3 Auxin Transport and the Integration of Gravitropic Growth
Gloria K. Muday* and Abidur Rahman473.1 Introduction to Auxins
Auxins have been implicated in controlling elongation, branching, and development of
plant organs (as reviewed in Woodward and Bartel 2005), as well as the asymmetric
growth known as tropisms, which is the focus of this chapter. Of the plant hormones,
auxin best resembles the canonical concept of a messenger, being synthesized in one
place and acting in another. Indole-3-acetic acid (IAA) is the most studied and abundant
natural auxin, yet in some plants indole-3-butyric acid (IBA) is at almost equivalent lev-
els (as reviewed in Ludwig-Muller et al. 1993). Auxins also include synthetic compounds
such as 1-naphthaleneacetic acid (1-NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D),
which have been used in many studies because they are not susceptible to photolysis from
blue and ultraviolet lights, like the native auxin IAA (Stasinopoulos and Hangarter 1990).
The synthetic auxins have differences in activity (as reviewed in Woodward and Bartel
2005) and in transport properties (Delbarre et al. 1996), which make them useful for de-
signing experiments to test specific aspects of auxin function.
3.2 Auxin transport and its role in plant gravity response
Auxin moves through plants by a unique cell-to-cell polar transport mechanism, from the
shoot meristem and young leaves (Ljung et al. 2002) toward the base of stems (as re-
viewed in Blakeslee et al. 2005). Figure 3.1 contains a diagram summarizing the move-
ments of IAA in a seedling. Polar auxin transport results in an auxin gradient down the
length of the stem or hypocotyl, with the highest auxin concentrations found in the re-
gions of greatest elongation (Ortuno et al. 1990). Auxin is also synthesized in the root tip
(Ljung et al. 2005), where auxin transport is more complex, with two distinct polarities.
Shoot-derived IAA moves acropetally (toward the root apex) through the central cylinder,
and basipetally (from the apex toward the base) through the outer layers of root cells
(Tsurumi and Ohwaki 1978). Arabidopsisroots also have a tip-focused IAA gradient
(Casimiro et al. 2001) and basipetal transport of radiolabeled auxin applied to the root tip
moves only within the apical centimeter of the root tip (Rashotte et al. 2000; Geisler et
al. 2005). It is this basipetal IAA transport movement that is specifically linked to root
gravitropism (Rashotte et al. 2000).
In addition to polar transport down the length of plant tissues, the Cholodny-Went hy-
pothesis suggests that the lateral transport of auxin across gravity-stimulated plant tissues
drives differential gravitropic growth, as indicated in Figure 3.1B (as reviewed in Evans
1991; Trewavas 1992; Muday 2001; see also Chapters 1 and 2). Such a lateral redistrib-
*Corresponding author