ring to give -ketobutyrate. In this case, the ethylene precursor ACC is
withdrawn from the pathway, again resulting in a reduced ethylene syn-
thesis. The genes targeted for manipulation of ethylene biosynthesis
comprise AdoMet hydrolase, ACC synthase, N-ACC malonyltrans-
ferase, ACC oxidase, and ACC deaminase.
A second aspect of ripening relates directly to the softening processes
in the fruit. Delay of softening has been achieved by gene suppression
of enzymes that degrade the structural integrity of the cell tissue. Cel-
lulose, hemicelluloses and pectins are cell wall components that con-
tribute to textural characteristics. One of the pectin-modifying enzymes
is polygalacturonase (PG), a pectinase, which hydrolyzes the -1,4 link-
ages in the polygalacturonic acid component of cell walls. PG is syn-
thesized specifically during ripening and is secreted into the intercellular
space of pericarp cells. The function of PG during the natural ripening
process is to break down the pectin of the middle lamella between the
fruit cell walls, thereby causing fruit softening.
The best known case of inhibition of pectin degradation by means of
genetic engineering is the antisense suppression of the PG enzyme gene,
which is the effective principle in the transgenic cultivar ‘FlavrSavr’ by
Calgene. As a different approach, cosuppression of the same enzyme
gene was applied by Zeneca/Petoseed for the production of a tomato
giving a higher-viscosity pulp, which was introduced into the British
market as tomato paste and purée in February 1996. Suppression of
pectin methylesterase (PE), which is also involved in pectin modifica-
tion, and of endo-1,4--glucanase, an enzyme thought to be involved in
fruit softening because it is known to degrade the major hemicellulosic
polymer, xyloglucan, was also patented as approaches to inhibit fruit
softening.
Benefits of Long-Shelf Life Tomatoes
The flavor of fresh market tomatoes is of continuous concern for con-
sumers. Initially, the development of genetically engineered “long-shelf
life” tomatoes was claimed to improve the taste of the fruit because the
tomatoes would ripen on the vine for a longer time and would, thus, ac-
cumulate more flavor than traditional cultivars, which are picked at the
mature green stage (Vanderpan, 1994). However, from the beginning
this was a matter of controversy because tomatoes properly harvested at
the mature green stage will ripen into a product indiscernible from vine-
ripened fruit. Yet, it is a problem to constantly hit the appropriate, rel-
atively shortlasting mature green ripening stage in large-scale tomato