Plant Responses to the Environment — Six Graduate Student Projects

A Lot of Gall

The grape phylloxera didn't get as much press in the United States as the Irish potato famine, but it probably caused as much social upheaval in the wine districts of Europe during the 1860s. "Vines just started to wither and die," says graduate student Brian Rehill. "No one knew what was happening. By the time people figured out what was causing the problem a minuscule gall-forming insect from American early half of France's vineyards were wiped out.

"Gall-forming insects like the phylloxera defy logic," Rehill says. "They don't just eat the plant and make it go away. They show up, do something to the plant, and, at least initially, there's more plant tissue than there used to be." After making the plant form a gall or tumor, the female phylloxera lays her eggs. Then, with syringe-like mouth parts, she plugs into the root's plumbing and begins sucking out the plant's juices. Areas of dead tissue form at the feeding sites. If too many dead areas form, the roots eventually die.

Over a century has passed and growers still don't have a good method of chemical or biological control for the grape phylloxera. In the 1800s, American entomologists proposed grafting the tastier European wine grape to the more resistant American rootstock, the method still used today. However, as with organ donors, a given wine grape isn't necessarily compatible with a given rootstock. "Grafting also is expensive, and these rootstocks consistently tend to fail after a while," says Rehill. "Recently, 30 to 40 thousand acres have started to fail in California's Napa and Sonoma Counties." These vineyards will need to be replanted in the next couple decades a cost somewhere on the order of $750 million to $1 billion.

"Viticulture has always been an applied artit's thousands of years old," Rehill says. "Everybody's wanted something that works, but they haven't cared why it works. The glaring omission over the years has been that nobody's figured out precisely what the insect does to the plant besides kill it. How does it kill it? How does it manipulate the plant? And what makes one plant more resistant than another? These are the kinds of questions I'm interested in. If we can figure out the physiological and biochemical mechanisms behind resistance, we'll have a target for a more rational breeding program."

To find the mechanisms, Rehill will grow tiny grape plants in tissue culture, infest them with phylloxera, then observe the biochemical changes that take place in the tissues in and around the gall. He will look for two groups of chemicals. The first group is the secondary compounds the tannins, phenolics, and stillness known to be important in grape responses to fungal diseases. The second group is the enzymes plants are known to make in higher quantities in the presence of pathogens or insects. Then he will see if he can correlate the presence of any of these compounds to phylloxera resistance. "Maybe the successful galling insect suppresses some of these chemicals," he says.

Finally, Rehill will conduct field trials with whole grape plants to explore insect management techniques. "Grapes are typically pruned severely," he explains. "Growers don't want the plants wasting resources on long vegetative branches. But if we limit pruning, the increased number of buds might compete with the insect for carbohydrates and other nutrients. Letting the leaves and branches grow more could be an easy way to suppress the insect. It would be a lot easier than tearing up an established vineyard."