microarray experiments identifying hundreds of genes whose expression is rapidly mod-
ified in response to auxin treatment (Pufky et al. 2003). The most rapidly expressed genes
are induced even in the absence of protein synthesis. These primary auxin-responsive
genes include SAURs(small auxin up-regulated), GH3s, and AUX/IAAs(Auxin/IAA in-
ducible genes) gene families (as reviewed in Hagen and Guilfoyle 2002). Consensus pro-
moter elements were found in these auxin-responsive genes and used to identify proteins,
named Auxin Response Factors (ARFs), that bind to them (as reviewed in Hagen and
Guilfoyle 2002). The in vivofunction of these proteins has been demonstrated by isola-
tion of Arabidopsismutants altered in expression or function of ARFandAUX/IAAgenes.
These mutants have a diversity of auxin-dependent phenotypes, including agravitropic
roots and/or hypocotyls (as reviewed in Liscum and Reed 2002). The nph4(nonpho-
totropic hypocotyl) mutant, which has a defect in the ARF7gene, exhibits reduced
hypocotyl phototropic and gravitropic responses (Stowe-Evans et al. 1998; Harper et al.
2000). Several mutants with defects in IAAgenes also have agravitropic phenotypes, in-
cludingiaa14/slr1-1,iaa17/axr3-1, and iaa19/msg2(as reviewed in Liscum and Reed
2002). ARF proteins contain DNA binding domains and dimerization domains, and have
been shown to act as transcriptional regulators (Tiwari et al. 2003). AUX/IAA proteins
have similar dimerization domains and form homo- and hetero-dimers with ARF pro-
teins, thereby modulating the ability of ARFs to bind to the promoter of auxin-responsive
genes (Kim et al. 1997).
Until relatively recently, it has been unclear how the auxin signal influences the for-
mation of transcription factor complexes that are needed to modulate the expression of
auxin-responsive genes. Recent experiments have demonstrated that auxin-dependent
proteolytic destruction of AUX/IAA proteins is a critical factor (as reviewed in Leyser
2006). Specifically, mutations in the TIR1, AXR1, andAXR6genes lead to auxin-resistant
plants. The proteins encoded by these genes have now been shown to be part of protein
complexes that ubiquitinylate substrate proteins, thereby targeting them for destruction
(as reviewed in Leyser 2006). The current model for this process is that TIR1 binding to
auxin facilitates formation of complexes with AUX/IAA proteins, resulting in their ubiq-
uitination. Ubiquitinylated proteins are then targeted for destruction by the proteosome.
Consequently, ARF proteins are released from inhibitory complexes with AUX/IAA and
bind to the promoter elements of auxin-responsive genes, regulating their expression
(Dharmasiri et al. 2005; Kepinski and Leyser 2005). Although this mechanism was rather
unexpected, the data clearly show that destruction of regulatory transcription factor sub-
units is an efficient system for rapid gene expression changes in response to changing
auxin levels.
An additional question that remains to be answered is: What kinds of genes are induced
in response to auxin? Microarray experiments have shown that expression of a large num-
ber of genes changes in response to auxin application (Pufky et al. 2003) or gravitropic re-
orientation (Moseyko et al. 2002; Kimbrough et al. 2004; Esmon et al. 2006). Auxin-
induced gene products include enzymes that may loosen the cell wall to facilitate growth,
such as expansins (Esmon et al. 2006) or invertase (Long et al. 2002), or transporters that
allow ion flow across the membrane to alter membrane polarization, turgor, and character-
istics of the cell wall (as reviewed in Becker and Hedrich 2002), such as H+-ATPase (as
reviewed in Hager 2003) and potassium channels (Philippar et al. 1999). The most strik-
frankie
(Frankie)
#1