6.3 Signal Transduction and Transmission
It is obvious that a mode of communication exists between the root tip cells that
sense gravity and the cells in the elongation zone that respond to the gravity signal by
expanding differentially on the opposite root flanks which leads to the downward
curvature. After conversion of the physical stimulus into a biochemical signal (signal
transduction) in the columella cells, the signal of all statocytes must be integrated,
enhanced, transmitted and received in the responding tissue.
6.3.1 Secondary Messengers
Signal transduction and transmission employs a plethora of secondary messengers
including calcium ions, InsP 3 and protons (Fasano 2001; Perera 2006).
Calcium is an essential secondary messenger in many signaling processes in
plants and has been considered as an important messenger in gravitropism. It was
noted early on that radioactively labeled
45 Ca
2+ is transported basally in Zea mays
roots after gravistimulation (Lee and Evans 1985). One-sided application of calcium
to the root leads to bending towards the source of calcium (Lee et al. 1983a).
Gravitropic bending is severely impaired by application of calcium chelators or by
inhibition of calmodulin or calcium-channels (Lee et al. 1983b; Vanneste and Friml
2013). These observations suggest that calcium plays a role in the apoplast of the
root (Toyota and Gilroy 2013). Calmodulins, prominent calcium binding proteins,
are highly expressed in the root tip (Stinemetz et al. 1987), which might suggest a
role of calcium also in gravity perception.
More recent experiments with the calcium sensor aequorin support a role for
calcium during early gravitropic responses (Toyota et al. 2007). Aequorin fluorescence is dependent on the availability of intracellular calcium (Shimomura et al.
1962). After reorientation experiments, two calcium waves were measured in
statocytes of the hypocotyl, the first after 4 s, the second after 1 min. The first
calcium wave was considered an effect of the mechanical reorientation, the second
as an effect of the gravitational response. It remains unclear whether calcium is part
of the signal perception or part of the later signaling events. The second wave
suggests an involvement in signal transduction and not signal perception. This is
supported by physiological changes, like membrane depolarization, that appear
earlier than 1 min (Behrens et al. 1985). More recently introduced calcium sensors,
like Cameleon YC3.6, have been used to detect a calcium wave arising on the lower
side of the root and moving towards the elongation zone like auxin (Monshausen
et al. 2011). Calcium could play different roles in early as well as progressed signal
transduction. Modern real-time measurements, e.g. in parabolic plane flights, promise more detailed results in the coming years (Neef et al. 2016).
Calcium intimately interplays with auxin. Auxin induces calcium levels and
signaling (Toyota and Gilroy 2013). Increases in calcium regulate auxin transport
6.3 Signal Transduction and Transmission
83
It is obvious that a mode of communication exists between the root tip cells that
sense gravity and the cells in the elongation zone that respond to the gravity signal by
expanding differentially on the opposite root flanks which leads to the downward
curvature. After conversion of the physical stimulus into a biochemical signal (signal
transduction) in the columella cells, the signal of all statocytes must be integrated,
enhanced, transmitted and received in the responding tissue.
6.3.1 Secondary Messengers
Signal transduction and transmission employs a plethora of secondary messengers
including calcium ions, InsP 3 and protons (Fasano 2001; Perera 2006).
Calcium is an essential secondary messenger in many signaling processes in
plants and has been considered as an important messenger in gravitropism. It was
noted early on that radioactively labeled
45 Ca
2+ is transported basally in Zea mays
roots after gravistimulation (Lee and Evans 1985). One-sided application of calcium
to the root leads to bending towards the source of calcium (Lee et al. 1983a).
Gravitropic bending is severely impaired by application of calcium chelators or by
inhibition of calmodulin or calcium-channels (Lee et al. 1983b; Vanneste and Friml
2013). These observations suggest that calcium plays a role in the apoplast of the
root (Toyota and Gilroy 2013). Calmodulins, prominent calcium binding proteins,
are highly expressed in the root tip (Stinemetz et al. 1987), which might suggest a
role of calcium also in gravity perception.
More recent experiments with the calcium sensor aequorin support a role for
calcium during early gravitropic responses (Toyota et al. 2007). Aequorin fluorescence is dependent on the availability of intracellular calcium (Shimomura et al.
1962). After reorientation experiments, two calcium waves were measured in
statocytes of the hypocotyl, the first after 4 s, the second after 1 min. The first
calcium wave was considered an effect of the mechanical reorientation, the second
as an effect of the gravitational response. It remains unclear whether calcium is part
of the signal perception or part of the later signaling events. The second wave
suggests an involvement in signal transduction and not signal perception. This is
supported by physiological changes, like membrane depolarization, that appear
earlier than 1 min (Behrens et al. 1985). More recently introduced calcium sensors,
like Cameleon YC3.6, have been used to detect a calcium wave arising on the lower
side of the root and moving towards the elongation zone like auxin (Monshausen
et al. 2011). Calcium could play different roles in early as well as progressed signal
transduction. Modern real-time measurements, e.g. in parabolic plane flights, promise more detailed results in the coming years (Neef et al. 2016).
Calcium intimately interplays with auxin. Auxin induces calcium levels and
signaling (Toyota and Gilroy 2013). Increases in calcium regulate auxin transport
6.3 Signal Transduction and Transmission
83