production. Fractions of fruit harvested immature may have an impact
on the consumer’s overall impression of taste and texture characteristics
of commercial tomatoes even when they have been treated with ethyl-
ene to red ripeness. Tomato varieties with delayed ripening are, how-
ever, not expected to become a remedy for a lack of flavor because the
new trait will be exploited by industry for extending shelf life after har-
vest rather than for longer ripening time on the vine. In addition, flavor
in any fruit is a multiple-factor trait dependent on the genetic background
of a cultivar and may, thus, not be expected to undergo substantial im-
provement by modifying a single gene. Consequently, for any of the
transgenic tomatoes that exhibit reduced ethylene biosynthesis or de-
layed softening, improved taste may be regarded as a secondary bene-
fit, far outweighed by the economics of improved production and
processing qualities.
Approaches to Improving Processing Characteristics
via Genetic Engineering
Processing characteristics are in part related to the aspects that also
apply to fresh marketability, e.g., genetic inhibition of PG expression as
described above to increase the viscosity and glossy appearance of
tomato paste. Solids content, which is important for processing of toma-
toes, is another quality trait that has been genetically modified by dif-
ferent approaches. Again, pectin esterase gene suppression has been
applied to increase soluble solids content by about 15%. A field release
was conducted first in 1993 sponsored by the company Heinz. Invertase
gene suppression (Ohyama et al., 1995; Klann et al., 1996) was ap-
proached by PetoSeeds to increase soluble solids that mainly consist of
polysaccharides. Since the balance of sugars and acids has a major im-
pact on the flavor of a fruit, attempts to modify the soluble solids are
also of interest for the fresh market. Another interesting approach is to
modify the expression pattern of the plant hormone cytokinin, which is
thought to be involved in the generation of a metabolic sink in young
reproductive tissues. As a consequence, assimilates of the plant are di-
rected towards such a sink. The formation of a cytokinin precursor,
isopentenyl AMP, is catalyzed by the enzyme isopentenyl transferase in
a side chain of the carotenoid pathway (see below). To widen the
source/sink relationship between the assimilating tissue and the grow-
ing tomato, fruit researchers at Calgene connected a gene for isopen-
tenyl transferase with a promotor specific for the ovary. As a result, the
sink situation in the growing fruit was pronounced, leading to signifi-
Genetic Engineering of Traits Affecting Quality 31