Regulatory Effect of Exogenous γ-Aminobutyric Acid on Respiratory Rate through the γ-Aminobutyric Acid Shunt in Malus baccata (L.) Borkh. Roots under Suboptimal Low Root-Zone Temperature

Abstract

Malus baccata (L.) Borkh. is one of the most widely used rootstocks in the apple-producing region of Northern China. However, in the early growing season, apple roots are often subjected to suboptimal low root-zone temperatures. The regulatory effects of exogenous γ-aminobutyric acid (GABA) on both the γ-aminobutyric acid shunt (GABA shunt) and the respiratory activity of roots under suboptimal low root-zone temperatures remain unknown. To explore the physiological basis for GABA alleviation of low-temperature stress in M. baccata Borkh. roots, the following treatments were examined: suboptimal low root-zone temperature (potted parts of the seedlings were maintained at 5 ± 0.5 °C; L); suboptimal low root-zone temperature + GABA (LG); and suboptimal low root-zone temperature + vigabatrin (VGB; LV), which is a specific active inhibitor of γ-aminobutyric acid transaminase (GABA-T). Each treatment was matched with a control (18 °C/8 °C day/night; CK) for comparison. Our results showed that the L treatment reduced the root vitality, increased malondialdehyde (MDA) content, promoted the accumulation of GABA, activated the GABA shunt, and inhibited the total root respiration rate (VTotal) by decreasing the respiratory rates of Embden–Meyerhof pathway (VEMP) and tricarboxylic acid cycle (VTCAC). The LG treatment significantly increased the content of endogenous GABA, accelerated the metabolism of the GABA shunt, enhanced root respiratory activity by increasing VTotal, VEMP, VTCAC, and increased the cytochrome pathway respiratory rate (VCP), thus alleviating the damage of low root-zone temperature stress. Meanwhile, contrasting results were observed in the LV treatment. These findings revealed that exogenous GABA improved the tolerance of apple rootstocks to suboptimal low temperatures in early spring by regulating the GABA shunt and root respiratory activity.