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 enzymatic digestion of leaves of Alternanthera tenella, a C-3-C-4 intermediate species, The intercellular distribution of selected photosynthetic, photorespiratory and respiratory (mitochondrial) enzymes in these mesophyll and bundle sheath cells was studied, The activity levels of photosynthetic 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 oxidase (EC 1.1.3.1) or NADH-hydroxypyruvate reductase (EC 1.1.1.29) were 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 (EC 4.2.1.2) were 2- to 3-fold higher in bundle sheath cells, On the other hand, the activity levels of mitochondrial photorespiratory enzymes, namely glycine decarboxylase (EC 2.1.2.10) and serine hydroxymethyltransferase (EC 2.1.2.1), were 6-9-fold higher in bundle sheath cells than in mesophyll protoplasts. Such preferential localization of mitochondria enriched with the glycine-decarboxylating system in the inner bundle sheath cells would result in efficient refixation of CO2 from not only photorespiration but also dark respiration before its exit from the leaf, We propose that predominant localization of mitochondria specialized in glycine decarboxylation in bundle sheath cells may form the basis of reduced photorespiration in this C-3-C-4 intermediate species.