Lentil root statoliths reach a stable state in microgravity

The kinetics of the movement of statoliths in gravity-perceiving root cap c ells of Lens culinaris L. and the force responsible for it have been analys ed under 1 g and under microgravity conditions (S/MM-03 mission of Spacehab 1996). At the beginning of the experiment in spice, the amyloplasts were g rouped at the distal pole of the statocytes by a root-tip-directed 1-g cent rifugal acceleration. The seedlings were then placed in microgravity for in creasing periods of time (13, 29, 46 or 122 min) and chemically fixed. Duri ng the first 29 min of microgravity there were local displacements (mean ve locity: 0.154 mu m min(-1)) of some amyloplasts (first at the front of the group and then at the rear). Nevertheless, the group of amyloplasts tended to reconstitute. After 122 min in microgravity the bulk of amyloplasts had almost reached the proximal pole where further movement was blocked by the nucleus. After a longer period in microgravity (4 h; experiment carried out 1994 during the IML 2 mission) the statoliths reached a stable position du e to the fact that they were stopped by the nucleus, The position was simil ar to that observed in roots grown continuously in microgravity. Treatment with cytochalasin D (CD) did not stop the movement of the amyloplasts but s lowed down the velocity of their displacement (0.019 mu m min(-1)). Initial movement patterns were the same as in control roots in water. Comparisons of mean velocities of amyloplast movements in roots in space and in inverte d roots on earth showed that the force responsible for the movement in micr ogravity (F-c) was about 86% less (F-c = 0.016 pN) than the gravity force ( F-g = 0.11 pN). Treatment with CD reduced F-c by two-thirds. The apparent v iscosity of the statocyte cytoplasm was found to be 1 Pa s or 3.3 Pa a for control roots or CD treated roots, respectively, Brownian motion or elastic forces due to endoplasmic reticulum membranes do not cause the movement of the amyloplasts in microgravity. II is concluded that the force transporti ng the statoliths is caused by the actomyosin system.