erated by these “rogue” amyloplasts could be amplified several-fold, leading to a stronger
gravitropic response. Alternatively, extended contact between amyloplasts and the vacuo-
lar membrane surface in endodermal cells with a disrupted actin network could be re-
sponsible for the enhanced gravity response in shoots (Palmieri and Kiss 2005).
Similar to shoots, latrunculin B has a promotive effect on root gravitropism. The en-
hanced gravity response in roots is clearly manifested as persistent curvature on a slowly
rotating clinostat after a short gravistimulus is provided (Hou et al. 2003). In contrast to
endodermal cells of shoots, actin disruption has consistently induced the enhancement of
amyloplast sedimentation in the root columella upon gravistimulation (Baluˇska et al.
1997; Yoder et al. 2001; Hou et al. 2004). This could be explained by smaller vacuoles in
the columella being less of an impediment to amyloplast movement. Nonetheless, like in
shoots, the increased sensitivity of roots to gravity may result from diminished system
noise and the amplified signals generated by rapidly falling amyloplasts (Hou et al.
2004). Although the above proposal is purely speculative at this point, the ability to ma-
nipulate cytoskeletal dynamics specifically in the gravity-sensing cells should provide a
significant step toward further testing the relationship between the cytoskeleton and
plastid-based gravity sensing in plants.
There have been other explanations as to how the cytoskeleton and amyloplasts in the
columella interact to generate a gravity signal, including a proposal that localized disrup-
tion of the actin network can produce a directional signal by altering the balance of forces
acting on plasma membrane receptors (Yoder et al. 2001). Moreover, a recent report
showing that actin disruption of decapped maize roots can partially restore gravitropism,
which points to the intriguing possibility that actin-dependent gravity sensing may occur
outside the root cap (Mancuso et al. 2006). Although it will require additional work to
tease apart the different possibilities, results from recent cellular and pharmacological ap-
proaches are leading to new, testable hypotheses on how the cytoskeleton mediates in
gravity perception in higher plants.
1.6 Concluding remarks and future prospects
Gravitropism is important for plant survival since it directs the growth of organs to loca-
tions that optimize the utilization of direct environmental resources such as light, water,
and soil nutrients. It seems clear that the process of gravity sensing is mainly accom-
plished through sedimenting amyloplasts in gravisensitive cells of the columella in roots,
the endodermis in dicotyledon shoots, and the pulvinus in monocotyledons. However,
one cannot totally discount the experimental evidence pointing to alternative gravity-
perceiving sites, or that other cellular structures (including the whole cell itself) could
function as gravitropic susceptors, in addition to falling plastids. Given the importance of
gravitropism in plant development, it might not be surprising to find some redundancy in
mechanisms of gravity sensing. Evolutionarily speaking, a single mutation that would
wipe out amyloplast formation could severely compromise survival if the plant relies en-
tirely on amyloplasts for gravity sensing. With this in mind, it is reasonable to imagine
that the starch-statolith hypothesis and the gravitational pressure model are not necessar-
ily conflicting, even though evidence for the starch-statolith hypothesis seems over-