Title : Metabolism and ROS dynamics are differentially regulated by aminobutyric acid in a concentration dependent manner to induce immunity against B. cinerea in Arabidopsis.
The non-proteinogenic amino acid γ-aminobutyric acid (GABA) rapidly accumulates in plant tissue in response to several biotic and abiotic stresses and has been associated with increased resistance. Besides its role in modulating the flux of carbon and energy in the TCA cycle through the GABA shunt, it is also proposed to be an effective signaling entity between cells. Previous data suggested that exogenous GABA application affected seedling development and that the expression of several genes involved in carbon, nitrogen and reactive oxygen species (ROS) metabolism were affected in a concentration dependent manner. This prompted us to investigate whether exogenous GABA application and subsequent infection with B. cinerea affected soluble sugar levels and nitrogen metabolism and their interplay with ROS dynamics. We found that exogenous GABA induced long-term resistance against the necrotrophic fungus B. cinerea in a concentration dependent manner. In contrast, lower concentrations of GABA reduced the levels of both glucose and total soluble sugars more severely compared to higher concentrations. Similarly, GABA negatively regulated both the total, and active nitrate reductase activities, and that lower concentrations affected the active form more severely. The level of H2O2 in plants were also reduced after GABA treatment in a concentration dependent manner by inducing the activities of both catalase and peroxidase, but not superoxide dismutase. In contrast, we showed that the bacterial elicitor flg22 induced ROS burst through NADPH oxidases were negatively affected in plants treated with low concentrations of GABA, and unaffected in those treated at higher concentrations. The level of MPK3 remained unaltered after treatment. Our data support the hypothesis that GABA differentially regulates metabolism and stress responses in a spatial and concentration dependent manner. At low concentrations, GABA most likely act as signal entity only, and at high concentrations the signaling role is most probably overshadowed by GABA metabolism through the GABA shunt. We propose that cells perceive low concentrations of extracellular GABA exported/produced by surrounding stressed cells, and subsequently reshuffle their metabolism and ROS dynamics towards stress responses.