Agriculture Reference
In-Depth Information
AXR1
UBQ
Auxin
?
s
?
RUB1
E2
ARF
AUX/IAA
ARF
SCF
TIR1
ARF
UBQ
AUX/IAA and
other early genes
AUX/IAA and
other early genes
AUX/IAA
Auxin-mediated
growth/development
Proteasome
Figure 1.7
Model for auxin response. AUX/IAA proteins repress the auxin-response pathway by
inhibiting ARF transcription factors. Auxin promotes the ubiquitination of AUX/IAAs by promoting
their interaction with the SCF
TIR1
ubiquitin ligase. The subsequent degradation of AUX/IAA proteins
results in activation of ARFs and derepression of the auxin-response pathway. Because AUX/IAA
genes themselves are rapidly induced by auxin, a negative-feedback loop exists with the newly
synthesized AUX/IAA proteins restoring repression upon the pathway. Reproduced from Gray
et al
.
(2001), with permission.
(Guilfoyle & Hagen, 2001). When auxin levels increase, the AUX/IAA repressors
are degraded via SCF
TIR1
-dependent ubiquitination (reviewed by Kepinski & Leyser,
2002). This releases ARF transcriptional regulators from their AUX/IAA-based in-
hibition and results in the derepression of the auxin response genes (Gray
et al
.,
2001; Zenser
et al
., 2001; Tiwari
et al
., 2004). It is unclear if or how the other part
of ARF family (the transcriptional repressors) functions in auxin signaling. They
may act on genes that are downregulated in response to auxin or have functions
that are not yet identified. Despite the considerable gaps in our knowledge on auxin
