7.10 The Arabidopsis Transcriptome Is Affected by Altered
Gravity
We still do not fully understand how gravity affects the expression of gravitysensitive genes and whether particular molecular signatures are elicited by changing the orientation of roots in the gravitational field. In particular it will be
important to clarify which genes are necessary to characterize different phases of
the gravity response. Also important is the question of whether changing gravity
causes a specific stress response against which cells may remodel their metabolic
pathways in order to compensate. For this, it will be crucial to identify the genes
which regulate the gravity response. The analysis of the gravitome, the complement of gravity regulated genes, is likely to improve our understanding of the
molecular mechanisms regulating perception, transduction and the response to
gravity and how this response is regulated (Aubry-Hivet et al. 2014). Although
microgravity can be simulated in ground-based facilities by averaging gravity to
zero levels, here the influence of gravity will never fully be neutralized (Briegleb
1992; Herranz et al. 2013a, b). Therefore, instead, experiments may only be
performed on appropriate space environment platforms such as on the International
Space Station (ISS), satellites, sounding rockets, drop towers, or aircrafts during
parabolic flight (cf Chap. 2). Parabolic flights are more easily accessible for experimentation and offer experimental scenarios that enable us to obtain independent
experimental replications for solid statistical evaluation which are typically elusive
in other space flight scenarios. Although in these experiments, microgravity phases
are followed by phases of hyper-g accelerations, carefully designed control experiments have been able to separate the effects of microgravity and hyper-g on the
biological samples (Paul et al. 2011; Herranz et al. 2013a, b; Aubry-Hivet et al.
2014; Herranz and Medina 2014). By using mutants of the auxin efflux pathway
(i.e. pin2, pin3) it has been possible to correlate, under parabolic flight conditions,
genetic relationships with remodeling of metabolic pathways. In roots lacking
columella-localized PIN3, changes in gene expression were more dramatic than
in those defective in the epidermis and cortex cell-specific PIN2 confirming a
critical function of PIN3 in mediating auxin-fluxes in response to altered gravity
(Aubry-Hivet et al. 2014). These and other studies have provided us with many
of the important insights which are needed if we are to understand signal transduction processes in altered gravity conditions. Similar transcriptome studies
performed on Arabidopsis seedlings or in vitro grown callus cultures which were
exposed to microgravity either during growth in suborbital flights or on the ISS
provided further evidence that space provides an environment which requires
novel adaptive processes (Paul et al. 2011). Interestingly comparison of knockout
mutants of ARG1 with wild type Arabidopsis revealed the engagement of unique
genes during physiological adaptation to the space flight environment.
104
7 Gravitropism in Higher Plants: Molecular Aspects
Gravity
We still do not fully understand how gravity affects the expression of gravitysensitive genes and whether particular molecular signatures are elicited by changing the orientation of roots in the gravitational field. In particular it will be
important to clarify which genes are necessary to characterize different phases of
the gravity response. Also important is the question of whether changing gravity
causes a specific stress response against which cells may remodel their metabolic
pathways in order to compensate. For this, it will be crucial to identify the genes
which regulate the gravity response. The analysis of the gravitome, the complement of gravity regulated genes, is likely to improve our understanding of the
molecular mechanisms regulating perception, transduction and the response to
gravity and how this response is regulated (Aubry-Hivet et al. 2014). Although
microgravity can be simulated in ground-based facilities by averaging gravity to
zero levels, here the influence of gravity will never fully be neutralized (Briegleb
1992; Herranz et al. 2013a, b). Therefore, instead, experiments may only be
performed on appropriate space environment platforms such as on the International
Space Station (ISS), satellites, sounding rockets, drop towers, or aircrafts during
parabolic flight (cf Chap. 2). Parabolic flights are more easily accessible for experimentation and offer experimental scenarios that enable us to obtain independent
experimental replications for solid statistical evaluation which are typically elusive
in other space flight scenarios. Although in these experiments, microgravity phases
are followed by phases of hyper-g accelerations, carefully designed control experiments have been able to separate the effects of microgravity and hyper-g on the
biological samples (Paul et al. 2011; Herranz et al. 2013a, b; Aubry-Hivet et al.
2014; Herranz and Medina 2014). By using mutants of the auxin efflux pathway
(i.e. pin2, pin3) it has been possible to correlate, under parabolic flight conditions,
genetic relationships with remodeling of metabolic pathways. In roots lacking
columella-localized PIN3, changes in gene expression were more dramatic than
in those defective in the epidermis and cortex cell-specific PIN2 confirming a
critical function of PIN3 in mediating auxin-fluxes in response to altered gravity
(Aubry-Hivet et al. 2014). These and other studies have provided us with many
of the important insights which are needed if we are to understand signal transduction processes in altered gravity conditions. Similar transcriptome studies
performed on Arabidopsis seedlings or in vitro grown callus cultures which were
exposed to microgravity either during growth in suborbital flights or on the ISS
provided further evidence that space provides an environment which requires
novel adaptive processes (Paul et al. 2011). Interestingly comparison of knockout
mutants of ARG1 with wild type Arabidopsis revealed the engagement of unique
genes during physiological adaptation to the space flight environment.
104
7 Gravitropism in Higher Plants: Molecular Aspects