Biology Reference
In-Depth Information
In contrast, oligocarrageenans, which are chemically similar to oligoagars, did not
lead to any defence response in
Gracilaria
sp.
“dura”
1
(Weinberger et al.
1999
).
In
Gracilaria
(
Gracilaria
“
sensu stricto
”) and the closely related
Hydropuntia
species (Gurgel and Fredericq
2004
), agar oligosaccharides induce a fast and transient
oxidative burst in which relatively high amounts of H
2
O
2
(
100 nmol g
1
min
1
)
are released into the medium after the first challenge. It is assumed that agar oligosac-
charides are presumably perceived by cell membrane-bound, specific receptors which
may trigger a subsequent signal transduction pathway. In fact, the lack of oxidative
burst due to the use of the protein kinase inhibitor staurosporine demonstrates
that phosphorylation of signal molecules and/or proteins seems to be crucial to relay
the “signal of an attack” toward a H
2
O
2
producing agent. ANADPHoxidase located in
the plasma membrane appears to be activated by the signal transduction pathway.
Inhibitor studies with DPI (diphenylene iodonium), which is a specific inhibitor of
NADH- and NADPH-dependent enzymes (Weinberger et al.
2005b
), showed that
H
2
O
2
was not released in species of these both genera. These findings suggest that
ROS production for pathogen defence by an activation of a NADPH oxidase requires
phosphorylation events in a signal transduction pathway which is triggered by
oligoagar-specific receptors.
An alternative mechanism leading to strong H
2
O
2
release in response to agar
oligosaccharides has been shown to be present in species related to
Gracilaria
chilensis
(
G. chilensis
clade) and those of the genus
Gracilariopsis
(Weinberger
et al.
2010
). In these species, fast H
2
O
2
production (
>
100 nmol g
1
min
1
) results
from direct oxidation of agar oligosaccharides by an agar oligosaccharide oxidase.
Inhibitor studies have proven that neither NADPH oxidases nor kinase-mediated
phosphorylation events are involved in H
2
O
2
release. Moreover, oxidation products
of agar oligosaccharides are able to activate expression of genes related to agar
oligosaccharide oxidase within the first 24 h after the challenge (Weinberger et al.
2010
). In contrast to the members of the
Gracilaria
“
sensu lato
” major clade
(
Gracilaria
“
sensu stricto
”and
Hydropuntia
) which show a refractory state of oxida-
tive burst after first challenge, H
2
O
2
is repeatedly released without any delay after a
previous challenge in species of the “
G. chilensis
clade” and the genus
Gracilariopsis
.
A comparative study of agar oligosaccharide responses within the family of the
Gracilariaceae revealed that the activation of a NADPH oxidase may be a relatively
“newly” acquired feature because it is only present in the
Gracilaria
“
sensu lato
”
major clade (Weinberger et al.
2010
). It was speculated by the authors of the study
that the activation of a NADPH oxidase can be coincident either with the acquisi-
tion of agar oligosaccharide-specific receptors or the signaling pathway that
interlinks the receptors with the oxidase (Weinberger et al.
2010
). In contrast,
direct oxidation of agar oligosaccharides by agar oligosaccharide oxidase seems
to be widely distributed in Gracilariaceae because of its activity in the whole
<
1
Gracilaria conferta
from Israel being used in the study of Weinberger et al. (
1999
) was recently
identified as
G
. sp. “
dura
” and can be classified as a
Hydropuntia
species. However, species
belonging to the
G. “sensu lato”
major cluster show the same response as
G
. sp. “
dura
” from Israel
(Weinberger et al.
2010
).
