Editors' ChoiceAuxin Signaling

PIN2 Polarity Requires Vesicular Cycling and Proteasomal Degradation

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Science's STKE  14 Mar 2006:
Vol. 2006, Issue 326, pp. tw94
DOI: 10.1126/stke.3262006tw94

PIN2 is part of the auxin efflux carrier that controls the asymmetric distribution of auxin, which is a key regulator of root gravitropism. Abas et al. show that root meristems reorient to point their tips down in response to being positioned horizontally in the gravitational field (gravistimulation) by generating an asymmetric distribution of PIN2 such that PIN2 was more abundant on the lower side of the root tip. Before positioning the root horizontally, PIN2 abundance was equal in the lateral sides of the root. Gravistimulation initially caused PIN2 to be removed from the upper root cells' plasma membrane and appear in intracellular vesicles. The decrease in PIN2 abundance (measured in isolated membrane fractions or from the upper cells by microscopic techniques) and gravitropic root bending was blocked if the root meristems were treated with an inhibitor of the proteasome (MG132) or brefeldin A (BFA), which inhibits vesicular trafficking. Plants expressing a mutant PIN2 (wav6-52) do not respond to gravity, and Abas et al. showed that wav6-52 root meristems had increased abundance of PIN2, which accumulated in intracellular vesicles and the plasma membrane in unstimulated roots and did not redistribute in response to gravistimulation. Furthermore, wav6-52 roots exhibited aberrant auxin activity in the lateral root cap cells and elongation zone, which is not seen in wild-type roots. Auxin also exhibited feedback onto PIN2, with application of an auxin decreasing PIN2 abundance. Uniform pretreatment of the roots with an auxin analog followed by gravistimulation caused the internalization of PIN2 from both the upper and lower root cells. Auxin would only accumulate in the lower cells and thus may have served as a signal to reset the system once the root bent and was oriented vertically again. The constitutive cycling of PIN2 provides an opportunity for the cells of the upper root to selectively trigger degradation of PIN2, thereby decreasing its abundance and decreasing auxin flow to the upper side of the root to mediate bending in response to gravity.

L. Abas, R. Benjamins, N. Malenica, T. Paciorek, J. Wiřniewska, J. C. Moulinier-Anzola, T. Sieberer, J. Friml, C. Luschnig, Intracellular trafficking and proteolysis of the Arabidopsis auxin-efflux facilitator PIN2 are involved in root gravitropism. Nat. Cell Biol. 8, 249-256 (2006). [PubMed]