Mitochondria as a site of C 4 acid decarboxylation in C 4-pathway photosynthesis

Abstract

One group of C 4, species utilize a NAD-malic enzyme to decarboxylate C 4 acids. This enzyme, together with a major isoenzyme of aspartate aminotransferase and a NAD-malate dehydrogenase, is localized in the mitochondria of the bundle sheath cells and the following pathway for C 4, acid decarboxylation has been proposed: aspartate → oxaloacetate → malate → CO 2 + pyruvate. The present study reports that mitochondria isolated from the bundle sheath cells of one of these species, Atriplex spongiosa, are capable of decarboxylating C 4, acids at rates between 5 and 8 μmol/min/mg chlorophyll. For maximum decarboxylating activities, these particles required aspartate, 2-oxoglutarate and phosphate as well as malate; in the absence of any one of these compounds, activity was reduced to 0.3–0.8 μmol/min/mg chlorophyll. Rates for C 4 acid decarboxylation were much greater than the respiratory activities of these particles, including the capacity to form citrate or to oxidize malate, succinate, pyruvate or 2-oxoglutarate (0.03–0.6 μmol/min/mg chlorophyll). A comparison of mitochondria prepared from leaves of various C 4, and C 3, species showed that only the mitochondria from the bundle sheath cells of plants with high NAD-malic enzyme have capacities for rapid C 4 acid decarboxylation. The effects of a variety of experimental conditions on C 4 acid decarboxylating activities are also reported. The role of these mitochondria in C 4 photosynthesis is discussed.