Auxin-regulated signaling in root development. a At low
auxin level, a co-repressor, TPL represses auxin-regulated transcription
by mediating the binding of AUX/IAA proteins to ARF. b At
high auxin level, SCFTIR1 complex binds auxin and targets AUX/IAA
proteins for degradation through ubiquitin–proteasome pathway.
These proteins are tagged with the small protein modifier ubiquitin.
The ubiquitin is covalently conjugated to AUX/IAA by the sequential
activity of three enzymes: the ubiquitin activating enzymes (E1), the
ubiquitin conjugating enzymes (E2) and the ubiquitin protein ligases
(E3). This tagged protein complex is recognized by proteasome and is