Chapter 4
Gravitropism in Tip-Growing Rhizoids
and Protonemata of Characean Algae
Markus Braun
Abstract Characean green algae provide two well-established model cell types
for gravitropic research. Experiments in the almost stimulus-free microgravity environments of parabolic flights, sounding rocket flights and Space Shuttle missions
have contributed greatly to the progress that has been made in the understanding of
cellular and molecular mechanisms underlying plant gravity sensing and gravityoriented growth responses. While in higher-plant statocytes the role of actin in gravity
sensing is still enigmatic, there is clear evidence that actin is intimately involved in
polarized growth, gravity sensing and the positive and negative gravitropic response
of characean rhizoids and protonemata. The apical tip-growth organizing structure,
the Spitzenkörper, and a steep gradient of cytoplasmic free calcium are crucial
components of a feedback mechanism that controls polarized growth. Microgravity
experiments provided evidence that actomyosin plays a key role in gravity
sensing by coordinating the position of statoliths, and, upon gravistimulation, directs
sedimenting statoliths to specific gravisensitive areas of the plasma membrane, where
they initiate the short gravitropic signalling pathways. In rhizoids, statolith sedimentation is followed by a local reduction of cytoplasmic free calcium resulting in
differential growth of the opposite subapical cell flanks—the downward bending.
The negative gravitropic response of protonemata is initiated by statolith sedimentation in the apical dome causing actomyosin-mediated relocation of the calcium
gradient and displacement of the center of maximal growth towards the upper flank.
Keywords Actin · Chara · Gravitropism · Rhizoid · Protonema · Statolith ·
Tip growth
4.1 Introduction
Charophytes are ubiquitous, filamentous green algae displaying a higher-plant
like stem- and leaf-like structure. The main axis is differentiated into nodes and
internodes. For characean green algae, gravitropically tip-growing rhizoids and
protonemata originating from nodal cells are critical for survival in a rough and
© The Author(s), under exclusive licence to Springer International Publishing AG,
part of Springer Nature 2018
M. Braun et al., Gravitational Biology I, SpringerBriefs in Space Life Sciences,
https://doi.org/10.1007/978-3-319-93894-3_4
47
Gravitropism in Tip-Growing Rhizoids
and Protonemata of Characean Algae
Markus Braun
Abstract Characean green algae provide two well-established model cell types
for gravitropic research. Experiments in the almost stimulus-free microgravity environments of parabolic flights, sounding rocket flights and Space Shuttle missions
have contributed greatly to the progress that has been made in the understanding of
cellular and molecular mechanisms underlying plant gravity sensing and gravityoriented growth responses. While in higher-plant statocytes the role of actin in gravity
sensing is still enigmatic, there is clear evidence that actin is intimately involved in
polarized growth, gravity sensing and the positive and negative gravitropic response
of characean rhizoids and protonemata. The apical tip-growth organizing structure,
the Spitzenkörper, and a steep gradient of cytoplasmic free calcium are crucial
components of a feedback mechanism that controls polarized growth. Microgravity
experiments provided evidence that actomyosin plays a key role in gravity
sensing by coordinating the position of statoliths, and, upon gravistimulation, directs
sedimenting statoliths to specific gravisensitive areas of the plasma membrane, where
they initiate the short gravitropic signalling pathways. In rhizoids, statolith sedimentation is followed by a local reduction of cytoplasmic free calcium resulting in
differential growth of the opposite subapical cell flanks—the downward bending.
The negative gravitropic response of protonemata is initiated by statolith sedimentation in the apical dome causing actomyosin-mediated relocation of the calcium
gradient and displacement of the center of maximal growth towards the upper flank.
Keywords Actin · Chara · Gravitropism · Rhizoid · Protonema · Statolith ·
Tip growth
4.1 Introduction
Charophytes are ubiquitous, filamentous green algae displaying a higher-plant
like stem- and leaf-like structure. The main axis is differentiated into nodes and
internodes. For characean green algae, gravitropically tip-growing rhizoids and
protonemata originating from nodal cells are critical for survival in a rough and
© The Author(s), under exclusive licence to Springer International Publishing AG,
part of Springer Nature 2018
M. Braun et al., Gravitational Biology I, SpringerBriefs in Space Life Sciences,
https://doi.org/10.1007/978-3-319-93894-3_4
47