Scientists turn tomatoes into a rich source for vitamin D

Tomatoes get riper and tastier in the summer Sun. Two studies now show that with a little help from gene editing, Sun-ripened tomatoes can also stockpile a precursor molecule to vitamin D, a vital nutrient normally found mainly in animal products.

“This could be a game changer” in nations where vitamin D deficiency is a problem, says Esther van der Knaap, a plant geneticist at the University of Georgia, Athens. Biofortified plants could also help vegans get enough of the nutrient. The finding “opens up a very exciting new era for vitamin D,” says nutritional scientist Susan Lanham-New of the University of Surrey.

Vitamin D helps regulate how the body uses calcium, for example, leading to stronger bones. And there’s some evidence that low levels are linked to higher risk of cardiovascular disease and other health problems.

Sunlight can cause your body to synthesize the vitamin, as ultraviolet B (UVB) radiation converts a precursor in the skin into a form that the liver and kidney transform into usable vitamin D. But people living at high latitudes often aren’t exposed to enough UVB, particularly in winter, to avoid deficiency. And age or darker skin can also slow vitamin D synthesis.

Eating animal products—fish, eggs, and liver—that contain precursors can help make up the deficit. In addition, milk sold in the United States and a few other countries is supplemented. For vegans, mushrooms and yeast are a less effective source. Another option is to take supplement pills, which are often made from lanolin, a waxy substance secreted by sheep. (Sheep can get vitamin D by licking the lanolin off their wool.)

Because tomatoes naturally make a key vitamin D precursor, two groups thought some genetic tweaking could turn them into an animal-free source of the vitamin.

Today in Nature Plants, a team led by Cathie Martin, a plant metabolic engineer at the John Innes Centre, reported that knocking out a single gene created tomatoes which could each provide 20% of the recommended daily allowance of vitamin D in the United Kingdom. And in a late March preprint, a group led by plant geneticist Sunghwa Choe of Seoul National University reported that by knocking out a related gene, it was able to produce tomatoes with even higher levels of a vitamin D precursor.

Normally, tomatoes and other plants in their Solanaceae family make a precursor called provitamin D3 but then convert it into other compounds using enzymes coded for by two genes, called 7-DR1 and 7-DR2. The researchers suspected that knocking out, or incapacitating, either of these genes would cause the plant to accumulate provitamin D3, which when exposed to sunlight transforms into a second precursor—previtamin D3—that people can use. “This seemed like a real opportunity,” Martin says.

It worked. Martin’s team decided to knock out 7-DR2, which helps the plant synthesize compounds plants use to deal with stress from pests and microbes. Thanks to the intact 7-DR1, the modified plants grew normally. And each ripe, sliced tomato, after exposure to sunlight, should offer as much previtamin D3 as two medium eggs. The content can be increased by slicing the tomato first, the researchers found, and likely even more by drying them in the Sun. The leaves and stems of fortified plants could be useful as well, Martin notes, because they could be used to manufacture vitamin D supplements.

Choe’s group knocked out the other gene, 7-DR1, involved in making growth hormones. In their preprint, posted at Research Square, the researchers estimate that a modified tomato can contain up to 100 micrograms of provitamin D3—more than seen in Martin’s experiments—after a month of freeze-dried storage. “We think that the molecule is pretty stable in the fruit,” Choe says.

So far, the modified tomatoes have been grown only in laboratory greenhouses. Martin will begin a field trial next month and Choe hopes to start one this summer. Field tests will be crucial to seeing whether the plants can thrive under real-world stress. Researchers will also need to show that the body can absorb the previtamin D3 in the tomatoes and convert it to vitamin D.

Another challenge could be consumer acceptance: Some people may not accept gene-edited tomatoes, notes Kevin Cashman, a nutritional scientist at University College Cork. If the biofortified tomatoes make it to market, plant physiologists Dominique Van Der Straeten and Simon Strobbe of Ghent University write in a commentary in Nature Plants, they could mark “a leap forward in decreasing our dependence on animal-based foods.”

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