7.6 Auxin Is an Early Gravistimulation Signal
‘The Power of Movement in Plants’, one of the early benchmarks for the study of
plant physiology, aimed to understand the principles of tropic growth (Darwin,
1880). This pioneering work underpinned the discovery that the asymmetric distribution of auxin across an organ is a common module for the transmission of an
external stimulus into a directional growth response. However, the most pressing
task arising from this work still remains: understanding how a gravitropic stimulus
leads to an asymmetric auxin distribution. By integrating high-resolution imaging
with computer vision-based analysis, several studies have suggested that ROS, pH,
and Ca
2+ -mediated signals all play crucial roles in the early gravity response (Scott
and Allen 1999; Fasano et al. 2001; Joo et al. 2001; Hou et al. 2004; Monshausen
et al. 2011; Salmi et al. 2011; Hayatsu and Suzuki 2015; Sato et al. 2015; Dummer
et al. 2016; Krieger and Shkolnik 2016; Singh et al. 2016; Ponce et al. 2017). A
surprising observation reported by Weerasinghe et al. (2009) places the release of
ATP from root tip cells downstream of mechanical stimulus-dependent rapid
changes in cytosolic Ca
2+ , but candidate genes playing roles in early gravitropic
signaling have not yet been found. Protein kinases are obvious candidates; for
example, members of the CrRLK1 receptor-like kinase family are thought to play
roles in the earliest stages of strain-activated Ca
2+ -signaling. Hypotheses which link
auxin- and calcium-based signals are strengthened by feedback regulation between
the two processes; the external application of auxin modulates gravitropic signaling
in a Ca
2+ -dependent fashion. Application of 100 nM auxin to the root tip triggered a
rise in cytosolic Ca
2+ within 7 sec eliciting a wave of Ca
2+ which moves shootward
back along the root axis (Monshausen et al. 2011).
The pathway resulting in elicitation of a similar wave of shootward-travelling auxin
seems now to be well understood. It is widely accepted that the products of two genes:
ARG1 (ALTERED RESPONSE TO GRAVITY) and its paralog ARG-LIKE2 (ARL-2)
link amyloplast sedimentation with auxin flux (Fukaki et al. 1997; Sedbrook et al.
1999; Guan et al., 2003; Harrison and Masson 2008). ARG1 is a type-II DnaJ-like
protein with a C-terminal coil-coiled domain which interacts with Hsp70 molecular
chaperones to help disassemble clathrin triskelia (a three-legged pinwheel-shaped
heteropolymer, which coats certain post-Golgi and ER vesicles) from clathrin-coated
vesicles during endocytosis. Mutations in ARG1 and ARL2 show reduced cytoplasmic alkalinization and a significantly reduced gravitropic response. After combining
arg1 and arl2 alleles with starch-less mutant pgm-1, it was demonstrated that the
proteins act in distinct pathways, with PGM affecting the mechanostimulatory
response and ARG1 and ARL2 playing roles in the endocytotic modulation of PIN
relocalization. Interestingly, AUX1, another polar auxin transport protein (see below),
seems not to be affected (Boonsirichai et al. 2003). Using a rotating stage image
system, Evans and coworkers were able to maintain the root tip at a constant angle and
observe at constant rate the gravitropic response regardless of the angle of tip
orientation (Mullen et al. 2000; Wolverton et al. 2002). WT and pin3–1 plants showed,
in contrast to pgm-1, increasing response rates as the tip was constrained at greater
98
7 Gravitropism in Higher Plants: Molecular Aspects
‘The Power of Movement in Plants’, one of the early benchmarks for the study of
plant physiology, aimed to understand the principles of tropic growth (Darwin,
1880). This pioneering work underpinned the discovery that the asymmetric distribution of auxin across an organ is a common module for the transmission of an
external stimulus into a directional growth response. However, the most pressing
task arising from this work still remains: understanding how a gravitropic stimulus
leads to an asymmetric auxin distribution. By integrating high-resolution imaging
with computer vision-based analysis, several studies have suggested that ROS, pH,
and Ca
2+ -mediated signals all play crucial roles in the early gravity response (Scott
and Allen 1999; Fasano et al. 2001; Joo et al. 2001; Hou et al. 2004; Monshausen
et al. 2011; Salmi et al. 2011; Hayatsu and Suzuki 2015; Sato et al. 2015; Dummer
et al. 2016; Krieger and Shkolnik 2016; Singh et al. 2016; Ponce et al. 2017). A
surprising observation reported by Weerasinghe et al. (2009) places the release of
ATP from root tip cells downstream of mechanical stimulus-dependent rapid
changes in cytosolic Ca
2+ , but candidate genes playing roles in early gravitropic
signaling have not yet been found. Protein kinases are obvious candidates; for
example, members of the CrRLK1 receptor-like kinase family are thought to play
roles in the earliest stages of strain-activated Ca
2+ -signaling. Hypotheses which link
auxin- and calcium-based signals are strengthened by feedback regulation between
the two processes; the external application of auxin modulates gravitropic signaling
in a Ca
2+ -dependent fashion. Application of 100 nM auxin to the root tip triggered a
rise in cytosolic Ca
2+ within 7 sec eliciting a wave of Ca
2+ which moves shootward
back along the root axis (Monshausen et al. 2011).
The pathway resulting in elicitation of a similar wave of shootward-travelling auxin
seems now to be well understood. It is widely accepted that the products of two genes:
ARG1 (ALTERED RESPONSE TO GRAVITY) and its paralog ARG-LIKE2 (ARL-2)
link amyloplast sedimentation with auxin flux (Fukaki et al. 1997; Sedbrook et al.
1999; Guan et al., 2003; Harrison and Masson 2008). ARG1 is a type-II DnaJ-like
protein with a C-terminal coil-coiled domain which interacts with Hsp70 molecular
chaperones to help disassemble clathrin triskelia (a three-legged pinwheel-shaped
heteropolymer, which coats certain post-Golgi and ER vesicles) from clathrin-coated
vesicles during endocytosis. Mutations in ARG1 and ARL2 show reduced cytoplasmic alkalinization and a significantly reduced gravitropic response. After combining
arg1 and arl2 alleles with starch-less mutant pgm-1, it was demonstrated that the
proteins act in distinct pathways, with PGM affecting the mechanostimulatory
response and ARG1 and ARL2 playing roles in the endocytotic modulation of PIN
relocalization. Interestingly, AUX1, another polar auxin transport protein (see below),
seems not to be affected (Boonsirichai et al. 2003). Using a rotating stage image
system, Evans and coworkers were able to maintain the root tip at a constant angle and
observe at constant rate the gravitropic response regardless of the angle of tip
orientation (Mullen et al. 2000; Wolverton et al. 2002). WT and pin3–1 plants showed,
in contrast to pgm-1, increasing response rates as the tip was constrained at greater
98
7 Gravitropism in Higher Plants: Molecular Aspects