reversibly reduced or even completely abolished graviresponsiveness in cress
roots (Audus 1979; Busch and Sievers 1990). Moreover, mutant analyses also
confirmed a connection between starch content of amyloplasts and gravitropic
responses. Mutants of PHOSPHOGLUCOMUTASE (PGM) in A. thaliana and
Nicotiana sylvestris that form starchless plastids in columella cells are impaired
in gravitropism, highlighted by a delayed and weak gravitropic response (Kiss and
Sack 1989; Kiss et al. 1989). It was assumed that a reduction in the mass of
the amyloplasts led to a delayed sedimentation and thereby to a delayed
graviperception (Kiss and Sack 1989). This was confirmed by experiments with
the Arabidopsis thaliana mutant starch excess 1 (sex1) which has amyloplasts with
higher starch content in the hypocotyl and requires a shorter presentation time
before bending can be observed (Vitha et al. 2007).
Auxin redistribution in response to plant reorientation is also severely disrupted
in the pgm 1 mutant as shown by the auxin sensor DII-Venus (Band et al. 2012).
The process of statolith sedimentation therefore appears to be fundamental for the
perception of gravity.
The starch-statolith hypothesis includes all perception mechanisms that are driven
by the sedimentation of statoliths. It is based on mechanisms in which the sedimentation of statoliths ultimately leads to a contact with or a movement along the ER or
plasma membrane and thereby to perception of the biophysical stimulus. This can be
achieved by the opening of mechanosensitive channels or via protein-protein interaction, which can both trigger a biochemical signaling cascade. Statolith sedimentation exerts pressure on the ER or plasma membrane. A proportion of amyloplasts is
usually already in close proximity to the distal ER and moves along the ER upon
gravistimulation. Both movement and pressure can cause membrane deformation
after sedimentation (Behrens et al. 1985; Leitz et al. 2009), which yields the potential
to open membrane-localized mechanosensitive ion channels (Hamilton et al. 2015).
The resulting ion current can trigger further signaling cascades (Toyota and Gilroy
2013). Calcium ions are suspected to be the ions that are transported, as the ER is a
prominent calcium store (Meldolesi and Pozzan 1998). Other ions that can pass
mechanosensitive channels are potassium and chloride (Toyota and Gilroy 2013).
The identity of the suspected mechanosensitive channels has not been revealed yet
(reviewed by Baldwin et al. 2013). A binding of receptor (in the ER membrane) and
ligand (in the statolith membrane) could also trigger gravitropic signaling cascades.
Experiments with Chara rhizoids offer results that could in part also be true for
higher plants (see Chap. 4). A transfer of results obtained with Chara to higher
plants, however, must consider that statocytes have a different architecture. Chara
rhizoids do not have a peripheral positioned ER. Contact between statolith and
membrane in Chara takes place with the plasma membrane, in higher plants with
the ER. If protein-protein interactions play a role in gravitropism in higher plants, an
evolutionary relocalization of proteins or new receptor ligand complexes would have
to be assumed.
It is worth noting that there is a significant difference in timing between the
duration of statolith sedimentation and first measurable responses. Complete sedimentation of statolith was reported to take about 5–10 min (Leitz et al. 2009;
6.2 Gravity Perception
81
roots (Audus 1979; Busch and Sievers 1990). Moreover, mutant analyses also
confirmed a connection between starch content of amyloplasts and gravitropic
responses. Mutants of PHOSPHOGLUCOMUTASE (PGM) in A. thaliana and
Nicotiana sylvestris that form starchless plastids in columella cells are impaired
in gravitropism, highlighted by a delayed and weak gravitropic response (Kiss and
Sack 1989; Kiss et al. 1989). It was assumed that a reduction in the mass of
the amyloplasts led to a delayed sedimentation and thereby to a delayed
graviperception (Kiss and Sack 1989). This was confirmed by experiments with
the Arabidopsis thaliana mutant starch excess 1 (sex1) which has amyloplasts with
higher starch content in the hypocotyl and requires a shorter presentation time
before bending can be observed (Vitha et al. 2007).
Auxin redistribution in response to plant reorientation is also severely disrupted
in the pgm 1 mutant as shown by the auxin sensor DII-Venus (Band et al. 2012).
The process of statolith sedimentation therefore appears to be fundamental for the
perception of gravity.
The starch-statolith hypothesis includes all perception mechanisms that are driven
by the sedimentation of statoliths. It is based on mechanisms in which the sedimentation of statoliths ultimately leads to a contact with or a movement along the ER or
plasma membrane and thereby to perception of the biophysical stimulus. This can be
achieved by the opening of mechanosensitive channels or via protein-protein interaction, which can both trigger a biochemical signaling cascade. Statolith sedimentation exerts pressure on the ER or plasma membrane. A proportion of amyloplasts is
usually already in close proximity to the distal ER and moves along the ER upon
gravistimulation. Both movement and pressure can cause membrane deformation
after sedimentation (Behrens et al. 1985; Leitz et al. 2009), which yields the potential
to open membrane-localized mechanosensitive ion channels (Hamilton et al. 2015).
The resulting ion current can trigger further signaling cascades (Toyota and Gilroy
2013). Calcium ions are suspected to be the ions that are transported, as the ER is a
prominent calcium store (Meldolesi and Pozzan 1998). Other ions that can pass
mechanosensitive channels are potassium and chloride (Toyota and Gilroy 2013).
The identity of the suspected mechanosensitive channels has not been revealed yet
(reviewed by Baldwin et al. 2013). A binding of receptor (in the ER membrane) and
ligand (in the statolith membrane) could also trigger gravitropic signaling cascades.
Experiments with Chara rhizoids offer results that could in part also be true for
higher plants (see Chap. 4). A transfer of results obtained with Chara to higher
plants, however, must consider that statocytes have a different architecture. Chara
rhizoids do not have a peripheral positioned ER. Contact between statolith and
membrane in Chara takes place with the plasma membrane, in higher plants with
the ER. If protein-protein interactions play a role in gravitropism in higher plants, an
evolutionary relocalization of proteins or new receptor ligand complexes would have
to be assumed.
It is worth noting that there is a significant difference in timing between the
duration of statolith sedimentation and first measurable responses. Complete sedimentation of statolith was reported to take about 5–10 min (Leitz et al. 2009;
6.2 Gravity Perception
81