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Why delay fruit ripening?
How does delayed fruit ripening work?

At present, only tomatoes have been marketed with GE delayed-ripening traits. Delaying the ripening process in fruit is of interest to producers because it allows more time for shipment of fruit from the farmer's fields to the grocer's shelf, and increases the shelf life of the fruit for consumers. Although ripening makes fruit edible and flavorful, it also begins the gradual decline of the fruit towards softening and rot, causing losses for producers and consumers.

The development of fruit in many plants follows a two-step process. First, after pollination, parts within the flower of the plant expand and develop into a full-sized fruit. Second, the full-sized fruit undergoes the process called ripening, a complex set of molecular and physiological changes in the fruit. The ripening process brings dramatic changes to the fruit-- softening of cell walls, production of color compounds, and changes in sugar content, flavor and aroma. In tomatoes and many other fruits, ripening begins when the fruit produces a volatile compound called ethylene. (Because ethylene is a gas released into the air, one ripe tomato can speed up the ripening of other tomatoes when they are stored in the same bag.)

To increase the shelf life of tomatoes (and to prevent bruising during shipping), it is a common practice to pick tomatoes while they are still green, and later, when the tomatoes are closer to the grocer's shelf, spray them with ethylene to induce ripening. However, it is widely believed that this makes the flavor of the tomatoes much more bland, particularly in winter tomatoes-- in the winter, tomatoes are shipped longer distances from warmer growing regions and are more likely to have been harvested green (refrigeration in shipping may also reduce flavor). This probably explains the popularity of vine-ripened tomatoes, left on the plant until the first patch of red appears (not necessarily until they are completely red). Fruit that are genetically engineered to have delayed ripening can be left to mature on the plant longer, will have longer shelf-life in shipping, and may last longer for consumers.
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Because ethylene is the main trigger for fruit ripening, several genetic engineering strategies involve the reduction or prevention of ethylene production. Tomato fruits that do not produce ethylene develop fully on the plant and then stop before ripening and turning red. These "paused" fruits can all be picked later in development and all at the same time, whereas the green-picking method requires the field to be constantly inspected to pick each fruit just before it begins to turn red (which isn't always easy to tell). The GE fruit must also be sprayed with ethylene to induce ripening. There are at least three methods employed to genetically-engineer a reduction in ethylene:

1) Decreased ACC synthase:
ACC synthase is an enzyme in tomato fruits that is responsible for the next-to-the-last step in the synthesis of ethylene in the fruit. Reducing levels of ACC synthase dramatically decreases ethylene production. Scientists discovered that when a "mirror-image" copy (antisense) of the ACC synthase gene is genetically engineered into tomatoes, a quirk of the plant's genetic editing machinery prevents the production of both a backwards ACC enzyme and the normal, "forward" version already in the plant. This "antisense" technique also has the advantage that no new protein is produced in the plant (see GE and Food Allergies). DNA Plant Technologies developed this kind GE tomatoes, and they are marketed as "Endless Summer."

2) Addition of ACC deaminase:
Scientists have added to the tomato a new gene from the soil bacteria Pseudomonas chlororaphis. The gene encodes an enzyme called ACC deaminase, which causes the breakdown of one of the precursors of ethylene synthesis (ACC). The reduction in levels of this precursor causes reduced ethylene production and delayed ripening. Monsanto developed this tomato, but it has not yet been marketed.

3) Addition of SAM hydrolase:
Like tomatoes with added ACC deaminase, tomatoes genetically engineered to produce SAM hydrolase (from the E. coli T3 bacteriophage) also break down one of the precursors of ethylene synthesis (SAM). This technology has been developed by Agritope, Inc. for a variety of cherry tomato.

Finally, some tomatoes have been genetically engineered to alter one particular aspect of fruit ripening: softening. The process of fruit softening is caused in part by the breakdown of pectins-- compounds which give support to the cell walls of fruit. Tomatoes have been engineered to have reduced levels of a pectin-breakdown enzyme called polygalacturonase (PG) using the same "antisense" technique applied to ACC synthase (see above). Not only does this increase shelf-life, but it also improves qualities of interest to tomato processors-- the tomato products are "thicker" (higher pectin to water ratio). This is the technique used in the well-known "FlavrSavr" tomato (developed by Calgene, now owned by Monsanto).
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