Washington, Mar 14 : Crop scientists have discovered and cloned a gene that is responsible for controlling the shape of fruits like tomatoes.
The discovery of this gene, dubbed SUN, could also aid in unravelling the mystery behind the huge morphological differences among edible fruits and vegetables, and also provide new insight into mechanisms of plant development.
This gene was discovered by Esther van der Knaap, assistant professor of horticulture and crop science at Ohio State University's Ohio Agricultural Research and Development Center (OARDC) in Wooster, who said that SUN is only the second gene ever found to play a significant role in the elongated shape of various tomato varieties.
Tomatoes have evolved from a very small, round wild ancestor into the wide range of cultivated varieties, some large and segmented, some pear-shaped, some oval, some resembling chili peppers, which are available through most seed catalogs and for sale in supermarkets. Still, not much is known about the genetic basis for such transformations in tomatoes, and almost nothing has been recognized about morphological changes in other fruits and vegetables.
"Tomatoes are the model in this emerging field of fruit morphology studies. 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. Once we know all the genes that were selected during that process, 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, cucumbers and gourds," said van der Knaap.
It was discovered that SUN was also commonly found in elongated heirloom varieties, such as the Poblano pepper-like "Howard German" tomato.
"After looking at the entire collection of tomato germplasm we could find, we noticed that there were some varieties that had very elongated fruit shape. By genetic analysis, we narrowed down the region of the genome that controls this very elongated fruit shape, and eventually narrowed down that region to a smaller section that we could sequence to find what kind of genes were present at that location," explained van der Knaap.
She added: "In doing that, 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. And after we confirmed that observation in several other varieties, we found that this gene was always very highly expressed in varieties that carry very elongated fruit."
After identifying SUN, the researchers then aimed at proving if this gene was actually responsible for causing changes in fruit shape. For this, the researchers conducted several plant-transformation experiments.
When the researchers introduced SUN gene into wild, round fruit-bearing tomato plants, they produced extremely elongated fruit. And when the gene was "knocked out" of elongated fruit-bearing plants, they ended up producing round fruit similar to the wild tomatoes.
"SUN doesn't tell us exactly how the fruit-shape phenotype is altered, but what we do know is that turning the gene on is very critical to result in elongated fruit. We can now move forward and ask the question: Does this same gene, or a gene that is closely related in sequence, control fruit morphology in other vegetables and fruit crops?" said van der Knaap.
The researchers also noticed that SUN encodes a member of the IQ67 domain of plant proteins, called IQD12, which they found to be self-sufficient to make tomatoes elongated rather than round during the plant transformation experiments.
While identifying and cloning SUN, the researchers also succeeded in tracing this gene's origin and the process that made it reside in the tomato genome. Also, SUN has a unique characteristic of influencing fruit shape after pollination and fertilization, with the most significant morphological differences found in developing fruit five days after plant flowering.
The discovery was reported, as the cover article, in the latest issue of the journal Science.