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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
58 Postharvest Diseases of Fruits and Vegetables
(see the chapter on Host Protection and Defense Mechanisms -
Inhibitors of Cell Wall Degrading Enzymes).
Many fungi are capable of utilizing cellulose for their growth, when it
is present as the sole source of carbon in the culture medium, thanks to
their ability to produce and secrete cellulases. Cellulose, which is a linear
polymer of D-glucose, the configuration being that of a 1,4-P-glucoside, is
the principal polysaccharide of plants, being an important component of
the plant cell wall, mainly of the secondary wall. Cellulose is by far the
largest reservoir of biologically utilizable carbon on the surface of the
earth, and its decomposition, in which fungi probably play a major role,
is of great ecological importance. Most fungal cellulases are induced
enzymes, the inducing substrate being cellulose in the medium. However,
various microorganisms, such as the fungi, R, stolonifer, Penicillium
digitatum and P. italicum, produce and secrete cellulase independently of
the presence of cellulose (Spalding, 1963; Barkai-Golan and Karadavid,
1992).
Although plant tissue decomposition and cell death during disease
development are strongly connected with the activity of extracellular
pectolytic enzymes, the pathogen cellulases, as well as hemicellulases
(the enzymes responsible for breakdown of hemicelluloses - plant cell
wall constituents, particularly polysaccharides and polyuronides), may
also play a role in this complex process. The description of the
involvement of cellulases in pathogenesis has generally addressed the
late stages, the saprophytic stages of pathogen development (Bateman
and Basham, 1976). However, studies of the Cx-cellulase (the enzyme
splitting the polymeric chain of cellulose to form cellobiose residues) of P.
digitatum and P. italicum in citrus fruits, revealed high levels of
enzymatic activity during the incubation period of the disease, before the
appearance of disease symptoms; moreover, a correlation was found
between cellulase activity of the pathogen in the fruit and the severity of
the disease symptoms (Barkai-Golan and Karadavid, 1992). These
findings led to the suggestion that the cellulolytic enzymes of the two
Penicillia may play an active role in the early stages of pathogenesis,
including the penetration of the fungus into the tissue. In tomato
genotypes a correlation was drawn between the level of cellulolytic
activity in the fruit prior to infection and the extent of cellulolytic activity
of Alternaria alternata in infected fruit tissues. Thus, the cellulolytic
activity of the pathogen in the ripening normal fruit was higher than
those in the non-ripening nor mutant and its hybrid at each stage of
http://arab2000.forumpro.fr