limiting step in ethylene synthesis is catalyzed by ACC synthase, which is encoded by the
ACS gene family with some family members being auxin-inducible in dark-grown
seedlings and plants (Abel et al. 1995; Yamagami et al. 2003). Recently, one ACC syn-
thase gene was shown to be asymmetrically induced across a gravity-stimulated snap-
dragon flower spike, suggesting that asymmetric synthesis of ethylene may be part of
gravitropic response in some tissues (Woltering et al. 2005). The regulation of auxin syn-
thesis by ethylene has been uncovered by the mutants designated weak ethylene insensi-
tive (wei2andwei7). The WE12andWE17genes encode ethylene-regulated enzymes of
Trp synthesis, whose activity positively regulates IAA synthesis (Stepanova et al. 2005).
Another explanation for the interaction between auxin and ethylene is that ethylene
may inhibit IAA transport. In some plant species, ethylene has been shown to inhibit the
polar IAA transport in shoot tissues (Morgan and Gausman 1966; Suttle 1988), but a di-
rect effect of ACC treatment on IAA transport was not detected in Arabidopsis
hypocotyls (Muday et al. 2006) or roots (Buer et al. 2006). Lateral IAA transport has also
been found to be inhibited in both shoots (Burg and Burg 1966) and gravity-stimulated
corn roots (Lee et al. 1990). These results suggest that ethylene-mediated inhibition of
auxin transport may play an important role in regulating the gravity response. A recent
article from Buer et al. (2006) asked whether ethylene might inhibit gravity response
through induction of flavonoid synthesis resulting in reduced IAA transport. Several al-
leles of the flavonoid-deficient mutant tt4exhibit a delayed gravity response in roots and
are insensitive to the inhibition of gravitropism at early time points. More interestingly,
ACC has been shown to induce flavonoid accumulation in Arabidopsisroots through a
mechanism that requires EIN2 and ETR proteins (Buer et al. 2006). Taken together, these
results suggest that the ethylene regulation of root gravity response may occur through al-
tering flavonoid synthesis, assuming that enhanced flavonoid accumulation will reduce
IAA transport. However, Buer et al. (2006) did not find any effect of ACC on root
basipetal auxin transport, indicating that this interaction may be more complex in nature
or too difficult to detect in the tips of the small roots of Arabidopsis.
The most direct evidence for interaction between ethylene signaling and auxin trans-
port comes from the studies of Arabidopsismutants having mutations in auxin transport
proteins. Both the auxin influx mutant, aux1,and the auxin efflux mutant, agr1/ eir1/pin
2/wav6, have ethylene-insensitive root elongation (Roman et al. 1995; Pickett et al.
1990). The restoration of ethylene sensitivity in both aux1andeir1by exogenous appli-
cation of NAA and IAA indicates that cytoplasmic auxin is needed at sufficient levels for
ethylene response (Rahman et al. 2001b).
3.7 Overview of the mechanisms of auxin-induced growth
This chapter has focused on the mechanisms by which asymmetries in auxin are estab-
lished in response to changing orientation of plants relative to gravity. Yet, to understand
how these auxin gradients control growth, a brief discussion of auxin signal transduction
is required, although this topic has been reviewed in greater detail elsewhere (Leyser
2002, 2006).
Plants respond to changing levels of auxin by dramatic changes in transcription, with