1.3.2 Amyloplast Sedimentation Is Influenced by the Environment and Developmental
Stage of the Plant
To optimize exposure to the available environmental resources, it would be beneficial for
the plant to regulate the gravitropic response of its organs to a certain degree. One can
imagine that under certain circumstances it is in the plant’s best interest to suppress the
gravitropic response. Indeed, evidence for such regulatory mechanisms does exist. For
example, a downward-growing root (i.e., positively responding to the gravity stimulus)
encountering a rock in the soil may benefit from temporarily suppressing its gravitropic
response so that it can grow around the encountered obstacle. As discussed in Chapter 5,
Massa and Gilroy (2003) have shown that the gravitropic response in Arabidopsisroots
is suppressed after tactile stimulation of the peripheral cap cells. Interestingly, it seems
that this suppression is correlated with the inhibition of normal amyloplast sedimentation
in the statocytes. This finding indicates a complex and direct interaction between mech-
anisms of thigmo- and gravitropism allowing the roots to navigate the soil and grow
around rocks and other obstacles.
Another example of down-regulation of the gravitropic response comes from experi-
ments in which it was shown that roots of Arabidopsisand radish show greater hydrotrop-
ism and reduced gravitropism after columella amyloplasts are degraded (Iino 2006, and
references therein). The interaction between these two tropisms allows the root to inhibit
the gravitropic response (by disintegrating amyloplasts) in favor of a search for water (re-
viewed in Chapter 6). Regulated gravitropism can also be found in lateral roots. Lateral
roots need to grow out horizontally before becoming plagio-gravitropic (i.e., assume an
oblique orientation relative to the gravity vector). It appears that lateral root gravitropism
is delayed until amyloplasts mature and accumulate numerous starch grains in the lateral
root columella cells (Kiss et al. 2002).
In line with these observations is a recent study by Ma and Hasenstein (2006) that ad-
dresses the question of when the embryonic root is capable of sensing gravity. To this end,
flax seeds were gravistimulated and allowed to germinate during clinorotation. The onset
of gravisensing was determined as the time after which 50% of the emerging roots bent
in the direction of the gravity vector during gravistimulation. It was found that the onset
of graviperception was established 11 hours before root emergence at the time of germi-
nation (and 8 hours after imbibition) and, interestingly, coincided with the development
of mature amyloplasts in embryonic columella cells (Ma and Hasenstein 2006).
Taken together, the role of starch-statoliths in gravity perception seems undeniable.
Almost all graviperceptive tissues and organs in higher plants (stems, leaves, roots,
gynophores, and pegs) display sedimenting amyloplasts in their gravisensitive cells. As
mentioned above, in some reported cases the development of mature amyloplasts coin-
cides with the ability of cells to perceive gravity and the plant seems to be able to down-
regulate the gravity-sensing ability by disintegration of amyloplasts.
1.4 The gravitational pressure model for gravity sensing
Despite general support for plastid-based gravity perception in higher plants, one cannot
discount the possibility that plants possess other means to sense gravity. An alternative