June 29, 2012
Scientists Find Gene Behind Ripe, Tasteless Tomatoes
redOrbit Staff & Wire Reports - Your Universe Online
Ironically, it turns out the same genetic change that causes the tomatoes to ripen uniformly also make them produce less sugar, making them somewhat tasteless, the scientists said in their study published Friday in the journal Science.
The scientists reached their conclusion after decoding a gene that contributes to the level of sugar, carbohydrates and carotenoids in tomatoes. They determined that this gene also influences how tomatoes ripen, and is used by commercial breeders to create tomatoes that develop into perfectly red, store-ready fruit, the researchers said.
"It's a gene that whether you realize it or not, most of your tomatoes have,” said James Giovannoni, a USDA plant molecular biologist with Boyce Thompson Institute for Plant Research at Cornell University, one of the study´s senior authors.
"Practically, it is a very important trait.”
However, this same trait reduces sugars and nutrients in the fruit.
The study has important implications for the $2 billion U.S. tomato industry, which annually harvests more than 15 million tons of the fruit for processing and fresh-market sales.
Natural tomatoes have an 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 growers 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 U.S. 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.
Ann Powell, a UC Davis biochemist and one of the study´s lead authors, and her colleagues began studying the genes influencing tomato fruit development and ripening after spending two summers screening tomato plants for transcription factors that might play a role in both fruit color and quality.
Transcription factors are proteins that regulate genes. These factors themselves are manufactured or expressed by genes. The researchers were particularly interested in tomatoes they observed in the field that were unusually dark green before they ripened.
Powell and her team partnered with researchers at Cornell University and in Spain, who were mapping regions of the tomato genome. They discovered two transcription factors -- GLK1 and GLK2 -- which control the development of chloroplasts, which are structures in the plant cells that enable photosynthesis, or the converting the energy from sunlight into sugars and other compounds that influence flavor and color.
The researchers found that the DNA change in the gene GLK2 causes a tomato's chloroplasts to be smaller, fewer and dispersed evenly throughout the fruit. Although this causes uniform ripening, fewer chloroplasts also mean less sugar and a less flavorful tomato.
In fact, the genetic change, which is found in all uniform-ripening tomatoes, causes a tomato to contain 10-15 percent less sugar, while reducing the amount of lycopene and other compounds that give a tomato its color, smell and nutritional benefits.
In other words, most commercial tomato growers breeding their fruit for visual attractiveness and ease of harvesting have been preventing their fruit from reaching its fullest flavor potential.
"This is an unintended consequence," said Giovannoni, explaining why commercial growers continued to select for the trait.
"Producers currently don't get a penny more for [flavor] quality."
"Nature presents numerous important genes and their variants, like uniform ripening, that breeders employ to facilitate the needs of growers, processors and consumers," Giovannoni said.
"Understanding the genes responsible for these characteristics facilitates the challenging process of breeding crops that meet the needs of all components of the food-supply chain."
The study´s findings have practical applications for commercial producers. Those who wish to grow 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.
Powell said 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."