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

FREEDOM PALESTINE FREEDOM PALESTINE FREEDOM PALESTINE
Chemical Control 185
since the end of the 20^^ century. However, the role of plant lectins is still
not well defined and understood. One of the theories proposed
hypothesizes that lectins act as recognition determinants in the
formation of symbiotic relations between leguminous plants and
nitrogen-fixing bacteria, a process of great importance in agriculture and
in the nitrogen cycle of terrestrial life. According to a second hypothesis,
these lectins can play a role in the defense of plants against various
animals, as well as against phytopathogenic fungi (Sharon, 1997).
Interactions of fungal hyphae with lectins were first demonstrated by
Mirelman et al. (1975), who found that wheat germ agglutinin (WGA),
which is specific to chitin oligosaccharides, binds to hyphal tips and
hyphal septa of Trichoderma viride - a fungus with a chitinous hyphal
surface. This interaction resulted in the inhibition of hyphal growth and
spore germination. Based on these findings it was suggested that WGA
has a role in the protection of wheat seedlings against chitin-containing
fungi.
To assess whether the binding of lectins to fungal surfaces and their
inhibiting effects on fungal growth are general phenomena, Barkai-Golan
et al. (1978) investigated the interaction of several lectins, characterized
by differing sugar specificities, with various fungi belonging to different
taxonomic groups. WGA was found to bind to young hyphal walls of all
the fungi examined except for the chitinless Phytopthora citrophthora
(Table 11). Soybean agglutinin (SBA), specific for D-galactose and
N-acetyl-D-galactose, and peanut agglutinin (PNA), specific for
D-galactose, were found to bind those fungi that contain galactose
residues in their chitinous cell walls. Among these were several species of
Penicillium and Aspergillus. Other fungi, such as Geotrichum candidum,
Botrytis cinerea and Fusarium moniliforme, exhibited binding with SBA
alone. The abilities of SBA and PNA to bind to fungal surfaces of
Penicillium and Aspergillus species is in good agreement with the
presence of galactose reported in some species of these genera, such as
Penicillium digitatum and P. italicum (Grisaro et al., 1968) or
Aspergillus niger (Bardalaye and Hordin, 1976). The binding of fungi to
concanavalin A (Con A) is more difficult to account for since this lectin
reacts only poorly with p-linked glucans and with chitin (Sharon and Lis,
1989). It is possible, however, that a positive reaction with Con A
indicates the presence of small quantities of a-linked D-glucose (or
D-mannose) residues on the fungal surface (Barkai-Golan et al., 1978).
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