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Glutamate is a major amino donor for the synthesis of amino acids and other nitrogen-containing compounds in plants. Glutamate is converted into ?-Aminobutyric acid (GABA) via glutamate decarboxylase (GAD). Enzymatic activity of GAD may be activated by Ca2+/calmodulin binding at the C-terminus in response to various stresses (Shelp et al., 1999; Bouché and Fromm, 2004). GABA is a non-protein amino acid present in all living things (Bown and Shelp, 1997; Akama et al., 2001). GABA in turn is converted to succinic semialdehyde (SSA) by GABA-transaminase (GABA-T). SSA is catabolized via SSA dehydrogenase (SSADH) to form succinate, which enters the TCA cycle. The route from glutamate to succinate is known as the GABA shunt. Glutamate is known to modulate calcium signaling (Dannison et al., 2000), root system architecture ( Forde & Walch-Liu, 2009), inhibits primary root growth and stimulates the outgrowth of lateral roots near the primary root tip in Arabidopsis (Walch-Liu et al., 2006). This phenomenon is specific to glutamate but other structurally or metabolically related amino acids e.g., Asp, and Gln do not have similar effects (Forde & Walch-Liu, 2009).
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