The anion of the amino acid glutamic acid. It functions as a neurotransmitter at excitatory synapses in the vertebrate central nervous system and at excitatory neuromuscular junctions in insects and crustaceans. In plants, glutamate is the initial acceptor molecule in the assimilation of the ammonium ion (NH4+), combining with it to form glutamine in a reaction involving ATP. See also glutamate receptor.