With summer right around the corner, people are likely to be spending more time in the sun, which may mean more vitamin D. However, some people still do not get enough vitamin D. Indeed, approximately one billion people worldwide suffer from vitamin D insufficiency.

Although the most well-known source of vitamin D is exposure of the skin to ultraviolet B (UVB) light, which synthesizes the vitamin from 7-dehydrocholesterol (7-DHC)—otherwise known as provitamin D3—the major source of the vitamin is dietary. That said, most foods contain little vitamin D and plants are very poor sources.

Now, researchers have developed a new dietary source of vitamin D in plants by engineering the accumulation of provitamin D3 in tomatoes. The team used genome editing to modify a duplicated section of phytosterol biosynthesis in nightshades. They say that this provides “a biofortified food with the added possibility of supplement production from waste material.”

The work is published in Nature Plants in the article, “Biofortified tomatoes provide a new route to vitamin D sufficiency.

“We’ve shown that you can biofortify tomatoes with provitamin D3 using gene editing, which means tomatoes could be developed as a plant-based, sustainable source of vitamin D3,” said Cathie Martin, PhD, group leader at the John Innes Centre in Norwich, U.K. “Forty percent of Europeans have vitamin D insufficiency and so do one billion people worldwide. We are not only addressing a huge health problem, but are helping producers, because tomato leaves which currently go to waste, could be used to make supplements from the gene-edited lines.”

Tomatoes naturally contain provitamin D3 in their leaves at very low levels. Researchers used CRISPR-Cas9 gene editing to turn off the Sl7-DR2 enzyme, which normally converts 7-DHC. This edit resulted in an increase in the levels of 7-DHC in the tomato plants, which was then converted to vitamin D3 through exposure to UVB light.

There was a substantial increase in levels of 7-DHC in both the leaves and fruit of the edited plants. The leaves of the edited plants contained up to 600 ug of provitamin D3 per gram of dry weight. The recommended daily intake of vitamin D is 10 ug for adults. After treatment with UVB light to turn the 7-DHC into vitamin D3, one tomato contained the equivalent levels of vitamin D as two medium-sized eggs or 28 g tuna—which are both recommended dietary sources of vitamin D.

The researchers found that the 7-DHC could be converted to vitamin D3 by shining UVB light on leaves and sliced fruit for one hour. They noted that vitamin D levels in ripe fruit might be increased further by extended exposure to UVB, for example during sun-drying.

When growing tomatoes, the leaves are usually waste material. However, the leaves of the edited plants could be used for the manufacture of vitamin D3 supplements, or for food fortification.

Blocking the enzyme in the tomato had no effect on growth, development, or yield of the tomato plants. Other closely related plants such as aubergine, potato, and pepper have the same biochemical pathway, so the method could be applied across these vegetable crops.