mechanisms of fungal spore morphology
are important pathogens for humans, animals and plants and can cause
considerable damage. Major causes for the spread of fungal diseases
are fungal spores. Fungal spores are specialized either for dispersal
or for dormant survival. They can be produced either asexually or during
the sexual part of a fungal life cycle. In the first case they are often
specialized for dispersal, while in the latter case they are often produced
for survival. Fungal spores come in a broad variety of shapes and sizes
that are closely related to spore function. While dormant spores often
utilize the mechanical stability of a ball-like shape, spores specialized
for dispersal can have very distinct shapes that are adapted to the
route of dispersal. Examples for such spores are the needle shaped spores
of many members of the genus Eremothecium (see Figure 1 for
Figure 1: Spore morphology of Ashbya gossypii.
A. A single needle-shaped spore produced by A. gossypii. The scale
bar represents 5 mm. B. Schematic drawing of an A. gossypii spore,
which represents the results from fluorescence microscopy shown
on the right side. C. Various fluorescent-staining procedures revealed
a composite spore structure. Actin staining was performed using
rhodamine-phalloidin; chitin was stained with calcofluor white;
chitosan was visualized using eosin Y; membranes were stained using
DiOC6(3); mitochondria were stained with Rhodamine B; dityrosine-autofluorescence
was observed using a CFP filter; and the position of the nucleus
was detected using a strain possessing a CFP-tagged version of the
histone AgHhf1. Note that, in the latter case, dityrosine-autofluorescence
is also visible. The asterisk indicates the position of the nucleus.
The arrows indicate the anterior side of the spore. The scale bars
represent 5 mm.
The special form of these spores allows these fungi to be transmitted
by insect vectors (mainly by bugs) to host plants, where the fungus
can cause considerable damage. Such fungal diseases with economic impact
are the yeast spot disease of soy bean caused Eremothecium ashbyi
and Eremothecium corylii, and the stigmatomycosis of cotton
caused by Eremothecium gossypii, in the following called by
its more common name Ashbya gossypii (Wikipedia
article). The molecular mechanisms underlying formation of such
specialized spore forms, like the needle shaped Eremothecium
spores are not well understood. To extend the knowledge about fungal
sporulation onto morphological aspects of formation of complex spores,
we recently established Ashbya gossypii as a model for studying
the processes that lead to the formation of needle shaped spores . We
found that many components of the process of spore formation described
for S. cerevisiae and S. pombe are conserved. However,
in contrast to the latter organisms, actin and actin regulatory proteins
have a higher importance for shaping the spore and for integrity of
the spore wall. The most significant example of this difference is the
existence of actin cables that are connected to the SPB (Kemper et al.,
2011), which help to form the tip segment of the spore (see Movie 1).
These actin cables are formed by a member of the formin protein
family that also localizes to the SPB. Such a structure was not
Figure 2: A. gossypii sporebundle:
Typical spore bundle of several spores connected via a
ligament and released from a sporangium.
Figure 3: Spores forming in an A. gossypii sporangium.
Top: schematic drawing of a spore bundle in a sporangium, middle:
DIC image, bottom: actin stained with Rhodamin-Phalloidine.
Figure 4: Electron microscopic image of
a cross-section through a sporangium displaying seven spores.
3D-reconstruction of an actin cable (green) lnked to a
nucleus (red) in a sporangium (requires Flash-Player).
The smaller red dots are mitochondria.
Figure 5: Different spore
phenotypes. Split spores, kinked spores and elongated
spores are observed with various mutants of actin regulatory
This project is in parts currently funded by the Deutsche Forschungsgemeinschaft (DFG
project SCHM23883-1, see
here for DFG project page).
2. The role of endocytic microcompartments in signal
transduction and fast apical growth.
Maintenance of the
growth machinery at one side of a growing cell is an essentiell factor
for directed cell growth. Endocytosis plays an important role in keeping
these proteins in place. We hypothesize that in fast growing growing
cells, the speed and efficiency of this process has to be adapted to the
fast growth speed. To test this hypothesis, we have started to
characterize the order of appearance and the lifetime of endocytic
proteins at the early endocytic site. For this purpose we have
established dual-color-TIRF-timelapse-videomicroscopy of hyphal tips
growing at maximum speed in cooperation with
Jacob Piehlers lab. Preliminary work on fifteen
proteins from the early, middle and late phase from formation of
the endocytic side until scission of the endocytic vesicle
revealed that, in general, the lifetime of endocytic events at
the endocytic side decreases with increasing growth speed. In
addition we found evidence, that in A. gossypii, a
clathrin coat is not involved in early endocytic events.
Figure 6: Simple schematic drawing
of the connections between exocytosis, tip growth,
diffusion and endocytosis in hyphal growth.
Movie 2: Simultaneous
TIRF-imaging of two endocytic proteins. Timeframe
between images is 1s. Flash Player required.