Transduction of Gravistimulus

The nature of the gravisensors being not yet known, it is difficult to determine which cellular structure contains the gravireceptors. These receptors are able to transform the mechanical effect of graviperceptors into a biochemical factor. In 1972, Sievers and Volkmann suggested that the endoplasmic reticulum, which is always located at the basal pole of the statocyte, could be involved in the first steps of the transduction of gravistimulus. For these authors, after gravistimulation, an asymmetrical message could be created in the root cap because the amyloplasts sediment on the endoplasmic reticulum in statocytes of the lower half of this organ, whereas almost no contact are seen in the upper half. This hypothesis was consistent with the fact that it is well established that the concentration of cytosolic calcium in the cytoplasm is very low (Sinclair and Trewavas 1997), whereas its concentration in the endoplasmic reticulum is much greater. It was therefore proposed that the amyloplasts exerting a pressure on the endoplasmic reticulum tubules could provoke an efflux of calcium and a locally increase in calcium concentration which could activate some calcium-dependent proteins as calmodulin (Evans et al. 1986).

Another hypothesis was proposed by Iversen and Larsen (1971) and Perbal and Perbal (1976), who have observed that the gravitropic reaction was stronger when the amyloplasts moved along the longitudinal wall. According to these authors, the pressure and the movement on a cellular structure located along the longitudinal wall could be responsible for graviperception. An analysis of the distance from the amyloplasts to the plasma membrane showed that these organelles could exert a pressure on the cytoskeleton located along the longitudinal wall (Perbal et al. 2004).

The actin network of root and shoot statocytes has been intensively studied (White and Sack 1990, Collings et al. 2001, Driss-Ecole et al. 2003). It has been shown that this network is thin which allows the sedimentation of statoliths. The involvement in of the actin filaments in the transduction of gravistimulus is disputed since treatments with cytochalasins or Latrunculin B, which perturb the polymerization of actin, does not suppress the gravitropic response and can even enhance gravisensitivity (Yamamoto and Kiss 2002, Hou et al. 2004).

It could be advanced that the mechanical effect of the amyloplasts can be the cause of the activation of stretch-activated ion channels (Ca channels probably) as proposed by Pickard and Ping Ding (1992). This hypothesis implies that there is a local enhancement of cytoplasmic calcium, which has never been demonstrated in roots (Legué et al. 1997). However, the changes in calcium within the cell could be subtle which could explain that despite some preliminary trials no calcium change has been detected yet on single organ (Sinclair and Trewavas 1997). To demonstrate that cytosolic calcium transients are indeed induced by gravitropic stimulation (Plieth and Trewavas 2002), measurements have been made in groups of young seedlings of Arabidopsis thaliana producing aequorin in the cytoplasm and reconstructed in vivo with cp-coelenterazine, a synthetic high-affinity luminophore. In such transgenic plants, cytoplasmic increase of calcium can be followed by fluorescence. After gravistimulation, there is an initial calcium spike (20-30 sec) in the cytoplasm followed by a much longer shoulder that peaks about 90 sec after the change in orientation of the seedlings (Figure 6-06). The spike could be related to the early steps of gravisensing, whereas the shoulder could be related to the movement of amyloplasts (which can take minutes). The calcium signature is peculiar since it is biphasic and lasts much longer than those provoked by the mechanical effect of wind for instance (Plieth and Trewavas 2002).

Calcium is the most widespread ionic second messenger in plants, but changes in cytoplasmic pH are also known to have regulatory effects on cell function (Scott and Allen 1999, Blancaflor and Masson 2003). The cytosolic2

2 The cytosol (as opposed to cytoplasm, which also includes the organelles) is the internal fluid of the cell.

concentration of protons is tightly controlled and the columella3 cytoplasmic pH of 7.2 corresponds to a concentration that is very close to that of calcium. However, protons diffuse rapidly within the cell, unlike calcium, which is trapped by calcium-binding proteins. Therefore, without any structure in the cytoplasm to slow their movement, protons should not regulate localized changes in cellular microdomains. However, the pH changes could serve as a regulatory system for the whole statocyte (Fasano et al. 2001).

"55 oi

"3 cd

Time (min)

Figure 6-06. Effect of the gravitropic stimulus on the calcium response of a group (hundreds) of transgenic Arabidopsis seedlings. These seedlings are producing apoaequorin in their cytoplasm. Aequorin is reconstructed in vivo with cp-coelenterazine, a synthetic high-affinity luminophore. Aequorin fluoresces in the presence of cytoplasmic calcium. In A, gravistimulation: the plate with seedlings was turned through 135 deg (the root tip being slightly upward) at time 3 min. In B, control: the plate was turned 135 deg and immediately placed back to vertical position. Adapted from Plieth and Trewavas (2002).

Several researchers have indicated that proton flux from the root cap shifts after stimulation. When the root is vertical, fluxes are low and variable but, after gravistimulation, substantial proton efflux can be detected from the upper flank of the cap (Fasano et al. 2002, Monshausen and Sievers 2002). This asymmetry could be associated with both a sustained apoplastic4 acidification and transient (10 min) cytoplasmic changes in the statocytes (Fasano et al. 2001, Scott and Allen 1999). Acidification of the statocyte apoplast is induced upon reorientation but the wall pH only returns to pre-stimulation levels as the root tip reaches the vertical position. In maize

3 The columella is the central part of a rootcap in which the parenchyma cells are arranged in a series of column.

4 Apoplast is the cell wall continuum of a plant or organ; the movement of substances via cell walls is called apoplastic movement or transport.

pulvinus (a large swollen region at the base of the petiole composed predominantly of parenchyma cells), gravistimulation induces an alkalinization of the base of the statocytes where the plastids5 accumulate (Johannes et al. 2001), which indicates that gravisensors might create restricted zones where pH changes can occur. The link between pH changes and calcium increase is not yet known, and the chain of events leading to the transduction of gravistimulus remains to be determined.

0 0

Post a comment