedi mind trick: Convince the bacteria they’ve already caused disease. But to stop these microscopic killers, scientists had to do some criminal profiling.

When Xylella get into a grape vine, they’re released in the vascular tissue — the plumbing of the plant that pumps water up from the roots. From there the bacteria use the tissue as hallways to invade the whole vine. They then start exploring and munching on the plant.

“We think that the exploratory phase involves rather promiscuous movement of bacteria,” Lindow said. But as they spread from place to place, there are only a few bacteria in each area. This is key, he said, because when Xylella populations start to get big, they run into each other and switch tactics.

Each bacterium constantly sends out a molecular beacon, similar to the ping of a submarine’s sonar. When many cells are stuck in part of the hallway at once, they’re aware of each other because they pick up the pings. Then they mob up.

“It’s kind of a switch — a lifestyle switch,” Lindow said. In a mob, they make themselves sticky — to each other and to a spot — so they can be sucked into a sharpshooter, their getaway car.

Lindow and his team of researchers realized that this beacon is the bacteria’s glaring weakness — without it, they wouldn’t make it into their next sharpshooter or kill the vine. So the researchers engineered transgenic grape vines to make the same beacon.

If the vines constantly produce the signal for clumping, the arriving bacteria will get confused and act as though they’ve already infected the plant. They won’t explore. They’ll stay right where they landed and wait for their getaway insect — even though they didn’t pull off a heist. Lindow thinks they’ll start seeing results in fields in the next few years, but in the meantime, he’ll explore other methods of treating Pierce’s disease.