8.4 Gravitropism
On Earth, the direction of the gravitational acceleration vector can indicate the location
of water/nutrients (i.e., underground) and sunlight (i.e., aboveground). Not surprisingly,
gravity is a relatively strong stimulus in terms of directing plant growth. Typically, pri-
mary roots grow toward the direction of gravitational acceleration (positive gravitro-
pism), and shoots grow away from the direction of gravitational acceleration (negative
gravitropism). Other organs in plants, such as inflorescence stems, leaves, and lateral
roots and branches also display gravitropic responses, but their orientation may be an in-
termediate angle relative to the gravity vector (Hangarter 1997; Kiss et al. 2002; Mano et
al. 2006). After reorientation, a plant organ will curve to reach the appropriate orienta-
tion, called the gravitational set point angle (Mullen and Hangarter 2003). The gravita-
tional set point angle in plants can be observed in the patterns of branching in trees and
orientation of lateral and primary roots.
The temporal steps of gravitropism can be separated into three main stages: percep-
tion, signal transduction, and response. Details of these stages are found in Chapters 1, 2,
and 3 and are described in Kiss (2000). Microgravity and simulated-microgravity envi-
ronments have been used to study each stage of gravitropism as well as gravity-induced
morphogenesis of plants and the straightening of a plant organ after a gravitropic re-
sponse (autotropism) (Stankovic et al. 1998). In the following sections, experiments that
use microgravity as a tool to study the elements of gravity perception, signal transduc-
tion, and response in gravitropism are described. Gravimorphogenesis of plants has been
reviewed in a paper by Takahashi (1997).
8.4.1 Gravitropism: gravity perception
Several models for gravity perception in plants have been proposed, although two mod-
els currently dominate (Perrin et al. 2005; Chapter 1). One model, the starch-statolith hy-
pothesis, suggests that perception of gravity is mediated by the settling of dense or-
ganelles (statoliths) after a plant is reoriented (Figure 8.2) (Kiss 2000). In flowering
plants, the amyloplasts in specialized cells function as statoliths; in roots, the amyloplasts
are in the columella cells; and in shoots they are located in the endodermal cells (re-
viewed in Masson et al. 2002). The other model for gravity perception in plants suggests
that the entire mass of the protoplast is involved in perceiving gravity (Staves et al. 1997).
To date, neither of these models has been excluded and both may be acting simultane-
ously. Alternatively, another as-yet undiscovered mechanism of gravity detection may be
involved in gravity perception in plants (reviewed in Wolverton et al. 2002; Chapter 1).
Microgravity has been a useful tool to explore the mechanisms of gravity perception
in plants. For several plant species, the statoliths (i.e., amyloplasts in plants) in root col-
umella cells of plants grown in microgravity were located in different regions of the cell
compared to the location of statoliths in cells of roots from 1gcontrols (Driss-Ecole et al.
2003). For example, most of the amyloplasts in Arabidopsisroots were located in the
proximal two-thirds of the cell in plants grown in microgravity compared to amyloplasts
from 1g-grown plants, where most of the amyloplasts were located in the distal one-third
of the cell (Briarty et al. 1995).