Plant Hormone Signaling - Wikispaces - Biol512

Plant Hormone Signaling and the
Ubiquitin-proteasome System
Serena J. Gregory
March 30th, 2011
• Introduction
• Overview of ubiquitin and the Ub/proteasome
• Hormone signaling
– Auxin signaling
– Ethylene signaling
– Abscisic Acid signaling
• Summary
Growth and development in plants
Hormone signaling through protein
The ubiquitin molecule
• 76 amino acid
• Contains a b-grasp
• Di-glycine tail on the
• Covalently attached
to target proteins
Vierstra, R.D. (2009)
Schematic overview of the ubiquitin
Vierstra, R.D. (2009)
The Ubiquitin Code
Ye Y and Rape M. (2009).
Predicted number of genes that
encode each UPS component in plants
• E1- 2 genes
• E2- 37 genes
• E3- >1400
Vierstra, R.D. (2009)
Target protein specificity is achieved
through E3s
• Three types of E3s
– U-box
• Structure can be
either single
subunit or multi
Santner, A. & Estelle, M.(2010)
• RING (Really Interesting
New Gene)
• 465 RING proteins
• 70 aa motif characterized
as the ring finger
• Zinc-binding motif that
binds to E2s
Vierstra, R.D. (2009)
Santner, A. & Estelle, M.(2010)
Multi subunit RING E3s
• Components
RING protein RBX1 (RING Box 1)
Cullin scaffold-like protein
Adaptor subunits
Additional substrate-recognition
• Types
– SCF (Skp-Cullin-F-box)
– CUL3-BTB (Broad-complex,
Tramtrack, Bric-a-Brac)
Santner, A. & Estelle, M.(2010)
U-box E3s
• Modified RING-finger
• 64 U-Box proteins
• 70 aa U-box domain
• Does not use zinc ions
to stabilize structure
Santner, A. & Estelle, M.(2010)
• Homology to E6-AP C
Terminus (HECT)
• Smallest E3 subfamily
• 20 HECT proteins
• 350 aa motif
• Contains an Ub-binding
and E2 binding site
Santner, A. & Estelle, M.(2010)
SCF complex assembly and hormone
• SCF-most common E3
• Discovered through
mutant screens
• Exposed to auxin or auxin
transport inhibitors
• Disruptions of components
of the SCF E3s
• CUL1 mutants were
discovered that are auxin
Mark Estelle
• Introduction
• Overview of ubiquitin and the
Ub/proteasome pathway
• Hormone signaling
– Auxin signaling
– Ethylene signaling
– Absicic Acid signaling
• Summary
Hormone signaling in plants
• Controls communication
– Internally
– Neighboring cells
– Long distance between different organ systems
• Three step process
– Signal perception, signal transduction and
• Phytohormones
– Produced by individual cells instead of gland
Structures of Phytohormones
Functional Interactions of
Jailais, Y. & Chory, J. (2010).
Plant hormone receptors
Ethylene: ETR1 (histidine kinases)
Cytokinins: CRE1 (histidine kinases)
Auxin: TIR1 and IAA proteins
GA: GID1 (regulates GID2 and DELLA)
Jasmonic acid: COI1 and JAZs
ABA: GTG1 and GTG2
Sites of plant hormone perception
Santner, A. & Estelle, M. (2009).
• Introduction
• Overview of the ubiquitin and the
Ub/proteasome pathway
• Hormone signaling
– Auxin signaling
– Ethylene signaling
– Absicic Acid signaling
• Summary
Auxin Signaling
• From the Greek auxein, meaning to grow.
• Described by Charles Darwin in 1880 as a growth
promoting substance that moves in plants
• Responsible for
– Cell division/elongation
– Organogenesis
– Prevents senescence
• Transported via influx transporter proteins and efflux
transporter proteins
• Act primarily through Auxin Response Transcription
Factors (ARFs)
Auxin signaling with ARFs
F-box protein TIR1 is the auxin
Vierstra, R.D. (2009)
Crystal structure of TIR1
Vierstra, R.D. (2009)
Jasmonic Acid and Gibberellic Acid
Target protein- JA-ZIM domain repressor
proteins (JAZ)
Transcription factor- MYB DOMAIN PROTEIN 2
F-box proteins- SLEEPY 1 AND SNEEZY 1
Target protein- Della repressor proteins
Vierstra, R.D. (2009)
Ethylene Signaling
• Gaseous hormone
– Promote fruit ripening and leaf abscission and
plant senescence
– Resistance to pathogens and stress response
• Triple Response Phenotype
– Dark grown seedlings exposed to ethylene
– Very distinctive phenotype
The Ethylene Triple-Response
The Ethylene Triple-Response
Mutant testing
Ethylene Signaling
J. M. Alonso , J. R. Ecker 2001
Abscisic Acid Signaling
• Isoprenoid compound
• Responsible for
– Control of seed dormancy
– Drought response
• Controversial receptors
– CHLH (GUN5 in Arabidopsis)
– GCR2
• Established receptors
– GTG1 and GTG2
The RNA-binding protein FCA is an
abscisic acid receptor
Razem FA, El-Kereamy A, Abrams SR, Hill RD. Department of Plant Science, University
of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
The phytohormone abscisic acid (ABA) regulates various physiological processes in
plants. The molecular mechanisms by which this is achieved are not fully
understood. Genetic approaches have characterized several downstream
components of ABA signaling, but a receptor for ABA has remained elusive.
Although studies indicate that several ABA response genes encode RNA-binding or
RNA-processing proteins, none has been found to be functional in binding ABA.
Here we show that FCA, an RNA-binding protein involved in flowering, binds ABA
with high affinity in an interaction that is stereospecific and follows receptor
kinetics. The interaction between FCA and ABA has molecular effects on
downstream events in the autonomous floral pathway and, consequently, on the
ability of the plant to undergo transition to flowering. We further show that ABA
binding exerts a direct control on the FCA-mediated processing of precursor
messenger RNA. Our results indicate that FCA is an ABA receptor involved in RNA
metabolism and in controlling flowering time.
FCA does not bind abscisic acid
Risk JM, Macknight RC, Day CL.
Biochemistry Department, University of Otago, Dunedin 9054, New
Zealand. [email protected]
The RNA-binding protein FCA promotes flowering in Arabidopsis.
Razem et al. reported that FCA is also a receptor for the
phytohormone abscisic acid (ABA). However, we find that FCA does
not bind ABA, suggesting that the quality of the proteins assayed
and the sensitivity of the ABA-binding assay have led Razem et al. to
erroneous conclusions. Because similar assays have been used to
characterize other ABA receptors, our results indicate that the ABAbinding properties of these proteins should be carefully reevaluated and that alternative ABA receptors are likely to be
Sites of plant hormone perception
Santner, A. & Estelle, M. (2009)
Abscisic Acid Signaling
• Two-fold process
• Blocks degradation of
ABI5 preventing
ubiquitination by KEG
• Promotes degradation
of ABI3 by increasing
AIP2 expression
Vierstra, R.D. (2009)
Effects of abscisic acid signaling on
• The ubiquitin-26S proteasome system plays a
prominent regulatory role in plant hormone
• E3 ubiquitin ligases represent a new class of
hormone signaling receptors
• E3 activities can be manipulated by small
molecules could lead to new pharmacological
• Elucidation of all E3 ligases will potentially
uncover other interesting signaling pathways and
• Vierstra, R.D. (2009). The ubiquitin-26S proteasome system at the
nexus of plant biology. Nat. Rev. Mol. Cell Biol. 10, 385–397.
• Ye Y and Rape M. (2009). Building ubiquitin chains: E2 enzymes at
work. Nat. Rev. Mol. Cell Biol. 10(11):755-64.
• Santner, A. & Estelle, M. (2009). Recent advances and emerging
trends in plant hormone signaling. Nature 459, 1071–1078.
• Santner, A. & Estelle, M.(2010). The ubiquitin-proteasome system
regulates plant hormone signaling. The Plant Journal. 61, 10291040.
• Jailais, Y. & Chory, J. (2010). Unraveling the paradoxes of plant
hormone signaling integration. Nat. Structural & Mol. Biol. 17(6):
• Benavente, L. M. & Alonso, J. M. (2006). Molecular mechanisms of
ethylene signaling in Arabidopsis. Mol. BioSyst. 2, 165-173.
• J. M. Alonso , J. R. Ecker , The Ethylene Pathway: A Paradigm for Plant Hormone
Signaling and Interaction. Science's STKE (2001),;2001/70/re1.
• Tan, X. et al. (2007). Mechanism of auxin perception by the TIR1
ubiquitin ligase. Nature 446, 640–645.
• Pandey, S., Nelson, D. C. & Assmann, S. M. (2009). Two novel GPCR-type
G proteins are abscisic acid receptors in Arabidopsis. Cell 136, 136–148.
• Christmann, Alexander & Grill, Erwin. (2009). Are GTGs ABA's biggest fans?.
Cell, 136. Retrieved from
• Risk J.M. Macknight R.C. (2008). FCA does not bind abscisic acid. Nature
456(7223): E5-6.

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