Plants adjust their growth according to gravity. Gravitropism involves gravity perception, signal transduction, and asymmetric growth response, with organ bending as a consequence [1]. Asymmetric growth results from the asymmetric distribution of the plant-specific signaling molecule auxin [2] that is generated by lateral transport, mediated in the hypocotyl predominantly by the auxin transporter PIN-FORMED3 (PIN3) [3-5]. Gravity stimulation polarizes PIN3 to the bottom sides of endodermal cells, correlating with increased auxin accumulation in adjacent tissues at the lower side of the stimulated organ, where auxin induces cell elongation and, hence, organ bending. A curvature response allows the hypocotyl to resume straight growth at a defined angle [6], implying that at some point auxin symmetry is restored to prevent overbending. Here, we present initial insights into cellular and molecular mechanisms that lead to the termination of the tropic response. We identified an auxin feedback on PIN3 polarization as underlying mechanism that restores symmetry of the PIN3-dependent auxin flow. Thus, two mechanistically distinct PIN3 polarization events redirect auxin fluxes at different time points of the gravity response: first, gravity-mediated redirection of PIN3-mediated auxin flow toward the lower hypocotyl side, where auxin gradually accumulates and promotes growth, and later PIN3 polarization to the opposite cell side, depleting this auxin maximum to end the bending. Accordingly, genetic or pharmacological interference with the late PIN3 polarization prevents termination of the response and leads to hypocotyl overbending. This observation reveals a role of auxin feedback on PIN polarity in the termination of the tropic response.

Institute of Science and Technology Austria 3400 Klosterneuburg Austria

Institute of Science and Technology Austria 3400 Klosterneuburg Austria; Department of Plant Systems Biology VIB and Department of Plant Biotechnology and Bioinformatics Ghent University 9052 Gent Belgium

Institute of Science and Technology Austria 3400 Klosterneuburg Austria; Key Laboratory of Protected Horticulture of Liaoning Province Shenyang Agriculture University Shenyang 110161 P R C

Mendel Centre for Genomics and Proteomics of Plants Systems CEITEC MU Central European Institute of Technology Masaryk University 625 00 Brno Czech Republic

References provided by Crossref.org

AGC kinases and MAB4/MEL proteins maintain PIN polarity by limiting lateral diffusion in plant cells

Strigolactones inhibit auxin feedback on PIN-dependent auxin transport canalization

Auxin canalization and vascular tissue formation by TIR1/AFB-mediated auxin signaling in Arabidopsis

Arabidopsis Flippases Cooperate with ARF GTPase Exchange Factors to Regulate the Trafficking and Polarity of PIN Auxin Transporters

The Nuts and Bolts of PIN Auxin Efflux Carriers

Pinstatic Acid Promotes Auxin Transport by Inhibiting PIN Internalization

The Inhibitor Endosidin 4 Targets SEC7 Domain-Type ARF GTPase Exchange Factors and Interferes with Subcellular Trafficking in Eukaryotes

PID/WAG-mediated phosphorylation of the Arabidopsis PIN3 auxin transporter mediates polarity switches during gravitropism

WRKY23 is a component of the transcriptional network mediating auxin feedback on PIN polarity