Summary: The glyoxalase system constitutes the major pathway for the detoxification of metabolically produced cytotoxin methylglyoxal (MG) into a non‐toxic metabolite d‐lactate. Glyoxalase I (GLY I) is an evolutionarily conserved metalloenzyme requiring divalent metal ions for its activity: Zn²⁺ in the case of eukaryotes or Ni²⁺ for enzymes of prokaryotic origin. Plant GLY I proteins are part of a multimember family; however, not much is known about their physiological function, structure and metal dependency. In this study, we report a unique GLY I (OsGLYI‐11.2) from Oryza sativa (rice) that requires Ni²⁺ for its activity. Its biochemical, structural and functional characterization revealed it to be a monomeric enzyme, possessing a single Ni²⁺ coordination site despite containing two GLY I domains. The requirement of Ni²⁺ as a cofactor by an enzyme involved in cellular detoxification suggests an essential role for this otherwise toxic heavy metal in the stress response. Intriguingly, the expression of OsGLYI‐11.2 was found to be highly substrate inducible, suggesting an important mode of regulation for its cellular levels. Heterologous expression of OsGLYI‐11.2 in Escherichia coli and model plant Nicotiana tabacum (tobacco) resulted in improved adaptation to various abiotic stresses caused by increased scavenging of MG, lower Na⁺/K⁺ ratio and maintenance of reduced glutathione levels. Together, our results suggest interesting links between MG cellular levels, its detoxification by GLY I, and Ni²⁺ – the heavy metal cofactor of OsGLYI‐11.2, in relation to stress response and adaptation in plants.

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