As if in response to the news earlier this month that watermelons contain more lycopene than tomatoes, traditionally seen as one of the best sources of the antioxidant, researchers from the USDA's Agricultural Research Centre and Purdue University have created a tomato which has nearly three-and-a-half times the usual amount of the carotenoid.
However, the researchers were not looking to create a tomato with high lycopene levels but rather to develop tomatoes for the food processing industry which would ripen later and have a higher quality. They achieved this, and found that the higher antioxidant levels were a happy side effect.
"We were quite pleasantly surprised to find the increase in lycopene," said Avtar Handa, professor of horticulture at Purdue. "This is one of the first examples of increasing the nutritional value of food through biotechnology. In fact, it may be the first example of using biotechnology to increase the nutritional value of a fruit."
Co-discoverer Autar Mattoo, head of the USDA's Vegetable Laboratory, said the increase in lycopene occurred naturally in the genetically modified tomatoes. "The pattern for the accumulation was the same as in the control tomatoes," he said. "The lycopene levels increased two to three-and-a-half times compared to the non-engineered tomatoes."
The research was announced this week in the June issue of Nature Biotechnology.
To develop the lycopene-rich tomato, the researchers inserted a gene, derived from yeast, fused to a promoter gene into tomato plants. The promoter gene helps turn on the yeast gene in the tomato.
"The promoter gene is like a ZIP code that tells the yeast gene when and where to turn on in tomato," Handa said. "For high-lycopene tomatoes we used a promoter that targeted expression of the introduced gene in fruits only."
The yeast gene itself produces an enzyme that affected the production of growth substances in the plants called polyamines, which are known to help prevent cell death. In plant cells, polyamines help build new, beneficial compounds.
"They may stabilise membrane networks that involve longevity of physical structures in the cells called chromoplasts," said Mattoo. "Because lycopene accumulates in chromoplasts in the tomato fruit cells, in this case the polyamines seem to have a positive effect."
The polyamines share a precursor with a plant hormone called ethylene that causes ripening in many fruits. The researchers thought that because ripening was delayed there must have been a decrease in ethylene, but found the opposite was true.
"That's not how we started out thinking, but that's why we do experiments," Mattoo said. "Now we know the change - i.e. allowing the accumulation of polyamines in the fruit - doesn't necessarily affect ethylene production, but ethylene action. We think the polyamines have changed the ethylene receptors on the cell membranes, but we are looking into that."
Handa said the technique used in this research might also be used to increase the amount of other antioxidants in foods. "We are excited about this approach, not only because it results in an increase in lycopene in tomato, but because we think this approach could be used to increase the phytonutrient content of other fruits and vegetables."