4.6.2.1. Auxin and GA Signaling.When auxin acts to promote cell division and
growth, it does so mainly by increasing the expression of genes that encode required pro-
teins for these processes. Thus, researchers have sought to understand the steps between
auxin perception and the final gene expression regulation. We now know that auxin signal-
ing involves ubiquitin-mediated protein turnover as way to control transcription of genes
that allow the plant to effect a response to auxin. Molecular studies revealed the first
players in auxin signaling as a group of genes encoding the IAA/AUX proteins whose
expression is rapidly upregulated in response to auxin within minutes. Most of the IAA/
AUX proteins are nuclear-localized and have a very short half-life. They can form hetero-
dimers with theauxin response factortranscriptional regulators (ARFs), and then bind to a
6bp (6-base-pair) auxin-responsive element (ARE) present in the promoters of auxin-
regulated genes. Further studies revealed that ARF:ARF homodimers were responsible
for activation of gene expression in response to auxin, while ARF:AUX/IAA heterodimers
blocked transcriptional activation (Quint and Gray 2006).
Genetic mutants that failed to respond to auxin in seedling growth assays identified
genes that are required for some of the plant’s responses to auxin. These genes, which
include theaxr1andtir1genes, encode proteins that function in the ubiquitin-mediated
protein turnover pathway in eukaryotes. The proteasome is a large, macromolecular struc-
ture that functions to degrade proteins within the cell. Proteins destined for the 26Sprotea-
some are modified by the addition of ubiquitin, itself a small protein (76 amino acids).
Thus, auxin signaling requires a functioning 26Sproteasome and enzymes necessary to
add ubiquitin to target proteins.
It has been shown that the F-box protein encoded by TIR1 becomes physically associ-
ated with auxin, and thus may function as an auxin receptor. After binding to auxin, TIR1
may stimulate the proteasome to specifically degrade some of the IAA/AUX proteins. Once
the IAA/AUX proteins are degraded, ARF:ARF homodimers form, bind the AREs in pro-
moters of auxin-responsive cells, and stimulate the transcription of these genes. In this way,
auxin can stimulate expression of genes required to carry out its physiological effects.
Figure 4.10.A paradigm signal transduction pathway. Signals from the outside of cells can be per-
ceived by receptors or other proteins present at or near the plasma membrane. Once activated, these
receptors can transmit signaling information (arrows) to the interior of the cell. Many signal transduc-
tion pathways converge on the stimulation of gene expression within the nucleus which results in the
production of new proteins in the cytoplasm that can affect specific biological responses.
104 PLANT DEVELOPMENT AND PHYSIOLOGY