Glycolate metabolism in mesophyll cells and bundle sheath cells isolated from crabgrass, Digitaria sanguinalis (L.) Scop., leaves

Photorespiration (glycolate metabolism) has been studied in isolated mesophyll cells and bundle sheath from crabgrass leaves and in mesophyll cells from spinach leaves. In these preparations the specific activity was determined for the glycolate pathway enzymes and the amount of CO₂ evolved and O₂ taken up from glycolate and glycine fed to each cell type was measured. With crabgrass and spinach all enzymes of the glycolate pathway were found in extracts from mesophyll cells or from bundle sheath cells. However, in crabgrass, the specific activity of glycolate oxidase, catalase, glyoxylate-glutamate aminotransferase, hydroxypyruvate reductase, and 3-phosphoglycerate phosphatase was three- to fivefold higher in the bundle sheath cells than in the mesophyll cells. The glycolate pathway enzymes in crabgrass were about one-fifth as active as in spinach, except for 3-phosphoglycerate phosphatase which was higher in crabgrass. Spinach cells evolved fivefold more CO₂ than bundle sheath cells of crabgrass when fed glycolate or glycine. With crabgrass the isolated bundle sheath cells evolved CO₂ about five times faster than mesophyll cells. With crabgrass and Bermudagrass the number of peroxisome, chloroplast, and mitochondrion profiles in electron micrographs of leaf cross sections were counted and in mesophyll cells the number of peroxisomes is only about one third of that in the bundle sheath cells. Key C₄-cycle enzymes, such as phosphoenolpyruvate carboxylase, and NADPH malate dehydrogenase, were predominantly in the mesophyll cells of crabgrass. In crabgrass leaf cells the amount of CO₂ fixed in the presence of an exogenous substrate, phosphoenolpyruvate, by mesophyll cells is equal to that evolved by bundle sheath cells fed glycolate. When isolated spinach mesophyll cells or crabgrass bundle sheath cells were fed glycolate in the dark an anomalous stoichiometry of O₂ uptake to CO₂ evolution of 10:1 was measured.It is concluded that in leaves of crabgrass and other C₄ plants the apparent lack of photorespiration is due to the ability of the active C₄-cycle enzymes in the mesophyll cells to assimilate all of the CO₂ released, which may come primarily from the bundle sheath cells, plus the unique spatial arrangement inside of C₄ plant leaves in which 70–80% of the organelles and the enzymes involved in photorespiration are localized in the bundle sheath cells.