A portly person described as "pear shaped" has an unmistakable contour - round at the bottom, narrowing toward the top. His or her two-phase profile may or may not be due to overeating. But what gives the edible pear (Pyrus communis) its uniquely descriptive form?
"One of the most common recurring themes is the shape of a pear," observed plant biologist Steven Tanksley at Cornell University in Ithaca, N.Y. He is senior author of a paper in today's Proceedings of the Academy of Sciences (PNAS), released online Sept. 17, 2002. Its title: "A new class of regulatory genes underlying the cause of pear-shaped fruit."
"We've discovered the genetic basis of how a pear-shaped fruit forms from spherical tomatoes," he told BioWorld Today. "The fundamental question we were asking was what kind of molecule could lead to diversification of fruit type, which has been a hallmark of domesticated fruit-bearing plants - whether tomatoes, eggplants, peppers or pears. Almost all of these plants proliferated tremendously in shape as humans domesticated them.
"So we were trying to find out what happened at the genetic and molecular levels when people began domesticating wild plants that could give rise to pear-shaped fruit. Our motivation for the work," Tanksley continued, "was to understand the historical events around taming of plants, and the diversification of fruit shape, which is associated with usability of fruits. Tomato is a nice round market-fresh fruit that you have in your salads and sandwiches. But the lengthier and more pear-shaped fruit are typically used for processing sauces or catsup.
"The primary reason for our work," Tanksley went on, "was as much trying to engineer fruit as to understand a series of historical events and molecular events. We found that what caused these round fruit to turn pear shaped was a single mutation in the OVATE gene we isolated. And that defect would cause premature truncation of the gene's presumably inactive protein. The result was that the protein apparently acted as some kind of negative growth regulator. When that protein is absent or dysfunctional, the developing fruit loses control of growth in all but one direction, and that's the direction toward the stem."
Tomato, Deadly Nightshade Are Related
"It's like taking a round balloon and squeezing it in your hand," Tanksley analogized, "so it would puff out at the weakest point to make a protrusion. In this case, the protrusion forms the neck of the pear shape.
"Tomato is not only a major vegetable crop," he pointed out, "with marketability and value. It's also the best-studied representative of a whole class of plants called the nightshade family. This has many plants of agricultural importance, such as potato, pepper, eggplant, petunias, tobacco and a number of lesser crops. The deadly nightshade [Atropa belladonna] is, in fact, very closely related to tomato. Another notorious branch is the jimson weed [Datura strommium], a hallucinogen similar to peyote, used by Native Americans.
"Tomato [Lycopersicon esculentum] itself produces lycopene, its red pigment," Tanksley explained. "It's one of the strongest antioxidants for preventing certain types of cancer. It's currently used as a nutriceutical supplement. Tomatoes are a New World crop," he recounted. "They were domesticated in South and Central America by Native Americans. It was only when the Europeans came over and began exploring that they took back some samples of tomato seeds to Europe. At first, people recognized them as looking like toxic deadly nightshade, and they would not eat them. Gradually, tomatoes worked their way to the cuisine of fruit all around the world.
"All we know right now," Tanksley observed, "is that in tomatoes the transition from a spherical berry to a pear shape is due to this single OVATE gene we report isolating in PNAS. We know that in a related plant, the eggplant, if we map its shape, there's a gene for pear shape that also maps the same region of the same chromosome. This provides circumstantial evidence that the OVATE gene may be involved in eggplant pear shape. We have no direct evidence that could support or refute whether it's the exact same gene that gave rise to pear shape in other plants. That remains to be tested.
"The absence of the functional protein seems to lead to growth, and its presence to stopping growth," Tanksley pointed out. "So when you stop growth in all directions but one in spherical fruit, you get a protrusion that is the neck of the pear-shaped fruit. The mutation of the OVATE gene," he explained, "is a premature stop codon that would lead to a truncated protein and probably inactivate it, thereby allowing the neck shape to form.
"We're now trying to discover, in our ongoing research, how this protein works," he noted, "looking at an in situ level of expression. We believe it's part of some pathway for regulation, so we're hunting for other protein factors that might interact with this one in trying to unravel the entire protein pathway."
Future Holds Multiple Fruit Features
"We also have a larger goal," Tanksley went on, "which is trying to grasp many of the factors that give rise to size and shape variation in tomato. So we're working on a number of other genes that have changed during domestication. One of the themes that's been unfolding is that often a very few genes give rise to major changes that are associated with domesticated plants. We imagine that in the future it might be possible to use this combined knowledge to domesticate undomesticated plants, and have them produce things they don't produce now. Large, edible, variable-shaped fruit could be induced to do that.
"I can just see it as something that might unfold in the future," Tanksley speculated. "For example, going back to the deadly nightshade, which is well adapted to growing around the world, in some cases under very severe habitats. It could theoretically be a tomato. It's the same plant as to its genetics, and theoretically a few switches could be turned to make it produce much larger, potentially edible fruit by shutting off some of the pathways that produce its toxin. And that," he concluded, "could become a domesticated plant."