Contribution of Putrescine and Glutamic Acid on γ-Aminobutyric Acid Accumulation of Malus baccata Borkh. Roots under Suboptimal Low Root-Zone Temperature

Abstract

GABA (γ-aminobutyric acid) is found in plants and accumulates rapidly under stresses. However, the contributions of glutamic acid and a (Glu)-derived pathway and polyamines (PAs) catabolism pathway on GABA accumulation and the regulatory effects of exogenous putrescine (Put) on a GABA shunt under suboptimal low root-zone temperatures remain unknown. Our results showed that suboptimal low root-zone temperatures (treatment L) significantly increased GABA contents and GABA transaminase (GABA-T) activities. The contribution rate of the PAs catabolism pathway increased from 20.60% to 43.31%. Treatment L induced oxidative stress in Malus baccata Borkh. roots. Exogenous Put increased the contents of endogenous Put, spermine (Spm), and spermidine (Spd), promoted the transformation of PAs, increased the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), and decreased the contents of hydrogen peroxide (H2O2), superoxide anion (O2∙−), and malondialdehyde (MDA). Meanwhile, contrasting results were observed after aminoguanidine (AG, an inhibitor of diamine oxidase) application. These findings revealed that the Glu-derived pathway is the main route of GABA synthesis. The contribution rate of the Pas catabolism pathway increased gradually with the extension of treatment time, and the treatment of exogenous Put significantly improved the tolerance of Malus baccata Borkh. Roots to suboptimal low temperature by regulating the transformation of Pas, GABA shunt, and the antioxidant system.