Hypoxia response protein HRM1 modulates the activity of mitochondrial electron transport chain in Arabidopsis under hypoxic stress.

We studied Arabidopsis HYPOXIA-RESPONSIVE MODULATOR 1 (HRM1), which belongs to a group of core hypoxia-responsive genes that are conserved among plant species across great evolutionary distance. The hrm1 mutants had lower survival rates and showed more damage than the wild-type (WT) plants under hypoxic stress. Promoter analyses showed that HRM1 is regulated by EIN3 and RAP2.2 during hypoxia. Fluorescence tracing and immunogold labeling assays showed that HRM1 protein was enriched in mitochondria. Co-immunoprecipitation coupled with mass spectrometry and bimolecular fluorescence complementation assays showed that HRM1 associates with the complex-I in mitochondria. Compared with the WT plants, metabolic activities related to the mitochondrial electron transport chain (mETC) were higher in hrm1 mutants during hypoxia. Loss of HRM1 caused de-repression of mETC complex I, II, and IV activities and higher basal and maximum respiration rates under hypoxia. Our results showed that through association with complex-I, HRM1 attenuates mETC activity and modulates the respiratory chain under low oxygen. Compared with the regulatory system in mammalian, adjustment of mitochondrial respiration to low oxygen helps plants decrease reactive oxygen species production and is also critical for the submergence survival.