starts even 30 min after the stems have been placed horizontally, however, only after
completion of meiosis (Kher et al. 1992).
While several organisms have been found to use heavy statoliths or a heavy
cytoplasm to exert pressure on an underlying gravireceptor, an opposite mechanism
has been suggested for several fungi. In the zygomycete Phycomyces blakesleeanus
some 200 lipid globuli are arranged in a spherical complex about 100 μm below
the growing tip of the vegetative sporangiophore (Grolig et al. 2004, 2006). The
complex is held in place by a dense framework of filamentous actin. The buoyancy
of these globuli could exert an upward pressure onto a gravireceptive structure.
Experimental inhibition of the globuli formation by growing the sporangiophores at
low temperatures reduces the gravitropic response. Similar lipid globuli have been
found in the gravitropically responding hyphae of the fungus Gigaspora margarita
and other fungi. In contrast, Eibel and coworkers suggest that gravitropism is
instrumentalized by octahedral protein crystals with a specific mass of 1.2 g cm
À3
located in the central vacuoles of the sporangiophore acting as statoliths (Eibel et al.
2000). Gravitropic mutants lack these protein crystals. Another publication offered a
combined hypothesis based on both buoyant lipid globuli and sedimenting protein
crystals. Molecular genetic approaches, magnetophoresis and laser ablation have
supported the hypothesis that the actin cytoskeleton is involved in the gravitaxis and
gravitropism sensory transduction chain (Kiss 2000).
5.3 Bryophytes
Dark-grown caulonemata and gametophores of the moss Physcomitrella patens
show a pronounced negative gravitropism (Jenkins et al. 1986). After being placed
horizontally, the caulonemata bend about 20
within 12 h and subsequently complete the 90
bending at a slower pace. Several mutants have been found which
show a partially or completely inhibited gravitropism; one mutant even shows
positive gravitropism. Negative gravitropism in wild-type protonema cells is
reversed after a period of growth on a clinostat. The same reversal of the growth
direction is observed during mitotic division (Knight and Cove 1991). Protoplast
fusion resulting in somatic hybrids showed that at least three genes are involved in
gravitropism. It is interesting to note that in none of these mutants gravitropism
of the gametophores is affected indicating that the mechanisms of graviperception
or transduction are different in caulonemata and gametophores. In contrast to
caulonemata, rhizoids show a pronounced positive gravitropism and a negative
phototropism (Glime 2017).
Amyloplasts have been discussed as possible statoliths in the protonemata
of Ceratodon purpureus (Walker and Sack 1990). In the tip, there is a cluster of
non-sedimenting amyloplasts with an amyloplasts-free zone below. The amyloplasts
below this zone seem to be anchored by (actin?) filaments as they do not sediment to
the basal wall, but to the lower cell wall in horizontal protonemata. This behavior
5.3 Bryophytes
71
completion of meiosis (Kher et al. 1992).
While several organisms have been found to use heavy statoliths or a heavy
cytoplasm to exert pressure on an underlying gravireceptor, an opposite mechanism
has been suggested for several fungi. In the zygomycete Phycomyces blakesleeanus
some 200 lipid globuli are arranged in a spherical complex about 100 μm below
the growing tip of the vegetative sporangiophore (Grolig et al. 2004, 2006). The
complex is held in place by a dense framework of filamentous actin. The buoyancy
of these globuli could exert an upward pressure onto a gravireceptive structure.
Experimental inhibition of the globuli formation by growing the sporangiophores at
low temperatures reduces the gravitropic response. Similar lipid globuli have been
found in the gravitropically responding hyphae of the fungus Gigaspora margarita
and other fungi. In contrast, Eibel and coworkers suggest that gravitropism is
instrumentalized by octahedral protein crystals with a specific mass of 1.2 g cm
À3
located in the central vacuoles of the sporangiophore acting as statoliths (Eibel et al.
2000). Gravitropic mutants lack these protein crystals. Another publication offered a
combined hypothesis based on both buoyant lipid globuli and sedimenting protein
crystals. Molecular genetic approaches, magnetophoresis and laser ablation have
supported the hypothesis that the actin cytoskeleton is involved in the gravitaxis and
gravitropism sensory transduction chain (Kiss 2000).
5.3 Bryophytes
Dark-grown caulonemata and gametophores of the moss Physcomitrella patens
show a pronounced negative gravitropism (Jenkins et al. 1986). After being placed
horizontally, the caulonemata bend about 20
within 12 h and subsequently complete the 90
bending at a slower pace. Several mutants have been found which
show a partially or completely inhibited gravitropism; one mutant even shows
positive gravitropism. Negative gravitropism in wild-type protonema cells is
reversed after a period of growth on a clinostat. The same reversal of the growth
direction is observed during mitotic division (Knight and Cove 1991). Protoplast
fusion resulting in somatic hybrids showed that at least three genes are involved in
gravitropism. It is interesting to note that in none of these mutants gravitropism
of the gametophores is affected indicating that the mechanisms of graviperception
or transduction are different in caulonemata and gametophores. In contrast to
caulonemata, rhizoids show a pronounced positive gravitropism and a negative
phototropism (Glime 2017).
Amyloplasts have been discussed as possible statoliths in the protonemata
of Ceratodon purpureus (Walker and Sack 1990). In the tip, there is a cluster of
non-sedimenting amyloplasts with an amyloplasts-free zone below. The amyloplasts
below this zone seem to be anchored by (actin?) filaments as they do not sediment to
the basal wall, but to the lower cell wall in horizontal protonemata. This behavior
5.3 Bryophytes
71