6.2 Gravity Perception
The gravitropic response can be subdivided into three distinct and sequential events:
signal perception, signal transduction and gravitropic response. The first step in
signal perception is the conversion of the gravitational stimulus into a detectable
physical change within the cell (Baldwin et al. 2013; Toyota and Gilroy 2013;
Schüler et al. 2015). Various models have been proposed of how the physical
stimulus of statolith sedimentation is transduced into a biochemical signaling
event. The leading hypotheses are the statolith-dependent starch-statolith hypothesis
and the tensegrity model (Yoder 2001; Zheng and Staehelin 2001) as well as the
statolith-independent gravitational pressure or protoplast pressure models (Wayne
and Staves 1996; Palmieri and Kiss 2007).
6.2.1 Tissue Localization of Graviperception
All plant cells experience the gravitational stimulus equally. However, one must
differentiate between a specific gravity sensing mechanism that is localized in
specific tissues, and general unspecific physical and physiological or metabolic
reactions that might be observable in all cells. For a gravitropic response, gravity
must be perceived by specialized tissues (Fig. 6.3). Graviperception in the shoot
occurs mainly in the starch sheath in the endodermis. In Arabidopsis thaliana, a
mutation of the gene PHOSPHOGLUCOMUTASE (PGM) prevents formation of the
starch sheath, phenotypically observable by an agravitropic behavior of the shoot.
This connection between defect and absent endodermis and gravitropism was further
confirmed by mutant screens. The mutant shoot gravitropism 1 (sgr1) has a mutation
in the gene SCARECROW (SCR), the mutant sgr7 has a mutation in gene SHORT
ROOT (SHR). In both mutants, the endodermis is not formed in the shoot and
consequently, in both mutants shoot gravitropism is no longer observable (Fukaki
et al. 1998). All shoot gravitropism mutants, although likewise defective in root
endodermis, still show normal root gravitropism which is easily explained because
in the root, gravity is sensed in the columella cells in the root cap.
Charles Darwin was one of the first naturalists and biologists who had shown that
the removal or damaging of the root tip leads to agravitropic growth and that these
roots regain the ability to grow downward again after regeneration of the root cap
(Darwin 1880). In particular the manipulation or removal of the columella cells
impairs gravitropic behavior (Juniper et al. 1966; Konings 1968; Tsugeki and
Fedoroff 1999). There are typically 4 layers of columella cells at the root tip,
S1-S4 (Fig. 6.3). By selective removal of individual columella cells using laser
ablation, it was shown that the innermost layers S1 and S2 are most important for
gravitropism (Blancaflor et al. 1998).
78
6 Gravitropism in Higher Plants: Cellular Aspects
The gravitropic response can be subdivided into three distinct and sequential events:
signal perception, signal transduction and gravitropic response. The first step in
signal perception is the conversion of the gravitational stimulus into a detectable
physical change within the cell (Baldwin et al. 2013; Toyota and Gilroy 2013;
Schüler et al. 2015). Various models have been proposed of how the physical
stimulus of statolith sedimentation is transduced into a biochemical signaling
event. The leading hypotheses are the statolith-dependent starch-statolith hypothesis
and the tensegrity model (Yoder 2001; Zheng and Staehelin 2001) as well as the
statolith-independent gravitational pressure or protoplast pressure models (Wayne
and Staves 1996; Palmieri and Kiss 2007).
6.2.1 Tissue Localization of Graviperception
All plant cells experience the gravitational stimulus equally. However, one must
differentiate between a specific gravity sensing mechanism that is localized in
specific tissues, and general unspecific physical and physiological or metabolic
reactions that might be observable in all cells. For a gravitropic response, gravity
must be perceived by specialized tissues (Fig. 6.3). Graviperception in the shoot
occurs mainly in the starch sheath in the endodermis. In Arabidopsis thaliana, a
mutation of the gene PHOSPHOGLUCOMUTASE (PGM) prevents formation of the
starch sheath, phenotypically observable by an agravitropic behavior of the shoot.
This connection between defect and absent endodermis and gravitropism was further
confirmed by mutant screens. The mutant shoot gravitropism 1 (sgr1) has a mutation
in the gene SCARECROW (SCR), the mutant sgr7 has a mutation in gene SHORT
ROOT (SHR). In both mutants, the endodermis is not formed in the shoot and
consequently, in both mutants shoot gravitropism is no longer observable (Fukaki
et al. 1998). All shoot gravitropism mutants, although likewise defective in root
endodermis, still show normal root gravitropism which is easily explained because
in the root, gravity is sensed in the columella cells in the root cap.
Charles Darwin was one of the first naturalists and biologists who had shown that
the removal or damaging of the root tip leads to agravitropic growth and that these
roots regain the ability to grow downward again after regeneration of the root cap
(Darwin 1880). In particular the manipulation or removal of the columella cells
impairs gravitropic behavior (Juniper et al. 1966; Konings 1968; Tsugeki and
Fedoroff 1999). There are typically 4 layers of columella cells at the root tip,
S1-S4 (Fig. 6.3). By selective removal of individual columella cells using laser
ablation, it was shown that the innermost layers S1 and S2 are most important for
gravitropism (Blancaflor et al. 1998).
78
6 Gravitropism in Higher Plants: Cellular Aspects