Does the GABA Shunt Regulate Cytosolic GABA?

The GABA shunt has long been known as a metabolic pathway that produces GABA in, and removes GABA from, the cytosol. There is no consensus regarding its function. The hypothesis presented here is that the GABA shunt regulates cytosolic GABA levels and GABA signaling.

Plant GABA: Not Just a Metabolite.

γ-Aminobutyric acid (GABA) accumulates rapidly when plants are exposed to stress. Whether GABA accumulation represents the regulation of metabolism in response to stress or an adaptive response to mitigate stress is unknown. Genetic manipulation of GABA levels has revealed that GABA accumulation functions in defense against drought and insect herbivory.

Gamma-aminobutyrate: defense against invertebrate pests?

Gamma-aminobutyrate (GABA) is a ubiquitous four-carbon, non-protein amino acid. In plants, stress-induced GABA accumulation is well documented. However, the role(s) of GABA accumulation is contentious. In this Opinion article, we argue that wounding due to herbivory and crawling by insect larvae causes rapid GABA accumulation via the disruption of cellular compartmentation and the release of the acidic vacuolar contents to the cytosol. The activity of glutamate decarboxylase, the cytosolic enzyme responsible for GABA synthesis, has an acidic pH optimum. Subsequent GABA ingestion has a plant defense function by directly acting on GABA-regulated invertebrate neuromuscular junctions. Plants with an enhanced GABA-producing capacity reduce herbivory by invertebrate pests. These findings suggest that GABA accumulation is a rapidly deployed, local resistance mechanism that constitutes a first line of defense in deterring herbivory.

Overexpression of glutamate decarboxylase in transgenic tobacco plants deters feeding by phytophagous insect larvae.

Gamma-aminobutyrate (GABA) is a ubiquitous four-carbon, non-protein amino acid synthesized by glutamate decarboxylase. Previous research suggests that the endogenous synthesis of GABA, a naturally occurring inhibitory neurotransmitter at neuromuscular junctions, serves as a plant resistance mechanism against invertebrate pests. In this study, two homozygous transgenic tobacco lines constitutively overexpressing a single copy of a full-length chimeric glutamate decarboxylase cDNA and possessing enhanced capacity for GABA accumulation (GAD plants), a homozygous transgenic line lacking the gene insert, and wild-type tobacco were employed. Tobacco budworm larvae were presented with plant attached wild type and transgenic leaves for 4 hr in a feeding preference study. Larvae consumed six to twelve times more leaf tissue from wild-type plants than from GAD plants. These results suggest that leaf GABA accumulation, which is known to occur in response to insect larval walking and feeding, represents a rapidly deployed local resistance mechanism.