Targeted quantitative profiling of metabolites and gene transcripts associated with 4-aminobutyrate (GABA) in apple fruit stored under multiple abiotic stresses

Abstract

4-Aminobutyrate accumulates in plants under abiotic stress. Here, targeted quantitative profiling of metabolites and transcripts was conducted to monitor glutamate- and polyamine-derived 4-aminobutyrate production and its subsequent catabolism to succinate or 4-hydroxybutyrate in apple (<i>Malus</i> x <i>domestica</i> Borkh.) fruit stored at 0 °C with 2.5 kPa O₂ and 0.03 or 5 kPa CO₂ for 16 weeks. Low-temperature-induced protein hydrolysis appeared to be responsible for the enhanced availability of amino acids during early storage, and the resulting higher glutamate level stimulated 4-aminobutyrate levels more than polyamines. Elevated CO₂ increased the levels of polyamines, as well as succinate and 4-hydroxybutyrate, during early storage, and 4-aminobutyrate and 4-hydroxybutyrate over the longer term. Expression of all of the genes likely involved in 4-aminobutyrate metabolism from glutamate/polyamines to succinate/4-hydroxybutyrate was induced in a co-ordinated manner. CO₂-regulated expression of apple <i>GLUTAMATE DECARBOXYLASE 2</i>, <i>AMINE OXIDASE 1</i>, <i>ALDEHYDE DEHYDROGENASE 10A8</i> and <i>POLYAMINE OXIDASE 2</i> was evident with longer term storage. Evidence suggested that respiratory activities were restricted by the elevated CO₂/O₂ environment, and that decreasing NAD⁺ availability and increasing NADPH and NADPH/NADP⁺, respectively, played key roles in the regulation of succinate and 4-hydroxybutyate accumulation. Together, these findings suggest that both transcriptional and biochemical mechanisms are associated with 4-aminobutyrate and 4-hydroxybutyrate metabolism in apple fruit stored under multiple abiotic stresses.