Several years ago, Esther van der Knaap isolated and cloned a gene that controls the shape of tomatoes, that holds all sorts of promise for fruit growers. But the Ohio State University researcher didn’t stop there. Because if you ask farmers about their tomatoes, they’ll tell you that size matters, too.

That’s why van der Knaap recently announced that her lab has isolated and cloned the gene that controls that. “It controls several important traits, actually,” she said.

In addition to how big a tomato gets, this gene — called SIKLUH — helps govern how many tomatoes a plant produces and how quickly they ripen. Van der Knaap said she imagines custom tomato plants ideally suited for different growing regions and markets. “Maybe it’s advantageous to have a little smaller fruit that ripens faster,” she said.

That being said, there are no freakishly giant tomatoes growing in the greenhouses at the university’s Ohio Agricultural Research and Development Center in Wooster. In fact, van der Knaap and her team of researchers went in the other direction and created progressively smaller tomatoes.

Her findings, recently published in the journal Proceedings of the National Academy of Sciences, open a window on the molecular mechanics that began thousands of years ago when the fruit was domesticated and centuries ago when cross-breeding began to produce different varieties.

The gene is the common key that links salad-size cherry tomatoes and plump and juicy beefsteaks. “It becomes progressively more abundant in the modern tomatoes,” said Manohar Chakrabarti, a lead researcher on van der Knaap’s team.

The gene regulates cell division. In larger tomatoes, the gene is likely to express a more specific enzyme to create more fruit cells. Further research will have to determine the actual ratio of enzyme to growth.

"A more active SlKLUH gene delays ripening — bigger tomatoes take longer to grow. Plants that grow bigger tomatoes also produce fewer branches and fruits," Chakrabati said.

Five years of cross-breeding the different tomato types helped van der Knaap narrow a list of 500 candidate genes to one. That was SlKLUH, which was tested using techniques that progressively deactivated it, or “knocked it down,” to produce smaller tomatoes.

The team did try to create a bigger tomato, she said, but attempts to over-express the gene produced no visible changes in size. The experiment might have triggered an immune response in the plant that suppressed growth. Future experiments will look at increasing tomato size, van der Knaap said, and explore the possibility of manipulating the gene to create plants that would better suit farmers.