How the tomato became a heavyweight mutant

Springtime for many gardeners brings the ritual of planting tomatoes in the backyard. They have the luxury of choosing seedlings from a wide array of variants, including big, plump varieties with names such as beefsteak, brandywine pink, and Cherokee purple.

These modern favourites are easily 1,000 times heavier than the fruit produced by the tomato plant’s wild Peruvian ancestor.

The gradual increase in size of tomatoes is in part the result of breeders, ancient and modern, continually selecting and crossbreeding to favour big-fruiting plants. 

Research from the University of Georgia also reveals a strong genetic component driving the increase in size.

A team led by Esther van der Knaap has found a mutation in a gene dubbed the Cell Size Regulator (CSR) that influences the size of cells in the pericarp – the fleshy part – of the fruit.

The genetic secrets of the tomato (Solanum lycopersicum) first began to be revealed in 2012 when more than 100 scientists – collectively known as the Tomato Genome Consortium – produced a high-density genome sequence for the plant, along with a draft genome for its closest wild relative, the Peruvian currant tomato (S. pimpinellifolium).

The consortium found the genomes of the two species, despite a massive difference in the size of their fruits, varied by just 0.6%.

Further research identified the CSR, positioned on chromosome 11, but ascribed it only a minor role influencing fruit size.

Van der Knaap and her colleagues, however, say the CSR found in cultivated versions is a mutation of its wild ancestor. In a paper published in the journal PLOS Genetics, they show that the mutated CSR results in a shortened variant of the protein produced by the non-domesticated version, and this likely affects cell differentiation and maturation in the fruit-flesh and vascular regions of the plant.

The mutation first appeared in cherry tomatoes, which produce fruits a little larger than wild currant tomatoes. Selection by generations of breeders ensured its effects were perpetuated and extended, leading to big beefsteaks popular with gardeners today.

Van der Knaap says the discovery of the mutated CSR has important economic implications. 

Commercial tomato farmers and their wholesale customers prefer large fruits because they are better able to fill the demands of  the food processing industry. All too often, however, efforts to create bigger tomatoes result in a blander affect on the palate.

“The knowledge of the gene,” she says, “will now open up avenues of research into how fruit size can be increased further without negatively impacting other important qualities such as disease resistance and flavour.”