Gene discovery provides insight into fruit shape and size

US crop scientists have cloned the gene, dubbed SUN, that is only the second ever identified for playing a significant role in the elongated shape of tomato varieties that have evolved from a very small, round wild ancestor.

Furthermore, the gene encodes a member of the IG67 domain of plant proteins, called IQD12 - a family of proteins whose discovery is relatively new.

"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, lead researcher in the study and assistant professor at Ohio State University.

"Once we know all the genes that were selected during that process, we will be able to… gain a better understanding of what controls the shape of other very diverse crops, such as peppers, cucumbers and gourds."

The study The new research appears in the March 14 issue of Science.

Scientists were interested in how the small ancestral tomato transformed into a large variety of cultivated varieties.

The SUN gene was named after the Sun 1642 tomato, an oval-shaped cultivated tomato with a pointy end, where it was found.

After identifying the gene that appeared in high levels in elongated fruit types, the team conducted several plant-transformation experiments.

When the gene was introduced into wild, round fruit-bearing tomato plants, it ended up producing extremely elongated fruit.

When the gene was removed from elongated fruit-bearing plants, they produced round fruit like wild tomatoes.

The discovery provides scope for future investigations into increasing fruit and vegetable size as well as altering their shape.

Van der Knaap said: "We can now move forward and ask the question: Does this same gene that is closely related in sequence, control fruit morphology in other vegetables and fruit crops?"

Another interesting discovery was the relationship between SUN and the plant protein IQD12, which is only the second IQ67 protein-containing domain whose function in plants has been identified.

The other is AtIQD1, discovered in the Arabidopsis thaliana plant, which is part of the same family as broccoli and cabbage.

In Arabidopsis, the protein was found to increase levels of glucosinolate, a metabolite that is currently being investigated for its possible role in inhibiting cancer.

"Unlike AtIQD1, SUN doesn't seem to be affecting glucosinolate levels in tomato, since these metabolites are not produced in plants of the Solanaceous family," said van der Knaap.

"Thus, SUN may be telling us more about the whole process of diversification in fruits and across plant species, perhaps through its impact on plant hormones and/or secondary metabolites levels."

Source: Science Published online ahead of print, doi: 10.1126/science.1153040 "A Retrotransposon-Mediated Gene Duplication Underlies Morphological Variation of Tomato Fruit" Authors: Han Xiao, Ning Jiang, Erin Schaffner, Eric Stockinger, Esther van der Knaap