Tomatoes gene-edited to boost vitamin D

A game changer in targeted gene editing is the technique called CRISPR-Cas9. This won a Nobel Prize in 2020, and allows researchers to target and alter precise parts of the DNA genetic code. Using CRISPR-cas9 to edit crops could lead to a revolution in food production. One of those foods is tomatoes augmented to make Vitamin D. Vitamin D is essential to human health, but we do not make it naturally. During the COVID19 pandemic, the UK government permitted the prescription of vitamin D tablets as its levels were correlated with reduced severity of illness. Having a food product packed with this vitamin may have huge knock-on implications for the health of the population. And Cathie Martin, a plant scientist at the John Innes Centre in Norwich, decided to use CRISPR-cas9 in attempts to generate such a product using the world's most popular fruit. Julia Ravey popped to the John Innes Centre to speak to Cathie and see these tomatoes for myself. Firstly, Jie Lie, a researcher in Cathie’s lab, explained why tomatoes were prime candidates for making a vitamin-D enriched food…

Jie - Because tomatoes naturally accumulate provitamin D3 in leaves, but at very low levels - in ripe fruit it is not detectable at all. We targeted a specific enzyme, which converts this provitamin D3 to other molecules. If we block the function of this enzyme, it should accumulate the substrate, which is provitamin D3.

Julia - So it's almost like if you think of the tomato as a factory, and you've got this enzyme, it's like a worker and they're doing a specific job to move this one thing on the line, along to the next. You've taken that worker out of the chain. And what you'll get is the provitamin D that would have been moved along the line to become the next substrate that it's going to be. That's just building up and building up because the worker isn't there to move it along.

Jie - Yeah, exactly. And the good thing is, this pathway is a partially duplicated pathway from brassinosteroid. Brassinosteroid is really important for plant growth. If you block or touch this pathway, you may get dwarfism.

Julia - And with the tomatoes, we're getting a buildup of provitamin D which is great for us, but what does it mean for the tomato? What does the tomato normally use that provitamin D for down the line that we are now taking for ourselves?

Jie - Oh yes. Vitamin D is a converted to cholesterol, and then to these anti-pathogen compounds. The good thing is we've reduced, but not eliminated this process. So we can still maintain the pathogen resistance of the plant.

Julia - After learning about the method behind how gene editing can be used to generate provitamin D3-enriched tomatoes, Cathie Martin, the plant scientist behind this project, took me to their greenhouse to see the resulting fruit with my own eyes. And they look like, well, tomatoes.

Cathie - I don't think you can tell the ones that are enriched in provitamin D compared to the wild type. So that one's a wild type and that one's a provitamin D enriched one as are these. That's a good thing in terms of getting something that people will grow and use, because we don't want to have a big yield impact or anything like that.

Julia - Are the conditions exactly the same? So you've just grown these in the exact same way as the regular wild type tomatoes?

Cathie - Yes. We had to make sure that following the edit that we made, that the machinery that we use to make the mutation has all segregated away. So these are the progeny of the initial transformation event. So we've lost all the machinery that we use to make the edit and just has the edit left in the lines. And that's very important that we can classify them as qualifying higher plants that don't contain any foreign DNA.

Julia - A big player in this process was the leaves and the plant itself. Also containing these enriched levels of provitamin D.

Cathie - I think that it's really important for any consumer trait that you have something that makes farmers, producers, growers want to grow it because otherwise it tends to be a boutique variety that disappears off the shelves. But if you have a producer trait, whether it's disease resistance or improved nitrogen use efficiency or improved water use efficiency, then you've got something that will make people grow it because you get lower inputs. And for these provitamin D enriched rich tomatoes, there's massive levels of provitamin D in the leaves. And although these plants don't look fantastic at the moment, that can be taken and you can make extracts of provitamin D or vitamin D from the leaves, and those can go to feed supplements. And so you can get value from the waste.

Julia - And side by side. I couldn't tell the difference.

Cathie - No, I can't either.

Julia - I have to look at the labels, because they're all mixed in with the wild type plants next to each other. But in terms of, they may look the same, but are these plants exactly the same, except for that one change? Can we see if there's any other knock-on impacts within the tomato itself?

Cathie - We've done quite a bit of metabolite characterisation to check that. I mean, that's part of the process of engineering a new step, you want to know whether there are any knockout effects and we have done a lot of analysis to show that there was no effect. For example, there was a risk that we might affect brassinosteroid production, which is a hormone that affects the height of the plants. But as you can see, they're normal height -these ones are a bit bigger than the wild type over there. As far as we can tell so far, we can say that that's the only change in the genome.

Julia - Have you tasted them? Are they allowed to be tasted yet?

Cathie - Yes. I think the one I tasted was a bit over ripe. But they taste like a tomato.

Julia - That's brilliant. So you couldn't tell the difference?

Cathie - I'd say squishy, but that was what was available.

Julia - And these tomatoes are enriched with provitamin D but what if we want vitamin D from them? What has to happen next?

凯蒂,在人类中,前维生素D3是欺诈verted to vitamin D3 and you need exposure to UV light to do that. So we've done it experimentally in the lab, we've taken leaves or fruit and exposed them to UV light. And you can get the conversion at about 30% efficiency to vitamin D3. In humans you're recommended to go out for half an hour, a day in sunlight to be able to convert your provitamin D3 to vitamin D3. So we're quite interested to test whether that works in tomatoes as well. Simple kind of idea but maybe we can get away from having to treat with UV by growing the plant outside. So that's why we have applied for a field trial to expose them to the sunlight. And we've been very lucky the last few weeks that there's been plenty of sunlight. So they're just growing down the corridor.

Julia - Cathie took me along the corridor to see the vitamin D three field trial.

Cathie - You Ready?

Julia - Ready for this. The field trial field trial! It's less of a field and more of a stoney patch in between greenhouses.

Cathie - That's right. And I don't know which ones which, again, you have to look at the label on the plants to know whether they're edited or not. So we've got wild type in here as well as provitamin D enriched ones, but we're waiting to see what the effect of sunlight, which is really quite strong.

Julia - Yeah. I'm glad I've got factor 50 on!

Cathie - They're doing quite well I think. And we just have to harvest all the leaves and fruit and see how much vitamin D we get.

Julia - And obviously here, it's out with the sun from the sky coming in and there isn't a control over that. So is there a way you're going to say, okay, well, this plant we know has had this much sunlight based on the weather forecast. So you can do a correlation of sun to vitamin D?

Cathie - Of course in the UK, we do not grow tomatoes commercially outside because they don't do very well. So we were hoping to do these experiments in Italy. But because the Italians are under EU regulations, it's going to take them about two years. So we've just actually got really lucky - as is everyone else - it's been sunny for the last few weeks. And so, we're dependent on what we can get, but I think that this would give us a minimum value of whether it works outside. Of course it may not work. I don't know the answer, but that's the most exciting thing about outside doing sciences when you don't know the answer.

Julia - It's really exciting! Well, I'm definitely getting my vitamin D out here today. Move the ginger inside! Fingers crossed the tomatoes are getting it as well.