compared to the controls (Perera et al. 2006). Together, these results suggest that InsP 3
synthesis may be a signal that modulates auxin transport to allow differential growth in
response to changes in the gravity vector.
3.6.6 Regulation of gravity response by ethylene
Increasing evidence suggests that ethylene may regulate gravitropism either directly or
through modulation of auxin’s role in this process. Exogenous application of ethylene gas
or the ethylene precursor 1-aminocyclo-propanecarboxylic acid (ACC) has been used to
test the role of ethylene in gravity response. In some experiments ethylene or ACC treat-
ment clearly reduced the early phase of gravitropic response of roots and shoots (Wheeler
and Salisbury 1981; Wheeler et al. 1986; Lee et al. 1990; Kiss et al. 1999; Madlung et al.
1999; Buer et al. 2006), whereas others showed no effect (Kaufman et al. 1985; Harrison
and Pickard 1986; Woltering 1991). A third set of experiments suggested that ethylene
positively regulated shoot gravitropism (Chang et al. 2004). The experiments that identi-
fied a role for ethylene in gravity response examined the initial rate of gravitropic curva-
ture (Wheeler and Salisbury 1981; Wheeler et al. 1986; Lee et al. 1990), suggesting that
the absence of kinetic data at early times after gravitropic stimulation in several of the ex-
periments may explain their negative results (Kaufman et al. 1985; Harrison and Pickard
1986; Woltering 1991). It has been shown that the application of ethylene inhibitors such
as AVG/AgNO 3 also reduced the initial gravitropic curvature (Wheeler et al. 1986; Lee
et al. 1990; Muday et al. 2006) both in roots and shoots. The similar effect observed by
the ethylene precursor and inhibitors suggests that ethylene may both positively and neg-
atively regulate gravitropism.
Mutants altered in ethylene signaling and/or synthesis have been used to examine the
role of ethylene in gravitropic curvature. The gravitropic responses of the etr1roots (Buer
et al. 2006) and hypocotyls (Muday et al. 2006) andein2-1roots are wild type (Roman
et al. 1995; Rahman et al. 2001b; Buer et al. 2006). The gravitropic response of shoots of
the tomato mutants, Never-Ripe(Nr) and epi, which have reduced ethylene response and
enhanced synthesis, respectively, were examined. Both mutants exhibit delays in shoot
gravitropic response but with only a small reduction in Nr(Madlung et al. 1999), consis-
tent with a role for ethylene in the early events of gravitropic response. The study of
Madlung et al. (1999) revealed a concentration-dependent modulation of shoot gravitro-
pism by ethylene, with Nrbeing insensitive to the effect of exogenous ethylene on
hypocotyl gravitropism. Similarly, etr1andein2roots and hypocotyls are insensitive to
the inhibition of gravitropism by ACC treatment (Buer et al. 2006; Muday et al. 2006).
These results suggest that ethylene negatively regulates gravity response, and that for
plants grown on agar the endogenous levels of ethylene are low enough that there are no
detectable differences between wild-type and ethylene-insensitive mutants. The one ex-
ception to this conclusion are the agravitropic hypocotyls of ein2, although this pheno-
type may be linked to EIN2 activities that are ethylene-independent (Muday et al. 2006).
One mechanism by which auxin and ethylene may interact is at the level of hormone
synthesis with auxin-inducing ethylene synthesis and/or ethylene-inducing auxin synthe-
sis. Auxin is a positive regulator of ethylene biosynthesis in many plants, including
Arabidopsis(Yang and Hoffman 1984; Woeste et al. 1999; Harper et al. 2000). The rate-