When Spanish explorers first brought domesticated tomatoes to Europe 500 years ago, the fruit was already gigantic compared with its olive-sized wild counterparts. Researchers trying to understand the genetic basis of this girth have uncovered a way to make other fruits larger as well. The team discovered this secret by studying two mutant tomato strains that had many branches coming off the upper part of the stem and that produced unusually fecund fruit. Fruit size and other plant traits depend on what happens in the uppermost part of the stem, a growing tip called the meristem. There, unspecialized cells called stem cells divide to make more of themselves or specialize into specific plant parts, like carpels, the flower organs that make seed compartments. Wild tomatoes have two; beefsteaks have eight or more. The scientists discovered a feedback loop involving two genes, one to stimulate stem cell production and the other to hold production in check. A shortage of the latter, a gene called CLAVATA3, leads to plumped up beefsteaks, the team reports today in Nature Genetics. This gene’s protein requires a chain of three sugar molecules to work right, and shortening that chain yields ever larger fruit, the scientists report. Because this feedback loop exists in most plants, the team suspects plant breeders can manipulate it to improve crops, and even increase the number of kernels on a corncob.