Tomatoes genetically modified to produce vitamin D, known as the sunshine vitamin, could be a simple and sustainable innovation to address a global health problem.
The researchers used gene editing to knock out a specific molecule in the plant’s genome that increased provitamin D3 in both the fruit and leaves of tomato plants. It was then converted to vitamin D3 by exposure to UVB light.
Vitamin D is made in our body after our skin is exposed to UVB light, but the main source is food. This new bio-fortified plant could help millions of people with vitamin D deficiency, a growing problem linked to an increased risk of cancer, dementia and many leading causes of death. Research has also shown that vitamin D deficiency is associated with increased severity of infection from Covid-19.
Tomatoes naturally contain one of the building blocks of vitamin D3, called provitamin D3 or 7-dehydrocholesterol (7-DHC), in their leaves in very small amounts. However, provitamin D3 does not normally accumulate in ripe tomato fruits.
Scientists in Professor Cathie Martin’s group at the John Innes Center used CRISPR-Cas9 gene editing to reengineer the genetic code of tomato plants so that provitamin D3 accumulates in the tomato fruit. The leaves of the processed plants contained up to 600 µg (micrograms) of provitamin D3 per gram dry weight. The recommended daily intake of vitamin D is 10 µg for adults.
When growing tomatoes, the leaves are usually waste material, but those from the processed plants could be used to make vegan-friendly vitamin D3 supplements or to fortify foods.
“We have shown that one can biofortify tomatoes with provitamin D3 through gene editing, meaning that tomatoes could be engineered as a plant-based, sustainable source of vitamin D3,” said Professor Cathie Martin, corresponding author of the study, which appears in Nature Plant.
“Forty percent of Europeans suffer from vitamin D deficiency, as do a billion people worldwide. Not only are we tackling a huge health problem, we are helping growers because tomato leaves that are currently wasted could be used to make supplements from genetically engineered lines.”
Previous research has examined the biochemical pathway by which 7-DHC is used in the fruit to make molecules and found that a particular enzyme, Sl7-DR2, is responsible for converting it into other molecules.
To take advantage of this, the researchers used CRISPR-Cas 9 to turn off this Sl7-DR2 enzyme in tomatoes, allowing the 7DHC to accumulate in the tomato fruit.
They measured how much 7-DHC was present in the leaves and fruits of these processed tomato plants and found that the 7-DHC levels increased significantly in both the leaves and fruits of the processed plants.
The 7-DHC accumulates both in the pulp and in the skin of the tomatoes.
The researchers then tested whether the 7-DHC in the processed plants could be converted into vitamin D3 by exposing leaves and cut fruit to UVB light for 1 hour. They found that it was and was very effective.
After exposure to UVB light to convert the 7-DHC into vitamin D3, one tomato contained the same amount of vitamin D as two medium-sized eggs or 28g of tuna — both recommended dietary sources of vitamin D.
The study states that vitamin D in ripe fruit could be further increased by prolonged exposure to UVB, for example during sun drying.
Blocking the enzyme in the tomato had no effect on tomato plant growth, development or yield. Other closely related plants such as aubergines, potatoes and peppers share the same biochemical pathway, so the method could be applied to these vegetable crops.
Earlier this month, the UK government announced an official review to look at whether food and drink should be fortified with vitamin D to address health inequalities.
Most foods are low in vitamin D, and plants are generally very poor sources. Vitamin D3 is the most bioavailable form of vitamin D and is produced in the body when the skin is exposed to sunlight. In winter and at higher latitudes, people need to get vitamin D through diet or supplements because the sun isn’t strong enough for the body to produce it naturally.
The first author of the study, Dr. Jie Li, said: “The Covid-19 pandemic has helped highlight the problem of vitamin D deficiency and its impact on our immune function and overall health. The provitamin D-fortified tomatoes we produce provide a much-needed plant-based source of the sunshine vitamin. This is great news for people who eat a plant-based, vegetarian or vegan diet and for the growing number of people suffering from vitamin D deficiency around the world.”
Reference: “Biofortified Tomatoes Offer a New Path to Vitamin D Sufficiency” by Jie Li, Aurelia Scarano, Nestor Mora Gonzalez, Fabio D’Orso, Yajuan Yue, Krisztian Nemeth, Gerhard Saalbach, Lionel Hill, Carlo de Oliveira Martins, Rolando Moran, Angelo Santino and Cathie Martin, May 23, 2022, nature plants.