Tomatoes That Age Gracefully
|Six years ago, the tomato industry thought it had a fresh-market winner—a tomato that could be left to ripen longer on the vine and still remain firm through a couple of weeks of shipping, handling, and sitting on the shelf. But the genetically engineered Flavr Savr tomato was poor tasting and costly to produce, so it shriveled on the vine.
With the growing competition in today's fresh-tomato market—worth nearly $1 billion in 1999—the time is ripe for a tomato that ages gracefully. Researchers in ARS' Horticultural Crops Quality Laboratory at Beltsville, Maryland, are closer to providing the industry with the needed tools—a clearer picture of some of the genes involved in turning a nice, firm tomato into an undesirable, mushy one.
In their first success, Kenneth C. Gross, who heads the lab, and molecular biologist David L. Smith have produced ripe tomatoes that are 40 percent firmer than unmodified siblings and stay firmer for at least 2 weeks. The researchers inserted the backward version of the gene for an enzyme that removes the sugar galactose from cell walls. Galactose is a component of pectins, a major part of the scaffolding of cell walls. Structurally sound cell walls are essential to tomato firmness, explains Smith, and the loss of galactose appears to play a key role in the loss of structural integrity.
The concept is similar to that used for the Flavr Savr tomato, but it targets a different component of pectin, says Smith. "We're focusing on galactose because it's the sugar that changes most throughout fruit development."
The two scientists identified and sequenced seven different genes that code for the galactose-removing enzyme—beta-galactosidase. U.S. and international patent applications on all seven genes have been filed for ARS, citing Gross and Smith as the inventors. The two have inserted five of those genes into the tomato genome. But so far they have tested tomatoes from only one of the reversed, or antisense, genes—number 4.
"We want to know why there are seven of these genes," says Gross. "All are turned on during development, but they have different patterns of expression. Maybe they have other functions," he says, noting that one may affect fruit color.
"We want to change one thing without changing anything else," Smith adds. He and Gross also want to find other genes involved in fruit softening in hopes of further improving firmness or viscosity. Tomato canners love viscous, or nonwatery, tomatoes because less cooking is needed to produce thick sauce.
Lessons learned from the tomato—the most popular model for studying the ripening process—could lead to firmer peaches or crisper apples, notes Smith.—By Judy McBride, Agricultural Research Service Information Staff.
Kenneth C. Gross and David L. Smith are with the USDA-ARS Horticultural Crops Quality Laboratory, 10300 Baltimore Blvd., Bldg. 002, Beltsville, MD 20705-2350; phone (301) 504-6128; fax (301) 504-5107.
"Tomatoes That Age Gracefully" was published in the December 2000 issue of Agricultural Research magazine.