plant continues to provide unprecedented opportunities for isolating mutants and
exploring gene function and signaling pathway regulation (Provart et al. 2016).
7.2 Plants Sense Gravity
Plants constantly undergo rhythmic changes in growth such as the diurnal waving of
the leaves or the wavy growth of roots (Barlow 2015). They are therefore exposed to
constant changes in their relative position to the gravitational field. The environment
also imposes changes in the direction of growth such as when, for example, primary
root growth is impeded by an impenetrable obstacle in the substrate which requires
the root to traverse the surface of the barrier. Such changes are sensed and integrated
into overarching developmental programs which control the overall architecture of
the plant (Noll 1900; Monshausen and Gilroy 2009). Plants are equipped with
exquisitely sensitive sensory machineries which are even able to sense the periodic
alterations of the gravitational force caused by movement of the Moon (Barlow and
Fisahn 2012; Barlow 2015). This lunar tidal acceleration affects stem elongation
growth, leaf movement, seed imbibition and germination (Zajaczkowska and Barlow
2017). Rhythmic variations in the elongation of Arabidopsis roots have been
described which correspond perfectly to the rise and fall of the gravimetric tide
(Fisahn et al. 2012). Plants that were grown on the International Space Station (ISS)
and were exposed to lunar gravity in a centrifuge have shown different movement
profiles (Fisahn et al. 2015).
7.3 Root Growth Responses
Root growth is heavily influenced by endogenous regulatory rhythms. These oscillatory responses give rise to two growth behaviors: skewing and waving (Roy and
Bassham 2014). Obtaining a molecular understanding of skewing and waving has
been thought to provide key insights into root growth strategies and in the mechanisms underlying the building of the root’s architecture (Braybrook 2017; Roy and
Bassham 2017). Oscillatory growth is observed in the laboratory by growing roots
on tilted impermeable agar surfaces or in microchips (Grossmann et al. 2011).
However, waving is also considered to be part of the roots growth strategy in soil
(Tan et al. 2015). In Arabidopsis, some but not all ecotypes show skewing behavior,
whereas root waving seems to be inherent to all Arabidopsis ecotypes. Differentially
expressed candidate genes have been identified from different skewing and
non-skewing Arabidopsis accessions by transcriptome profiling. Genes found to
influence skewing or waving using this approach play roles in diverse cellular
processes including sugar transport, salt signaling, cell wall organization and hormone signaling (Schultz et al. 2017).
In more than 300 publications, waving and skewing have been considered to be
part of the gravitropic response (Roux 2012). However, imaging experiments
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7 Gravitropism in Higher Plants: Molecular Aspects
exploring gene function and signaling pathway regulation (Provart et al. 2016).
7.2 Plants Sense Gravity
Plants constantly undergo rhythmic changes in growth such as the diurnal waving of
the leaves or the wavy growth of roots (Barlow 2015). They are therefore exposed to
constant changes in their relative position to the gravitational field. The environment
also imposes changes in the direction of growth such as when, for example, primary
root growth is impeded by an impenetrable obstacle in the substrate which requires
the root to traverse the surface of the barrier. Such changes are sensed and integrated
into overarching developmental programs which control the overall architecture of
the plant (Noll 1900; Monshausen and Gilroy 2009). Plants are equipped with
exquisitely sensitive sensory machineries which are even able to sense the periodic
alterations of the gravitational force caused by movement of the Moon (Barlow and
Fisahn 2012; Barlow 2015). This lunar tidal acceleration affects stem elongation
growth, leaf movement, seed imbibition and germination (Zajaczkowska and Barlow
2017). Rhythmic variations in the elongation of Arabidopsis roots have been
described which correspond perfectly to the rise and fall of the gravimetric tide
(Fisahn et al. 2012). Plants that were grown on the International Space Station (ISS)
and were exposed to lunar gravity in a centrifuge have shown different movement
profiles (Fisahn et al. 2015).
7.3 Root Growth Responses
Root growth is heavily influenced by endogenous regulatory rhythms. These oscillatory responses give rise to two growth behaviors: skewing and waving (Roy and
Bassham 2014). Obtaining a molecular understanding of skewing and waving has
been thought to provide key insights into root growth strategies and in the mechanisms underlying the building of the root’s architecture (Braybrook 2017; Roy and
Bassham 2017). Oscillatory growth is observed in the laboratory by growing roots
on tilted impermeable agar surfaces or in microchips (Grossmann et al. 2011).
However, waving is also considered to be part of the roots growth strategy in soil
(Tan et al. 2015). In Arabidopsis, some but not all ecotypes show skewing behavior,
whereas root waving seems to be inherent to all Arabidopsis ecotypes. Differentially
expressed candidate genes have been identified from different skewing and
non-skewing Arabidopsis accessions by transcriptome profiling. Genes found to
influence skewing or waving using this approach play roles in diverse cellular
processes including sugar transport, salt signaling, cell wall organization and hormone signaling (Schultz et al. 2017).
In more than 300 publications, waving and skewing have been considered to be
part of the gravitropic response (Roux 2012). However, imaging experiments
94
7 Gravitropism in Higher Plants: Molecular Aspects