Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

BIOCHEMISTRY OF FRUITS 43
components of fruits. In citrus fruits, these include components such as limonene, myrcene,
and pinene occurring in various proportions. Derivatives of monoterpenes such as geranial,
neral (aldehydes), geraniol, linalool, terpineol (alcohols), geranyl acetate, and neryl acetate
(esters) are also ingredients of the volatiles of citrus fruits. Citrus fruits are especially rich
in monoterpenes and derivatives. α-Farnesene is a major sesquiterpene (C15) component
evolved by apples. The catabolism of α-farnesene in the presence of oxygen into oxidized
forms has been implicated as a causative feature in the development of the physiological
disorder superficial scald (a type of superficial browning) in certain varieties of apples such
as red Delicious, McIntosh, and Cortland (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
infection, etc. In tomato fruits, the level of hmg1 expression is high during early stage
of fruit development when cell division and expansion processes are rapid 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 membranous and cytosolic fractions of apple
fruit skin tissue extract. HMGR is a membrane-localized enzyme, and the activity is detectable
in the endoplasmic reticulum, 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 considerable degree of interaction
between the different enzymes responsible for the biosynthesis of isoprenoids, which may
exist as multienzyme complexes referred to as metabolons. The enzyme farnesyl pyrophosphate
synthase, responsible for the synthesis of farnesyl pyrophosphate, is a cytosolic enzyme.
Similarly, farnesene synthase, the enzyme that converts farnesyl pyrophosphate to
α-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
hmg1 and hmg2, which are differentially expressed during storage. The expression of hmg1
was constitutive and the transcripts (mRNA) were present throughout the storage period. By
contrast, the expression of hmg2 increased during storage in parallel with the accumulation
of α-farnesene. Ethylene production also increased during storage. Ethylene stimulates
the biosynthesis of α-farnesene as evident from the inhibition of α-farnesene biosynthesis
and the expression of hmg2 by the ethylene action inhibitor 1-methylcyclopropene. Thus,
biosynthesis of isoprenoids is a highly controlled process.
Carotenoids, which are major isoprenoid components 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. NADPHmediated
reduction of DOXP leads ultimately to the formation of IPP. Subsequent condensations
of IPP and DMAPP are similar as in the classical mevalonate pathway. Carotenoids