Metabolic regulation of glycolate synthesis, photorespiration, and net photosynthesis in tobacco by L-glutamate

Experiments were undertaken to identify and characterize control mechanisms in tobacco leaf tissue which decrease the relative contribution of photorespiratory CO₂ release and thereby increase net photosynthetic CO₂ fixation. A number of metabolites were supplied to illuminated leaf discs and their effect on the inhibition of glycolate synthesis was measured. Glycolate accumulation, in the presence of α-hydroxy-2-pyridinemethanesulfonic acid, was inhibited in leaf discs previously floated on 30 mM solutions of either L-glutamate, L-aspartate, phospho-enolpyruvate, or glyoxylate. The effect of glutamate on glycolate synthesis, which was investigated in detail, was concentration- and time-dependent. Glycolate synthesis was inhibited about 40% by treating leaf discs with 30 mM glutamate, and the inhibition continued for more than 4 hours after the glutamate solution was removed.