Detoxification of succinate semialdehyde in Arabidopsis glyoxylate reductase and NAD kinase mutants subjected to submergence stress

Succinate semialdehyde (SSA) is a mitochondrially generated intermediate in the metabolism of gamma-aminobutyrate (GABA), which accumulates in response to a variety of biotic and abiotic stresses. SSA can be reduced to gamma-hydroxybutyrate (GHB) in plants exposed to various abiotic stress conditions. Recent evidence indicates that distinct cytosolic and plastidial glyoxylate reductase isoforms from Arabidopsis thaliana (L.) Heynh (GLYR1 and GLYR2, respectively) catalyze the in vitro conversion of SSA to GHB, as well as glyoxylate to glycolate, via NADPH-dependent reactions. In the present study, recombinant Arabidopsis GLYR1 was demonstrated to catalyze the NADPH-dependent reduction of both glyoxylate and SSA simultaneously to glycolate and GHB, respectively. Six-hour time-course experiments with intact vegetative wild-type Arabidopisis plants subjected to submergence demonstrated that GHB accumulates in rosette leaves, and this is accompanied by increasing levels of GABA and alanine, NADH/NAD(+) and NADPH/NADP(+) ratios, and GLYR1 and GLYR2 transcript abundance. The use of GLYR (glyr1 or glyr2 knockout) and NAD kinase1 (NADK1 suppression or overexpression) mutants demonstrated that under submergence the production of GHB is mediated via both GLYR isoforms, the loss of either GLYR1 or GLYR2 activity influences redox status and the levels of GABA and alanine, and the manipulation of NADP(H) availability, specifically in the cytosol, influences the production of GHB. These results suggest that biochemical mechanisms are more important than transcriptional mechanisms in the regulation of GLYR activity and SSA detoxification in plants during the onset of submergence-induced oxygen deficiency.