506 Part V: Fruits, Vegetables, and Cereals
between the stimulation of the ethylene receptor and
phospholipase D activation is not fully understood,
but it could involve the release of calcium and mi-
gration of PLD to the membrane. PLD-alpha ap-
pears to be the key enzyme responsible for the ini-
tiation of membrane lipid degradation in tomato
fruits. Antisense inhibition of PLD-alpha in tomato
fruits resulted in the reduction of PLD activity and,
consequently, an improvement in the shelf life, firm-
ness, soluble solids, and lycopene content of the ripe
fruits (Pinhero et al 2003, Oke et al. 2003). There
are other phospholipid-degrading enzymes such as
phospholipase C and phospholipase A 2. Several roles
of these enzymes in signal transduction processes
have been extensively reviewed (Wang 2001, Meijer
and Munnik 2003).
Lipoxygenase exists in both soluble and membra-
nous forms in tomato fruits (Todd et al. 1990). Very
little information is available on phosphatidate phos-
phatase and lipolytic acyl hydrolase in fruits.
PROTEOLYSIS ANDSTRUCTUREBREAKDOWN IN
CHLOROPLASTS
The major proteinaceous compartments in fruits are
the chloroplasts, which are distributed in the epider-
mal and hypodermal layers of fruits. The chloroplasts
are not very abundant in fruits. During senescence,
the chloroplast structure is gradually disassembled,
with a decline in chlorophyll levels due to degrada-
tion and disorganization of the grana lamellar stacks
of the chloroplast. With the disorganization of the
thylakoid, globular structures (plastoglobuli) that are
rich in degraded lipids accumulate within the
chloroplast stroma. The degradation of chloroplasts
and chlorophyll results in the unmasking of other
colored pigments and is a prelude to the state of
ripening and the development of organoleptic quali-
ties. Mitochondria, which are also rich in protein,
are relatively stable and undergo disassembly during
the latter part of ripening and senescence.
Chlorophyll degradation is initiated by the en-
zyme chlorophyllase, which splits chlorophyll into
chlorophyllide and the phytol chain. The phytol
chain is made up of isoprenoid units (methyl-1,3-
butadiene), and its degradation products accumulate
in the plastoglobuli. Flavor components such as 6-
methyl-5-heptene-2-one, a characteristic component
of tomato flavor, are also produced by the catabo-
lism of the phytol chain. The removal of magnesium
from chlorophyllide results in the formation of pheo-
phorbide. Pheophorbide, which possesses a tetra-
pyrrole structure, is converted to a straight-chain
colorless tetrapyrrole by the action of pheophorbide
oxidase. Action of several other enzymes is neces-
sary for the full catabolism of chlorophyll. The pro-
tein complexes that organize the chlorophyll, the
light-harvesting complexes, are degraded by the ac-
tion of several proteases. The enzyme ribulose-bis-
phosphate carboxylase/oxygenase (rubisco), the key
enzyme in photosynthetic carbon fixation, is the
most abundant protein in chloroplast. Rubisco levels
also decline during ripening/senescence due to pro-
teolysis. The amino acids resulting from the cata-
bolism of proteins may be translocated to regions
where they are needed for biosynthesis. In fruits,
they may just enrich the soluble fraction with amino
acids.SECONDARY PLANT PRODUCTS
AND FLAVOR COMPONENTSSecondary plant products are regarded as metabolites
that are derived from primary metabolic intermed-
iates through well-defined biosynthetic pathways.
The importance of the secondary plant products to
the plant or organ in question may not readily be
obvious, but these compounds appear to have a role
in the interaction of the plant with the environment.
The secondary plant products may include nonpro-
tein amino acids, alkaloids, isoprenoid components
(terpenes, carotenoids, etc.), flavonoids and antho-
cyanins, ester volatiles, and several other organic
compounds with diverse structures. The number and
types of secondary plant products are enormous, but
from the perspective of fruit quality, the important
secondary plant products include isoprenoids, antho-
cyanins, and ester volatiles.ISOPRENOIDBIOSYNTHESISIn general, isoprenoids possess a basic five-carbon
skeleton in the form of 2-methyl-1,3-butadiene (iso-
prene), which undergoes condensation to form larg-
er molecules. There are two distinct pathways for
the formation of isoprenoids, the acetate/mevalonate
pathway (Bach et al. 1999) localized in the cytosol,
and the DOXP pathway (Rohmer pathway; Rohmer
et al. 1993), localized in the chloroplast (Fig. 21.7).
The metabolic precursor for the acetate/mevalonate