Newly Identified Tomato Gene Appears Key To Fruit Softening

May 27, 1997

A gene known to control production of proteins believed to allow plant cells to elongate during growth also appears to play a key role in fruit ripening, according to researchers at the University of California, Davis.

The researchers, who identified and cloned the gene in tomatoes, suspect the gene could be useful for genetically engineering tomatoes, as well as other fragile and perishable fruit, for which shipping quality and shelf life are critical. The University of California has applied to patent the gene, and the researchers will report their findings in the May 26 issue of the Proceedings of the National Academy of Sciences.

The gene, LeExp1, is a member of a family of genes formerly shown to coordinate production of proteins appropriately named "expansins." These expansin proteins, identified so far in cucumbers, rice, tomatoes and the research plant Arabidopsis, work by loosening the plant cell walls and allowing the cells to elongate during growth.

"The plant cell wall is composed of a complex matrix of compounds," said Jocelyn Rose, a plant biology doctoral candidate, who identified and cloned the newly discovered gene. "A major goal for many scientists in this field has been to find one gene that can be identified as being the major player in causing tissue disintegration that occurs in softening fruit," he explained.

Rose works in the laboratory of Alan Bennett, a professor in the vegetable crops department and associate dean of the plant sciences division of the UC Davis College of Agricultural and Environmental Sciences.

Conventional wisdom among scientists studying plant cells walls has held that pectin was the wall component critical to fruit softening. In fact, a genetically engineered tomato was commercialized in recent years based on that theory. However, subsequent molecular genetics studies have indicated that while pectin may determine other aspects of fruit quality, it is not the primary determinant of fruit softening, according to Rose, who hopes that the expansins may hold the key to controlling fruit softening.

Rose and Bennett suspected a process similar to the growth-related cell wall degradation might be occurring during fruit softening, but they were surprised to find the expansin proteins being produced so quickly and at such high levels in ripening fruit, which are no longer growing.

"In ripening fruit, the cells aren't expanding so you wouldn't expect to find expansins," said Rose. "It appears, then, that expansins may have a totally different role during ripening."

The researchers found the LeExp1 gene only in the fruit, not in the vegetative parts, of the tomato plant. And the gene was found only when ethylene -- a hormone known to induce and coordinate fruit ripening -- was either present in the plant or applied externally.

The researchers also identified genes closely related to LeExp1 in ripening melons and strawberries, an indication that this gene may be common in fruits that soften rapidly as they ripen.

They believe that the expansin proteins produced by the LeExp1 gene may act by disrupting the hydrogen bonds between key compounds in the cell wall matrix. Once those bonds are broken, the compounds are vulnerable to destruction by enzymes in the plant cell wall.

"It might be possible to turn off the gene in any fruit such as tomatoes, strawberries or bananas, which undergo rapid deterioration, so that they would soften later during the ripening process and have a longer shipping and shelf life," said Rose.

He and colleagues are currently testing this possibility in genetically engineered tomato plants.

This work to date on the LeExp1 gene was supported by the University of California. New investigation of the action of the gene will be funded by a UC Biotechnology-STAR grant, which seeks to develop the commercial potential of UC research for California-based companies.

University of California - Davis

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