All aerobic organisms require O2 for survival. When their O2 is limited (hypoxia), a response is required to reduce demand and/or improve supply. A hypoxic response mechanism has been identified in flowering plants: the stability of certain proteins with N-terminal cysteine residues is regulated in an O2-dependent manner by the Cys/Arg branch of the N-degron pathway. These include the Group VII ethylene response factors (ERF-VIIs), which can initiate adaptive responses to hypoxia. Oxidation of their N-terminal cysteine residues is catalyzed by plant cysteine oxidases (PCOs), destabilizing these proteins in normoxia; PCO inactivity in hypoxia results in their stabilization. Biochemically, the PCOs are sensitive to O2 availability and can therefore act as plant O2 sensors. It is not known whether oxygen-sensing mechanisms exist in other phyla from the plant kingdom. Known PCO targets are only conserved in flowering plants, however PCO-like sequences appear to be conserved in all plant species. We sought to determine whether PCO-like enzymes from the liverwort, Marchantia polymorpha (MpPCO), and the freshwater algae, Klebsormidium nitens (KnPCO), have a similar function as PCO enzymes from Arabidopsis thaliana. We report that MpPCO and KnPCO show O2-sensitive N-terminal cysteine dioxygenase activity toward known AtPCO ERF-VII substrates as well as a putative endogenous substrate, MpERF-like, which was identified by homology to the Arabidopsis ERF-VIIs transcription factors. This work confirms functional and O2-dependent PCOs from Bryophyta and Charophyta, indicating the potential for PCO-mediated O2-sensing pathways in these organisms and suggesting PCO O2-sensing function could be important throughout the plant kingdom.
