performed on the ISS in which growth patterns of Arabidopsis roots were analyzed
in a microgravity environment, clearly demonstrated that waving and skewing were
not guided by gravity, but instead by light. These experiments, in which factors such
as acceleration vectors, airflow or other directional environmental factors were
accounted for, revealed that roots grew away from light whereas their 1-g controls
in a centrifuge oriented positively gravitropically in parallel to the acceleration vector
(Paul et al. 2012). From this, it is justified to conclude that skewing and waving are
driven by plant endogenous rhythms independent of microgravity. Interestingly this
phenomenon was reanalyzed using roots from Medicago trunculata; here the speed of
growth and size of the root enabled an easy nondestructive examination of geometric
parameters during root coiling, waving, skewing and gravity mediated curvature (Tan
et al. 2015). Data showed that gravity-influenced developmental switches control the
root’s growth direction and are governed by the root’s ability to measure the direction
of gravity with some precision. Analysis suggests that, in a similar manner to bacterial
chemotaxis, Medicago roots are apparently able to find the path of steepest descent by
sensing their orientation relative to gravity. This grow-and-switch gravitropism may
have provided Medicago with an evolutionary favorable trait which allows it to thrive
in highly obstructed environments.
7.4 Root Systems Architecture Is Built by Periodic Growth
Responses
Lateral roots, organs crucial for exploring the soil, extract nutrients and communicate with the soil microbiome and are initiated post-embryonically in response to
environmental cues. Their growth therefore largely defines the complexity of a root
system. How then does gravity trigger the periodic growth responses of lateral roots?
It was shown in Arabidopsis thaliana that lateral root initiation is induced by
gravitropic curvature (Ditengou et al. 2008). At the site of lateral root induction
auxin accumulated before formation of the primordium, an accumulation which is
correlated with a subcellular relocalization of the auxin efflux carrier PIN1 in a single
protoxylem cell (Fig. 7.1). This relocalization preceded auxin-dependent gene transcription and defined a competence zone in which lateral root primordia formation
became possible (Ditengou et al. 2008).
Root bending bypassed ARF1/19-dependent nuclear auxin signaling. These
transcription factors are normally necessary for lateral root formation as shown by
analysis of arf7/19 double knock-out mutants which normally form no lateral roots.
However, lateral root formation proceeded upon bending when the root tip was
removed (Ditengou et al. 2008). Another feature of lateral root induction is periodic
oscillatory changes in auxin levels revealed by either using fluorescent or luminescent auxin reporters. Several studies have identified dynamic alterations of auxin at
positions where future founder cells for lateral root primordia will be formed
(De Smet et al. 2007; Moreno-Risueno et al. 2010; Xuan et al. 2015). Depending
7.4 Root Systems Architecture Is Built by Periodic Growth Responses
95
in a microgravity environment, clearly demonstrated that waving and skewing were
not guided by gravity, but instead by light. These experiments, in which factors such
as acceleration vectors, airflow or other directional environmental factors were
accounted for, revealed that roots grew away from light whereas their 1-g controls
in a centrifuge oriented positively gravitropically in parallel to the acceleration vector
(Paul et al. 2012). From this, it is justified to conclude that skewing and waving are
driven by plant endogenous rhythms independent of microgravity. Interestingly this
phenomenon was reanalyzed using roots from Medicago trunculata; here the speed of
growth and size of the root enabled an easy nondestructive examination of geometric
parameters during root coiling, waving, skewing and gravity mediated curvature (Tan
et al. 2015). Data showed that gravity-influenced developmental switches control the
root’s growth direction and are governed by the root’s ability to measure the direction
of gravity with some precision. Analysis suggests that, in a similar manner to bacterial
chemotaxis, Medicago roots are apparently able to find the path of steepest descent by
sensing their orientation relative to gravity. This grow-and-switch gravitropism may
have provided Medicago with an evolutionary favorable trait which allows it to thrive
in highly obstructed environments.
7.4 Root Systems Architecture Is Built by Periodic Growth
Responses
Lateral roots, organs crucial for exploring the soil, extract nutrients and communicate with the soil microbiome and are initiated post-embryonically in response to
environmental cues. Their growth therefore largely defines the complexity of a root
system. How then does gravity trigger the periodic growth responses of lateral roots?
It was shown in Arabidopsis thaliana that lateral root initiation is induced by
gravitropic curvature (Ditengou et al. 2008). At the site of lateral root induction
auxin accumulated before formation of the primordium, an accumulation which is
correlated with a subcellular relocalization of the auxin efflux carrier PIN1 in a single
protoxylem cell (Fig. 7.1). This relocalization preceded auxin-dependent gene transcription and defined a competence zone in which lateral root primordia formation
became possible (Ditengou et al. 2008).
Root bending bypassed ARF1/19-dependent nuclear auxin signaling. These
transcription factors are normally necessary for lateral root formation as shown by
analysis of arf7/19 double knock-out mutants which normally form no lateral roots.
However, lateral root formation proceeded upon bending when the root tip was
removed (Ditengou et al. 2008). Another feature of lateral root induction is periodic
oscillatory changes in auxin levels revealed by either using fluorescent or luminescent auxin reporters. Several studies have identified dynamic alterations of auxin at
positions where future founder cells for lateral root primordia will be formed
(De Smet et al. 2007; Moreno-Risueno et al. 2010; Xuan et al. 2015). Depending
7.4 Root Systems Architecture Is Built by Periodic Growth Responses
95