support for the starch-statolith hypothesis comes from the observation that gravitropism
is impaired in shoots of the eal1mutant but not in roots. Further compelling support
comes from the laser ablation experiments mentioned earlier, where it was shown that a
strong correlation exists between the maximum amyloplast sedimentation rates in the
different cell layers of the root cap (S1, S2, and S3: Figure 1.2) and their involvement in
the gravitropic response (Blancaflor et al. 1998). Moreover, a series of elegant experi-
ments employing a high-gradient magnetic field to displace amyloplasts statoliths mim-
icked a gravitropic response (i.e., induced curvature) in roots and shoots, providing fur-
ther strong support for the starch-statolith hypothesis (Kuznetsov and Hasenstein 1996;
Weise et al. 2000).
Recently, other less-studied plant organs have been shown to exploit sedimenting amy-
loplast for the purpose of gravity sensing. For example, Arabidopsisplants shift their
leaves upward against the gravity vector when kept in the dark. This movement was
shown to be a combination of nastic and gravitropic movement (Mano et al. 2006). Cells
with sedimenting amyloplasts were observed in several cell layers around the vasculature
of the proximal region of the petiole. Two mutants in amyloplast formation (pgmand
sgr2-1) showed that abnormal distribution or absence of amyloplasts in the petioles re-
sulted in reduced upward bending of Arabidopsis leaves, indicating that sedimenting
amyloplasts are in part responsible for this process (Mano et al. 2006).
An unusual example of gravitropism can be found in peanut gynophores (Moctezuma
and Feldman 1999). The gynophore is a specialized organ which ensures that developing
fruits are buried in the soil. This is a step that is essential to the life cycle of the peanut
plant. Moctezuma and Feldman (1999) explored gravitropism in the peanut gynophore
and found that sedimenting amyloplasts are present in the starch sheath cells that sur-
round the vasculature in the gynophore. De-starching the plastids by incubation with gib-
berellic acid and kinetin in the dark did not affect overall growth but abolished 82% of
the gravitropic response. Gravitropism in the peanut gynophore is interesting because the
gravity-sensing mechanism seems similar to gravity sensing in shoots, yet the induced
growth response is opposite (i.e., the gynophore grows down rather than upward). Thus,
studies on the peanut gynophore might prove extremely helpful in addressing the ques-
tion on how the positive versus negative gravitropic response is determined in roots and
shoots.
Another example where sedimenting plastids appear to mediate a particular gravi-
tropic response is in peg formation in cucumber hypocotyls. Cucumber seedlings form a
specialized protuberance called a peg on the concave side of the bending site between the
hypocotyl and the root, which assists in shedding of the seed coat upon germination. The
formation of the peg is gravity-dependent and the amyloplast-containing sheath cells of
the vascular bundles in this area are responsible for gravity sensing (Saito et al. 2004). In
this case, gravity is responsible for the inhibition of peg formation on the convex side of
the bending zone as, under microgravity conditions, two pegs (one on either side of the
bending site) will form (Takahashi et al. 2000).
From the above, it appears that not only roots and shoots rely on sedimenting amylo-
plasts to perceive gravity, but that other plant organs do as well. These observations are
consistent with, and testify to, the generality of the starch-statolith hypothesis proposed
more than a century ago.
frankie
(Frankie)
#1