Post harvest diseases fruits and vegetables - Xavier University ...

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
56 Postharvest Diseases of Fruits and Vegetables
diffusion of nutrients which can serve as a medium for pathogen
development (Mount, 1978).
Two groups of pectolytic enzymes take part in the disintegration of the
pectic substances. The first group includes pectin methylesterase (PME),
which breaks the ester bonds and separates the methyl groups from the
carboxylic groups of the pectin or pectinic acid, to yield pectic acid and
methanol. Enzymes of this group do not break the pectin chain. The
second group includes pectolytic enzymes that break the 1,4-glycoside
bonds between the galacturonic acid subunits. The glycoside bonds may
be broken by the hydrolytic activity of polygalacturonases (PG), which
act on pectic acid chains, or by that of pectin methylgalacturonases
(PMG), which act on chains of pectin or pectinic acid. The glycosidic
bonds may also be broken by the action of pectin or pectate lyase (PL),
also called transeliminase (PTE). These enzymes cleave the 1-4-glycoside
linkages by unsaturating the ring between the fourth and the fifth
carbon atoms in the subunits of the chain (Bateman and Millar, 1966).
Thus, both the hydrolytic enzymes and the lyases attack the
galacturonide links of the polymeric chain and cause it to break, but they
differ from one another in their mode of action: the polygalacturonases
act hydrolytically, while the lyases function in a lytic way by
transelimination.
According to the point of attack in the pectic compound, the
polygalacturonases and the lyases are classified as endo-enzymes, that
break the galacturonide connections at random while producing
oligogalacturonides, or as exo-enzymes, which attack only the terminal
linkages. Disintegration and softening of the tissues (maceration),
resulting in the formation of soft to watery rot caused by species of
Rhizopus, Penicillium, Geotrichum and Sclerotinia, and by the bacterium
Erwinia carotovora in many fruits and vegetables during storage, is
generally related to the extracellular activity of endo-PG (Rombouts and
Pilnik, 1972). However, the involvement of exo-PG in Penicillium
digitatum pathogenesis in citrus fruits has been reported (Barash and
Angel, 1970; Barmore and Brown, 1979). Enzymes from the
endo-pectin-lyase group have been described as the main cause of the
maceration of citrus fruits infected by Penicillium italicum or P.
digitatum (Bush and Codner, 1970). However, in more recent studies the
enzymes causing maceration of citrus fruit peel by these fungi were
identified by Barmore and Brown (1979, 1980) as endo-PG and exo-PG,
respectively.
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