Metabolic Regulation of Glycolate Synthesis, Photorespiration, and Net Photosynthesis in Tobacco by L-Glutamate

Abstract

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. The glutamate inhibition of glycolate synthesis was accompanied by a marked decrease in the rate of photorespiratory CO₂ release and by maximal increases of about 25% in net photosynthetic CO₂ fixation. The products of ¹⁴CO₂ fixation in leaf discs previously treated with glutamate showed a decrease in glycine (26%), serine (12%), and the stronger acids (18%), and an increase in the neutral compounds (26%) in comparison with discs floated only on water. Data are presented which question whether a catabolite of glutamate or the amino acid itself is responsible for the results observed. These experiments support the view that a genetic selection strategy based on the metabolic control of photorespiration would result in large increases in net photosynthetic CO₂ assimilation in species with high rates of photorespiration.