Type one protein phosphatases (TOPPs) catalyze EIN2 dephosphorylation to regulate ethylene signaling in Arabidopsis

Abstract

TOPPs (Type one protein phosphatases) widely modulate plant hormone signaling and stress responses in plants, but their roles in ethylene signal transduction remain unknown. This study reveals a reciprocal regulatory relationship between TOPPs and EIN2 (Ethylene insensitive 2)-mediated ethylene signaling. We identified that ethylene can induce TOPPs expression, and topp1/4/5 triple mutants exhibited partial ethylene insensitivity with reduced EIN3 protein levels and ERF1 (Ethylene response factor 1) expression. Mechanistically, TOPPs genetically act upstream of EIN2, physically interacting with its carboxyl-terminal domain (EIN2-C) to dephosphorylate the critical S655 residue. This site-specific dephosphorylation promotes EIN2 stability and EIN2-C nuclear accumulation, thereby activating ethylene responses. Notably, transgenic plants expressing phosphorylation-deficient EIN2S655A displayed constitutive ethylene responses and improved salt tolerance. Further investigation showed that EIN3/EIL1 (EIN3 like 1) transcriptionally activates TOPPs expression by binding to their promoter, amplifying ethylene signaling accordingly. Together, our finding establishes TOPPs play an important regulatory role in ethylene signal transduction and elucidate a dephosphorylation-switch mechanism in controlling EIN2 function, providing critical insight into the role of EIN2 post-translational modifications in plant stress adaptation.