508 Part V: Fruits, Vegetables, and Cereals
varieties of apples such as red Delicious, McIntosh,
Cortland, and others (Rupasinghe et al. 2000, 2003).
HMGR is a highly conserved enzyme in plants
and is encoded by a multigene family (Lichtenthaler
et al. 1997). The HMGR genes (hmg1, hmg2, hmg3,
etc.) are nuclear encoded and can be differentiated
from each other by the sequence differences at the
3’-untranslated regions of the cDNAs. There are
three distinct genes for HMGR in tomato and two in
apples. The different HMGR end products may be
localized in different cellular compartments and are
synthesized differentially in response to hormones,
environmental signals, pathogen infections, and so
on. In tomato fruits, the level of hmg1expression is
high during the early stage of fruit development
when cell division and expansion processes are rap-
id and require high levels of sterols for incorporation
into the expanding membrane compartments. The
expression of hmg2, which is not detectable in
young fruits, increases during the latter part of fruit
maturation and ripening.
HMGR activity can be detected in both membra-
nous and cytosolic fractions of apple fruit skin tissue
extract. HMGR is a membrane-localized enzyme,
and the activity is detectable in the endoplasmic
reticular, plastid, and mitochondrial membranes. It is
likely that HMGR may have undergone proteolytic
cleavage, releasing a fragment into the cytosol, which
also possesses enzyme activity. There is a consider-
able degree of interaction between the different
enzymes responsible for the biosynthesis of iso-
prenoids, which may exist as multienzyme complex-
es referred to as metabolons. The enzyme farnesyl
pyrophosphate synthase, responsible for the synthe-
sis of farnesyl pyrophosphate, is a cytosolic enzyme.
Similarly, farnesene synthase, the enzyme that con-
verts farnesyl pyrophosphate to alpha-farnesene in
apples, is a cytosolic enzyme. Thus several enzymes
may act in concert at the cytoplasm–endoplasmic
reticulum boundary to synthesize isoprenoids.
HMG-CoA reductase expression and activities in
apple fruits are hormonally regulated (Rupasinghe
et al. 2001, 2003). There are two genes for HMGR
in apples, designated as hmg1and hmg2,which are
differentially expressed during storage. The expres-
sion of hmg1was constitutive, and the transcripts
(mRNA) were present throughout the storage peri-
od. By contrast, the expression of hmg2increased
during storage in parallel with the accumulation of
alpha-farnesene. Ethylene production also increased
during storage. Ethylene stimulates the biosynthesis
of alpha-farnesene, as is evident from the inhibition
of alpha-farnesene biosynthesis and the expression of
hmg2by the ethylene action inhibitor 1-methylcy-
clopropene (MCP). Thus, biosynthesis of isopre-
noids is a highly controlled process.
Carotenoids, which are major isoprenoid compo-
nents of chloroplasts, are biosynthesized through the
Rohmer pathway. The precursors of this pathway
are pyruvate and glyceraldehyde-3-phosphate, and
through a number of enzymatic steps, 1-deoxy-D-
xylulose-5-phosphate (DOXP), a key metabolite of
the pathway, is formed. NADPH-mediated reduction
of DOXP leads ultimately to the formation of
isopentenyl pyrophosphate (IPP). Subsequent con-
densation of IPP and DMAPP are similar as in the
classical mevalonate pathway. Carotenoids have a
stabilizing role in photosynthetic reactions. By virtue
of their structure, they can accept and stabilize ex-
cess energy absorbed by the light-harvesting com-
plex. During the early stages of fruit development,
the carotenoids have a primarily photosynthetic fun-
ction. As the fruit ripens, the composition of caro-
tenoids changes to reveal the colored xanthophyll
pigments. In tomato, lycopene is the major caro-
tenoid pigment that accumulates during ripening.
Lycopene is an intermediate of the carotene biosyn-
thetic pathway. In young fruits, lycopene formed by
the condensation of two geranylgeranyl pyrophos-
phate (C 20 ) moieties, mediated by the enzyme phy-
toene synthase, is converted to beta-carotene by the
action of the enzyme sesquiterpene cyclase. How-
ever, as ripening proceeds, the levels and activity of
sesquiterpene cyclase are reduced, leading to the
accumulation of lycopene in the stroma. This leads
to the development of red color in ripe tomato fruits.
In yellow tomatoes, the carotene biosynthesis is not
inhibited, and as the fruit ripens, the chlorophyll
pigments are degraded, exposing the yellow caro-
tenoids. Carotenoids are also major components that
contribute to the color of melons. Beta-carotene is
the major pigment in melons with an orange flesh. In
addition, the contribution to color is also provided
by alpha-carotene, delta-carotene, phytofluene, phy-
toene, lutein, and violaxanthin. In red-fleshed melons,
lycopene is the major ingredient, whereas in yellow-
fleshed melons, xanthophylls and beta-carotene pre-
dominate. Carotenoids not only provide a variety of