Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

40 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
that act downstream from phospholipase D, for the phospholipids. Thus, phosphatidate phosphatase,
lipolytic acyl hydrolase, and lipoxygenase do not directly act on phospholipids,
though there are exceptions to this rule. Therefore, the degree of membrane lipid catabolism
will be determined by the extent of activation of phospholipase D.
The membrane lipid catabolic pathway is considered as an autocatalytic pathway. The
destabilization of the membrane can cause the leakage of calcium and hydrogen ions from
the cell wall space, as well as the inhibition of calcium and proton ATPases, the enzymes
responsible for maintaining a physiological calcium and proton concentration within the
cytoplasm (calcium concentration below micromolar range, pH in the 6–6.5 range). Under
conditions of normal growth and development, these enzymes pump the extra calcium and
hydrogen ions that enter the cytoplasm from storage areas such as apoplast and the ER
lumen, in response to hormonal and environmental stimulation using ATP as the energy
source. The activities of calcium and proton ATPases localized on plasma membrane, endoplasmic
reticulum, and the tonoplast are responsible for pumping the ions back into the
storage areas. In fruits (and other senescing systems), with the advancement in ripening and
senescence, there is a progressive increase in leakage of calcium and hydrogen ions. Phospholipase
D is stimulated by low pH and calcium concentration over 10 μM. Thus, if the
cytosolic concentrations of these ions progressively increase during ripening or senescence,
the membranes are damaged as a consequence. However, this is an inherent feature of the
ripening process in fruits, which results in the development of ideal organoleptic qualities
that makes them edible. The uncontrolled membrane deterioration can result in the loss of
shelf life and quality in fruits.
The properties and regulation of the membrane degradation pathway are increasingly
becoming clear. Enzymes such as phospholipase D (PLD) and lipoxygenase (LOX) are very
well studied. There are several isoforms of phospholipase D designated as PLD-α, PLD-β,
PLD-γ , etc. The expression and activity levels of PLD-α are much higher than that of the
other PLD isoforms. Thus, PLD-α is considered as a housekeeping enzyme. The regulation
of PLD activity is an interesting feature. PLD is normally a soluble enzyme. The secondary
structure of PLD shows the presence of a segment of around 130 amino acids at the N-
terminal end, designated as the C2 domain. This domain is characteristic of several enzymes
and proteins that are integral components of the hormone signal transduction system. In
response to hormonal and environmental stimulation and the resulting increase in cytosolic
calcium concentration, C2 domain binds calcium and transports PLD to the membrane where
it can initiate membrane lipid degradation. The precise relation between the stimulation of
the ethylene receptor and phospholipase D activation is not fully understood, but could
involve the release of calcium and migration of PLD to the membrane. PLD-α appear to
be the key enzyme responsible for the initiation of membrane lipid degradation in tomato
fruits. Antisense inhibition of PLD-α in tomato fruits resulted in the reduction of PLD
activity and, consequently, an improvement in the shelf life, firmness, soluble solids, and
lycopene content of the ripe fruits (Oke et al., 2003; Pinhero 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 as both soluble and membranous forms in tomato fruits (Todd et al.,
1990). Very little information is available on phosphatidate phosphatase and lipolytic acyl
hydrolase in fruits.