In a paper published in Nature Genetics, scientists trace the size of tomatoes to a set of genes that govern stem cells in the growing tip of the plant, called the meristem. The bigger the meristem, the bigger the resulting tomato. Normally, two genes balance the growth of the meristem: WUSCHEL makes it grow and CLAVATA makes it stop.
In larger tomatoes, this balance is thrown off because the tomato is also missing a proper gene for an enzyme that adds a sugar called arabinose. Why does arabinose matter? CLAVATA needs a chain of three arabinose molecules to function properly. Without it, WUSCHEL goes into overdrive, the meristem grows bigger, and finally the tomatoes grew bigger.
A similar system exists in most plants. The geneticists in this study used a genome editing technique called CRISPR/Cas to knock out the genes they studied. CRISPR/Cas can actually be used with pretty much any other crop as well, which could mean more supersized fruits in the future.
[Nature Genetics via Science]