AMMONIUM ION SALVAGE BY GLUTAMATE-DEHYDROGENASE DURING DEFENSE RESPONSE IN MAIZE

When phytochemical defence response was elicited by treatment of maize with chitosan, the binomial isoenzymes of L-glutamate dehydrogenase ( GDH, EC 1.4. 1.2) isomerized. The amination electrode potential of the GDH charge isomers decreased by 10 mV in the chitosan-treated maize. GDH isomerization and amination are therefore spontaneous reactions. T he free L-Glu and L-Gln were 3.84 and 3.68 mu Lmol, respectively, per 10 g of the chitosan-treated maize. The amination activities of GDH sh owed that the 10 mV decrease in electrode potential accounted fully fo r the 3.84 mu mol free L-Glu content of the chitosan-treated maize. Th erefore the binomial GDH synthesized all the L-Glu, thereby salvaging at least 51% of the NH4+ liberated during phytochemical defence respon se, while glutamine synthetase salvaged the remaining NH4+. The chitos an-elicited isomerization of maize GDH and the accompanying 10 mV decr ease in electrode potential widened the plant-pathogen amination energ y barrier, thereby minimizing the amount of NH4+ the pathogen can abso rb from the plant.