Usually when one thinks of tomatoes, images of nice round fruits with a bright red color spring to mind. Heirloom tomatoes, on the other hand, typically don’t enjoy the same patterns of uniform redness. However, despite their lackluster color, a distinction in their genetics allows them to exhibit a higher level of sweetness than their traditional red counterparts.
In a June study conducted by UC Davis and Cornell plant biologists, researchers gained new insight into these differences.
The amount of selective breeding devoted to creating a perfect color has lead to the unintentional reduction in tomato flavor. Farmers have traditionally selected varieties of tomatoes that are lighter green before ripening. This allowed for easier identification of ripe tomatoes for harvest but also gradually lowered the overall sweetness of the fruits harvested. The lighter green color is due to a reduced amount of chloroplasts, the sugar-producing part of the plant.
Ann Powell, a UC Davis biochemist, and her team sought to better understand how this information specifically pertains to fruit growth and development. In their efforts, they identified a series of transcription factors that could lead to higher sugar levels in tomatoes.
“Sugar is a flavor enhancer; as we increase sugar levels, we increase the flavor of raw tomatoes,” Powell said.
The research effort focused on finding sequences in the tomato genome responsible for how the fruits ripened. Specifically, researchers isolated segments of DNA where expression would result in differences in color or fruit quality. Researchers identified two important transcription factors, genetic material responsible for regulating the expression of other genes.
According to the study, expression of the transcription factors GLK1 and GLK2 resulted in tomatoes exhibiting a darker shade of green prior to ripening. This color was the end product of the transcription factors increasing production of chloroplasts in the fruits compared to their traditional processed counterparts.
Powell, the coauthor of the study, said that tomato breeders had unintentionally selected strains of tomatoes exhibiting a missense mutation of the GLK2 gene by breeding for tomatoes with their appealing uniform color.
A missense mutation is a change in the DNA sequence that prevents a gene from being properly translated into a protein.
This premature stop sequence cleaves the resulting protein early and renders it ineffective. In the case of GLK2, it no longer functioned to increase sugar levels. However, the higher chloroplast concentrations in the darker green tomatoes that don’t exhibit the mutation result in higher sugar concentrations in the fruit.
Traditionally, tomatoes are harvested at a specified percentage brix, a by-weight concentration measurement for dissolved sugars, and then cooked to artificially increase these concentrations. By utilizing the increased chloroplasts found in the darker strains of tomatoes, tomato farmers have the potential to harvest higher quality tomatoes per unit time.
“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.”
The increased efficiency or quality of the tomatoes harvested could potentially translate into cheaper, higher-quality tomatoes for everyone.
ALAN LIN can be reached at science@theaggie.org.
