Engineering a better tomato

October 23, 1997|By Dennis T. Avery

CHURCHVILLE, Va. -- For centuries, plant breeders have tried to raise crop yields by looking at whole plants. They would take two of their best plants and breed them, hoping the offspring would be better than either parent.

Whole plants were all they had to work with. They couldn't work with just the best parts. Now biotechnology has opened up a whole new world to plant breeders.

Two Cornell University plant breeders, Steven Tanksley and Susan McCouch, recently demonstrated that plant breeders have been wasting most of the best genes in the world's seed banks.

They write in the Aug. 22 issue of Science magazine that plant breeders must shift from finding high-yielding whole plants to finding the best individual genes or groups of genes -- and then combining them into the best total plant that can be assembled.

Mr. Tanksley and Ms. McCouch demonstrated the validity of their approach by collaborating with Chinese rice breeders. Together, they produced new lines that yield 20 percent to 40 percent more than the best current Chinese rice hybrids.

They did it by capturing genes from ancient, low-yielding rice varieties that showed little sign of useful potential. The Cornell researchers focus on quantitative trait loci, or QTL -- groups of genes that together control an inherited trait.

While the highest-yielding plants obviously have some of the good QTL, it's almost impossible for the relatively random process of plant breeding to deliver all of the best genes in one plant variety. That would be like throwing ''snake eyes'' on a pair of dice 20 times in a row.

Out of dozens of beneficial genes (each with several possible variants), traditional breeding techniques may miss half or even more.

This means plant researchers must now examine not only the genes in the latest high-yielding varieties but also all of the genes in wild relatives and in the old heirloom varieties farmers stopped growing decades ago.

Thankfully, the world's governments years ago heeded the suggestions of farsighted plant breeders and conservationists and created the many seed banks that are now our genetic Fort Knox, keeping our precious genetic resources safe.

The genes of most of the major crop species have been ''mapped'' already. That's true for a number of minor species, too. These maps allow researchers to study the individual genes that -- when acting together in plants -- control complex traits like yield.

With sophisticated techniques, these individual genes can be isolated and recombined with our highest-yielding modern varieties. The possibilities for significant crop improvement have already been demonstrated.

In tomatoes, where the most extensive gene mapping has been done, taking key genes from a low-yielding wild relative of the tomato has produced tomatoes with 48 percent more yield and 22 percent more soluble solids.

Researchers even increased the redness of a modern processing tomato with genes from a green wild relative.

Four wild tomato species have been studied in this way -- and more than half of the good genes appear to be unique. This indicates continued sampling of wild plants is likely to produce a global cornucopia of unused genetic variations!

In rice, the genes from one low-yielding wild ancestor increased yield by about 17 percent in one of China's most productive rice varieties. And rice is a crop where yields had recently stagnated.

''We are fortunate to be living at a time when genetic engineering holds so much promise for modifying crop performance,'' the Cornell researchers write.

''More than 50 years ago, [the Russian plant breeder Nikolai] Vavilov predicted the value to agriculture of collecting and maintaining the wild relatives of crop plants in gene banks,'' the researchers write.

''Owing to the advent of molecular mapping and the ability to scan the genomes of wild species for new and useful genes, we may now be in a position to unlock the genetic potential of these resources.''

For decades, pessimists have told us our farm technology would soon run out and we would reach the limits to crop yields. It's a good thing they are still proving to be radically wrong.

Dennis T. Avery is editor of the Global Food Quarterly.

Pub Date: 10/23/97

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