Figure 2. The GABA shunt and its relationship with other metabolic pathways.

The GABA shunt metabolites and enzymes are indicated as blue and orange balloons respectively, and the TCA cycle is shown as black and pink balloons. Glutamate, the precursor to GABA, is supplied predominantly via the glutamine synthetase/glutamate synthase cycle or the TCA cycle. The polyamine putrescine has been proposed as an alternative GABA precursor, although in planta contribution in mature leaves is likely to be minor. There are two branching routes for GABA catabolism via SSA: one to succinate and the TCA cycle and the other to GHB via SSA reductase (a yellow balloon, shown as GLYR in Figure 3), although the products of GHB catabolism in plants are currently unknown.