of elegant experiments has helped to shed light onto the relationship between the
mechanisms behind lateral root initiation and growth (Kircher and Schopfer 2016).
Experiments designed to differentiate between bending-induced local auxin accumulation and clock-type oscillations in auxin-induced gene expression revealed that
the frequency of lateral roots was promoted by auxin in the mature root; positioning,
however, followed a pre-formed pattern determined by previous bending (Kircher
and Schopfer 2016).
7.5 Gravitropism Follows Grow-and-Switch or Tipping
Point Mechanisms
In the root cap, specialized cells named statocytes sense gravity (cf. Chap. 6).
Analysis of gravisensitivity determined a threshold acceleration at between 10
À3
and 10
À4 g for roots and 10
À2 and 10
À3 g for shoots and a perception time ranging
between 1 and 10 s (Perbal et al. 1997; Hejnowicz et al. 1998; Perbal and DrissEcole 2003). However, dose-response-based studies, in which it has been suggested
that the curvature response varies linearly with the logarithm of the gravity stimulus,
may have to be revisited using up-to-date technologies (Perbal et al. 2002).
The weight of an object is the product of gravitational acceleration and its mass.
Therefore, all organelles which have a different density than the surrounding
cytoplasm and are not fixed in place by cytoskeletal elements may be involved
in gravisensing. In Arabidopsis roots, relatively dense starch-filled amyloplasts
sediment to the bottom of statocytes, where they might stimulate mechanosensors
which initiate a chain of physiological events that ends in the reorientation of root
growth. In this way, a root which has been turned upside-down regains its previous
position. Analysis of starchless phosphoglucomutase loss-of-function (pgm)
mutants (introduced in Chap. 6) respond to gravity at one-third the rate of wildtype (WT) roots; intermediate mutants (acg20, acg27) showed responses proportional to their starch content (Kiss et al. 1996). Amyloplasts seem to undergo
cage-confined diffusion and cage-breaking motions characteristic of intracellular
microenvironments which determine their sedimentation dynamics (Zheng et al.
2015). But which signals and responses are elicited by sedimentation and subsequent mechanical stimulation? While the primary signal is still elusive and no
mechanosensitive receptors have been identified to date, interesting candidate
genes may be hidden in the glutamate-receptor-like gene family which is homologous to mammalian ionotropic glutamate receptors (Roy et al. 2008; De Bortoli
et al. 2016). GLR3.3 for example is such a ligand-gated, Ca
2+ -permeable channel
worth further detailed study as, when mutated, careful dissection of the
gravitropic response showed a range of intriguing growth phenotypes (Miller
et al. 2010).
7.5 Gravitropism Follows Grow-and-Switch or Tipping Point Mechanisms
97
mechanisms behind lateral root initiation and growth (Kircher and Schopfer 2016).
Experiments designed to differentiate between bending-induced local auxin accumulation and clock-type oscillations in auxin-induced gene expression revealed that
the frequency of lateral roots was promoted by auxin in the mature root; positioning,
however, followed a pre-formed pattern determined by previous bending (Kircher
and Schopfer 2016).
7.5 Gravitropism Follows Grow-and-Switch or Tipping
Point Mechanisms
In the root cap, specialized cells named statocytes sense gravity (cf. Chap. 6).
Analysis of gravisensitivity determined a threshold acceleration at between 10
À3
and 10
À4 g for roots and 10
À2 and 10
À3 g for shoots and a perception time ranging
between 1 and 10 s (Perbal et al. 1997; Hejnowicz et al. 1998; Perbal and DrissEcole 2003). However, dose-response-based studies, in which it has been suggested
that the curvature response varies linearly with the logarithm of the gravity stimulus,
may have to be revisited using up-to-date technologies (Perbal et al. 2002).
The weight of an object is the product of gravitational acceleration and its mass.
Therefore, all organelles which have a different density than the surrounding
cytoplasm and are not fixed in place by cytoskeletal elements may be involved
in gravisensing. In Arabidopsis roots, relatively dense starch-filled amyloplasts
sediment to the bottom of statocytes, where they might stimulate mechanosensors
which initiate a chain of physiological events that ends in the reorientation of root
growth. In this way, a root which has been turned upside-down regains its previous
position. Analysis of starchless phosphoglucomutase loss-of-function (pgm)
mutants (introduced in Chap. 6) respond to gravity at one-third the rate of wildtype (WT) roots; intermediate mutants (acg20, acg27) showed responses proportional to their starch content (Kiss et al. 1996). Amyloplasts seem to undergo
cage-confined diffusion and cage-breaking motions characteristic of intracellular
microenvironments which determine their sedimentation dynamics (Zheng et al.
2015). But which signals and responses are elicited by sedimentation and subsequent mechanical stimulation? While the primary signal is still elusive and no
mechanosensitive receptors have been identified to date, interesting candidate
genes may be hidden in the glutamate-receptor-like gene family which is homologous to mammalian ionotropic glutamate receptors (Roy et al. 2008; De Bortoli
et al. 2016). GLR3.3 for example is such a ligand-gated, Ca
2+ -permeable channel
worth further detailed study as, when mutated, careful dissection of the
gravitropic response showed a range of intriguing growth phenotypes (Miller
et al. 2010).
7.5 Gravitropism Follows Grow-and-Switch or Tipping Point Mechanisms
97