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family. Thus, two mechanisms of H
2
O
2
release seem to be coexist in the
Gracilaria
“
sensu lato
” major clade. This pattern reveals that defence response by direct
oxidation of agar oligosaccharides is more ancient than oxidative burst triggered
by oligoagar (Weinberger et al.
2010
).
ROS formation due to perception of PIMPs also occurs in brown macroalgae. In
Laminariales, oxidative burst is triggered by recognition of specific degradation
products of alginates, being the major polysaccharides in their cell walls. The
addition of oligoguluronates, which are homopolymeric blocks of poly-
-1,4-
L
-
guluronic acid, to
L. digitata
induces a transient and strong release of H
2
O
2
into the
medium within a few minutes. Similar to the
Gracilaria“sensu lato”
major clade,
the oxidative burst is a result from signal transduction chains triggered by the
recognition of the elicitor. The activation of phospholipases and kinases as well
as ion fluxes across the plasma membrane is apparently involved in the signaling
pathways (K
a
upper et al.
2001
). Moreover, perception of bacterial lipopolysac-
charides (i.e., MAMPs) may also induce early events of oxidative burst and fatty
acid oxidation (K
€
upper et al.
2006
).
An analysis of the pattern of gene expression in response to oligoguluronate
elicitation showed that 50 genes were highly regulated within the first 24 h in
L. digitata
(Cosse et al.
2009
). The number of differentially expressed genes which
were upregulated was maximal (38 genes) after 6 h and decreased continuously
after 12 h (21 genes) and 24 h (12 genes) after challenge. However, six of these
differentially expressed genes remained upregulated over the first 24 h. By the use
of various inhibitors affecting signal transduction and ROS formation, the expres-
sion pattern of ten genes was affected differently. Three trends of changes in
transcript levels observed led to the conclusion that three different signal transduc-
tion pathways might be involved in oligoguluronate-induced oxidative burst and set
the stage for subsequent gene regulation (Cosse et al.
2009
). A coexistence of
multiple pathways leading to H
2
O
2
release due to PIMP perception was shown in
the
Gracilaria“sensu lato”
major clade as well (Weinberger et al.
2010
). One
signal transduction pathway appears to be dependent on early phosphorylation
events by kinases and has a great impact on expression of
vIPO1
(vanadium-
dependent iodoperoxidase 1),
trx
(thioredoxin like 5), and
hsp70
(heat-shock
protein 70) genes. A second signaling pathway seems to be fully controlled by
the oxidative burst: H
2
O
2
production regulates transcript levels of
vBPO3
(vanadium-dependent bromoperoxidase 3) and
g6pd
(glucose-6-phosphate dehy-
drogenase). The third signal transduction chain apparently depends on both ROS
production (oxidative burst) and the activation of putative MAP kinases or other
downstream kinases. This pathway led to an upregulation of the
vIPO3
gene
whereas the
vBPO1
gene was concurrently downregulated. Based on this finding,
Cosse et al. (
2009
) hypothesized that ROS produced by oxidative burst might also
be an internal signal to induced expression of genes with defence-related function.
The rapid induction of the
vIPO3
gene encoding a novel vanadium-dependent
iodoperoxidase upon oligoguluronate elicitation suggests that the iodine metabo-
lism might be specifically activated on the molecular level during early defence
response (Cosse et al.
2009
). This is in line with the fast and massive iodide release
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