within seconds to minutes after gravistimulation. As discussed in Chapter 2, the role for
InsP 3 as a likely universal signaling molecule in the gravity response was recently
demonstrated by constitutively overexpressing a human type I polyphosphate
5-phosphatase in Arabidopsis. This enzyme specifically hydrolyzes InsP 3. This experi-
ment resulted in a 90% reduction in InsP 3 levels and a 30% decline in the gravity re-
sponse of roots, hypocotyls, and inflorescence stems (Perera et al. 2006). Genetic and
biochemical studies such as these are yielding important information regarding the sig-
nal transduction pathways activated upon gravistimulation. However, demonstrating
how the cells that sense gravity via sedimenting plastids convert this directional infor-
mation into biochemical signals that include InsP 3 transients remains a major challenge
in gravitropism research.
1.3 The starch-statolith hypothesis
It is clear from the previous section that a common denominator of gravisensitive plant
organs is the presence of statoliths/amyloplasts which sediment to the bottom of the
graviperceptive cells and that, upon reorientation of these cells within the gravity field,
the amyloplasts rapidly resediment to the new bottom of the cell. The starch-statolith hy-
pothesis poses that the sedimentation of the statoliths within the gravisensitive cells is the
event that translates the gravity-driven mechanical stimulus into a chemical signal that
further regulates differential growth of the flanks of the reorienting organ. How the
“falling” of the statoliths exactly converts into a chemical signal is not clear yet, but it is
thought that possible interactions of amyloplasts with other cell components such as the
endoplasmic reticulum, cytoskeleton, or vacuole are important. The starch-statolith hy-
pothesis is now widely accepted as the major gravity-sensing mechanism despite the lack
of knowledge about some details.
1.3.1 A variety of plant organs utilize sedimenting amyloplasts to sense gravity
Evidence in support of a role for the starch-filled amyloplasts in the gravitropic response
comes from several Arabidopsismutants that are impaired in amyloplast formation. For
example, the phosphoglucomutasemutant (pgm) is impaired in starch synthesis and as a
result shows reduced gravitropic responses in both roots and shoots (Kiss et al. 1989;
Kiss et al. 1997). Some controversy about the starch-statolith hypothesis came from a
paper published in 1989 by Caspar and Pickard (1989) showing that an Arabidopsismu-
tant lacking plastid phosphoglucomutase activity still was able to respond to gravistimu-
lation, albeit in a reduced manner. From this study it was concluded that starch is unnec-
essary for gravitropism. However, upon careful ultrastructural examination of this
mutant, it appeared that not all starch formation was inhibited. Apparently, some starch
grains were formed in a subset of the amyloplasts, explaining the reduced but not com-
pletely abolished gravitropic response (Saether and Iversen 1991).
More recently, Fujihara et al. (2000) isolated another mutant with impaired amylo-
plasts. The endodermal-amyloplastless 1(eal1) mutant has no amyloplasts in the hypo-
cotyl endodermal cells while retaining normal plastids in the columella cells. Strong