Studies with downstream elements in the signal-transduction cascade of tropisms in
microgravity may provide significant contributions to our understanding of gravity ef-
fects on plant growth and movement. Currently, plants have been developed with green
fluorescent proteins that are expressed along with proteins in the downstream compo-
nents of gravity signaling (e.g., PIN2-GFPplants) (Abas et al. 2006). Studies monitoring
the expression and localization of these proteins in microgravity may prove useful to
identify how downstream elements in gravity-induced signaling respond to different
gravitational accelerations.
8.4.3 Gravitropism: the curving response
As the gravity signal-transduction process has reached a significant level of propagation
downstream, the response or curvature of the organ can be detected. Without gravity as a
cue to direct plant growth, plants appear to grow randomly. For example, roots of cress
(Lepidium sativum) plants and hypocotyls of Arabidopsisshow a vertical orientation in
1 gbut appear to grow randomly in microgravity (Figure 8.3) (Johnsson et al. 1996a;
Johnsson et al. 1996b; Kiss et al. 1998; Kiss et al. 1999; Kiss 2000). Roots and hypo-
cotyls also grew randomly when grown in simulated microgravity, such as on clinostats
or random positioning machines (Kraft et al. 2000). After microgravity-grown plants are
exposed to acceleration, their organs will curve in response to the direction of the accel-
eration vector (Kiss et al. 1998; Kiss et al. 1999). The following questions have been pro-
posed to explore the sensitivity of plants to gravity:
- What is the minimum time required for a plant to sense and respond to gravity (thresh-
old duration or presentation time)?
CHAPTER 8 SPACE-BASED RESEARCH ON PLANT TROPISMS 169
Figure 8.3. Images of roots of lentil seedlings grown in a 1gcontrol (A) and in microgravity (B) during
spaceflight. Roots of the 1gcontrol are straight and oriented relative to gravity (toward the bottom of the fig-
ure). In contrast, roots of the microgravity-grown seedlings appear to have a more random orientation.
Photographs are courtesy of Dr. Dominique Driss-Ecole.