The Story of Auxin – 1890 to present
Charles Darwin (1890s) studied phototropism – movement towards light
Darwin and others studied
coleoptiles –tissues that
protect monocot leaves
during germination
Darwin, C., and Darwin, F. (1881) The power of movement in plants. Appleton and Co., New York.; Photos courtesy of Dr. R.L. Nielsen
© 2013 American Society of Plant Biologists
Cutting off or covering the coleoptile
tip interferes with the response
Coleoptiles with tips
shielded from light or
removed do not bend
“We must therefore conclude
that when seedlings are freely
exposed to a lateral light some
influence is transmitted from
the upper to the lower part,
causing the latter to bend.”
Untreated
coleoptile
bends
signal
What is the signal
that carries
information from
tip to base?
These experiments showed that the light signal is
perceived at the tip, although the bending occurs at
the base – some signal must move from tip to base
© 2013 American Society of Plant Biologists
Boysen-Jensen (1913) showed that
the transmitted influence can move
through a gelatin block
Before
After
The signal cannot
move through a
solid block or butter,
demonstrating that
it is a water- soluble
chemical.
© 2013 American Society of Plant Biologists
Repositioning the tip can induce
bending in uniform light
Tip removed and replaced
to one side
Tip
removed
Control
Before
Asymmetric tip placement
causes bending
Paal (1919) showed
that removing the tip
and replacing it on
one side of the base
is sufficient to cause
bending.
After
Tip removed
and replaced
© 2013 American Society of Plant Biologists
In the 1930s, auxin was purified and
shown to promote growth
Angle of curvature
is proportional to
amount of auxin in
block
Frits Went collected auxin
from shoot tips into agar
blocks...
Indole-3-acetic
acid, IAA
...and showed that
the material collected
in the agar blocks
was the growthpromoting substance.
This bending assay for the growthpromoting effect of auxin was used
as a basis for its purification.
Redrawn from Went, F.W. (1935) Auxin, the plant growth-hormone. Bot. Rev. 1: 162-182.
© 2013 American Society of Plant Biologists
Auxin: a 21st century perspective
•Auxin homeostasis
•Tools in auxin research
•Polar auxin transport
•Perception and signaling
•Auxin action in whole-plant
processes
•Interactions with other
signals
© 2013 American Society of Plant Biologists
Auxin synthesis and homeostasis,
transport and signaling
Catabolism
Synthesis
IAA
Conjugation
Transport
Perception
(receptor)
ABP1
Perception
(receptor)
TF activation/
inactivation
Cell
surface
proteins
Biological
Functions
Target
genes
Biological
Functions
Auxin’s effects depend upon its synthesis, transport,
perception, signaling, and target gene responses. Most of
these functions are controlled by many genes with differing
cell specificities.
Adapted from Kieffer, M., Neve, J., and Kepinski, S. (2010). Defining auxin response contexts in plant development. Curr. Opin. Plant Biol.13: 12-20.
© 2013 American Society of Plant Biologists
Auxin is produced through several
pathways
Indole
Light, nutrients, and
other hormones all
regulate auxin
synthesis
Trp
IAOx
IAN
The IAOx pathway
may be restricted to
Arabidopsis and its
close relatives
IAOx pathway
Trp-dependent pathways
IAA is produced from
tryptophan (Trp) via
several pathways,
and one Trp-independent
pathway
IAM
Catabolism
IAM pathway
IAA
TAM
IAAld
Conjugation
TAM pathway
IPA
IAOx pathway
The pool of
active auxin is
also regulated
by catabolism
and conjugation
Trp-independent pathway
Adapted from Rosquete M.R., Barbez, E. and Kleine-Vehn, J. (2011) Cellular auxin homeostasis: gatekeeping is housekeeping. Mol. Plant 5: 772–
786. See also Mashiguchi, K.,et al. (2011) The main auxin biosynthesis pathway in Arabidopsis. Proc. Natl. Acad. Sci. U.S.A. 108: 18512–18517.
.
© 2013 American Society of Plant Biologists
Polar Auxin Transport
Auxin moves long distances
through the phloem.
Auxin also moves via auxin
transport proteins.
Auxin normally moves from the tip
of the shoot towards the tip of the
root. At the root tip, auxin changes
direction and moves short
distances up the root again
(basipetally).
Reprinted with permission from Macmillan Publishers, Ltd. Robert, H.S., and Friml, J. (2009) Auxin and other signals on the move in plants. Nat. Chem. Biol. 5: 325-332.
Reprinted from Muday, G.K., and DeLong, A. (2001). Polar auxin transport: Controlling where and how much. Trends Plant Sci. 6: 535–542, with permission from Elsevier.
© 2013 American Society of Plant Biologists
The Cholodny-Went theory of tropic
curvature
The Cholodny-Went theory of tropic curvature states that perception of
a stimulus (e.g. asymmetric light or gravity) initiates a lateral relocation
of auxin, resulting in differential growth.
© 2013 American Society of Plant Biologists
Coleoptiles or shoots move auxin to
the shaded side
IAA accumulates on
the shaded side of
phototropically
stimulated Brassica
oleracea hypocotyls.
Increased auxin promotes
cell elongation on the
shaded side, causing
bending toward the light.
DR5::GUS
Cell length
Auxin
concentration
Auxin-induced
transcription of an
auxin-sensitive
promoter is increased
on the dark side of this
phototropicallystimulated Arabidopsis
hypocotyl.
Esmon, C.A. et al. (2006) A gradient of auxin and auxin-dependent transcription precedes tropic growth responses. Proc. Natl. Acad. Sci. USA 103: 236–241. Reprinted by
permission from Macmillan Publishers, Ltd: Friml, J., Wisniewska, J., Benkova, E., Mendgen, K., and Palme, K. (2002) Lateral relocation of auxin efflux regulator PIN3
mediates tropism in Arabidopsis. Nature 415: 806-809.
© 2013 American Society of Plant Biologists
Auxin transport – chemiosmotic
model
Indole-3-acetic acid is a charged
anion (IAA-) in the cytoplasm (pH 7).
In the more acidic cell wall (pH 5.5)
some is uncharged (IAAH). The
uncharged form crosses the plasma
membrane into the cell where it is
deprotonated and unable to exit
other than through specific
transporters.
Cell wall
pH 5.5
A proton ATPase
maintains the
differential pH
gradient
Cytoplasm
pH 7
H+
IAA-
IAAH
IAA- + H+
Cell-to-cell
polar
auxin
transport
IAAH
IAA- + H+
Redrawn from Robert, H.S., and Friml, J. (2009) Auxin and other signals on the move in plants. Nat. Chem. Biol. 5: 325-332.
© 2013 American Society of Plant Biologists
Auxin moves through efflux and
influx carrier proteins
ABCB
ABCB
The AUX1/LAX
influx carriers
contribute to
movement of
IAAH into the
cytoplasm.
The PIN family of
proteins contributes to
directional movement
of auxin out of the cell.
Cell-to-cell
polar
auxin
transport
ABCB
The ABCB
transporters
contribute to auxin
transport in a diverse
ways. (These were formerly
referred to as MDR or PGP
proteins).
Reprinted with permission from Macmillan Publishers, Ltd. Robert, H.S., and Friml, J. (2009) Auxin and other signals on the move in plants. Nat. Chem. Biol. 5: 325-332.
© 2013 American Society of Plant Biologists
PIN proteins orient asymmetrically
in plant cells
PIN1 localizes to
the lower
surface of root
cortex cells
Root
Base
PIN1 is
responsible for
auxin flow from
shoot apex to
root apex.
Root
Apex
Reprinted by permission from Macmillan Publishers, Ltd. Dhonukshe, P., et al. (2008). Generation of cell polarity in plants links endocytosis, auxin distribution and cell fate
decisions. Nature 456: 962-966. Reproduced with permssion from Dolan, L., et al. (1993). Cellular organisation of the Arabidopsis thaliana root. Development 119: 71-84.
© 2013 American Society of Plant Biologists
The distribution of PIN proteins
contributes to the auxin gradients
Křeček , P., Skůpa , P., Libus, J., Naramoto, S., Tejos, R., Friml J., and Zažímalová, E. (2009) The PINFORMED (PIN) protein family of auxin transporters. Genome Biology 10: 249.
© 2013 American Society of Plant Biologists
Auxin’s effects are mediated through
at least two types of receptor
Perception
(receptor)
ABP1
Perception
(receptor)
SCFTIR1
ABP1 was first identified in the
1970s, but only recently are its
functions becoming clear…
IAA
© 2013 American Society of Plant Biologists
ABP1 mediates rapid auxin
responses at the plasma membrane
AUXIN
Perception of auxin
by ABP1 at the outer
face of the plasma
membrane initiates
signals that lead to
proton-pump
activation, wall
acidification and wall
loosening.
Reprinted from Tromas, A., Paponov, I., and Perrot-Rechenmann, C. (2010). AUXIN BINDING PROTEIN 1: functional and evolutionary aspects.
Trends Plant Sci. 15: 436–446 with permission from Elsevier.
© 2013 American Society of Plant Biologists
Auxin acts like a molecular glue that
holds coreceptor proteins together
TIR1 is an F-box
protein, part of the
SCFTIR1 ubiquitin
ligase complex
It forms a
coreceptor
complex with
Aux/IAA proteins
Aux/IAA
IAA
TIR1
SCFTIR1
complex
Tan, X., Calderon-Villalobos, L.I.A., Sharon, M., Zheng, C., Robinson, C.V., Estelle, M. and Zheng, N. (2007). Mechanism of auxin
perception by the TIR1 ubiquitin ligase. Nature. 446: 640-645. Jena Library
© 2013 American Society of Plant Biologists
The auxin signaling pathway
Auxin promotes the association of
Aux/IAA proteins and the SCFTIR1
ubiquitin ligase complex
Aux/IAA
ARF
LOW AUXIN
LEVELS
SCFTIR1
IAA
IAA
At low auxin levels, Aux/IAA proteins
and ARF proteins associate and
interfere with ARF action
Aux/IAA
…leading to
proteolysis of
Aux/IAA
HIGH AUXIN
LEVELS
ARF
Elimination of Aux/IAA proteins stops
them from interfering with ARF
proteins, which can activate or
repress transcription
© 2013 American Society of Plant Biologists
Auxin action in whole-plant
processes
Auxin in
action!
Promote lateral organ
initiation at the shoot
apical meristem
Light responses
Cell elongation
Responses to pathogens
Inhibit
branching in
the shoot
Integrate growth
signaling pathways
Control patterning
and vascular
development
Maintain stemPromote
cell fate at the
branching
in the root
Integrate growth root apical
meristem
Response to nutrient
signaling pathways
distribution and
Responses to symbionts –
abundance
nodule formation
Reprinted by permission from Macmillan Publishers, Ltd: NATURE Wolters, H., and Jürgens, G. (2009). Survival of the flexible:
Hormonal growth control and adaptation in plant development. Nat. Rev. Genet. 10: 305–317. Copyright 2009.
© 2013 American Society of Plant Biologists
Patterns of auxin accumulation
contribute to developmental
patterning
It has recently become clear that one of
auxin’s key roles involves establishing
and conveying positional information, and
that it can act both as a morphogen and
developmental trigger.
Reprinted by permission from Macmillan Publishers, Ltd. Sorefan, K. et al. (2009) A regulated auxin minimum is required for seed dispersal in Arabidopsis. Nature 459: 583-586. Dubrovsky, J.G., et al., (2008) Auxin acts as a local morphogenetic
trigger to specify lateral root founder cells. Proc. Natl. Acad. Sci. 105: 8790–8794, copyright © by the National Academy of Sciences. Petersson, S.V., et al. (2009) An auxin gradient and maximum in the Arabidopsis root apex shown by highresolution cell-specific analysis of IAA distribution and synthesis. Plant Cell 21:1659-1668. Reprinted from Heisler, M.G., et al. (2005). Patterns of auxin transport and gene expression during primordium development revealed by live imaging of the
Arabidopsis inflorescence meristem. Curr. Biol. 15: 1899–1911, with permission from Elsevier.Reproduced with permission from Petrášek, J., and Friml, J. (2009) Auxin transport routes in plant development. Development 136: 2675-2688.
© 2013 American Society of Plant Biologists
Auxin acts like a morphogen at the
root apex
Auxin amount
Cells experiencing
a low auxin level
elongate and
differentiate
Cells near the tip
experience
intermediate auxin
levels and divide
frequently
Cells at the
quiescent center
experience the
highest auxin
concentrations and
remain quiescent.
Quiescent
center
Petersson, S.V., Johansson, A.I., Kowalczyk, M., Makoveychuk, A., Wang, J.Y., Moritz, T., Grebe, M., Benfey, P.N., Sandberg, G., and Ljung, K.(2009) An auxin
gradient and maximum in the Arabidopsis root apex shown by high-resolution cell-specific analysis of IAA distribution and synthesis. Plant Cell 21:1659-1668.
© 2013 American Society of Plant Biologists
Auxin also acts like a trigger,
initiating developmental events
The experimental increase
of auxin synthesis in one cell
is sufficient to trigger a
lateral root formation.
A localized auxin maximum (or minimum) can
be sufficient to initiate a developmental event.
Here the red indicates cells triggered to
differentiate by elevated auxin levels.
Dubrovsky, J.G., et al., (2008) Auxin acts as a local morphogenetic trigger to specify lateral root founder cells. Proc. Natl. Acad. Sci. 105: 8790–8794,
copyright © by the National Academy of Sciences.
© 2013 American Society of Plant Biologists
Summary
Catabolism
Synthesis
IAA
Conjugation
Transport
Perception
(receptor)
ABP1
Perception
(receptor)
TF activation/
inactivation
Cell
surface
proteins
Biological
Functions
Target
genes
Biological
Functions
In the past 30 years we have identified many of the molecular characters
in the auxin story, and have a pretty good idea of its major themes, but
the story is far from complete.
Adapted from Kieffer, M., Neve, J., and Kepinski, S. (2010). Defining auxin response contexts in plant development. Current Opinion in Plant Biology 13: 12-20.
© 2013 American Society of Plant Biologists