Synthesis of glutamine and glutamate by intact bundle-sheath cells of maize (Zea mays L.).

Intact bundle-sheath cells with functional plasmodesmata were isolated from leaves of Zea mays L. cv. Mutin, and the capacity of these cells to synthesize glutamine and glutamate was determined by simulating physiological substrate concentrations in the bathing medium. The results show that glutamine synthetase can operate at full rate in the presence of added 8 mM ATP. At lower concentrations of ATP a higher rate of glutamine synthesis was found in the light than in darkness. Glutamate-synthase activity, on the other hand, was strictly light dependent. It appears that in bundle-sheath cells of maize the nitrate-assimilatory capacities of glutamine synthetase (located mainly in the cytosol) and of glutamate synthase (located in the stroma) are high enough to meet the demands of whole maize leaves.