Plant Gravitropism What is Known and What is to be Done

Microgravity has represented a very useful tool to analyze gravitropism since it was the only opportunity to clearly measure parameters such as the presentation time, the presentation dose, and the threshold acceleration for the gravitropic reaction. Although clinostats have been used for a century, their action remains not clear. Clinostat works (Aarrouf et al. 1999) have shown that the rotation about a horizontal axis on a clinostat could eventually produce a slight but continuous stimulation, so that the estimate of presentation time and dose calculated with this device should be questionable. However, the values obtained in space for these parameters are of the same order as those obtained on the clinostat, which validates to a certain extent the use of this device for studying these parameters. The re-examination of these parameters in the frame of the space experiments led Perbal et al. (2002) to demonstrate that the way of estimating the presentation dose (which was used for decades) was not the best one. A new model for fitting the data has been proposed and it has been shown that the presentation dose did not measure gravisensitivity but dealt with the minimal stimulus to provoke a differential growth in the upper and lower halves of the organs. For stimulation doses less than the presentation dose, the stimulus is transduced but does not provoke a curvature. This can be due to a kind of resistance to curve of the growing organ (Piclcard 1973). The perception dose (or perception time) should be the only parameter directly linked to the phase of perception and should be measured in space.

This result is important since it implies that the perception and the transduction phases can be very short, less than 1 sec at 1 g. In such a short period of time, the sedimentation of the amyloplasts should be very limited (for a rate of displacement equals to 1 pm x min"1 it should be 1/60 urn). In this case the potential energy dissipated by one amyloplast should not be greater than the thermal noise.

The level of acceleration that can be perceived by the organs is about 5 x 10"4 g for roots and 10"3 g for shoots (Shen-Miller et al. 1968), but these values were obtained on clinostat with a background of 1 g. The estimates obtained in space by Merkys and Laurinavicius (1990) were extrapolated from data obtained on plants placed on a centrifuge and subjected to 0.1 g or 0.01 g. These estimates are too high if the threshold acceleration (5 x 10"4 g) is about 20 times less than the lowest value (0.01 g) of acceleration the organs were subjected to. To confirm this, an experiment on the threshold acceleration will be performed on the International Space Station in the frame of the European mierogravity program.

The main result obtained in space deals with the role of the endoplasmic reticulum (ER). Volkmann and Sievers (1979) have proposed that the pressure of the amyloplasts on the ER membrane should lead to the stimulus and to an asymmetrical signal coming from the cap. The fact that the statocytes are more sensitive in 0-g grown plants than in 1-g grown plants does not confirm this hypothesis (Perbal et al. 2004). In mierogravity, the amyloplasts are situated near the nucleus, whereas in 1 g they are sedimented on the ER tubules. When a centrifugal force is applied to the organs, the probability of having contacts between amyloplasts and the ER tubules is therefore much less in 0 g than in 1 g, although the response is greater in 0 g than in 1 g. Thus, experiments performed in space brought a strong argument against the hypothesis based on a role of ER in the transduction of gravity stimulus. This conclusion is also supported by experiments carried out on the ground in which the ER tubules were displaced by centrifugal forces (Wendt et al. 1987).

The analysis of the statocyte polarity in space showed that the amyloplasts were in majority located in the center of the statocyte close to the nucleus (Perbal et al. 1987). The transfer from gravity to mierogravity induces a movement of the amyloplasts toward the nucleus (Volkmann et al. 1991, 1999, Lorenzi and Perbal 1990, Driss et al. 2000a), which shows that these organelles are not free in the statocyte. Treatment by cytochalasin B or D can strongly slow down this movement, which indicates that actin filaments could be responsible for the movement of the amyloplasts in microgravity (Buchen et al. 1993, Driss et al. 2000a). This result obtained in space led to a new hypothesis about the signal transduction of gravity stimulus. According to Volkmann et al. (1991), the amyloplasts could exert tension in the actin network, which becomes asymmetrical when the organ is placed horizontally (see Figure 6-11). However, there is a controversy between the authors who think that the amyloplasts are the gravisensors and those who think that the whole cell is the gravisensor (see Sack 1997). It may happen that both can play this role, the amyloplasts being more efficient than the protoplast. If the nature of gravisensor is still disputed (Barlow 1995), one must recognize that space experiments have brought new data about gravisensing, which forced plant physiologists to change their view on how plants sense gravity (Perbal and Driss-Ecole 2003).

Another finding concerns the regulation of root curvature by gravity. After a slight stimulation on a centrifuge, the roots show autotropism, i.e., straightening after several hours. On the opposite, a strong stimulation induces a curvature, which can lead the root to overshoot the direction of the stimulus. This overshooting does not occur on the ground or on a centrifuge, which demonstrates that gravity regulates the curvature. The mechanism of this regulation is not yet known but could depend on the amyloplast sedimentation (Perbal et al. 2004). On the ground, these organelles can move along the longitudinal wall in gravistimulated roots during the bending of the root, whereas when the root are placed in microgravity after stimulation on a centrifuge the amyloplasts are pulled away from the longitudinal wall, i.e., away from the mechanoreceptors.

To some extend the transduction pathway of gravistimulation could be analyzed in space by using transgenic plant expressing the aequorin gene in order to observe calcium responses under different stimulus conditions (see Figure 6-06). One experiment will be performed soon to examine calcium redistribution by the means of a special chemical fixation: the glutaraldehyde contains potassium antimonite, which reacts with calcium to form a precipitate that can be observed in electron microscopy. Such a technique was already used in space (Hilaire et al. 1995), but only on plants grown in microgravity and not subjected to gravistimulus.

With new tools that have been developed recently as Arabidopsis plants harboring a DR5::GUS construct (see Figure 6-16), it should be possible to analyze auxin distribution during and after gravistimulation on a 1 -g centrifuge in space. The analysis of the distribution of the PIN and AUX proteins, which are auxin transporters, should also be investigated.

Figure 6-23. Astronaut Car1E. Waiz holds a plant in the Russian Zvezda Module on board the International Space Station. Photo courtesy of NASA.
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