One of the mechanisms by which signalling molecules regulate cellular behaviour is modulating subcellular protein translocation. This mode of regulation is often based on specialized vesicle trafficking, termed constitutive cycling, which consists of repeated internalization and recycling of proteins to and from the plasma membrane1. No such mechanism of hormone action has been shown in plants although several proteins, including the PIN auxin efflux facilitators, exhibit constitutive cycling2,3. Here we show that a major regulator of plant development, auxin, inhibits endocytosis. This effect is specific to biologically active auxins and requires activity of the Calossin-like protein BIG. By inhibiting the internalization step of PIN constitutive cycling, auxin increases levels of PINs at the plasma membrane. Concomitantly, auxin promotes its own efflux from cells by a vesicle-trafficking-dependent mechanism. Furthermore, asymmetric auxin translocation during gravitropism is correlated with decreased PIN internalization. Our data imply a previously undescribed mode of plant hormone action: by modulating PIN protein trafficking, auxin regulates PIN abundance and activity at the cell surface, providing a mechanism for the feedback regulation of auxin transport.
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We thank C. Bellini, M. Bennett, M. Estelle, M. Grebe, W. Michalke, D. Robinson and Y. Zhao for sharing material, and E. Benková, P. Brewer, J. Eder, J. Malbeck, C. Oecking, M. Sauer, H. Stransky and D. Weijers for technical assistance and discussions. This work was supported by the Volkswagenstiftung (J.F. and T.P.), the F. Ebert Stiftung (J.K.-V.), the Deutsche Forschungsgemeinschaft (N.G., G.J. and Y.-D. S.), the Grant Agency of the Academy of Sciences of the Czech Republic (J.P. and E.Z.) and the Royal Society of London (D.A.M.).
Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.
Auxins inhibit BFA-induced internalisation of constitutively cycling PM markers. (JPG 337 kb)
Inhibitors of protein expression and degradation confirm BFA-visualized constitutive cycling of PIN1. (JPG 66 kb)
Other plant hormones do not inhibit internalisation of constitutively cycling proteins. (JPG 109 kb)
5 µM concentrations of NAA and 2,4-D inhibit BFA-induced internalisation of constitutively cycling proteins. (JPG 336 kb)
IAA is unstable in Arabidopsis medium but, if stabilised, it inhibits BFA-induced internalisation at 5 µM concentrations. (JPG 178 kb)
Auxins down-regulate but do not completely block BFA-induced internalisation of constitutively cycling proteins. (JPG 127 kb)
NAA does not influence BFA uptake in Arabidopsis root tissues. (JPG 73 kb)
Auxins do not affect the morphology of selected subcellular structures. (JPG 146 kb)
Auxins inhibit the BFA-induced internalisation of PM proteins but do not affect BFA-induced aggregation of endosomes. (JPG 160 kb)
Auxin efflux in suspension-cultured BY-2 and VBI-0 tobacco cells. (JPG 72 kb)
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Paciorek, T., Zažímalová, E., Ruthardt, N. et al. Auxin inhibits endocytosis and promotes its own efflux from cells. Nature 435, 1251–1256 (2005). https://doi.org/10.1038/nature03633
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