Gravitropic Response in Microgravity

The gravitropic response of lentil seedling roots was first analyzed during the Spacelab S/MM05 mission (Perbal et al. 1999). The lentil seedlings were grown in microgravity for 26 h and then placed on the centrifuge for 22 min. The gravitropic response was followed by time-lapse photography. In microgravity, the tip of gravistimulated roots could overshoot the direction of the acceleration after 3 h (Figure 6-13), whereas roots stimulated on the ground or in space continuously did not (see Figure 6-02). On Earth, there must be a regulation (inhibition of gravistimulus) that is gravity dependent. This regulation is seen when in some roots there is a kind of counteraction that reduces the bending. The regulation of the gravitropic response appears to be linked to the actin filaments, since Arabidopsis roots treated with Latrunculin B (which perturbs actin polymerization) are more responsive than control roots (Hou et al. 2004). According to Stanlcovic et al. (1998a), the straightening of the root is part of the gravitropic reaction chain.

Tewinkel and Volkmann (1996a) have shown that Lepidium roots were more sensitive to gravistimulus when the seedlings were grown in microgravity than when the seedlings were grown on a 1-g centrifuge. During the S/MM05 flight, Perbal et al. (2004) have also shown higher sensitivity of lentil roots grown in microgravity than grown in space on the 1-g centrifuge. The response for the same stimulation dose was in effect much greater for the former than for the latter. Perbal et al. (2004) have proposed that this should be due to the position of the amyloplasts within the statocyte and to the direction of their movement upon gravistimulus.

It must be noticed that in the case of lentil roots grown in 1 g as for roots grown in microgravity, no counteraction was observed even after a period of 6 h in microgravity at least for strong stimulations.

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Figure 6-13. Gravitropic curvature of lentil roots grown for 26 h on a 1-g centrifuge (1G) or in microgravity (0G) and subjected to a dose of stimulation of 14.7gx min (22 min at 0.67 g). The curvature rate is almost constant and the root tip overshoots the direction of the acceleration (90 deg) in the microgravity sample after 150 min. the curvature is slower in the 1-g sample so that it takes more times to observe the over-shooting (not shown).

This counteraction must not be mistaken for autotropism, which is a complete different phenomenon (Stankovic et al. 1998b). Autotropism, i.e., the fact that after curving the organs straighten out, has been studied by Chapman et al. (1994) on Avena coleoptiles. The oat seedlings were grown on a 1-g centrifuge in space and were then stimulated laterally by variable centrifugal forces in such a way that acceleration and duration of the stimulus varied. Stimulus doses were thus comprised from 1.8 to 25.2 g x min. This analysis showed that after gravitropic bending of the coleoptiles, these organs straightened and the curvature disappeared after about 5 h. Stankovic et al. (1998a) proposed to limit the use of autotropism to the straightening occurring in microgravity (or on the clinostat) after gravitropic stimulation. Recently,

Stanlcovic et al. (1998b) studied autotropism on Lepidium roots. In this species, autotropic straightening also occurred on the clinostat.

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