PREDOMINANT LOCALIZATION OF MITOCHONDRIA ENRICHED WITH GLYCINE-DECARBOXYLATING ENZYMES IN BUNDLE-SHEATH CELLS OF ALTERNANTHERA-TENELLA, A C-3-C-4 INTERMEDIATE SPECIES

Mesophyll protoplasts and bundle sheath cells were prepared by enzymat ic digestion of leaves of Alternanthera tenella, a C-3-C-4 intermediat e species, The intercellular distribution of selected photosynthetic, photorespiratory and respiratory (mitochondrial) enzymes in these meso phyll and bundle sheath cells was studied, The activity levels of phot osynthetic enzymes such as PEP carboxylase (EC 4.1.1.31) or NAD-malic enzyme (EC 1.1.1.39) and photorespiratory enzymes such as glycolate ox idase (EC 1.1.3.1) or NADH-hydroxypyruvate reductase (EC 1.1.1.29) wer e similar in the two cell types, The activity levels of mitochondrial TCA cycle enzymes such as citrate synthase (EC 4.1.3.7) or fumarase (E C 4.2.1.2) were 2- to 3-fold higher in bundle sheath cells, On the oth er hand, the activity levels of mitochondrial photorespiratory enzymes , namely glycine decarboxylase (EC 2.1.2.10) and serine hydroxymethylt ransferase (EC 2.1.2.1), were 6-9-fold higher in bundle sheath cells t han in mesophyll protoplasts. Such preferential localization of mitoch ondria enriched with the glycine-decarboxylating system in the inner b undle sheath cells would result in efficient refixation of CO2 from no t only photorespiration but also dark respiration before its exit from the leaf, We propose that predominant localization of mitochondria sp ecialized in glycine decarboxylation in bundle sheath cells may form t he basis of reduced photorespiration in this C-3-C-4 intermediate spec ies.