Approaches to Modification of Ripening Characteristics
via Genetic Engineering
The ripening stages in the tomato fruit are referred to as immature,
mature green, breaker, pink and red. Because of their tolerance to rough
handling, tomatoes harvested at the mature green stage constitute the
major fraction of the commercial fresh market tomato crop. Fruit har-
vested mature green hold the longest in storage, shipping, and on the su-
permarket shelf, which is of major interest for producers and industry.
Ripening behavior is a compound trait and is the result of complex phys-
iological pathways. Different approaches have been suggested to mod-
ify genes related to the ripening process. These may be grouped into
efforts to reduce internal ethylene production as the physiological basis
of ripening, and to delay fruit softening at the final steps of ripening.
Some of the targeted genes were expected to have relevance to the major
breeding goals identified by the tomato industry, i.e., viscosity, handling
characteristics, soluble solids, color and taste. Moreover, from these stud-
ies, which were primarily directed toward an understanding of the ripen-
ing process in general, it was expected that approaches to target the
complex trait of “ripening” would be transferable to other fruit and veg-
etable crops including apples, bananas, mangoes, melons, nectarines,
sweet peppers, peaches, pineapples, raspberries and strawberries.
Tomato belongs to the group of climacteric fruits together with, e.g.,
apples, pears, melons and bananas, which develop an autocatalytic re-
lease of ethylene at the beginning of maturation. The physiological ef-
fects of the plant hormone ethylene are manifold and include seed
germination, stimulation of ripening in fruits and vegetables, leaf ab-
scission, fading in flowers, flower wilting, leaf yellowing and leaf
epinasty.
Different approaches have been tested to manipulate the plant’s eth-
ylene biosynthesis (Figure 2.3). The molecular pathway of ethylene for-
mation begins with methionine and ATP to form S-adenosylmethionine,
which is converted into aminocyclopropane carboxylate (ACC) by ACC
synthase. At the last step in ethylene formation, ACC oxidase is an es-
sential enzyme in plants. Since a decrease in the rate-limiting enzymes
ACC synthase or ACC oxidase should result in a concurrent decrease
of ethylene production, these two enzymes have been targets in differ-
ent transgenic antisense or cosuppression approaches. The transgenic
tomato cultivar FreshWorld Farms Endless Summer® (produced by
DNAP), which is a genetically engineered version of the FreshWorld