POLARIS is a copper-binding peptide that interacts with ETR1 to negatively regulate ethylene signaling in Arabidopsis.

Abstract

Ethylene signalling represents one of the classic hormonal pathways in plants, with diverse roles in development and stress responses. The dimeric ethylene receptor localises to the endoplasmic reticulum (ER) and contains Cu(I) ions essential for ethylene binding and signal transduction. We previously discovered that mutants in the Arabidopsis gene POLARIS (PLS), encoding a 36 amino acid peptide, exhibit enhanced ethylene signalling responses, suggestive of reduced receptor activity, but the role and activity of the peptide in this signalling cascade has not been defined. Here we report PLS binds copper as a 1:2 thiol-dependent Cu(I):PLS₂ complex, with an affinity of 3.79 (± 1.5) x10¹⁹ M^-2, via two cysteine residues also found in the related species Camelina sativa. These residues are also essential for biological function. This affinity precludes PLS as a cytosolic Cu chaperone. We demonstrate that PLS localises to endomembranes and interacts with the transmembrane domain of receptor protein ETR1. PLS-ETR1 binding is increased in the presence of copper, and this interaction provides a Cu-dependent mechanism for mediating a repression of ethylene responses. PLS transcription is up-regulated by auxin and down-regulated by ethylene, and so PLS-ETR1 interactions also provide a mechanism to modulate ethylene responses in high auxin tissues.