A Better Tomato

David Pacchioli
May 01, 2002

"In a sense," says Italian nutrition researcher Francesco Branca, "the future is to find a good combination of tradition and technology." Penn State's Majid Foolad has done just that, and come up with a nutritionally potent strain of cherry tomato.

cherry tomatoes hanging
Majid Foolad

The Penn State Cherry Tomato: Its deep red color means lots of lycopene.

Foolad, an associate professor of plant genetics and breeding, has been working for seven years to develop tomatoes more attuned to Pennsylvania growing conditions. Tomatoes are Pennsylvania's second biggest vegetable crop (behind sweet corn), with an annual harvest worth more than $16 million, but most current cultivars were developed for the warmer climates of California and Florida, Foolad says. These varieties "never reach their full genetic potential here," where the growing season is shorter and cooler. In particular, the fungal disease known as early blight is the bane of the state's growers.

Foolad's breeding focus, therefore, has been on early-blight resistance. At the same time, however, he has been working on boosting lycopene content.

Tomatoes, Foolad notes, are our number one dietary source of lycopene, an antioxidant reputed to offer powerful protection against heart disease and several types of cancer. But compared to wild varieties of the plant, current tomato cultivars are relatively low in this important phytochemical. "Average lycopene content in cultivated varieties is 50 micrograms per gram of fresh tissue, compared to 200-250 micrograms in some wild types," he says.

The reason for this discrepancy, he says, is that "the cultivated tomato has a very narrow germplasm base." The wild tomato is native to South America, he explains, "but the plant was domesticated in Mexico, and the early breeding was done in Europe." For each geographical transfer, only a limited number of tomatoes was chosen, and the gene pool shrank accordingly.

To breed in more lycopene, then, Foolad had to go back to wild strains, which he located in two tomato genebanks, one maintained by the University of California at Davis, and the other by the U.S.D.A. in Geneva, New York. Thanks to the diligent efforts of plant collectors during the early part of the 20th century, he says, he had quite a selection to choose from.

At first, narrowing the list was easy. The tomato has eight wild species, Foolad explains, but only one, Lycopersicon pimpinellifolium, bears red fruit. And since lycopene is the pigmenting compound that accounts for redness, color is a dead giveaway.

Within L. pimpinellifolium alone, however, he found over 300 possible candidates. To select among these, Foolad grew samples of each in a test plot at the University's Rock Springs research area, and evaluated fruit color by photospectrometry and liquid chromatography.

"From these [varieties] we identified a handful that were rich in lycopene, and one or two that had over 200 micrograms per gram," Foolad says. "Then we started crossing these with cultivated tomatoes."

Traditional plant breeding is a laborious process. In this case, however, there was no choice. Lycopene content, like blight resistance, is a complex trait, Foolad explains, which means that more than one gene is involved in its expression. And researchers don't know exactly which genes or how many. So extracting a lycopene gene and cloning it for insertion in other plants was out of the question.

Foolad did, however, make use of a molecular technique known as gene mapping to speed the process. By comparing stretches of DNA from high-lycopene tomatoes against those from low-lycopene varieties, and finding places where the code differed, he identified the genes likely to be responsible for lycopene content. ("So far we've found eight, but there may be more," he says.) Then he crossed only those plants possessing the high-lycopene genes, thus forgoing a lot of trial and error. He used the same technique to find and accentuate disease resistance.

The result of his efforts, dubbed the Penn State Cherry Tomato, possesses both resistance to early blight and a lycopene content two to three times higher than that found in other cultivated strains. Distinguishable by its deep red color, Foolad's prize should be available to backyard gardeners within two years.

Majid Foolad, Ph.D., is associate professor of plant genetics and breeding in the College of Agricultural Sciences, 217 Tyson Building, University Park, PA 16802; 814-865-5408; mrf5@psu.edu.

Last Updated May 01, 2002