Type one protein phosphatases (TOPPs) catalyze EIN2 dephosphorylation to regulate ethylene signaling in <i>Arabidopsis</i>.

Abstract

Type one protein phosphatases (TOPPs) widely modulate phytohormone signaling and stress responses, but their roles in ethylene signaling remain unknown. This study reveals a reciprocal regulatory relationship between TOPPs and ethylene insensitive 2 (EIN2)-mediated ethylene signaling. We identified that ethylene can induce <i>TOPPs</i>' expression, and <i>topp1/4/5</i> mutants exhibited partial ethylene insensitivity with reduced EIN3 protein. Mechanistically, TOPPs function upstream of EIN2 and interact with its carboxyl-terminal domain (CEND) to dephosphorylate the S655 residue. This site-specific dephosphorylation promotes EIN2 stability and EIN2 CEND nuclear accumulation, thereby activating ethylene responses. Notably, <i>EIN2^S655A-YFP/ein2-5</i> plants displayed constitutive ethylene responses and improved salt tolerance. Further investigation showed that EIN3/EIN3 like 1 (EIL1) activates <i>TOPPs</i>' expression by binding to their promoters, amplifying ethylene signaling accordingly. Together, our finding establishes TOPPs as key regulators in ethylene signaling and reveal a dephosphorylation switch mechanism governing EIN2 function, providing critical insight into how EIN2 posttranslational modifications mediate plant stress adaptation.