provides a localized assessment of mast cell re-
sponses, maintaining the factor of 100 times the
2NP concentration relative to that of DNP. At
30 min after exposure to DNP or 2NP, vascular
permeability as measured by extravasation of
Evans blue dye (see supplementary materials) was
significantly higher in DNP- than 2NP-treated
mice (fig. S6A). Consistent with this result, more
mast cells were degranulated in animals treated
with DNP. Ear swelling was significantly differ-
ent 30 min after DNP or 2NP treatment but nar-
rowed with time (fig. S6B), and the increase in
the thickness of the dermis was similar at 3 hours
after stimulation (fig. S6C). Immune cell infiltra-
tion was similarly increased in animals exposed
to DNP or 2NP (fig. S6D). By 12 hours after
stimulation, the thickness of the dermis returned
to that of control mice (fig. S6E) but immune cell
infiltrates were still elevated relative to control mice
(fig. S6F). Given that an inflammatory response
was initiated by either DNP or 2NP, we explored
the cell types involved. Gr-1+ CD11c– CD11b+
cells were distinguished on the basis of the mye-
loid marker 7/4 [Ly-6B.2, which is somewhat
more highly expressed by recently generated in-
flammatory macrophages (20)] from neutrophils
as marked by Ly-6G (Fig. 4A). Exposure to DNP
caused increased numbers of neutrophils relative
to inflammatory macrophages, whereas this ratio
was reversed in animals treated with 2NP, con-
sistent with the increased secretion of monocyte-
or macrophage-attracting chemokines, such as
CCL2, CCL3, and CCL4, after treatment of mast
cells with 2NP (Fig. 1D). Whole-mount immuno-
histochemical analysis of the skin also revealed
these differences (Fig. 4, B and C), and skin mast
cells from 2NP-treated mice produced greater
amounts of CCL2 than did those in the skin of
DNP-treated mice (fig. S7). Thus, low-affinity
stimulation of FceRI results in an inflammatory
response marked by a shift in the monocyte or
Collectively, our findings demonstrate that
differences in the affinity of antigen and antibody interactions are discriminated by receptors
through qualitative changes in molecular signals
resulting in distinct outcomes. This discriminatory ability of receptors may extend beyond the
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Acknowledgments: Supported by the intramural research
program of the National Institute of Arthritis and Musculoskeletal
and Skin Diseases and by its Laboratory Animal Care and
Use Section and Flow Cytometry Group, Office of Science
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Materials and Methods
Figs. S1 to S7
Movies S1 to S6
8 October 2013; accepted 17 January 2014
Published online 6 February 2014;
Cell Surface ABP1-TMK
Auxin-Sensing Complex Activates
ROP GTPase Signaling
Tongda Xu,1,2,3 Ning Dai,4 Jisheng Chen,1 Shingo Nagawa,1,3 Min Cao,2 Hongjiang Li,1,5,6
Zimin Zhou,2 Xu Chen,5,6 Riet De Rycke,5 Hana Rakusová,5,6 Wuyi Wang,3,7† Alan M. Jones,8
Jiří Friml,5,6,9 Sara E. Patterson,7 Anthony B. Bleecker,4‡ Zhenbiao Yang1,3,10§
Auxin-binding protein 1 (ABP1) was discovered nearly 40 years ago and was shown to be
essential for plant development and morphogenesis, but its mode of action remains unclear.
Here, we report that the plasma membrane–localized transmembrane kinase (TMK) receptor–like
kinases interact with ABP1 and transduce auxin signal to activate plasma membrane–associated
ROPs [Rho-like guanosine triphosphatases (GTPase) from plants], leading to changes in the
cytoskeleton and the shape of leaf pavement cells in Arabidopsis. The interaction between ABP1
and TMK at the cell surface is induced by auxin and requires ABP1 sensing of auxin. These findings
show that TMK proteins and ABP1 form a cell surface auxin perception complex that activates
ROP signaling pathways, regulating nontranscriptional cytoplasmic responses and associated
Auxin regulates nearly all aspects of plant development and behavior and impinges on a great variety of responses involving
cell polarization, expansion, division and differen-
tiation. Exactly how this small-molecule hormone
achieves this multitude of diverse roles is largely
unexplained, although it may be perceived by
multiple functionally distinct auxin perception
and signaling systems (1–6). Members of the nu-
clear TIR1/AFB F-box protein auxin receptor
and AUX/ IAA co-receptor families modulate
nuclear gene transcription in response to various
auxin concentrations (1–4).
Independently of the TIR1 family, auxin-binding protein 1 (ABP1) was proposed to perceive extracellular auxin to regulate a plethora of
plasma membrane or cytoplasmic responses not
necessarily involving gene transcription (6–18).
ABP1 may also coordinate with the TIR1/AFB
pathway to regulate gene transcription (16, 19).
ABP1 is essential for early embryogenesis, root
development, leaf expansion, cell morphogenesis, and subcellular distribution of PIN auxin
transporters (6, 8, 9, 12, 13, 15–18, 20, 21). ABP1
is required for the auxin-dependent activation
of ROPs [Rho-like guanosine triphosphatases
(GTPases) from plants] at the plasma membrane,
1Center for Plant Cell Biology, Department of Botany and Plant
Sciences, University of California, Riverside, CA 92521, USA.
2Temasek Life Sciences Laboratory, 1 Research Link, National
University of Singapore, 117604 Singapore. 3Shanghai Center for Plant Stress Biology, Shanghai Institutes for Biological
Sciences, The Chinese Academy of Sciences, Shanghai 200032,
China. 4Department of Botany, University of Wisconsin, Madison,
WI 53706, USA. 5Department of Plant Systems Biology, VIB
and Department of Plant Biotechnology and Bioinformatics,
Ghent University, 9052 Gent, Belgium. 6Bertalanffy Foundation Building, Institute of Science and Technology Austria,
Am Campus 1, 3400 Klosterneuburg, Austria. 7Department
of Horticulture, University of Wisconsin, Madison, WI 53706,
USA. 8Departments of Biology and Pharmacology, University
of North Carolina, Chapel Hill, Chapel Hill, NC 27599, USA.
9Mendel Centre for Plant Genomics and Proteomics, Masaryk
University, CEITEC MU, CZ-625 00 Brno, Czech Republic. 10Shanghai
Institute of Plant Physiology and Ecology, Shanghai Institutes
for Biological Sciences, The Chinese Academy of Sciences,
Shanghai 200032, China.
*Present address: Department of Molecular Biology, Massachusetts General Hospital, Boston, MA 02114, USA.
†Present address: Ceres, 1535 Rancho Conejo Boulevard, Thousand Oaks, CA 91320, USA.
§Corresponding author. E-mail: firstname.lastname@example.org