Several parts of the auxin signal transduction pathway are still not understood, and much
research is underway to delineate the pathway. Interestingly, proteasomal degradation of a
transcriptional repressor may be a common theme in plant hormone signal transduction
pathways. Such a repressor, containing a DELLA protein domain, represses GA-regulated
genes and is degraded by the proteasome after GA addition to plant cells. Thus, GA signal-
ing may share the same general regulation in stimulating expression of genes required for
the physiological responses to GA. The GA signal transduction pathway also has an ident-
ified protein, GID1, which may be the GA receptor and function as the initial step in GA
perception. GID1 is a nuclear and cytosol-located protein that is homologous to the animal
hormone-sensitive lipases and that binds to different GAs with saturable kinetics. This last
fact is an important test that helps support the idea that a protein directly and specifically
interacts with a hormone (Pimenta-Lange and Lange 2006).
4.6.2.2. Cytokinin and Ethylene Signaling. Plant cells utilize elements of the
two-component signaling pathwaysin their responses to cytokinin and ethylene. The
two-component systems function in microbes, yeast, and plants to convey signals
between a histidine kinase receiver and a phosphorylated response receiver. These two com-
ponents are joined by an intermediate in plant cells termed thephosphorelay intermediate.
Both cytokinin and ethylene have been shown to bind to specific histidine kinases con-
tained in the plasma membrane. This binding is thought to stimulate a phosphorylation
cascade wherein the activated histidine kinase phosphorylates an intermediate protein,
which then phosphorylates a specific aspartate residue on the response receiver. The
response receiver must act to stimulate downstream functions that are currently uncharacter-
ized for cytokinin signaling.
In ethylene signaling, downstream targets of the two-component signaling system have
been identified, mainly through the genetic isolation of mutants altered in their responses to
ethylene application. These genetic screens identified the CTR kinase that functions as a
negative regulator of ethylene signaling, and the EIN and ERF proteins that function as tran-
scriptional activators of ethylene-regulated genes. In a scenario that seems reminiscent of
auxin and GA-mediated signaling, the EIN3 transcriptional activator is subjected to protea-
somal degradation in the absence of ethylene. This fact implies that one critical step in
ethylene perception is the increased stability of EIN3 that allows for new transcription of
ethylene regulated genes.
4.6.2.3. Brassinosteroid Signal Transduction. The brassinosteriod, brassinolide
(Br), is the last example of plant hormone signaling that we will consider. As was
carried out for the other plant hormones, genetic mutant screens were performed to find
Br-insensitive mutants. The Bri gene was identified and shown to be required for seedling
responses to exogenously added Br. The BRI protein encodes a leucine-rich repeat (LRR)-
containing serine/threonine protein kinase. This fact is important since these types of sig-
naling kinases are abundant in animal cells and often serve as receptors for animal peptide
hormones such as insulin. The BRI protein is predicted to span the plant cell plasma mem-
brane, making the LRR domain accessible to the outside of the plant cell, with the kinase
domain contained on the interior of the cell. This arrangement led to an integral domain-
swapping experiment between the BRI protein and the XA1 protein that confers resistance
to rice blast fungus. Researchers produced transgenic plants containing the outside LRR
domain from BR1 and the interior kinase portion of XA1. The resulting plants could be
4.6. HORMONE PHYSIOLOGY AND SIGNAL TRANSDUCTION 105