Granny is right: Tomatoes are not like they used to be

Would you rather have tomatoes that look good, or taste good? Most people, no doubt, would swear that they prefer taste to looks when it comes to buying fruit and vegetables. But that is not how they behave. Years of retailing experience have shown that what actually gets bought is what looks good. And, unfortunately, for tomatoes at least, that is not well correlated with taste. A uniformly red skin – the sort preferred by consumers – is associated with a “cardboardy” flavour. But until now, nobody knew why.

The answer is provided by a paper in Science, written by by Ann Powell of the University of California, Davis, and her colleagues: a single gene mutation.

The mutation arose as breeders cultivated tomatoes to ripen evenly, a trait that makes harvesting cheaper and more efficient. As pretty as they look, though, mutated tomato fruits are less efficient at photosynthesizing, found a new study. As a result, they make less sugar and other compounds, which means they often taste far worse than tomatoes that may look blotchy but are full of explode-in-your-mouth sweetness.

For some 70 years, tomato breeders have worked to create fruits that are uniformly light green before they ripen. The advantage of this even colouring is that fruits become ripe all at once, instead of at the top end first. That makes it much easier for farmers to tell when it’s time to harvest.

Motivated more by the question of why tomatoes bother being green in the first place than by the question of why tomatoes so often taste terrible, Powell went on a hunt for proteins inside the fruit called transcription factors, which direct the genes that code for various traits.

That search led her to a type of protein in tomatoes called GLK. When intact, the protein makes pre-ripened fruits appear dark green at the shoulder, where flesh meets stem. With the mutation, on the other hand, fruits are light green all over before they turn red.

Scientists have known for a long time that two GLK proteins in the leaves of tomato and other plants direct the production of chloroplasts, which are responsible for photosynthesis, turning sunlight into sugars, and some of those sugars travel into the fruit to add sweetness.

Now it appears that the fruits of tomato plants also contain a GLK protein, which boosts sugar production just enough to make a real flavour difference. The dark green shoulder is a sign that extra photosynthesis is happening. Chemical analyses showed that tomatoes with normal GLK proteins also contained more lycopene, an antioxidant that gives tomatoes their red colour.

Once tomatoes turn red, though, it becomes impossible to tell which fruits have the mutation, making it highly unlikely that a supermarket shopper would be able to pick out tomatoes with naturally higher levels of sugar.

In fact, the chances of unmutated tomatoes showing up in any major grocery store are extremely slim. When Powell and colleagues looked at 25 commercial varieties of tomatoes from all over the world, they found the exact same mutation in all of them.

“The mutation they describe in their paper is in literally 100 percent of modern breeds sold in grocery stores today,” said Harry Klee, a molecular geneticist at the University of Florida, Gainesville, who studies the chemistry and genetics of flavour in fruits and vegetables. “It’s a really good illustration of some of the problems with modern breeding of tomatoes.”

The GLK mutation isn’t the only reason why supermarket tomatoes are so often tasteless, Klee added. But it’s an important reason, and it demonstrates how focusing on aesthetics can end up sacrificing other important qualities in an entire generation of produce.

Whether this discovery actually helps is moot. Any tinkering that brought back the flavour by manipulating the transcription factor would probably also bring back the original uneven colouring. But at least you now know that when your grandmother tells you that tomatoes tasted better when she was a girl, science will back her up.

“When you focus on one thing and neglect the other — the other being flavour — you can have some really bad unintended consequences,” Klee said. “The consumer is going to have to realize that their tomatoes may not look perfect. There may be a patch of green around the top of the fruit. But to me, I would say, if I see that a fruit is not perfectly red and perfectly uniformly ripened, maybe it’s going to taste better.”

Sources: Science, The Economist, MSNBC

Additional reading:

Tomato genome gets fully sequenced

The scientific search for the essence of a tasty tomato