(Robert and Offringa 2008). Calcium elevations change cell wall pH which regulates
elongation via acid growth (Monshausen et al. 2011). The proton concentration
(pH) of a cellular compartment has a strong effect on all containing proteins
and metabolites. A change in pH can have a modulating influence on various
cellular activities. Using a pH sensor, a movement of protons from the cytoplasm
to the apoplast was observed in the columella in Arabidopsis thaliana upon
gravistimulation (Fasano 2001). Alkalization of the cytoplasm occurs within the
first 2 min with a change from pH 7.2 to pH 7.6, while the pH in the apoplast changes
from pH 5.5 to 4.5. Later pH changes in the cell wall in the elongation zone were
attributed to tropic growth. Similar observations of pH changes were made in Zea
mays pulvini in the statocytes-containing endodermis (Johannes et al. 2001). With
the help of caged protons that were released upon UV irradiation, the beginning of
tropic growth could be delayed by manual manipulation of the pH value (Fasano
2001).
Mutants defective in columella cell alkalization fail to relocate auxin transporters
of the PIN family, thereby affecting auxin redistribution (Boonsirichai et al. 2003;
Harrison and Masson 2008).
Phosphatidylinositol-4,5-bisphosphate (PIP 2 ) is cleaved via hydrolysis by phospholipase C into diacylglycerol and InsP 3 . Both hydrolysis products play a role in
downstream signaling cascades. Mutants of Phosphatidylinositol-monophosphate5-kinase (PIP5K) are delayed in their gravitropic response and impaired in polar
auxin transport (Mei et al. 2012). One of the products of PIP5K, InsP 3 modulates
intracellular calcium signals in animal cells. InsP 3 opens ER-localized calcium
channels which trigger downstream signaling cascades (Berridge 2009). InsP 3
could have a similar influence on calcium signaling in plants during gravitropism.
A role for InsP 3 in gravitropism is supported by several experiments. A fivefold
increase in InsP 3 concentration in the lower pulvinus, the area of bending growth
in Zea mays, was measured 10 s after reorientation (Perera et al. 1999). The increase
in InsP 3 correlates with membrane depolarization after 8 s (Behrens et al. 1985)
and suggests that InsP 3 plays a role during early gravitropism signaling or even
graviperception. The InsP 3 concentration is still increased after 2 hours, suggesting
an involvement also in later signaling processes. Following the gravitropic response
(8–10 h), the InsP 3 concentration reverts to its initial level. In plants overexpressing
human INOSITOL-POLYPHOSPHATE-5-PHOSPHATASE, catalyzing constitutive hydrolysis of InsP 3 , gravitropic responses were present but reduced (Perera
2006). Inhibition of InsP 3 synthesis by blocking PHOSPHOLIPASE C in
Arabidopsis thaliana leads to reduced gravitropic responses in roots and shoot
(Andreeva et al. 2010).
It is noteworthy that gravitropic responses in InsP 3 -deficient plants were never
abolished but always reduced. InsP 3 can carefully be considered modulating but not
essential for gravitropism. InsP 3 may have to be considered in the context of other
secondary messengers. A direct connection between InsP 3 and calcium and InsP 3 -
activated calcium channels is a point of future research.
While calcium, InsP 3 , and pH play a role as secondary messengers in gravitropism
signaling, the resulting hormone distribution leads to the final curvature responses.
84
6 Gravitropism in Higher Plants: Cellular Aspects
elongation via acid growth (Monshausen et al. 2011). The proton concentration
(pH) of a cellular compartment has a strong effect on all containing proteins
and metabolites. A change in pH can have a modulating influence on various
cellular activities. Using a pH sensor, a movement of protons from the cytoplasm
to the apoplast was observed in the columella in Arabidopsis thaliana upon
gravistimulation (Fasano 2001). Alkalization of the cytoplasm occurs within the
first 2 min with a change from pH 7.2 to pH 7.6, while the pH in the apoplast changes
from pH 5.5 to 4.5. Later pH changes in the cell wall in the elongation zone were
attributed to tropic growth. Similar observations of pH changes were made in Zea
mays pulvini in the statocytes-containing endodermis (Johannes et al. 2001). With
the help of caged protons that were released upon UV irradiation, the beginning of
tropic growth could be delayed by manual manipulation of the pH value (Fasano
2001).
Mutants defective in columella cell alkalization fail to relocate auxin transporters
of the PIN family, thereby affecting auxin redistribution (Boonsirichai et al. 2003;
Harrison and Masson 2008).
Phosphatidylinositol-4,5-bisphosphate (PIP 2 ) is cleaved via hydrolysis by phospholipase C into diacylglycerol and InsP 3 . Both hydrolysis products play a role in
downstream signaling cascades. Mutants of Phosphatidylinositol-monophosphate5-kinase (PIP5K) are delayed in their gravitropic response and impaired in polar
auxin transport (Mei et al. 2012). One of the products of PIP5K, InsP 3 modulates
intracellular calcium signals in animal cells. InsP 3 opens ER-localized calcium
channels which trigger downstream signaling cascades (Berridge 2009). InsP 3
could have a similar influence on calcium signaling in plants during gravitropism.
A role for InsP 3 in gravitropism is supported by several experiments. A fivefold
increase in InsP 3 concentration in the lower pulvinus, the area of bending growth
in Zea mays, was measured 10 s after reorientation (Perera et al. 1999). The increase
in InsP 3 correlates with membrane depolarization after 8 s (Behrens et al. 1985)
and suggests that InsP 3 plays a role during early gravitropism signaling or even
graviperception. The InsP 3 concentration is still increased after 2 hours, suggesting
an involvement also in later signaling processes. Following the gravitropic response
(8–10 h), the InsP 3 concentration reverts to its initial level. In plants overexpressing
human INOSITOL-POLYPHOSPHATE-5-PHOSPHATASE, catalyzing constitutive hydrolysis of InsP 3 , gravitropic responses were present but reduced (Perera
2006). Inhibition of InsP 3 synthesis by blocking PHOSPHOLIPASE C in
Arabidopsis thaliana leads to reduced gravitropic responses in roots and shoot
(Andreeva et al. 2010).
It is noteworthy that gravitropic responses in InsP 3 -deficient plants were never
abolished but always reduced. InsP 3 can carefully be considered modulating but not
essential for gravitropism. InsP 3 may have to be considered in the context of other
secondary messengers. A direct connection between InsP 3 and calcium and InsP 3 -
activated calcium channels is a point of future research.
While calcium, InsP 3 , and pH play a role as secondary messengers in gravitropism
signaling, the resulting hormone distribution leads to the final curvature responses.
84
6 Gravitropism in Higher Plants: Cellular Aspects