Studies of Gamma-Hydroxybutyrate and Gamma-Aminobutyrate Metabolism in Apple and Arabidopsis
γ-Hydroxybutyrate (GHB) is an intermediate of γ-aminobutyrate (GABA) catabolism in plants subjected to abiotic stress and its formation is catalyzed by two NADPH-dependent glyoxylate/succinic semialdehyde reductases (GLYRs). GABA and/or GHB accumulation in ‘Honeycrisp’ and ‘Empire’ apple fruit stored under controlled atmosphere (CA) conditions (i.e., low temperature, low O2, elevated CO2) in the presence or absence of the ethylene-antagonist 1-methylcyclopropene, coincided with the onset of physiological injury, suggesting an association with cellular disruption. Salinity and chilling stresses differentially influenced the expression of GABA pathway genes and the levels of GHB among various GABA pathway mutants of Arabidopsis. Furthermore, the occurrence of GHB in glyr1/glyr2 double knockout mutants indicates the presence of an additional pathway for GHB production. Evidence for GHB oxidation was not detectable in cell-free leaf extracts, suggesting the existence of a novel enzyme for GHB turnover.