resembles that of the barium-sulfate filled statoliths in characean rhizoids. When
placed horizontally, wild-type C. purpureus protonemata shortly bend downwards,
which also occurs prior to cytokinesis (Wagner et al. 1997). UV-induced mutants
also show negative gravitropism with kinetics similar to wild-type protonemata.
Also Funaria caulonemata show upward bending (Schwuchow et al. 1995). The tip
cells have a broad subapical zone, where plastid sedimentation has been observed.
Under real microgravity on the Space Shuttle mission STS-87 and under simulated
microgravity, protonemata cultures showed a radial outgrowth followed by a clockwise spiral growth (Kern et al. 2005). The protonemata of C. purpureus also show
phototropism in red light. At irradiances ! 140 nmol m
À2 s
À1 gravitropism was
quenched, but amyloplast sedimentation was still observed (Kern and Sack 1999).
These results show that both stimuli compete and that light regulates the gravitropic
response.
In roots of higher plants, the gravitropic signal is relayed from the statenchyma
in the root tip to the elongation zone via the plant hormone auxin, which is guided
by several auxin transporters (cf. Chap. 6). Auxins and cytokinins have also been
found in mosses and liverworts, where they regulate morphological development
(Sabovljević et al. 2014). In Marchantia polymorpha, auxin is involved in the
establishment of a dorsiventral polarity. Gemmae cups usually respond negative
gravitropically, but after external application of auxin, they became positive
gravitropic (Flores-Sandoval et al. 2015). These results indicate that the auxin
regulation of growth and morphological development has been established in these
earliest land plants.
5.4 Ferns
Trophophylls (sterile leafs) of the eusporangiate fern Danaea wendlandii show
negative gravitropism, while sphorophylls (fertile leafs) grow horizontally (Sharpe
and Jernstedt 1990). This behavior has been attributed to a statolith mechanism
since sedimenting amyloplasts have been detected in the cells of the petiole and
rachis. Also dark-grown gametophytes of Ceratopteris richardii show negative
gravitropism while light inhibits gravitropism (Kamachi and Noguchi 2012). Also
in most Selaginella species light modulates the gravitropic response of rhizoids
during the early developmental stage (Liu and Sun 1994). Later the effect of light
is reduced while gravitropism dominates. Adult plants of the fern Ceratopteris
richardii show pronounced graviperception and gravitropism. Single-celled spores
of this fern were exposed to microgravity during a shuttle space flight (STS-93).
cDNA microarray and Q RT-PCR analysis of spores germinating in microgravity
showed significant changes in the mRNA expression of about 5% of the analyzed
genes (Salmi and Roux 2008). Similar changes in gene expression are found in
animal and plant cells.
72
5 Gravitropism in Fungi, Mosses and Ferns
placed horizontally, wild-type C. purpureus protonemata shortly bend downwards,
which also occurs prior to cytokinesis (Wagner et al. 1997). UV-induced mutants
also show negative gravitropism with kinetics similar to wild-type protonemata.
Also Funaria caulonemata show upward bending (Schwuchow et al. 1995). The tip
cells have a broad subapical zone, where plastid sedimentation has been observed.
Under real microgravity on the Space Shuttle mission STS-87 and under simulated
microgravity, protonemata cultures showed a radial outgrowth followed by a clockwise spiral growth (Kern et al. 2005). The protonemata of C. purpureus also show
phototropism in red light. At irradiances ! 140 nmol m
À2 s
À1 gravitropism was
quenched, but amyloplast sedimentation was still observed (Kern and Sack 1999).
These results show that both stimuli compete and that light regulates the gravitropic
response.
In roots of higher plants, the gravitropic signal is relayed from the statenchyma
in the root tip to the elongation zone via the plant hormone auxin, which is guided
by several auxin transporters (cf. Chap. 6). Auxins and cytokinins have also been
found in mosses and liverworts, where they regulate morphological development
(Sabovljević et al. 2014). In Marchantia polymorpha, auxin is involved in the
establishment of a dorsiventral polarity. Gemmae cups usually respond negative
gravitropically, but after external application of auxin, they became positive
gravitropic (Flores-Sandoval et al. 2015). These results indicate that the auxin
regulation of growth and morphological development has been established in these
earliest land plants.
5.4 Ferns
Trophophylls (sterile leafs) of the eusporangiate fern Danaea wendlandii show
negative gravitropism, while sphorophylls (fertile leafs) grow horizontally (Sharpe
and Jernstedt 1990). This behavior has been attributed to a statolith mechanism
since sedimenting amyloplasts have been detected in the cells of the petiole and
rachis. Also dark-grown gametophytes of Ceratopteris richardii show negative
gravitropism while light inhibits gravitropism (Kamachi and Noguchi 2012). Also
in most Selaginella species light modulates the gravitropic response of rhizoids
during the early developmental stage (Liu and Sun 1994). Later the effect of light
is reduced while gravitropism dominates. Adult plants of the fern Ceratopteris
richardii show pronounced graviperception and gravitropism. Single-celled spores
of this fern were exposed to microgravity during a shuttle space flight (STS-93).
cDNA microarray and Q RT-PCR analysis of spores germinating in microgravity
showed significant changes in the mRNA expression of about 5% of the analyzed
genes (Salmi and Roux 2008). Similar changes in gene expression are found in
animal and plant cells.
72
5 Gravitropism in Fungi, Mosses and Ferns