Biotechnological Approaches to Enhancing Tropical Fruit Quality - Postharvest Biology and Technology of Fruits, Vegetables, and Flowers - page 378

One example of the above is grapevines. The fruit and wine industries need to main-
tain and potentially expand production despite increasing constraints in the form of pests,
diseases, and temperature stress. However, this must be done without compromising the
quality traits of the crop. Using the CBF genes described above, grapevines tolerant to low
temperatures were recently generated (Fischer et al., 2004).
18.4.3 Protein metabolism
Research in this area has included the increase of essential amino acid content, the expression
of storage protein genes in plant organs other than seeds, and the reduction of the content
of allergenic proteins, by genetic manipulation. Work has been done, for example, on the
transfer of genes-encoding proteins rich in essential amino acids (i.e., methionine and
lysine) from other species. To improve the nutritional quality of soybean, a methionine-rich
2S albumin from the Brazil nut ( Betholletia excelsa ) has been introduced into transgenic
soybeans (Nordlee et al., 1996). However, since the Brazil nut is a known allergenic food, the
resultant transgenic soybean turned out to be allergenic as well. For that reason, methionine-
rich proteins from sources not containing known allergens have been employed in similar
experiments. A maize gene encoding the protein zein increased methionine content by over
80% in transgenic soybean seeds (Baldoni and Rugini, 2001). An amaranth ( Amaranthus
hypochondriacus ) 11S globulin, one the most abundant storage proteins (amarantin) of
the seed, was transferred into tropical maize (Rascon-Cruz et al., 2004). Total protein
and essential amino acids of the best expressing maize lines increased 32 and 8-44%,
respectively, compared to nontransformed plants. To increase the content of lysine in potato,
Sevenier et al. (2002) employed two approaches: Introduction of a feedback-insensitive gene
from Escherichia coli (lysCM4) involved in biosynthesis of the aspartate family of amino
acids resulted in a sixfold increase of the lysine content, whereas introduction of a mutated
form of the key plant enzyme of lysine biosynthesis (dihydrodipicolinate synthase) led to a
15-fold increase in lysine. Allergenicity could also be reduced by genetic transformation.
The 14-16-kDa allergenic proteins from rice have been reduced by using the antisense
technology (Tada et al., 1996) and the same strategies could be applied in fruit crops.
18.4.4 Flavor and carbohydrate metabolism
Considering the importance of flavor in fruits, it is surprising that no major advances have
been made to identify flavor components and enzymes responsible for their biosynthesis in
fruits. This might be a reflection of the complexity of this trait. Since many fruits contain
high quantities of carbohydrates, particularly sucrose, genetic manipulation of the sucrose-
metabolizing enzymes might provide a way to alter sugar content and, in turn, sweetness
of fruit.
There are a large number of reports on the manipulation of different enzymes in-
volved with carbohydrate metabolism. Transgenic plants have been produced containing
invertase in the sense and antisense orientations (Roitsch and Gonzalez, 2004), antisense
granule-bound starch synthase (Liu et al., 2003), antisense phosphorylase (Duwenig et al.,
1997), sense and antisense ADP-glucose pyrophosphorylase (Rober et al., 1996; Weber
et al., 2000), sense and antisense sucrose synthase (Fernie et al., 2002), antisense uridine
diphosphate-glucose pyrophosphorylase (Zrenner et al., 1993), and sense and antisense