Baldwin et al. 2013). However, with intermittent stimulation of cress roots on a
clinostat, the perception time was calculated to be in a range of 0.5 s, which strongly
indicates that a minor movement of statoliths already settled on ER-cisternae is
sufficient for graviperception to occur (Heinowicz et al. 1998). Measurements of
membrane potentials following a 5–15 s reorientation show membrane depolarizations already 8 s after reorientation (Behrens et al. 1985). An increase in cytoplasmic
inositol-1,4,5-triphosphate (InsP 3 ) was observed in Zea mays 10 s after reorientation
(Perera et al. 1999). Measurements of cytosolic calcium in parabolic flight experiments also suggest changes within seconds of the microgravity stimulus (Neef et al.
2016).
6.2.3 Tensegrity Model and the Role of the Actin in Gravity
Sensing
Tensegrity models describe the connection between graviperception and the cytoskeleton of the cell. Tensegrity is a composite of the words tension and integrity and
describes biologically the connection between mechanical force and mechanisms or
functions of the cell (Ingber et al. 2014). Statoliths are surrounded by an only very
delicate actin microfilament meshwork (Collings et al. 2001). First tensegrity models
described that the actin cytoskeleton surrounding the statoliths is connected to
statolith and ER cisternae or the plasma membrane. Sedimentation of a statolith
would put tension on the cytoskeleton and open mechanosensitive channels in
the plasma membrane (Sievers et al. 1989). One prediction from this model was
that a disruption of the actin cytoskeleton would prevent graviperception. But in
fact, experiments in which the actin filaments were disrupted by Latrunculin B
did not lead to an agravitropic phenotype and application of Latrunculin B and
Cytochalasin B, another actin depolymerizing drug, even increased statolith sedimentation velocity and gravitropic responses in cress roots (Sievers et al. 1989) and
in Zea mays roots (Blancaflor 2002; Hou et al. 2003, 2004). Furthermore, the
promoted graviresponse that was found in roots, which were gravistimulated for
only a short time and then rotated on a clinostat, was attributed to the inability of
fragmented actin microfilaments to terminate the curvature response (Hou et al.
2004). Thus, the actin cytoskeleton likely does not play a crucial role in the gravitysensing process and it is tempting to speculate that actin is not essential for gravity
sensing per se but is required for controlling and fine-tuning an appropriate and
functional resting position as well as unimpeded sedimentation of statoliths. In fact,
actomyosin forces may even increase the energetic noise level of the sensing
mechanism by increasing the random movements of statoliths, making statocytes
less susceptible for unfavorable fast responses to quickly changing or only transient
gravistimulation, e.g. when the wind repeatedly bends crop stalks (Braun and
Limbach 2006).
82
6 Gravitropism in Higher Plants: Cellular Aspects
clinostat, the perception time was calculated to be in a range of 0.5 s, which strongly
indicates that a minor movement of statoliths already settled on ER-cisternae is
sufficient for graviperception to occur (Heinowicz et al. 1998). Measurements of
membrane potentials following a 5–15 s reorientation show membrane depolarizations already 8 s after reorientation (Behrens et al. 1985). An increase in cytoplasmic
inositol-1,4,5-triphosphate (InsP 3 ) was observed in Zea mays 10 s after reorientation
(Perera et al. 1999). Measurements of cytosolic calcium in parabolic flight experiments also suggest changes within seconds of the microgravity stimulus (Neef et al.
2016).
6.2.3 Tensegrity Model and the Role of the Actin in Gravity
Sensing
Tensegrity models describe the connection between graviperception and the cytoskeleton of the cell. Tensegrity is a composite of the words tension and integrity and
describes biologically the connection between mechanical force and mechanisms or
functions of the cell (Ingber et al. 2014). Statoliths are surrounded by an only very
delicate actin microfilament meshwork (Collings et al. 2001). First tensegrity models
described that the actin cytoskeleton surrounding the statoliths is connected to
statolith and ER cisternae or the plasma membrane. Sedimentation of a statolith
would put tension on the cytoskeleton and open mechanosensitive channels in
the plasma membrane (Sievers et al. 1989). One prediction from this model was
that a disruption of the actin cytoskeleton would prevent graviperception. But in
fact, experiments in which the actin filaments were disrupted by Latrunculin B
did not lead to an agravitropic phenotype and application of Latrunculin B and
Cytochalasin B, another actin depolymerizing drug, even increased statolith sedimentation velocity and gravitropic responses in cress roots (Sievers et al. 1989) and
in Zea mays roots (Blancaflor 2002; Hou et al. 2003, 2004). Furthermore, the
promoted graviresponse that was found in roots, which were gravistimulated for
only a short time and then rotated on a clinostat, was attributed to the inability of
fragmented actin microfilaments to terminate the curvature response (Hou et al.
2004). Thus, the actin cytoskeleton likely does not play a crucial role in the gravitysensing process and it is tempting to speculate that actin is not essential for gravity
sensing per se but is required for controlling and fine-tuning an appropriate and
functional resting position as well as unimpeded sedimentation of statoliths. In fact,
actomyosin forces may even increase the energetic noise level of the sensing
mechanism by increasing the random movements of statoliths, making statocytes
less susceptible for unfavorable fast responses to quickly changing or only transient
gravistimulation, e.g. when the wind repeatedly bends crop stalks (Braun and
Limbach 2006).
82
6 Gravitropism in Higher Plants: Cellular Aspects