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 CO(2) release and thereby increase net photosynthetic CO(2) 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 alpha-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 CO(2) release and by maximal increases of about 25% in net photosynthetic CO(2) fixation. The products of (14)CO(2) 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 CO(2) assimilation in species with high rates of photorespiration.