Tomato breeding for color exacts toll in lost flavor

Gina Kolata / New York Times News Service /

Plant geneticists say they have discovered an answer to a near-universal question: Why are tomatoes usually so tasteless?

Yes, they are often picked green and shipped long distances. Often they are refrigerated, which destroys their flavor and texture. But now researchers have discovered a genetic reason that diminishes a tomato’s flavor even if the fruit is picked ripe and coddled.

The unexpected culprit is a gene mutation that occurred by chance and was discovered by tomato breeders. It was deliberately bred into almost all tomatoes because it conferred an advantage: It made them a uniform luscious scarlet when ripe.

Now, in a paper published in the journal Science, researchers report that the very gene that was inactivated by that mutation plays an important role in producing the sugar and aromas that are the essence of a fragrant, flavorful tomato. And these findings provide a road map for plant breeders to make better-tasting, evenly red tomatoes.

The discovery “is one piece of the puzzle about why the modern tomato stinks,” said Harry Klee, a tomato researcher at the University of Florida in Gainesville who was not involved in the research. “That mutation has been introduced into almost all modern tomatoes. Now we can say that in trying to make the fruit prettier, they reduced some of the important compounds that are linked to flavor.”

The mutation’s effect was a surprise, said James Giovannoni of the U.S. Department of Agriculture Research Service, an author of the new paper. He called the widespread adoption of tomatoes that ripen uniformly “a story of unintended consequences.”

Breeders stumbled upon the variety about 70 years ago and immediately saw its commercial potential. Consumers like tomatoes that are red all over, but ripe tomatoes normally had a ring of green, yellow or white at the stem end.

The variety also had an advantage for producers of tomatoes used in tomato sauce or ketchup. Growers harvest this crop all at once, Giovannoni said, and “with the uniform ripening gene, it is easier to determine when the tomatoes are ripe.”

Then, about 10 years ago, Ann Powell, a plant biochemist at the University of California, Davis, happened on a puzzle that led to the new discovery.

Powell, a lead author of the Science paper, was studying weed genes. Her colleagues had put those genes into tomato plants, which are, she said, the lab rats of the plant world. To Powell’s surprise, tomatoes with the weed genes turned the dark green of a sweet pepper before they ripened, rather than the insipid pale green of most tomatoes today.

“That got me thinking,” Powell said. “Why do fruits bother being green in the first place?”

The green is from chloroplasts, self-contained energy factories in plant cells, where photosynthesis takes place. The end result is sugar, which plants use for food. And, Powell said, the prevailing wisdom said sugar travels from a plant’s leaves to its fruit. So chloroplasts in tomato fruit seemed inconsequential.

Still, she said, the thought of those dark green tomatoes “kind of bugged me.” Why weren’t the leaves dark green too?

About a year ago, she and her colleagues, including Giovannoni, decided to investigate. The weed genes, they found, replaced a disabled gene in a tomato’s fruit but not in its leaves. With the weed genes, the tomatoes turned dark green.

The reason the tomatoes had been light green was that they had the uniform ripening mutation, which set up a sort of chain reaction. The mutation not only made tomatoes turn uniformly green and then red, but it also disabled genes involved in ripening. Among them are genes that allow the fruit to make some of its own sugar instead of getting it only from leaves. Others increase the amount of carotenoids, which give tomatoes a full red color and, it is thought, are involved in giving them their flavor.

To test their discovery, the researchers used genetic engineering to turn on the those disabled genes while leaving the uniform ripening trait alone. The fruit was evenly dark green and then red and had 20 percent more sugar and 20 to 30 percent more carotenoids when ripe.

But were the tomatoes the researchers genetically engineered more flavorful? It turns out that no one was allowed to taste them. Department of Agriculture regulations forbid the consumption of experimental produce.

And, Giovannoni says, do not look for those genetically engineered tomatoes at the grocery store. Producers would not dare to make such a tomato for fear that consumers would reject it.

“No one is interested in taking the arrows,” he said.