Powdery mildew, or oïdium, has been tormenting grapegrowers for almost two centuries. A fungal disease that creates powdery gray splotches on leaves, it weakens vines and keeps berries from ripening. To stop it from growing and spreading, vintners spray fungicides or copper sulfate on their vines.
Now a team of scientists at the University of Missouri has isolated a gene in Cabernet Sauvignon that makes it vulnerable to oïdium, raising the possibility of developing vines that could resist the disease and reducing the need for spraying chemicals in vineyards. The researchers had unlikely helpers in their study: frog eggs.
Like most Vitis vinifera vines, Cabernet is vulnerable to powdery mildew, especially in humid environments. Many native grapevine species, such as the Show-Me State's prolific Norton grape, are immune to the fungus. So what is it about the vinifera genome that makes so many cultivars succumb to it? Walter Gassmann, professor in the division of plant sciences at the University of Missouri and the lead author of the study, suspected that a gene called NPF, or nitrate and peptide transporter family, might be responsible.
"Nitrate transporters are proteins that can let [nitrate] molecules through a membrane," Gassmann told Wine Spectator. It’s like a door around the cell that opens only for nitrates. The more nitrate a plant produces, the more it can support pathogens that have infiltrated it, like powdery mildew.
So where do the frog eggs come in? It's difficult to observe the behavior of any single protein in a grapevine, commingled as it is with a host of other biological material. Oocytes, as unfertilized frog eggs are known, can be used as genetic petri dishes.
Gassmann injected the unfertilized frog eggs with DNA strands that encode the nitrate transporter protein. "That frog oocyte makes the protein for us, and then we can study the protein," he explained. The oocyte provides a clear platform for observing the protein without the confounding noise of all the grapevine’s other ingredients. By observing how the nitrate transporter behaves in frog eggs, his team could understand how it behaves in grapevines.
The results showed that the eggs "with this grapevine gene take up more nitrate; it was very specific for nitrate," said Gassmann. And because Cabernet has the NPF gene, powdery mildew can trick the vine into providing it with nutrients.
Gassmann said his research provides a clear avenue for making vinifera less vulnerable. "Either by breeding or by genetic modification, we'll be able to get more resistant grapevine cultivars." To genetically modify vinifera in this case would be as simple as injecting a single gene into the grapevine; the quality of the grape would remain otherwise unchanged. The advantage of genetic resistance to powdery mildew is clear: fewer fungicides or copper sulfate in vineyards.
"The more we know about how these two organisms interact, [the more] we'll know how to interfere," Gassmann said.