How Sweet It Is: Tomato Researchers Discover Link Between Ripening, Color And Taste

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For many grocery shoppers, those perfect, red tomatoes from the store just can’t match the flavor from the home garden. Now, researchers at Boyce Thompson Institute for Plant Research at Cornell University, USDA and the University of California at Davis have decoded a gene that contributes to the level of sugar, carbohydrates and carotenoids in tomatoes. (Science, June 29, 2012)

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Cuong Nguyen, a Cornell graduate student in plant breeding working at the Boyce Thompson Institute (BTI), along with colleagues at BTI, USDA, UC Davis, Universidad Politecénica de Valencia (Spain), Universidad de Málaga (Spain) and University of Suleyman Demiral (Turkey) revealed the gene that underlies the uniform ripening mutation.

This gene also influences how tomato fruits ripen and is used by commercial breeders to create tomatoes that develop into perfectly red, store-ready fruit. “Practically, it is a very important trait,” says James Giovannoni, a plant molecular biologist with BTI and the U.S. Department of Agriculture, Agricultural Research Service, who is a senior author on the paper. “It’s a gene that whether you realize it or not, most of your tomatoes have.” However, this same trait reduces sugars and nutrients in the fruit.

Naturally, tomatoes have uneven ripening, showing darker green patches when unripe and variable redness when ripe — traits that still show up in garden-variety and heirloom breeds. However, in the late 1920s, commercial breeders stumbled across a natural mutation that caused tomatoes to ripen uniformly—from an even shade of light green to an even shade of red. This mutation, known by plant biologists as ‘uniform ripening’, has become indispensable to the $2 billion a year US commercial tomato market, showing up in almost all tomatoes produced for grocery stores. The uniform redness makes it ideal for the grocery sector, which has to appeal to customer expectations of evenly colored, red fruit.

Nguyen conducted positional cloning and, with access to solgenomics.net, an online, public database hosted at BTI, he determined that the uniform ripening gene was located at a specific location on chromosome 10. With this location now known, the team could decipher the gene coding for the protein that controls photosynthesis levels in tomato fruit. While leaves are the primary photosynthesis factories in a plant, developing tomato fruit can contribute up to 20 percent of their own photosynthesis, yielding high sugar and nutrient levels in fully ripe fruit. The uniform ripening mutation, which commercial breeders select for, eliminates this protein in the fruit, therefore reducing sugar levels. “This is an unintended consequence,” says Giovannoni, explaining why commercial growers continued to select for the trait. “Producers currently don’t get a penny more for [flavor] quality.”

This discovery has practical applications. Commercial producers – who wish to produce uniform red fruit over multi-colored, flavorful ones – can now do an early test on seedling DNA for the uniform ripening mutation, rather than waiting to observe the mature fruit. Conversely, those who don’t care about appearances can make sure of the opposite—that their plants are mutation free and thus may have better-tasting fruit.

Ann Powell, a research biochemist who led the UC Davis team’s efforts on the research, says that the study “is a rare chance to translate scientific findings to the real world…it provides a strategy to re-capture quality characteristics that had been unknowingly bred out of modern cultivated tomatoes.”

The journal article, “Uniform ripening Encodes a Golden 2-like Transcription Factor Regulating Tomato Fruit Chloroplast Development, “ appears in Science, June 29, 2012. Funding for the Giovannoni lab at BTI is supported by the USDA and the National Science Foundation.

Contacts and sources:
Cornell University