Plants adjust their growth according to gravity. Gravitropism involves gravity perception, signal transduction, and asymmetric growth response, with organ bending as a consequence . Asymmetric growth results from the asymmetric distribution of the plant-specific signaling molecule auxin  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 , 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. © 2016 Elsevier Ltd
We thank Dr. Jie Li (Key Laboratory of Plant Molecular Physiology, Chinese Academy of Science, China) for the pPIN3::PIN3-GFP/DII::VENUS line and Martine De Cock for help in preparing the manuscript. This work was supported by the European Research Council (project ERC-2011-StG-20101109-PSDP), by the Czech Science Foundation GAČR (GA13-40637S) to J.F., and by the Ministry of Education, Youth and Sports of the Czech Republic under the project CEITEC 2020 (LQ1601) to H.S.R. H.R. is indebted to the Agency for Innovation by Science and Technology (IWT) for a predoctoral fellowship.
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Rakusová H, Abbas M, Han H, Song S, Robert H, Friml J. Termination of shoot gravitropic responses by auxin feedback on PIN3 polarity. Current Biology. 2016;26(22):3026-3032. doi:10.1016/j.cub.2016.08.067
Rakusová, H., Abbas, M., Han, H., Song, S., Robert, H., & Friml, J. (2016). Termination of shoot gravitropic responses by auxin feedback on PIN3 polarity. Current Biology, 26(22), 3026–3032. https://doi.org/10.1016/j.cub.2016.08.067
Rakusová, Hana, Mohamad Abbas, Huibin Han, Siyuan Song, Hélène Robert, and Jirí Friml. “Termination of Shoot Gravitropic Responses by Auxin Feedback on PIN3 Polarity.” Current Biology 26, no. 22 (2016): 3026–32. https://doi.org/10.1016/j.cub.2016.08.067.
H. Rakusová, M. Abbas, H. Han, S. Song, H. Robert, and J. Friml, “Termination of shoot gravitropic responses by auxin feedback on PIN3 polarity,” Current Biology, vol. 26, no. 22, pp. 3026–3032, 2016.
Rakusová H, Abbas M, Han H, Song S, Robert H, Friml J. 2016. Termination of shoot gravitropic responses by auxin feedback on PIN3 polarity. Current Biology. 26(22), 3026–3032.
Rakusová, Hana, et al. “Termination of Shoot Gravitropic Responses by Auxin Feedback on PIN3 Polarity.” Current Biology, vol. 26, no. 22, Cell Press, 2016, pp. 3026–32, doi:10.1016/j.cub.2016.08.067.