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considered as an important component of cellular signal transduction pathways
(Mackerness et al.
2001
), although there is only scant information obtained from
seaweeds in this respect. As a consequence of oxidative stress, most organisms, and
seaweeds as well, typically increase the expression of specific cell repair-related
proteins. Protoplasts of
L. digitata
sporophytes have been shown to massively
upregulate heat-shock proteins HSP-70 and HSP-90, which play a crucial role in
the recovery of cells from stress, in the prevention of protein aggregation and in the
refolding of denaturated proteins (Roeder et al.
2005
).
6.4 Oxidative Bursts: Reactive Oxygen and Interspecific
Interaction
The significance of reactive oxygen radicals is by no means restricted to be resultant
from abiotic stress resulting in a malfunction of photosynthesis. A fast, light-
independent induction of massive ROS formation, which is referred to as an
“oxidative burst”, has been recognized as a tool involved in interspecific interaction
like e.g., pathogen defence in marine macroalgae (see Potin
2008
and Weinberger
and Potin
2010
for review). In these complex systems, oxygen radicals (O
2
-
,H
2
O
2
,
or OH) are transiently formed by the infected or assaulted alga in response to
stimuli originating from either the pathogen and parasite or the host's cell wall.
These stimuli are often represented by chemical elicitors, as extensively studied in a
few red and brown macroalgal species within the last decade.
Chondrus crispus
exhibits oxidative bursts upon challenging with its frequent
parasite, the green alga
Acrochaete operculata
(Weinberger et al.
2005a
; see Dring
2005
and Weinberger
2007
for review). The mechanisms behind ROS and, more
specifically, H
2
O
2
production by the host upon infection by a parasite is well
understood in the
Chondrus/Acrochaete
system (Weinberger et al.
2005a
):
L
-aspar-
agine, which is released from the parasite serves as substrate for
L
-amino oxidase in
the apoplasm of the host. Thus,
C. crispus
is able to perceive microbe-associated
molecular patterns (MAMPs) which are also referred to as general or exogenous
elicitors. The excreted H
2
O
2
will result in the inhibition of the parasite's settlement,
but may also be quenched again by the presence of host-specific peroxidases.
Red and brown marine macroalgae are able to perceive pathogen-induced
molecular patterns (PIMPs) or endogenous elicitors. These elicitors are degradation
products of their own cell wall due to the action of lytic enzymes secreted by
attacking organisms (Weinberger
2007
). In a study in which 17 red macroalgal
species of Gracilariaceae were exposed to agar oligosaccharides it was shown that
two different mechanisms of defence-related responses to this chemical elicitor
exist in this family: (1) oligoagar-activated oxidative burst and (2) oxidation of agar
oligosaccharides. In nature, agar oligosaccharides arise when pathogenic organisms
cleave the cell wall matrices of a red macroalga enzymatically. In laboratory studies
however, this elicitor is used to add to algal media to study their defence responses.
