Biology Reference
In-Depth Information
Peumans et al. (1997) were the first investigators to perform any molecular
characterization on nectar proteins. They identified two defence-related
proteins in the nectar of leeks. The mannose-specific lectin from nectar is a
13 kDa protein that was present at about 150 µg/ml of nectar. It belongs to
the structurally and evolutionarily conserved protein family of monocotyledon
mannose-binding lectins. Other proteins from this family have antibiotic
properties, particularly against nematodes and insects with piercing-sucking
mouthparts (Hilder et al., 1995; Powell et al., 1995; Rabhé et al., 1995).
Because the monocotyledon mannose-binding lectins occur in the families
Amaryllidaceae, Alliaceae, Orchidaceae, Araceae, and Liliaceae, Peumans
et al. (1997) also tested nectar from these families. Using a mannose aggluti-
nation assay, these authors found that several
Allium
species (onion, shallot)
along with snowdrop (
Galanthus nivalus
) contained mannose agglutination
positive material. However, no mannose agglutination activity was found in
the nectars of other Amaryllidaceae, orchids and lilies. By feeding leek nectar
to honeybee colonies, they demonstrated that the mannose-binding lectin
is apparently not toxic to honeybees, even though it is toxic to aphids and
nematodes (Hilder et al., 1995; Powell et al., 1995;
Rabhé et al., 1995).
Alliinase (alliin lyase) is a pyridoxal-phosphate-containing enzyme involved
in the production of the organosulphur compounds responsible for both the
odour and pungency of
Allium
plants, as well as for their antimicrobial prop-
erties (Ankri et al., 1997; Perez-Giraldo et al., 2003). Alliinase acts on the
raw sustrate alliin to produce allicin, an active abiotic agent against a wide
variety of organisms, including both Gram-positive and Gram-negative bac-
teria, fungi such as
Candida albicans
, parasites such as
Entamoeba
histolytica
and
Giardia lamblia
, and viruses (Ankri & Mirelman, 1999).
Both of these proteins, mannose-binding lectin and alliinase, are found at
relatively high levels in phloem stalk exudates. In addition, Peumans et al.
(1997) also examined the levels of chitinase in leek nectar. Although present
at significant levels in leek stalk exudates, this enzyme is not significantly
transferred into the nectar of leeks. Mannose-binding lectin and alliinase are
also found in honey produced by bees working leek umbels; the amounts are,
however, reduced by 10-fold and 25-fold, respectively, despite the fact that
honey is concentrated nectar (Peumans et al., 1997).
6.2
Nectar redox cycle
The composition of nectar makes it an excellent microbial growth medium
(Sugiyama et al., 1991; Bubán et al., 2003; Raguso, 2004). Because visiting
