Loxodidae). In the roots and shoots of higher plants, amyloplasts are responsible as
sedimenting statoliths (Blancaflor and Masson 2003; cf. Chap. 6). The molecular
nature of the receptor, which senses the force of the accelerating statoliths, has not
yet been revealed.
In cases where no heavy statoliths have been detected, the whole cytoplasmic
content of a cell is thought to exert pressure onto the lower cell membrane where
it could activate mechanosensitive channels which gate an influx of Ca
2+ ions when
stimulated (Häder and Hemmersbach 2017; cf. Chap. 3), a principle obviously
used in some free-swimming ciliates. Here, the sedimenting cell content activates
polarily distributed mechano-(gravi-)sensitive ion channels in the cell membrane,
thereby, initiating signaling pathways which finally result in gravitactic orientation
(Hemmersbach et al. 1996).
Due to the small size of the statolith and the low difference in the density of the
cell content and the outer medium the force exerted on the gravireceptor is minute
and close to the physical limit for detection given by Brownian movement. E.g. in
the algae Euglena the cytoplasm of the cell has a specific density of about 1.045 g/mL.
Using the cell volume V allows calculating the force F from the gravitational
acceleration on Earth and the specific density difference.
F ¼ V g δρ
For this flagellate this yields a force of 0.49–1.23 pN. For Arabidopsis root cells a
force of 0.017 pN has been calculated, for the Chara rhizoid barium sulfate vesicle
0.018 pN and for the ciliate Bursaria 11.8 pN, while the 100 times larger ciliate
Paramecium caudatum produces a force of 128 pN (Häder et al. 2005).
In order to terminate in a physiological, biochemical or behavioral response, the
signal has to be relayed from the gravireceptor to the effector in a sensory transduction chain (Häder and Hemmersbach 1997). This can involve the activation of
proteins, enzymes or genes (Häder et al. 2017). In the case of motile microorganisms,
this chain of events results in the reorientation of cilia or flagella which is instrumental
in a gravitactic reorientation (Hemmersbach and Häder 1999). In higher plants, the
situation is even more complicated since the receptor and the effector are located in
different cells which are located at a considerable distance. While the gravireceptors
are organized in the root tip columella, the growth response in the form of differential
growth of the opposite root flanks (gravitropic bending) is elicited in the elongation
zone well above the root tip. The messenger has been identified as the plant growth
hormone auxin transported by specific PIN proteins (Friml et al. 2002). Depending on
the complexity of the response and the distances over which the signal has to be
relayed, the signal transduction chains can be of different lengths.
As in other stimulus signaling pathways regarding environmental stimuli, the
gravity-related one consists of a number of individual steps: perception, transduction
with amplification and response (Fig. 1.3).
The following chapters provide examples demonstrating common principles
and differences in graviperception and gravisignaling pathways between evolutionary diverse organisms, ranging from unicellular plant and animal systems to
1.5 How Do Organisms Detect and Respond to Gravity?
7
sedimenting statoliths (Blancaflor and Masson 2003; cf. Chap. 6). The molecular
nature of the receptor, which senses the force of the accelerating statoliths, has not
yet been revealed.
In cases where no heavy statoliths have been detected, the whole cytoplasmic
content of a cell is thought to exert pressure onto the lower cell membrane where
it could activate mechanosensitive channels which gate an influx of Ca
2+ ions when
stimulated (Häder and Hemmersbach 2017; cf. Chap. 3), a principle obviously
used in some free-swimming ciliates. Here, the sedimenting cell content activates
polarily distributed mechano-(gravi-)sensitive ion channels in the cell membrane,
thereby, initiating signaling pathways which finally result in gravitactic orientation
(Hemmersbach et al. 1996).
Due to the small size of the statolith and the low difference in the density of the
cell content and the outer medium the force exerted on the gravireceptor is minute
and close to the physical limit for detection given by Brownian movement. E.g. in
the algae Euglena the cytoplasm of the cell has a specific density of about 1.045 g/mL.
Using the cell volume V allows calculating the force F from the gravitational
acceleration on Earth and the specific density difference.
F ¼ V g δρ
For this flagellate this yields a force of 0.49–1.23 pN. For Arabidopsis root cells a
force of 0.017 pN has been calculated, for the Chara rhizoid barium sulfate vesicle
0.018 pN and for the ciliate Bursaria 11.8 pN, while the 100 times larger ciliate
Paramecium caudatum produces a force of 128 pN (Häder et al. 2005).
In order to terminate in a physiological, biochemical or behavioral response, the
signal has to be relayed from the gravireceptor to the effector in a sensory transduction chain (Häder and Hemmersbach 1997). This can involve the activation of
proteins, enzymes or genes (Häder et al. 2017). In the case of motile microorganisms,
this chain of events results in the reorientation of cilia or flagella which is instrumental
in a gravitactic reorientation (Hemmersbach and Häder 1999). In higher plants, the
situation is even more complicated since the receptor and the effector are located in
different cells which are located at a considerable distance. While the gravireceptors
are organized in the root tip columella, the growth response in the form of differential
growth of the opposite root flanks (gravitropic bending) is elicited in the elongation
zone well above the root tip. The messenger has been identified as the plant growth
hormone auxin transported by specific PIN proteins (Friml et al. 2002). Depending on
the complexity of the response and the distances over which the signal has to be
relayed, the signal transduction chains can be of different lengths.
As in other stimulus signaling pathways regarding environmental stimuli, the
gravity-related one consists of a number of individual steps: perception, transduction
with amplification and response (Fig. 1.3).
The following chapters provide examples demonstrating common principles
and differences in graviperception and gravisignaling pathways between evolutionary diverse organisms, ranging from unicellular plant and animal systems to
1.5 How Do Organisms Detect and Respond to Gravity?
7