For other plants, such as white clover (Trifolium repens) and lentil (Lens culinaris)
roots, amyloplasts were grouped near the center of the cell in microgravity whereas amy-
loplasts remained close to the distal half in 1gcontrols (Perbal and Driss-Ecole 1989;
Lorenzi and Perbal 1990; Smith et al. 1997). The grouping of amyloplasts was also found
in plants grown on clinostats, but to a lesser extent (Smith et al. 1997). Nonrandom
grouping of amyloplasts in apical cells from moss (Ceratodon purpureus) was also found
in clinostat- and microgravity-treated plants (Kern et al. 2001). Although the mechanisms
of the grouping of amyloplasts are unknown, it appears that the statocytes do not move
as a group in microgravity (Driss-Ecole et al. 2000). It also appears that amyloplasts re-
locate within 6 minutes of microgravity treatments (Volkmann et al. 1991; Driss-Ecole et
al. 2000). Future experiments in microgravity may help reveal the kinetics and mecha-
nisms of amyloplasts’ movement in response to changes in acceleration and provide a bet-
ter understanding of the grouping effect of amyloplasts in microgravity.
Other studies on gravity perception mechanisms have been performed using plants that
have reduced starch content and therefore smaller amyloplasts. Plants grown in micrograv-
ity that were then treated to 1gaccelerations for 60 minutes showed greater curvature in
roots from wild-type plants compared to roots from starch-deficient mutants (Kiss et al.
1998). These results further suggest that the amyloplasts are involved in the mechanisms
of gravity perception. However, restoration of the gravitropic response in these mutants
could be accomplished by treating plants with hypergravity, indicating that the starch-
filled amyloplasts are not required for a gravitropic response (Fitzelle and Kiss 2001).
The detailed mechanisms of gravity perception in plants are still unclear (Chapter 1).
However, studies in microgravity have shown that amyloplast location in root columella
cells moves in response to free fall in a potentially nonrandom orientation. It appears that
CHAPTER 8 SPACE-BASED RESEARCH ON PLANT TROPISMS 167
Figure 8.2. Electron micrographs of root cap columella cells from Arabidopsisseedlings grown in a 1g
control (A) and in microgravity (B) during spaceflight. The amyloplasts (arrows) in the 1gcontrol are in the
distal (“bottom”) part of the cell, and the gravity vector is toward the bottom of the figure. In contrast, the
amyloplasts (arrows) in the microgravity sample are dispersed throughout the cytoplasm. N = nucleus.