angles. Interestingly, wild type roots expressing the auxin response reporter DR5::GFP
displayed a graded GFP response with a maximum along the lower flank of wild type
roots, whereas pgm-1 roots formed a GFP maximum in the central columella but
lacked any observable gradient at up to 6 h after reorientation (Wolverton et al. 2011).
Based on the quantitative analysis of plastid sedimentation, a relationship between
root cap angle and gravitropic response was found.
These data are consistent with the idea of several overlapping sensory response
networks involved in controlling gravitropism, with PIN3 performing a rate-limiting
early response but becoming less important for sustained differential growth (Friml
et al. 2002; Wolverton et al. 2011). Rapid gravity-stimulated changes in PIN3
polarity have been observed; however, PIN3 polarization apparently does not require
secretion of de novo synthesized proteins or protein degradation but rather uses
(probably ARG1/ARL2-dependent) clathrin-dependent endocytosis for rapid cellular relocalization. This pathway, which distributes auxin asymmetrically during the
response to gravity, has been suggested to require Brefeldin A-sensitive recycling
and recruitment of an ARF-GEF (guanine nucleotide exchange factor for ARF
GTPases) for polar targeting and rapid transcytotic relocation to different sides of
graviresponsive cells (Steinmann et al. 1999; Friml et al. 2002; Kleine-Vehn et al.
2010; Naramoto et al. 2010).
Flux of auxin through plant organs, tissues and cells not only requires efflux from
cells but also influx and intracellular fluxes as recently demonstrated (Middleton
et al. 2018). The first auxin influx carrier prototype was cloned from the agravitropic
aux1 mutant (Bennett et al. 1996). Over the years genetic and functional analysis
revealed important aspects of its function. Surprisingly AUX1 is not only a major
auxin import carrier for auxin (indole-3-acetic acid) uptake but also mediates
membrane depolarization and correlates with a long-distance Ca
2+ wave that modulates the auxin response (Dindas et al. 2018). This effect has found additional
support from studies on root hairs, in which the external or internal application of
auxin caused Ca
2+ changes (observed with R-GECO1) which were propagated as
long-distance waves. These IAA-triggered local and systemic calcium signals were
blocked by treatment with the SCFTIR1/AFB 46 -signaling inhibitor 47 auxinole
and appeared strongly impaired in the tir1afb2afb3 triple mutant (Dindas et al.
2018). It can be hypothesized that AUX1 operates as the major auxin re-uptake
route after auxin is released from cells by different PIN efflux carriers in the
gravisensing region of the root cap. Ca
2+ waves may also well operate in this region
just as they do in root hairs.
7.7 Gravitropic Signaling by Ca
2+ /Calmodulin-Dependent
Kinase CRK5
It is therefore not surprising that a Ca
2+ /calmodulin-dependent kinase, CRK5, has
been found in this central gravisensing zone (Rigo et al. 2013). CRK5 belongs to an
eight-member CRK family in Arabidopsis, and shares structural and functional
7.7 Gravitropic Signaling by Ca
2+ /Calmodulin-Dependent Kinase CRK5
99
displayed a graded GFP response with a maximum along the lower flank of wild type
roots, whereas pgm-1 roots formed a GFP maximum in the central columella but
lacked any observable gradient at up to 6 h after reorientation (Wolverton et al. 2011).
Based on the quantitative analysis of plastid sedimentation, a relationship between
root cap angle and gravitropic response was found.
These data are consistent with the idea of several overlapping sensory response
networks involved in controlling gravitropism, with PIN3 performing a rate-limiting
early response but becoming less important for sustained differential growth (Friml
et al. 2002; Wolverton et al. 2011). Rapid gravity-stimulated changes in PIN3
polarity have been observed; however, PIN3 polarization apparently does not require
secretion of de novo synthesized proteins or protein degradation but rather uses
(probably ARG1/ARL2-dependent) clathrin-dependent endocytosis for rapid cellular relocalization. This pathway, which distributes auxin asymmetrically during the
response to gravity, has been suggested to require Brefeldin A-sensitive recycling
and recruitment of an ARF-GEF (guanine nucleotide exchange factor for ARF
GTPases) for polar targeting and rapid transcytotic relocation to different sides of
graviresponsive cells (Steinmann et al. 1999; Friml et al. 2002; Kleine-Vehn et al.
2010; Naramoto et al. 2010).
Flux of auxin through plant organs, tissues and cells not only requires efflux from
cells but also influx and intracellular fluxes as recently demonstrated (Middleton
et al. 2018). The first auxin influx carrier prototype was cloned from the agravitropic
aux1 mutant (Bennett et al. 1996). Over the years genetic and functional analysis
revealed important aspects of its function. Surprisingly AUX1 is not only a major
auxin import carrier for auxin (indole-3-acetic acid) uptake but also mediates
membrane depolarization and correlates with a long-distance Ca
2+ wave that modulates the auxin response (Dindas et al. 2018). This effect has found additional
support from studies on root hairs, in which the external or internal application of
auxin caused Ca
2+ changes (observed with R-GECO1) which were propagated as
long-distance waves. These IAA-triggered local and systemic calcium signals were
blocked by treatment with the SCFTIR1/AFB 46 -signaling inhibitor 47 auxinole
and appeared strongly impaired in the tir1afb2afb3 triple mutant (Dindas et al.
2018). It can be hypothesized that AUX1 operates as the major auxin re-uptake
route after auxin is released from cells by different PIN efflux carriers in the
gravisensing region of the root cap. Ca
2+ waves may also well operate in this region
just as they do in root hairs.
7.7 Gravitropic Signaling by Ca
2+ /Calmodulin-Dependent
Kinase CRK5
It is therefore not surprising that a Ca
2+ /calmodulin-dependent kinase, CRK5, has
been found in this central gravisensing zone (Rigo et al. 2013). CRK5 belongs to an
eight-member CRK family in Arabidopsis, and shares structural and functional
7.7 Gravitropic Signaling by Ca
2+ /Calmodulin-Dependent Kinase CRK5
99