Biology Reference
In-Depth Information
exposed to herbivore pressure when associated with a chemically defended host
(Hay
1986
; Amsler et al.
2009
) and endophytes may survive by avoiding grazing
when embedded in host tissues (Amsler et al.
2009
). Allelopathy occurs through the
exudation of antifouling compounds such as phenolics by the two species of
Sargassum
in the Sargasso Sea (Sieburth and Conover
1965
) or iodinated
compounds by species of
Laminaria.
In comparison with epiphytism, alga-parasite or pathogen interactions are gen-
erally highly specific and characterized by typical symptoms (for reviews, see
Andrews
1976
,
1977
; Fujita
1990
; Goff
1982
; Correa
1996
; Eggert et al.
2010
).
Some of the biochemical bases of the responses of seaweeds toward algal endo-
phyte infection have been uncovered recently with either biochemical emphasis
(Potin et al.
2002
; Weinberger
2007
; Cosse et al.
2008
; Potin
2008
; Weinberger and
Potin
2010
) or ecological emphasis (Pohnert
2004
; Amsler and Fairhead
2006
).
This contribution does not aim to add one more review on a specific aspect of this
research field but rather provide further understanding of the role of cross-talk
mechanisms involved in host specificity as well as host chemical defenses that
impact the diversity of associations.
11.3.1 Cross-Talk Involved in Host Colonization
Host specificity by several epiphytic or endophytic algal species is a dramatic example
of nonrandom settlement and recruitment (Ducker and Knox
1984
). In terrestrial
plant-fungus mutualistic or pathogenic associations, surface compounds of the host,
such as wax and other cell wall compounds, were shown to provide signals to the
pathogen in the early stages of colonization. Cell wall polysaccharides from the host
are also a source of settlement cues for successful colonization of red seaweeds by
swimming spores of pigmented endophytes. Interestingly, the causative agent of green
patch disease in the red alga
Mazzaella laminarioides,
the endophytic green alga
Endophyton ramosum,
colonizes carrageenan-producing red algae (carrageenophyte),
whereas agar producers (agarophytes) remain uninfected (Sanchez et al.
1996
).
Surprisingly, another green algal endophytic pathogen,
Acrochaete operculata,
infects
carrageenophytes containing lambda-carrageenan more successfully than carragee-
nophytes containing kappa-carrageenan. When zoospores of
A. operculata
settle and
germinate, the vegetative filaments of the parasite completely invade sporophytic
fronds of
C. crispus
, which contain lambda-carrageenan. In contrast, gametophytic
fronds—containing kappa-carrageenan—are not infected beyond the epidermis and
outer cortex (Correa and McLachlan
1991
). Recognition by the pathogen of the
carrageenan oligomeric degradation products of the host cell walls governs
this interaction (Bouarab et al.
1999
). Another example of host recognition through
cell wall polysaccharides was also partly elucidated in the case of
Olpidiopsis
(
Pythium) porphyrae
, the oomycete agent of red rot disease in
Porphyra yezoensis
.
Porphyran, the sulfated agar that characterized the cell wall of the red algal Bangiales,
was shown to enhance zoospore attachment, encystment, and appressorium formation.
