Production of γ-Aminobutyric Acid and Its Supplementary Role in the TCA Cycle in Rice (Oryza sativa L.) Seedlings

Abstract

Germination of rice (Oryza sativa L.) results in rapid accumulation of γ-aminobutyric acid (GABA), but its precise role in the process remains unclear. In this study, we analyzed GABA shunt-related amino acids [glutamate (Glu), alanine (Ala), and GABA] and tricarboxylic acid (TCA)-cycle-related organic acids (pyruvate, citrate, α-ketoglutarate, succinate, fumarate, and malate) in rice, under dark and anaerobic conditions, to investigate the role of GABA in rice germination. The change in the GABA content before and after germination was compared using 2 cultivars selected from 18 varieties of Korean rice. Correlation analysis indicated that the metabolism of GABA in germinating rice might be related to Ala metabolism. In a two-way stress study, GABA content was found to be positively related to TCA-cycle organic acid contents (α-ketoglutarate, succinate, fumarate, and malate) but show a negative relation with GABA shunt-related amino acid contents (Glu, Ala). In the dark, accumulation of TCA-cycle organic acids was observed, but Glu and Ala levels were reduced. GABA treatment (1 mM) in dark conditions produced TCA-cycle organic acids at levels similar to those in light conditions. In anaerobic conditions, TCA-cycle-related compounds accumulated at a high level, but Glu and Ala were remarkably reduced compared with normal conditions. As a result, GABA shunt is connected with TCA-cycle organic acids, but not amino acids. In conclusion, the present research suggests that GABA, accumulated in certain stress conditions, supplies organic acids such as succinate to the TCA cycle.