The plant-specific protein IQD22 interacts with calcium sensors to activate anaerobic respiration during hypoxia in Arabidopsis.

Abstract

Louis Pasteur first reported that living cells switch from aerobic to anaerobic metabolism under low-oxygen conditions, but the underlying regulatory mechanism remains to be fully elucidated. ALCOHOL DEHYDROGENASE 1 (ADH1) encodes a key enzyme in ethanolic fermentation and is upregulated under hypoxia. Here, we searched for Arabidopsis thaliana mutants with defects in hypoxia-induced ADH1 expression. This screen identified a mutant in IQ DOMAIN containing protein 22 (IQD22). The iqd22 mutants were hypersensitive to submergence and hypoxic stress, whereas IQD22 overexpressors were more tolerant of both compared to wild type. Under hypoxia, IQD22 modulated the interaction of the calcium-dependent protein kinase CPK12 with the ERF-VII-type transcription factor RELATED TO AP2.12 (RAP2.12) to upregulate hypoxia-responsive genes, including ADH1. Moreover, IQD22 interacted with calmodulins (CaMs) in vivo and facilitated their association with ADH1, stimulating its abundance, in response to hypoxia. Metabolic profiling of the iqd22-2 mutant revealed that hypoxia caused significant increases of glycolytic metabolites, but significantly lower ethanol accumulation compared to the wild type. Furthermore, deleting ADH1 suppressed the improved hypoxia-tolerance phenotype of IQD22 overexpressors. Our findings thus demonstrate that IQD22 functions in the CaM-ADH1 and CPK12-RAP2.12 regulatory modules, which coordinately mediate calcium-dependent activation of anaerobic respiration to control metabolic flux during hypoxia.