Farms®tomato, was put on the test market in 1995 and had been engi-
neered by cosuppression of the ACC synthase gene (Transwitch®tech-
nology) leading to an extension of shelf life to 30–40 days postharvest.
The delayed-ripening fruit ripen as usual when exogenous ethylene is
applied. An alternative approach was successfully tried by introducing
enzyme genes that are not originally present in tomato: In one such
system (Ferro et al., 1995) exploited by AGRITOPE Inc., a gene for
S-adenosylmethionine hydrolase (AdoMet hydrolase) was isolated from
the bacteriophage T3 and transferred to tomato. This enzyme hydrolyzes
the intermediate AdoMet by which homoserine and MTA are released.
MTA in turn is a potent inhibitor of ACC synthase. Thus, the principle
is again to decrease ACC activity. In another case, the enzyme gene for
ACC deaminase from Pseudomonas chloraphiswas introduced at Mon-
santo into tomato, which catalyzes the opening of the ACC cyclopropane
Genetic Engineering of Traits Affecting Quality 29FIGURE 2.3Ethylene biosynthesis in higher plants and enzymes targeted or introduced
by genetic engineering. ACC, 1-aminocyclopropane-1-carboxylic acid; Adomet,
S-adenosylmethionine; M-ACC, 1-(malonylamino_cyclopropane-1-carboxylic acid;
G-ACC, 1-(-L-glutamylamino)-cyclopropane-1-carboxylic acid; MTA, 5-methylthio-
adenosine; ,, overexpression and suppression of targeted genes, respectively.