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May 26, 2010

Tomato gene impacts taste, yield

by Marlene Cimons

American and Israeli researchers have discovered a gene that prompts hybrid tomatoes into producing exceptionally large yields, and with a surprisingly sweet taste.

“This is the first concrete example of a mutant hybrid in a flowering gene that is giving this dramatic increase in yield,’’ says Zach Lippman, one of the scientists who identified the gene.

Potentially, the newly found gene could have a significant impact both on the billion dollar tomato industry, as well as on agricultural practices engineered to get the most yield from other flowering crops, according to Lippman, an assistant professor in the Cold Spring Harbor Laboratory in New York.

“There’s a lot of amazing biology just in understanding what this gene is doing,’’ Lippman said. “Usually, when you get more yield, you get less sugar. The fact that there is more sugar tells you there is something going on in the leaves, where the sugar is made. They are maintaining a longer period of youth. In some respects, these plants experience a fountain of youth--that’s why you see these dramatic effects.’’

The research team, which also included Israeli scientists Uri Krieger and Dani Zamir of Hebrew University, set out to hunt for genes that boost hybrid vigor, a breeding principle that enhanced hybrid corn and rice production a century ago. Hybrid vigor, also known as heterosis, involves intercrossing two varieties of plants to produce more vigorous offspring with higher yields.

Naturalist Charles Darwin first observed heterosis in 1876, a process that 30 years later received renewed interest from Cold Spring Harbor Laboratory corn geneticist George Shull, whose studies suggested that harmful, vigor-killing gene mutations that accumulate naturally in every generation are exposed by inbreeding, but hidden by crossbreeding.

“But there is still no consensus as to what causes heterosis,” Lippman said. “Another theory for heterosis, supported by our discovery, postulates that improved vigor stems from only a single gene, an effect called “superdominance” or “overdominance.”

To find overdominant genes, the team combed through a vast tomato “mutant library,” a collection of 5,000 plants, each of which has a single mutation in a single gene that causes defects in various aspects of tomato growth, such as fruit size, leaf shape, etc. Selecting a diverse set of mutant plants, most of which produced low yield, the team crossed each mutant with its normal counterpart, searching for hybrids with improved yield.

“We have this large collection of mutants in tomatoes, so I proposed we cherry-pick 50 mutants, representing all different categories of mutant effects on plants, and let’s see if we take those mutants and cross them back to the non-mutant (normal) parents, whether we see these hybrid vigor effects,’’ Lippman explained. “Let’s see if we can find a case of a single mutant gene that gives you mutant vigor.’’

They found several. “The one we published was the strongest, but there were others,’’ he said.

The most dramatic example increased yield by 60 percent. This hybrid, the team found, produced greater yields because there was one normal copy and one mutated copy of a single gene that produces a protein called florigen. This protein, first discovered about five years ago, tells plants when to stop making leaves and start making flowers, which in turn produce fruit.

In plants such as tomatoes, flowering, which translates into yield, is controlled by a delicate balance between the florigen protein, which promotes flowering, and another related protein, that delays flowering. A mutation in only one copy of the florigen gene causes the hybrid to produce more flowers in less time--the key to improved yield.

“It’s the Goldilocks concept,” Lippman said. “What we find is that to maximize yield, you can’t have too much or too little florigen. A mutation in one copy of the gene results in the exact dose of florigen required to cause heterosis.”

The scientists found the gene’s heterosis effect in different varieties of tomatoes, and in plants grown in different climate and soil conditions, both in Israel and in New York at CSHL and the Cornell Horticultural Experiment Station at Riverhead, N.Y.

The researchers hope to find out whether genes related to florigen in other crops also can cause heterosis and improve yield. “Mutant plants are usually thrown away because of the notion that mutations would have negative effects on growth,” Lippman said. “Our results indicate that breeding with hybrid mutations could prove to be a powerful new way to increase yields, not only in tomato, but all crops.”

He added: “Usually, I am reserved and conservative about what biology means for the real world. But this is the first time I feel extremely comfortable saying that this has great potential, not just for tomato yield, but in other flowering crops. We would love this to be put into trials, and ultimately come to market.’’

US News & World Report

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