All About Galls

Walking through a woods or field, most of us probably wouldn’t recognize a gall if we saw one. “Galls are weirdlooking structures that are essentially non-normal organs on a plant,” says entomologist Jack Schultz. He and graduate student Sarah Melissa Witiak are studying the development of galls, which grow on plants in response to the presence of insects. Galls form on a plant when an insect sends a chemical signal to the plant to create a home and food source.

“Basically, the insect tells the plant, ‘Instead of making normal tissue here, make this organ for me,’” says Schultz. Galls are specific to the insects that live in them: It’s possible to identify the insect species living in a gall simply by looking at its host plant and its shape. Scientists know that galls develop only in response to an insect’s presence—if the insect is removed, the gall stops growing because the plant is no longer receiving a chemical signal.

“How do they do all this? We really want to know,” says Schultz. He and Witiak are studying a gall caused by an insect pest of grapes— an insect that causes the plant to make something that’s not a leaf from cells that should be forming a leaf. While the scientists haven’t identified the chemical involved, they know that certain genes are turned on and off to produce a leaf. They are looking for clues by examining which genes are being turned on and off in the gall, as contrasted with the genes in the leaf tissue.

All galls have a very specific organization. They almost always have the insect in the center, surrounded by nutritive tissues and external protective layers. Witiak, in her studies of galls on grape leaves, hypothesized that the organization of galls is like a fruit, with the seed in the middle, protected by spongy tissue and skin. “Sarah Melissa guessed that the insect is getting a fruit to develop mistakenly on a leaf, and she’s right,” says Schultz. “She’s finding genes in galls that are specific to seed and fruit development in grapes, genes that should never be on a leaf. So whatever signal the insect is using, it’s capable of turning on fruit development genes in the wrong place. This is the first time anyone has come remotely close to figuring it out. So we’re excited about it and can’t wait to learn more. The gall story is one of the craziest interactions between plants and insects.”

— Krista Weidner