2 Signal Transduction in Gravitropism
Benjamin R. Harrison, Miyo T. Morita, Patrick H. Masson*, and
Masao Tasaka212.1 Introduction
As discussed in Chapter 1, most plant organs use gravity as a growth guide. However, dif-
ferent organs will interpret that information in different ways. Shoots grow upward toward
light, aboveground environments in order to optimize photosynthesis, exchange gases, and
perform their reproductive functions. Most roots, on the other hand, grow downward, into
the soil, where they anchor the plant and take up water and nutrients necessary for plant
growth, development, and reproduction. To understand how these organs interpret differ-
ently the information provided by gravity, we first need to understand the molecular mech-
anisms that govern gravity signal transduction in gravity-sensing cells, termed statocytes.
Before we describe the current state of our knowledge on the mechanisms that gov-
ern gravity signal transduction in plants, it is important to understand that the gravity-
responding organs of higher plants are diversified in their tissue structure and develop-
mental origin. In cereal grasses, the graviresponsive coleoptile of seedlings is a hollow
cylindrical sheath, whereas the pulvini of adult plants are swellings at the base of each
internode. In dicots, hypocotyls, and epicotyls of young seedlings and leaf petioles and
stems of adult plants are all graviresponsive. Similarly, both primary and lateral roots
are graviresponsive in monocots and dicots, even though the site of root gravicurvature
does not contain obviously differentiated statocytes. Hence, the morphology and cytol-
ogy of different plant organs may affect the machinery that modulates their gravitropic
responses. In spite of such diversity, all graviresponsive organs share two common fea-
tures: they contain graviperceptive cells with sedimentable amyloplasts (Sack 1997),
and they develop asymmetry in auxin concentration between their upper (lower concen-
tration) and lower (higher concentration) flanks upon gravistimulation (Philippar et al.
1999; Muday and DeLong 2001; Friml et al. 2002; Long et al. 2002). Thus, within these
organs, a gravitational signal perceived through the relocalization of amyloplasts within
differentiated statocytes is converted into biochemical signal(s) that is (are) transmitted
to adjacent cells, leading to the formation of a lateral gradient of auxin at the elongation
zone, responsible for the gravitropic curvature (see also Chapter 3).
Although seemingly similar in global terms, the physiological and biochemical events
that accompany gravitropism in aboveground organs and roots differ substantially in the
details. For instance, the site of gravity perception and signal transduction (endodermal
cells) overlaps with the site of curvature response in shoots, which has been proposed to
occur simultaneously and uniformly along the organs (Firn and Digby 1980). Roots, on
the other hand, show a physical separation between the primary site of gravity perception
and signal transduction (the root cap columella) and the site of curvature response (the
*Corresponding author