The cellular levels of methylglyoxal (MG), a toxic byproduct of glycolysis, rise under various abiotic stresses in plants. Detoxification of MG is primarily through the glyoxalase pathway. The first enzyme of the pathway, glyoxalase I (GLYI), is a cytosolic metalloenzyme requiring either Ni2+ or Zn2+ for its activity. Plants possess multiple GLYI genes, of which only some have been partially characterized; hence, the precise molecular mechanism, subcellular localization and physiological relevance of these diverse isoforms remain enigmatic. Here, we report the biochemical properties and physiological role of a putative chloroplast-localized GLYI enzyme, OsGLYI-8, from rice, which is strikingly different from all hitherto studied GLYI enzymes in terms of its intracellular localization, metal dependency and kinetics. In contrast to its predicted localization, OsGLYI-8 was found to localize in the nucleus along with its substrate, MG. Further, OsGLYI-8 does not show a strict requirement for metal ions for its activity, is functional as a dimer and exhibits unusual biphasic steady-state kinetics with a low-affinity and a high-affinity substrate-binding component. Loss ofAtGLYI-2, the closest Arabidopsis ortholog ofOsGLYI-8, results in severe germination defects in the presence of MG and growth retardation under salinity stress conditions. These defects were rescued upon complementation withAtGLYI-2orOsGLYI-8. Our findings thus provide evidence for the presence of a GLYI enzyme and MG detoxification in the nucleus.Significance Statement
Methylglyoxal, a toxic byproduct of glycolysis, increases under abiotic stress and is detoxified primarily by glyoxalases. Previously studied glyoxalase I (GLYI) enzymes are cytoplasmic metalloproteins. Here, we demonstrate a nucleus-localized rice glyoxalase I, OsGLYI-8, that detoxifies methylglyoxal in a metal-independent but Zn2+/Mn2+-stimulated manner. As Arabidopsis mutant of its homolog exhibits severe growth retardation in the presence of methylglyoxal or salinity stress, we suggest that nuclear detoxification of methylglyoxal might protect DNA from damage, especially under stress conditions.