Tomatoes altered genetically to control ripening

October 19, 1991|By San Francisco Chronicle

Government scientists reported yesterday that they have genetically altered tomatoes in such a way that they can control ripening and delay overripening.

The discovery, especially if it proves applicable to other fruits and vegetables, could have broad implications.

If marketers can manipulate the ripening of fruit, varieties with more flavor and better texture may displace those that are on supermarket shelves now mainly because they are so tough they can take the rough and tumble of mass marketing without bruising or splitting.

The study of genes that control ripening and its aftermath -- plant senescence, or aging and rot -- may even lead to flowers that stay fresh longer after cutting.

For years, commercial tomato growers have picked their crops green, when chances of bruising or crushing are minimal, and then artificially ripened them by dosing them with the gas ethylene -- the same gas that many fruits generate naturally when ripening.

However, such artificially ripened tomatoes also produced their own ethylene and often turned soft and overripe within a few days, cutting the amount of time that markets have to sell them and that consumers have to eat them. Further, because they are picked green, mass-market tomatoes often do not have full mature flavor.

In yesterday's issue of the journal Science, researchers at the Plant Gene Expression Center, operated by the U.S. Department of Agriculture in Albany, Calif., in conjunction with the University of California at Berkeley, said they had found a way to prevent tomatoes from making their own ethylene.

See TOMATOES, 8A, Col. 6TOMATOES, from 1A"[When] you pick a normal tomato and you turn it on with ethylene so it ripens, then it will quickly rot.

"This one [the altered tomato] will last much longer," said Athanosios Theologis, the molecular biologist in whose laboratory the work was done.

Co-authors of the research include graduate student Paul Oehler, who did most of the work, plus Lu Min-Wong, Loverine Taylor and Deborah Pike.

The team has left fully developed tomatoes on the vine for as long as three months, with no spoilage. Treated with ethylene, the fruit turns red and ripe and then stops ripening when the gas is no longer around.

The tomatoes smell and look just like normal ones. Regulations bar the scientists from eating their experimental fruits until they conduct further tests, but sources at the lab said the tomatoes taste fine.

The researchers said there is no reason to think the gene-altered fruit will pose any threat to consumers. No unnatural chemicals or compounds are involved, they said, just manipulation of the raw material that nature puts into the fruit in the first place.

The team used a biotechnology technique called "anti-sense RNA" to short-circuit production of ethylene within the tomato.

They chose as their target the gene for an enzyme, called ACC synthase, that is critical for production of ethylene. To block the enzyme, the team inserted copies of the enzyme's gene, but reversed.

In normal cells, the blueprints for proteins are carried from the genes to manufacturing organelles by a messenger substance called RNA. In the altered tomato cells, however, the message for the ethylene enzyme was canceled by messenger RNA with the opposite message.

The importance of such work, Mr. Theologis said yesterday, is not just that big agricultural companies can put red, longer-lasting tomatoes or firm, yellow bananas in American supermarkets for less money.

"It is going to be very important to Third World countries, for it may lead to fruit that stays good without refrigeration," he said.

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