Differentiation of Photorespiratory Activity between Mesophyll and Bundle Sheath Cells of C4 Plants I. Glycine Oxidation by Mitochondria

Abstract

Mitochondria were isolated from mesophyll protoplasts and bundle sheath protoplasts or strands which were obtained by enzymatic digestion of six C4 species: Zea mays, Sorghum bicolor, Panicum miliaceum, Panicum capillare, Panicum maximum and Chloris gayana, representative of three C4 types. Photorespiratory glycine oxidation and related enzyme activities of mesophyll and bundle sheath mitochondria were compared. Mesophyll mitochondria showed good P/O ratios with malate and succinate as substrate but lacked the ability to oxidize glycine. On the other hand, mitochondria isolated from bundle sheath protoplasts of P. miliaceum and bundle sheath strands of Z. mays possessed glycine oxidation activity similar to that of mitochondria from C3 plant leaves. The two enzymes involved in glycine metabolism in mitochondria, serine hydroxymethyltransferase and glycine decarboxylase, were also assayed in the mitochondria of the two cell types. The activities of the two enzymes in bundle sheath mitochondria were in the range found in C3 mitochondria. In contrast, the activities in mesophyll mitochondria were either not detectable or far lower than those in bundle sheath mitochondria and ascribed to contaminating bundle sheath mitochondria. The present results indicate the deficiency of a complete glycine oxidation system in mesophyll mitochondria and also a differentiation between mesophyll and bundle sheath cells of C4 plants with respect to the photorespiratory activities of the mitochondria.