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coat suggested that neither thickness nor the levels of compounds such as tannic acid, tannins,
or HCN were important for the seed resistance. On the other hand, the authors showed that
phaseolin is detected in the seed coat by Western blotting and N-terminal amino acid
sequencing and that this protein was detrimental to the development of
C. maculatus
. Studies
have suggested that the resistance of some
Vigna unguiculata
cultivars seeds to
C. maculatus
attack is due to variant forms of vicilins which are refractory to hydrolysis by midgut
proteinases, what possibly leads to low dietary amino acid supply to the larvae (Xavier-Filho
et al. 1991; Sales el al. 1992; Macedo et al. 1993). The mechanism of action of variant
vicilins seems to be also linked to their chitin-binding power (Sales et al. 1996; Firmino et al.
1996; Yunes el al. 1998). This mechanism may be similar to the one attributed to the action of
chitin-binding proteins (N-acetylglucosamine-specific lectins, chitinases, hevein and
antimicrobial peptides) which are involved in defense mechanisms of plants against insects
and pathogens (Chrispeels & Raikhel, 1991).
A water-soluble polysaccharide from the Jack bean [
Canavalia ensiformis
(L) DC] seed
coat that was shown to be highly detrimental to larval development of the cowpea weevil
C.
maculatus
was also isolated (Oliveira et al. 2001). Determination of the composition and
structure of this polysaccharide showed that it is a galactorhamnan with an M
w
of 883.0,
containing 92 % rhamnose and 8 % galactose. The polymer is formed by a main chain of
rhamnose (1→2) substituted at
O
-4 by galactose non-reducing end units. Immunolocalisation
by light and electron microscopy showed that this polysaccharide is localized in the innermost
cell layer of the seed coat and also in the cytoplasmic space of cotyledonary tissues.
Soybean seed coat proteins have been previously identified, among these a 41 kDa
peroxidase enzyme (Buttery & Buzzel, 1968; Gijzen, 1997), a 32 kDa class I chitinase
(Gijzen et al. 2001), a 21 kDa trypsin inhibitor (Kunitz, 1945; Koide & Ikenaka, 1973) and an
8 kDa hydrophobic protein (Gijzen et al. 1999a); however the functions of these proteins are
not completely understood. Soybean seed coat peroxidases (SBPs) were shown to be very
stable at high temperatures, extremes of pH and in organic solvents (Nissum et al. 2001). The
mature protein showed higher than 70% amino acid sequence identity to peroxidases from
other legumes recruited in various defense response processes (Henriksen et al. 2001).
Examples of such plant defense relation with peroxidases are found in Hammerschmidt et al.
(1982) and El-Turk et al. (1996). Hammerschmidt et al. (1982) showed the association of
enhanced peroxidase activity with induced systemic resistance of cucumber to
Colletotrichum
lagenarium
. El-Turk et al. (1996) reported the nucleotide sequences of four pathogen-induced
alfalfa peroxidase-encoding cDNAs. Gijzen et al. (2001) showed that a 32 kDa soybean seed
coat protein presented an N-terminal cysteine-rich hevein domain typically found in class I
chitinases and in other chitin-binding proteins. The protein was abundant in soluble extracts
from soybean seed coats. We have recently shown that a soybean seed coat protein fraction
was able to inhibit the growth of
Fusarium lateritium
and
Fusarium oxysporum
phytopathogenic fungi. The antifungal fraction revealed the presence of peroxidase, vicilin
and of a 24 kDa protein, homologous to acid phosphatases. Germination experiments revealed
that both acid phosphatase and peroxidase were exuded during seed imbibition what might be
an indicative of a protective role for these proteins during seed germination (Santos et al.
2008).
The expression of the
C. maculatus
detrimental compounds in the seed coats of the non-
host seeds may have been important for the evolutionary discrimination of legume seeds by
this bruchid. Observations done by our group have shown that a great percentage of the
C.
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