Further evidence for the role of vacuolar membrane dynamics in shoot gravitropism
comes from the analysis of another mutant in Arabidopsis, namely the sgr3 mutant (Yano
et al. 2003). The SGR3gene encodes a syntaxin (AtVAM3) that also has been implicated
in vacuolar transport. The SGR3 protein is localized to the prevacuolar compartment and
the vacuole itself. The sgr3mutant shows abnormal appearance of vacuoles in endoder-
mal cells with irregular curves and aberrant membranous structures. This mutant also dis-
plays abnormal sedimentation of amyloplasts in the shoot endodermis, coupled with al-
tered gravitropism in inflorescence stems (Yano et al. 2003). This study provides
additional evidence for a role of vacuolar membrane dynamics and vacuolar biogenesis
in proper amyloplast sedimentation and, by extension, graviperception.
Further evidence for a role of membrane dynamics comes from yet another mutant.
Thegravitropism defective 2mutant (grv2) in Arabidopsisexhibits a defect in shoot and
hypocotyl gravitropism. The responsible gene, GRV2, appears to code for a protein that
is similar to the RME-8 protein in Caenorhabditis elegans, which is important for endo-
cytosis (Silady et al. 2004). Since the endodermal cell layers in shoots of grv2mutant dis-
play defects in amyloplast sedimentation, endocytosis might be involved in the initial
gravity perception steps as well as the membrane dynamics necessary for targeting of
auxin efflux carriers such as PIN3 to their correct position within the columella cell
(Friml et al. 2002; Chapter 3 in this book).
1.2.3 Cereal pulvini (monocotyledons)
In contrast to roots and shoots in dicotyledons, the shoots of grasses have a specialized
tissue that mediates gravitropism, namely the pulvinus. The pulvinus is a cushion-like
swelling at the base of each internode and the vascular bundles within it are surrounded
by bundle sheath cells that contain sedimenting amyloplasts. Upon reorientation of the
monocotyledon plant within the gravity field, the amyloplasts sediment to the bottom cell
wall in the same way as described for columella and endodermal cells (Allen et al. 2003).
Also, in pulvini it is this sedimentation that is thought to set off the signal transduction
cascade that ultimately regulates the reorientation of the stem of the plant. However, in
contrast to the primary root, the processes of gravity perception, signal transduction,
and growth response (i.e., the establishment of a gradient of cell elongation) all occur in
the same tissue (Collings et al. 1998). In order for the cells in the pulvini to be gravi-
competent, it appears that they must delay maturation. Maturation of the surrounding
cells in the tissue of the stems involves lignification of the cell wall and rearrangement
of microtubules from transverse to oblique after elongation is completed. Once these cel-
lular processes have taken place, the pulvinus is no longer capable of responding to grav-
istimulation. During the onset of the gravitropic curvature, maturation occurs only on the
upper side of the pulvinus, whereas the elongating cells in the lower side mature only
after maximum bending capacity of the stem has been reached (about 30 degrees upward
bending) (Collings et al. 1998).
The pulvini are amenable to biochemical studies, as relatively large amounts of cells
can be harvested. In maize and oat, upper and lower pulvinus tissue can even be col-
lected and analyzed separately. Using these techniques, Perera et al. (1999, 2001) have
been able to show that inositol 1,4,5-trisphosphate (InsP 3 ) levels increase transiently