rates (Häder and Hansel 1991). The pseudoplasmodia are also remarkable since they
show positive thermotaxis when exposed to a gradient of 0.2
C cm
À1 ignoring the
gravistimulus. Irradiances > 10
À3 W m
À2 canceled the response with respect to
gravity.
In contrast to the cellular slime mold Dictyostelium, Physarum polycephalum
belongs to the acellular slime molds, which form undivided plasmodia with millions
of nuclei (Guttes et al. 1961). The plasmodia consist of interconnected strands with a
tough ectoplasm and a low-viscosity endoplasm which moves back and forth at
regular intervals controlled by a Ca
2+ oscillator (Coggin and Pazun 1996). When the
shuttle streaming occurs more in one direction than in the other the whole plasmodium migrates in that direction. Also these myxomycetes respond to environmental
stimuli including light, chemicals and gravity (Sauer 1982; Wolke et al. 1987;
de Lacy Costello and Adamatzky 2013). On a vertical surface Physarum plasmodia
show predominantly positive gravitaxis (68%), while 29% were indifferent and 13%
showed negative gravitaxis (Wolke et al. 1987). When the vertical agar plate was
rotated stepwise by 90
, the plasmodia reoriented accordingly. This graviresponse
was investigated in real microgravity during the IML-1 mission on the Space Shuttle
(Block et al. 1992) as well as under simulated microgravity on a fast rotating
clinostat (Block et al. 1994). While no obvious statoliths can be detected in the
plasmodia, mitochondria have been suggested to exert the pressure on a putative
gravireceptor. It is interesting to note that the signal transduction chain involves
cAMP which has been demonstrated in graviresponses of other organisms such
as ciliates and flagellates as well (Block et al. 1998). Exposure to a few days of
microgravity during a space experiment (STS-69) decreased the cAMP concentration in the plasmodia which was, thus, identified as a secondary messenger.
Fungal zoospores show a pronounced negative gravitaxis. The discussion,
whether the spores of Phytophthora respond to an oxygen gradient, a light signal
or a chemical clue, could be solved by proving that they reacted to gravity (Cameron
and Carlile 1977). The cells also show chemotaxis, which was shown to be
controlled by changes in the membrane potential, which regulates the flagellar
activity; however, proof is still lacking, if the same mechanism is responsible for
graviorientation (Cameron and Carlile 1980).
Mushrooms show a number of different responses to various environmental
stimuli at different times of their development (Moore 1991). They display thigmotropism, gravitropism, anemotropism and phototropism. Young fruiting bodies grow
perpendicular away from the substratum even in the presence of increased accelerations. Afterwards, the fruiting body shows positive phototropism and subsequently
negative gravitropism; this shift is correlated with the initiation of spore formation.
One interpretation for the onset of gravitropism is the assumption that the hymenia
(forming to tubes or gills) in the fruiting body should be oriented vertically to
facilitate the liberation of the spores falling downward in the free space. In addition,
gravity is required for the morphogenesis of the fruiting body (Corrochano and
Galland 2006). On an orbiting space station, Polyporus brumalis failed to initiate
fruiting. Clinostat experiments also indicated that sporulation in Lentinus tigrinus
and P. brumalis is prevented in simulated microgravity and karyogamy was
5.2 Slime Molds and Fungi
69
show positive thermotaxis when exposed to a gradient of 0.2
C cm
À1 ignoring the
gravistimulus. Irradiances > 10
À3 W m
À2 canceled the response with respect to
gravity.
In contrast to the cellular slime mold Dictyostelium, Physarum polycephalum
belongs to the acellular slime molds, which form undivided plasmodia with millions
of nuclei (Guttes et al. 1961). The plasmodia consist of interconnected strands with a
tough ectoplasm and a low-viscosity endoplasm which moves back and forth at
regular intervals controlled by a Ca
2+ oscillator (Coggin and Pazun 1996). When the
shuttle streaming occurs more in one direction than in the other the whole plasmodium migrates in that direction. Also these myxomycetes respond to environmental
stimuli including light, chemicals and gravity (Sauer 1982; Wolke et al. 1987;
de Lacy Costello and Adamatzky 2013). On a vertical surface Physarum plasmodia
show predominantly positive gravitaxis (68%), while 29% were indifferent and 13%
showed negative gravitaxis (Wolke et al. 1987). When the vertical agar plate was
rotated stepwise by 90
, the plasmodia reoriented accordingly. This graviresponse
was investigated in real microgravity during the IML-1 mission on the Space Shuttle
(Block et al. 1992) as well as under simulated microgravity on a fast rotating
clinostat (Block et al. 1994). While no obvious statoliths can be detected in the
plasmodia, mitochondria have been suggested to exert the pressure on a putative
gravireceptor. It is interesting to note that the signal transduction chain involves
cAMP which has been demonstrated in graviresponses of other organisms such
as ciliates and flagellates as well (Block et al. 1998). Exposure to a few days of
microgravity during a space experiment (STS-69) decreased the cAMP concentration in the plasmodia which was, thus, identified as a secondary messenger.
Fungal zoospores show a pronounced negative gravitaxis. The discussion,
whether the spores of Phytophthora respond to an oxygen gradient, a light signal
or a chemical clue, could be solved by proving that they reacted to gravity (Cameron
and Carlile 1977). The cells also show chemotaxis, which was shown to be
controlled by changes in the membrane potential, which regulates the flagellar
activity; however, proof is still lacking, if the same mechanism is responsible for
graviorientation (Cameron and Carlile 1980).
Mushrooms show a number of different responses to various environmental
stimuli at different times of their development (Moore 1991). They display thigmotropism, gravitropism, anemotropism and phototropism. Young fruiting bodies grow
perpendicular away from the substratum even in the presence of increased accelerations. Afterwards, the fruiting body shows positive phototropism and subsequently
negative gravitropism; this shift is correlated with the initiation of spore formation.
One interpretation for the onset of gravitropism is the assumption that the hymenia
(forming to tubes or gills) in the fruiting body should be oriented vertically to
facilitate the liberation of the spores falling downward in the free space. In addition,
gravity is required for the morphogenesis of the fruiting body (Corrochano and
Galland 2006). On an orbiting space station, Polyporus brumalis failed to initiate
fruiting. Clinostat experiments also indicated that sporulation in Lentinus tigrinus
and P. brumalis is prevented in simulated microgravity and karyogamy was
5.2 Slime Molds and Fungi
69