Six years ago, the tomato industry thought it
had a fresh-market winnera 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 marketworth nearly
$1 billion in 1999the 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 toolsa 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 enzymebeta-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, genesnumber 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 tomatothe most popular model for studying the
ripening processcould lead to firmer peaches or crisper apples, notes
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
issue of Agricultural Research magazine.