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New Crops News, Spring 1993, vol. 3 no. 1

Genetically Engineered High Solid Tomato

Once a leader in processing tomatoes, Indiana has gradually lost its share to California. Tomato acreage has dropped from 97,000 acres in 1935 to 9,000 acres in 1991. One of the reasons for this downhill slide has been the development of new cultivars and technologies which made California the most efficient tomato producer. Breeders have developed new varieties with concentrated fruit set and uniform ripening adapted to mechanical harvesting, creating a niche for Indiana in whole peeled, stewed and diced tomatoes. But more needs to be done to further improve tomato varieties suitable for growing in Indiana's climate.

With the advent of molecular technology, a significant change in agriculture technology is taking place. Molecular technology adds a new dimension to the overall quest of improving agricultural productivity. This technology is based on identifying the molecular unit responsible for the desired agronomic trait and then isolating this unit by the recombinant DNA technology. Once this unit is isolated it can be used to modify any existing variety of a crop plant. A cultivar which has been selected for desirable traits can be improved by introducing a single trait. Thus, this technology provides a continuity between the previous work by plant breeders and the present modification by genetic engineering. Additionally, the cloned gene can be introduced not only to the plant species from which it was isolated, but to other agriculturally important species.

Using tools of molecular engineering, Avtar Handa of the Department of Horticulture at Purdue University has created a new tomato variety with significantly enhanced overall fruit quality. The genetically engineered fruit contain an average increase in soluble solids of 5 to 7%. A recent estimate indicates that each 1% increase in tomato solids could save about $70 million a year in processing costs.

Additionally, chemistry of pectins in the genetically engineered fruits has been modified, resulting in tomatoes with much improved processing characteristics. Several genes involved in fruit development and ripening have been cloned and characterized. One of these genes, encoding pectin methylesterase, was used to genetically engineer tomato fruit. Pectin methylesterase demethoxylates pectins to form pectins with a lower degree of methylesterification, which impairs gel-forming characteristics of fruit pectins. An antisense gene for pectin methylesterase in tomatoes has been introduced to create high solid fruits. Expression of an antisense RNA gene in the genetically engineered fruit caused over 95% reduction in the enzymatic activity of pectin methylesterase.

Under greenhouse conditions, the genetically engineered fruits ripened normally but showed a marked influence on pectins present in fruit. Lower pectin methylesterase activity in the transgenic fruits caused increases in both the size and degree of methylesterification of pectins. The genetically engineered fruits contained significantly higher levels of soluble solids, and this trait was maintained in subsequent generations.

In the summer of 1992, the genetically engineered high solids tomatoes were evaluated for their agronomic performance at the O'Neall Memorial Research Farm with a grant supported by Purdue's New Crops Center. Fruits from the genetically engineered tomato plants contained significantly higher levels of both soluble and total solids than normal 'Rutgers' plants with similar fruit size. However, the total fruit yield was significantly higher in the genetically engineered plant than in 'Rutgers'. No deleterious effect of the introduced gene was observed in the transgenic plant.

Field-grown tomatoes from genetically engineered and normal 'Rutgers' plants have been processed using several different processing methods. The processed juice has been evaluated for various processing characteristics, including pH, titratable acidity, precipitate weight ratio, total solids, serum viscosity, efflux viscosity, and color. Additionally, catsup is being made from the processed juice of both the genetically engineered and wild type fruits. Results indicate a marked improvement in processing characteristics of juice from the genetically engineered fruits.