which mutant phenotypes include altered gravity response, suggest other targets of phos-
phorylation beyond the PIN proteins.
3.6.4 Regulation of auxin transport by flavonoids
Prime candidates for endogenous auxin transport inhibitors are flavonoids. These pheno-
lic compounds displace the binding of synthetic IAA efflux inhibitors, such as NPA, dur-
ingin vitroassays (Jacobs and Rubery 1988). The flavonols quercetin and kaempferol
had the greatest activity, suggesting that specific members of this chemical family func-
tion as auxin transport inhibitors (Jacobs and Rubery 1988). The role of flavonoids as
regulators of auxin transport have been examined by in vivostudies in tt4mutants, which
have a defect in the CHSgene encoding chalcone synthase, the first enzyme in flavonoid
synthesis. These tt4mutants have elevated auxin transport in young seedlings, roots, or
inflorescences, consistent with the absence of an endogenous negative auxin transport
regulator (Murphy et al. 2000; Brown et al. 2001; Buer and Muday 2004; Peer et al. 2004;
Buer et al. 2006). Roots of multiple alleles of tt4mutants exhibit a lag in gravitropic cur-
vature compared to wild-type roots (Buer and Muday 2004; Buer et al. 2006). Chemical
complementation of tt4(2YY6) by naringenin reinstated flavonoid production and re-
stored a wild-type gravity response, consistent with a role for flavonoids in controlling
the flow of auxin needed for root gravitropism (Buer and Muday 2004).
Mutations that alter flavonoid synthesis affect the abundance of the mRNA encoding
members of the PINgene family (Peer et al. 2004; Lazar and Goodman 2006), suggest-
ing that flavonoids may regulate synthesis of auxin transport proteins, not just the activ-
ity of existing proteins. Consistent with flavonoid abundance affecting transcription, re-
cent evidence indicated that flavonoid biosynthetic enzymes and flavonoid products
accumulate in the nucleus (Saslowsky et al. 2005).
Changing environmental conditions modulate flavonoid synthesis (Winkel-Shirley
2002) and these changes in flavonoid accumulation may regulate plant growth and devel-
opment, including gravity responses (as reviewed in Taylor and Grotewold 2005).
Reorientation of plants relative to gravity leads to enhanced flavonoid accumulation in
the epidermal tissues of Arabidopsisroot tips (Buer and Muday 2004), which are the site
of basipetal IAA transport. Images of roots and the time course of this induction are
shown in Figure 3.4. This induction is on both the upper and lower sides of gravistimu-
lated roots, suggesting that its function may be to uniformly reduce the activity or abun-
dance of a set of efflux carriers, and thereby accentuate the formation of a gradient of
IAA across the root resulting from enhanced transport on the lower side. Therefore, in-
duction of flavonoid synthesis in response to environmental stimuli may alter auxin trans-
port to facilitate plant gravity response.
3.6.5 Regulation of auxin transport by other signaling pathways
Most of the mechanisms for altering IAA transport polarity during root gravitropism uti-
lize IAA as a signal to enhance the gradient, consistent with multiple levels of positive
feedback. It is critical to consider the gravity-signaling events that precede any IAA
asymmetries and act as the initial signals to initiate auxin transport changes. As discussed