What shape is a tomato? Most people would probably say round, but the fact is, tomatoes evolved from a very small, round wild ancestor into the wide array of cultivated varieties we know today. Some are large and segmented, some pear-shaped, some oval, and some resemble chili peppers.
But very little has been known about the genetic basis for such transformations in tomatoes. And virtually nothing has been discovered about shape changes in other fruits and vegetables. According to a report this week in the journal Science, crop scientists have now succeeded in isolating and characterizing a gene that controls tomato shape. Besides answering that age-old question, the researchers say the discovery could also provide new insight into how plants develop.
"We are trying to understand what kind of genes caused the enormous increase in fruit size and variation in fruit shape as tomatoes were domesticated," said Esther van der Knaap, an assistant professor of horticulture and crop science at Ohio State University and the lead researcher on the study.
"Once we know all the genes that were selected during that process," van der Knaap said, "we will be able to piece together how domestication shaped the tomato fruit, and gain a better understanding of what controls the shape of other very diverse crops, such as peppers and the cucumber and squash family."
One of the first pieces in van der Knaap's fruit-development puzzle is a gene called SUN, which takes its name from the "Sun 1642" cultivated tomato variety in which it was found. Sun 1642 is an oval-shaped, roma-type tomato with a pointed end. But the gene also turned out to be very common in elongated heirloom varieties, such as the poblano pepper-like "Howard German" tomato.
By genetic analysis, van der Knaap and her team first identified the region of the genome that controls this very elongated fruit shape. They eventually narrowed down that region to a smaller section they sequenced to find the genes present in the DNA at that location.
"In doing that," van der Knaap said, "we identified one key candidate gene that was turned on at high levels in the tomato varieties carrying the elongated fruit type, while the gene was turned off in round fruit."
Once SUN was identified, the next step involved proving whether the chromosome 7 gene was actually responsible for causing changes in fruit shape. To do that, van der Knaap and her team conducted several plant-transformation experiments. When they introduced the SUN gene into wild, round fruit-bearing tomato plants, they ended up producing extremely elongated fruit. And when the gene was "knocked out" of elongated fruit-bearing plants, they produced round fruit similar to the wild tomatoes.
A tomato genome consists of 12 pairs of chromosomes. By comparison a female ant has one pair, a dog has 39 pairs and a fern has a whopping 630 pairs. It turns out the tomato genome has two copies of the SUN gene—one on chromosome 10 and one on chromosome 7. The researchers discovered their suspect gene appears to have made a copy of itself on chromosome 10, which then jumped into chromosome 7. Such "jumping genes" are fairly common in plants.
The SUN gene is only the second found to play a significant role in the elongated shape of various tomato varieties, said van der Knaap. But the SUN gene may be unique because of its unusual structure, which allows it to manufacture large amounts of protein that in turn affect shape. Early studies indicate the gene may affect plant hormones or metabolites.
This research is funded by grants from the National Science Foundation.
—By Mauricio Espinoza/Ohio State University
This report is provided by the National Science Foundation, an independent federal agency that supports fundamental research and education across all fields of science and engineering, in partnership with U.S. News and World Report.