Glutamate decarboxylase.

Glutamate decarboxylase (GAD) catalyses the decarboxylation of glutamate to produce γ-aminobutyric acid (GABA). In plants, its activity is modulated by pH and calcium. The regulation of GAD activity by calcium is through a calmodulin binding domain that seems to be specific for the plant kingdom, having resulted from an ancient event during evolution. Plant GAD is usually encoded by several genes and these show different expression patterns, including tissue specificity and response to stress, which suggest that there are specialization phenomena within this gene family. Although the role of GABA as a neurotransmitter is well known in animals, several roles for GABA and its metabolism have been proposed in plants over the last few decades. The roles for GABA in plants include acting as a biochemical pH-stat, in temporary nitrogen storage, as a compatible osmolyte, in defence against biotic stress, in the control of reactive oxygen species (ROS) and in the metabolism of photorespiratory intermediates. In addition, GABA is considered to be a signal controlling the expression of regulatory genes. However, the available information supporting the involvement of GAD and/or GABA in many processes is very limited, as is the contribution of polyamine metabolism to the cellular levels of GABA. Studies on the regulation of GABA levels are required for a better understanding of the involvement of GAD and GABA in plant development.