- y-Aminobutyric acid (GABA) is a four-carbon non-protein amino acid conserved from bacteria to plants and vertebrates. In the latter it is mainly known as a neurotransmitter. The enzymes that synthesize and catabolize GABA constitute a metabolic pathway known as the GABA shunt which bypasses two steps of the tricarboxylic acid cycle. Functional genomics tools using Arabidopsis as a model system revealed that the GABA shunt is imperative for normal plant development and for response to stress, and suggest roles for GABA as an important metabolite as well as a potential signaling molecule. Moreover, y-hydroxybutyrate, a by-product of the GABA shunt and a neurotransmitter in animals, was recently discovered in plants. Here we discuss the possible roles of these two neurotransmitters in plants with focus on components that underlie their roles as signaling molecules.