June 29th, 2012 by James Ayre
Over the past 90 years, the breeding programs of the commercial tomato industry have slowly resulted in the bland supermarket tomatoes of today. But now, in new research, one of the genes responsible for that change in the levels of sugars, carbohydrates, and carotenoids in tomatoes has been discovered.
This gene is one of the main influences on how tomatoes ripen, and has been used by breeders to develop tomatoes that turn into perfectly red, store-ready fruit. “Practically, it is a very important trait,” says senior author on the paper James Giovannoni, a plant molecular biologist with BTI and the U.S. Department of Agriculture, Agricultural Research Service. “It’s a gene that whether you realize it or not, most of your tomatoes have.”
It’s this gene that leads to a more uniform and controllable ripening, but that also causes the reduction of sugars, taste, and nutrients, in commercial varieties.
“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.”
This finding, of course, has major implications for the U.S. commercial tomato industry, which annually produces more than 15 million tons of tomatoes for fresh sales and for processing.
“This information about the gene responsible for the trait in wild and traditional varieties provides a strategy to recapture quality characteristics that had been unknowingly bred out of modern cultivated tomatoes,” said Ann Powell, a biochemist in UC Davis’ Department of Plant Sciences and one of the lead authors of the study.
“Now that we know that some of the qualities that people value in heirloom tomatoes can be made available in other types of tomatoes, farmers can have access to more varieties of tomatoes that produce well and also have desirable color and flavor traits,” she said.
“For decades, plant breeders in the tomato industry have selected varieties that are uniformly light green before they ripen, in order to produce tomatoes that can be harvested at the same time.”
The research team began exploring the genes that influence tomato development and ripening after working two summers to screen tomato plants for transcription factors that might influence both fruit color and quality. A transcription factor is a protein that regulates genes, making them active or inactive. Genes, themselves, are the manufacturers of these transcription factors.
The researchers were focused primarily on the tomatoes in the field that were very unusually dark green before ripening.
“Partnering with researchers at Cornell University and in Spain, who were mapping regions of the tomato genome, the scientists discovered two transcription factors, called GLK1 and GLK2, that control the development of chloroplasts. Chloroplasts are the structures in the plant cells that enable plants to photosynthesize, converting the energy of sunlight into sugars and other compounds that influence flavor and color.”
“The researchers scoured a collection of mutant and wild species of tomatoes at UC Davis established at UC Davis by the late Professor Charles Rick beginning in the 1950s. They discovered that dark green tomatoes that naturally express GLK2 produced ripe fruit with increased levels of sugars or soluble solids, important for processing tomatoes, as well as higher levels of the health-promoting compound lycopene.”
“Nature presents numerous important genes and their variants, like uniform ripening, that breeders employ to facilitate the needs of growers, processors and consumers,” said Jim Giovannoni, a USDA plant molecular biologist with the Boyce Thompson Institute at Cornell University.
“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.”
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