2017). Like in the photoresponses, gravity can induce changes in the swimming
velocity of a microorganisms which is termed gravikinesis (Machemer et al. 1991;
Machemer 1996). Microorganisms sediment due to their higher specific density than
the surrounding medium and the sedimentation velocity is vectorially added to the
swimming velocity. Thus, the net swimming velocity is lower in upward direction
than in downward direction. However, in some ciliates, such as Paramecium,
Didinium, Tetrahymena and Loxodes, a directional-dependent speed regulation—
accelerated upward swimming versus decelerated speed during downward swimming—results in compensation of sedimentation and, thus, they aggregate in a water
layer with optimal environmental conditions (Hemmersbach-Krause et al. 1991b;
Machemer et al. 1991; Ooya et al. 1992).
1.4 Gravity Responses of Sessile Plants
Movements of organs of sessile plants are called tropisms. Branches, fronts, stems,
flowers and leaves can move with respect to light (phototropism; Briggs 2014;
Liscum et al. 2014). Apical stems often bend toward the light source which is called
positive phototropism (Briggs 2014; Liscum et al. 2014) while Arabidopsis inflorescences (flowering stems) show negative phototropism (Sato et al. 2015). With a
few exceptions, roots of higher plants do not bend with respect to the light direction
which in Arabidopsis have been found to be mediated by the universal photoreceptors in higher plants, phytochromes (Ruppel et al. 2001; Kiss et al. 2003). The young
seedlings of Gramineae (coleoptiles) such as Avena have been studied for a long time
and show a remarkable behavior: in unilateral light at low irradiances they show a
positive phototropism, at higher a negative one and at an even higher one they bend
again towards the light source (Buder 1920; Everett and Thimann 1968). Plants use
proprioreceptors to sense their own growth. E.g., the expression of the PtaZFP2 gene
is closely related with the bending angle of a poplar stem (Hamant 2013). In
addition, roots of several plants have been shown to react to gradients in humidity
(hydrotropism; Eapen et al. 2005).
Higher plants can also detect the presence of other plants or objects in their
neighborhood and respond accordingly (Baldwin 2010). For this purpose, they use
volatile substances derived from terpenoids, fatty acid catabolites, aromatics and
amino acids, which are released into the air. Plants that lose the ability to detect
ethylene also lack the ability to sense the location of other plants and object in their
surroundings. In addition, plants show tactile responses (thigmo- or haptotropism)
when they touch an object. This is of an advantage for climbing plants as it facilitates
the search for a support. The receptors are specific pits in the outer cell wall of the
epidermis and they can even distinguish between different materials which
e.g. prevents a vine to entwine a water jet (Isnard and Silk 2009).
All organs of sessile plants detect the gravity vector of the Earth but respond
differently. Main stems generally grow upward (negative gravitropism; Yamamoto
et al. 2002) while principal roots grow downward (positive gravitropism; Konings
1.4 Gravity Responses of Sessile Plants
5
velocity of a microorganisms which is termed gravikinesis (Machemer et al. 1991;
Machemer 1996). Microorganisms sediment due to their higher specific density than
the surrounding medium and the sedimentation velocity is vectorially added to the
swimming velocity. Thus, the net swimming velocity is lower in upward direction
than in downward direction. However, in some ciliates, such as Paramecium,
Didinium, Tetrahymena and Loxodes, a directional-dependent speed regulation—
accelerated upward swimming versus decelerated speed during downward swimming—results in compensation of sedimentation and, thus, they aggregate in a water
layer with optimal environmental conditions (Hemmersbach-Krause et al. 1991b;
Machemer et al. 1991; Ooya et al. 1992).
1.4 Gravity Responses of Sessile Plants
Movements of organs of sessile plants are called tropisms. Branches, fronts, stems,
flowers and leaves can move with respect to light (phototropism; Briggs 2014;
Liscum et al. 2014). Apical stems often bend toward the light source which is called
positive phototropism (Briggs 2014; Liscum et al. 2014) while Arabidopsis inflorescences (flowering stems) show negative phototropism (Sato et al. 2015). With a
few exceptions, roots of higher plants do not bend with respect to the light direction
which in Arabidopsis have been found to be mediated by the universal photoreceptors in higher plants, phytochromes (Ruppel et al. 2001; Kiss et al. 2003). The young
seedlings of Gramineae (coleoptiles) such as Avena have been studied for a long time
and show a remarkable behavior: in unilateral light at low irradiances they show a
positive phototropism, at higher a negative one and at an even higher one they bend
again towards the light source (Buder 1920; Everett and Thimann 1968). Plants use
proprioreceptors to sense their own growth. E.g., the expression of the PtaZFP2 gene
is closely related with the bending angle of a poplar stem (Hamant 2013). In
addition, roots of several plants have been shown to react to gradients in humidity
(hydrotropism; Eapen et al. 2005).
Higher plants can also detect the presence of other plants or objects in their
neighborhood and respond accordingly (Baldwin 2010). For this purpose, they use
volatile substances derived from terpenoids, fatty acid catabolites, aromatics and
amino acids, which are released into the air. Plants that lose the ability to detect
ethylene also lack the ability to sense the location of other plants and object in their
surroundings. In addition, plants show tactile responses (thigmo- or haptotropism)
when they touch an object. This is of an advantage for climbing plants as it facilitates
the search for a support. The receptors are specific pits in the outer cell wall of the
epidermis and they can even distinguish between different materials which
e.g. prevents a vine to entwine a water jet (Isnard and Silk 2009).
All organs of sessile plants detect the gravity vector of the Earth but respond
differently. Main stems generally grow upward (negative gravitropism; Yamamoto
et al. 2002) while principal roots grow downward (positive gravitropism; Konings
1.4 Gravity Responses of Sessile Plants
5