Light-induced formation of enzymes of the C4-dicarboxylic acid pathway of photosynthesis in detached leaves

Abstract

A comparison was made of the light-induced formation of enzymes of photosynthetic CO2-fixation in two plants, maize and sorghum, with the C4-dicarboxylic acid pathway and two, wheat and oats, with the Calvin cycle. Enzymes of both pathways were examined in the four species. Illumination of detached, etiolated leaves of these plants resulted in increases of chlorophyll and Calvin cycle enzymes, but not of respiratory enzymes. In maize and sorghum, enzymes of the C4-dicarboxylic pathway, pyruvate, Pi dikinase, phosphopyruvate carboxylase and adenylate kinase also increased, but light had little effect on the activity of phosphopyruvate carboxylase in wheat and oats. When etiolated leaves of maize were detached and illuminated, there was parallel development of enzymes unique to the C4-dicarboxylic acid pathway, Calvin cycle enzymes common to both pathways and also ribulose-1,5-diphosphate carboxylase. The light-induced increases in activity of enzymes of both the C4-dicarboxylic acid pathway and the Calvin cycle were inhibited by chloramphenicol. This was consistent with the hypothesis that the increases reflected synthesis of new enzyme on the 70S chloroplast ribosomes. The concentration of chloramphenicol required for inhibition varied with the different plants, wheat being the most sensitive and maize the least sensitive. Cycloheximide at concentrations of around 1 μg/ml also inhibited. This suggested that chloroplast development in leaves involves a close integration with cytoplasmic protein synthesis. Continuous light was not essential for synthesis of enzymes of the C4-dicarboxylic acid pathway. Irradiation of dark-grown maize with red light (660 nm) of low intensity for 3 min daily on five successive days increased the activity of pyruvate, Pi dikinase by 10·4-fold and phosphopyruvate carboxylase by 7·5-fold compared with non-irradiated plants.