Summary:
During the last decades, the ecological and commercial interest of the algae
has led to
increased scientific research on this group of organisms. Algae are at the
basis of most aquatic
food webs, while their photosynthetic activity has impact on global climate. In
parallel, global
seaweed production in aquaculture systems has increased exponentially.
Nowadays, seaweeds
are harvested and cultivated for food, the pharmaceutical industry, and
increasingly as a
possible biofuel source.
Macroalgae, like all other terrestrial and marine organisms, face a constant
onslaught from
pathogens, including viruses, bacteria and fungi. It has been estimated that
the seaweed
culture industry sustains losses of annual production between 10 and 30 % as a
result of
diseases caused by pathogens and more specifically, partly by marine oomycetes.
This provides
a renewed incentive to the study of interactions between algae and these
parasites, which had
hitherto been largely ignored.
Eurychasma dicksonii is a biotrophic, intracellular marine oomycete, capable to
infect at
least 45 species of brown seaweeds in laboratory cultures. The main stages of
E. dicksonii
infection cycle have been described previously and include: zoospore
encystment, attachment
of the spore to the host cell wall, penetration of the host cell wall and
transfer of the parasite’s
protoplast into the host cell, formation of a multinucleate, coenocytic thallus
and
zoosporogenesis. However, until now, there is no information related to the E.
dicksonii
infection mechanism during the encystment and penetration, the cytoskeleton
organization,
the cross-talk mechanisms between the parasite and its host and the potential
algal defensive
responses against the oomycete.
The present doctoral thesis aimed to present new findings on the cytoskeleton
organization of both host and parasite, cover the gaps of knowledge on the E.
dicksonii
infection mechanism and shed light onto the potential host-parasite
interactions and host
defenses. The most important observations of the present study are summarized
below:
1. Ultrastructural study of E. dicksonii attachment and penetration mechanism
The present part of the doctoral thesis focuses on the mechanism used by the
pathogen to
attach on the host cell wall and force its way into algal cells.
Ultrastructural examination
revealed a needle-like structure, which develops within the attached spore and
extends along
its main axis. Particular cell wall modifications are present at the basal part
of the spore
(adhesorium pad) and guide the needle-like tool to penetrate perpendicularly
the host cell wall.
The injection mechanism is shared with Haptoglossa sp. which suggests that this
is an
important characteristic of early diverging oomycetes. Furthermore, the
encystment and
adhesion mechanism of E. dicksonii shows significant similarities with other
oomycetes, some
of which are plant pathogens.
2. Cytoskeleton plays a key role in the algal defense and in E. dicksonii
different
developmental stages
Immunofluorescence imaging of tubulin revealed how the development of the
intracellular
biotrophic pathogen E. dicksonii impacts on microtubule (MT) organization of
three brown algal
species, Pylaiella littoralis, Ectocarpus siliculosus, and Ectocarpus
crouaniorum. The host MT
cytoskeleton remains normal and organized by the centrosome until the very late
stages of the
infection. Additionally, the organization of the parasite’s cytoskeleton was
examined. During
mitosis of the E. dicksonii nucleus, the MT focal point (microtubule
organization centre, MTOC,
putative centrosome) duplicates and each daughter MTOC migrates to opposite
poles of the
nucleus. Moreover, actin labeling with rhodamine-phalloidin in E. dicksonii
revealed typical
images of actin dots connected by fine actin filament bundles in the cortical
cytoplasm.
3. Host response to infection with β-1,3-glucan deposition on the host cell wall
Staining and immunolabeling techniques showed the deposition of β-1,3-glucans
on the host
cell wall at the pathogen penetration site, a strategy similar to physical
responses previously
described only in infected plant cells. It is assumed that the host defense in
terms of callose-like
deposition is an ancient response to infection.
4. Cell wall component analysis sheds light on the crosstalk between
filamentous brown
algae and E. dicksonii
In order to gain insight in the composition and plasticity of the algal and
pathogen cell walls
during infection, twelve host strains from four orders of Phaeophyceae
(Ectocarpales,
Laminariales, Discosporangiales, Tilopteridiales) and their pathogen E.
dicksonii, were scanned
by ten antibodies targeted at different cell wall components. The results
revealed that there are
no modifications occurred on the host cell wall, apart from the β-1,3 glucan
deposition at the
penetration site. Additionally, it was found that sulfated fucans exist on the
E. dicksonii cell
wall, which is a novel finding for oomycetes.
5. Incubation of infected material with medium including monosaccharides and
hormones
Incubation of material infected by E.dicksonii with D-galactose and L-fucose
during a complete,
14-day infection cycle, showed significant decrease of pathogenicity. It is
assumed that, due to
the long incubation with monosaccharides, host defensive strategies are
triggered. In addition,
incubation of infected algae with salicylic acid revealed significant decrease
of the infections
and increase of the algal hypersensitive response. Salicylic acid is known to
play an important
role in the defensive biochemical pathways of plants against oomycetes.
