that similar gene constructs will have impacts on parthenocarpic fruit
production in other crops such as cucumber.
After the initial genetic engineering work in major crops, the future
will certainly bring about a broadening of the range of crops that are
subject to transgenic approaches. Softening inhibition, e.g., with poly-
galacturonase or pectin methylesterase as targets will probably be con-
ducted in more fruits such as strawberries, grapes and possibly carrots.
Similarly, ethylene biosynthesis will be altered by genetically modify-
ing the expression of ACC synthases, deaminases, oxidases, or AdoMet
hydrolases in additional climacteric fruits such as melons, bananas,
pears, apples, and peaches as well as vegetables like broccoli to increase
the shelf life of the produce. The fact that the effects of ethylene are
quite broad opens the possibility of modifying several characters at a
time by altering the plant’s ethylene production. On the other hand, it
means that for a given crop the transgenic manipulation may have to be
fine-tuned to avoid undesirable side effects. This caution also applies to
other approaches to modifying complex quality traits. For instance, ini-
tial expectations that suppression of pectin esterase would improve re-
sistance of tomatoes to rough handling could not be verified. Even
suppression of PE down to 10% of the original level did not result in
reduced bruising susceptibility. The reason probably is that tissue tex-
ture is dependent on the presence of Ca^2 chelate complexes, which
form if a sufficiently high number of adjacent carboxylic residues are
present in the tissue. A high esterification state caused by reduced PE,
however, results in an adverse situation (R. Carle, personal communi-
cation). The physiological complexity of most quality traits may con-
stitute a major problem of genetic engineering, which for methodogical
limits has been confined to the modification of single or few genes. Thus,
limits in the extent of control of biosynthetic trafficking by genetic en-
gineering are still significant.
Fine-tuned manipulation of biosynthetic pathways may become fea-
sible as the control elements of these pathways are worked out in more
detail. In the case of ethylene, e.g., a promising approach would be to
regulate the effect of ethylene instead of turning down its abundance in
the plant. As a possible way to this end the ethylene receptors of the tar-
geted plant tissue or developmental stage may be modified to abolish
their sensitivity to the plant hormone. Thus, ethylene-sensitive processes
not related to the quality trait to be modified would remain untouched.
An ethylene receptor and a related gene, etr1and ers, together with a
number of downstream signal transduction components have already
been identified in Arabidopsis(Fluhr and Matoo, 1996). Homologous
Future Prospects 39