GAD1 and infection: On the contrary, the GABA shunt pathway was impaired suggesting that succinate is most likely synthesized through the TCA/glyoxylate shunt, but not from GABA in P. viticola. This non-proteinogenic amino acid is synthesized from glutamate by the action of the glutamic acid decarboxylase (GAD) enzyme that is strongly expressed in P. viticola during the course of infection.