plants which develop in microgravity are still responsive to gravitational acceleration,
suggesting that components of gravity perception do not need gravity to be expressed.
Future studies in microgravity may identify new mechanisms of gravity perception in
plants and identify downstream elements in the gravity signal transduction cascade.
8.4.2 Gravitropism: signal transduction
Once the gravity signal is perceived in plants, downstream elements in the signaling cas-
cade begin to react while the response (curvature) phase begins. Elements downstream of
sensing include changes in calcium, pH, flavanoids, ethylene, relocalization of auxin car-
riers (PIN proteins), and auxin redistribution which results in differential elongation of a
plant organ (Masson et al. 2002; Chapter 2). Since all tropisms result in the differential
growth of an organ, many of the downstream elements involved in gravitropism overlap
with other plant tropisms. Few studies have been performed in microgravity on the down-
stream elements in plant tropisms because these elements are difficult to measure on
Earth, even without the additional constraints imposed in the space environment.
Some of the studies that have been performed using microgravity to identify down-
stream elements in signal transduction are briefly described here. An experiment with
lentil roots was performed to identify the role of actin cytoskeleton in amyloplast move-
ment in microgravity (Driss-Ecole et al. 2000). Amyloplasts from roots treated with a
drug that blocks actin polymerization (cytochalasin D) did move in response to micro-
gravity, but the rate of movement was slowed compared to nontreated roots (Driss-Ecole
et al. 2000). These authors proposed that the microfilament bundles were not completely
depolymerized with the cytocalasin D treatment, which allowed for limited movement of
amyloplasts. The limited rate of movement of the amyloplasts in the cytochalasin D sam-
ples suggests that the cytoskeleton is involved, at some level, in gravity-induced re-
sponses. Other ground-based studies with drugs that disrupt the actin-cytoskeleton have
also demonstrated that the cytoskeleton is involved in gravity responses, although results
from these studies are often conflicting and depend on the organ, plant species, drug
dosage, and experimental conditions (Friedman et al. 2003; Palmieri and Kiss 2005).
Auxin transport is another downstream element in tropisms that has been studied using
microgravity. Polar auxin transport was inhibited in the internode segments of pea but
was promoted in maize (Zea mays) coleoptiles (Ueda et al. 1999). The mechanisms of
auxin transport during tropism in microgravity have not been studied but, since seedlings
can curve in response to stimuli in microgravity, it appears that auxin transport during tro-
pisms does not need gravity to function. Other studies with maize noted similar amounts
of auxin in plants grown in microgravity and 1gcontrols (Schulze et al. 1992).
Calcium redistribution is believed to be involved in signal transduction during tro-
pisms. Free calcium was redistributed in sweet clover (Melilotus alba) root columella
cells in microgravity compared to 1gcontrols and clinorotated seedlings (Hilaire et al.
1995). Fewer calcium precipitates were found with the nucleus and amyloplasts in cells
exposed to microgravity compared to 1gand clinorotated controls. The causes of the re-
distribution of the calcium-associated precipitates in microgravity are unclear. Additional
studies on calcium redistribution in microgravity may help further characterize its role in
gravity signaling.