Endogenous Photophosphorylation by Mesophyll and Bundle Sheath Chloroplasts from Setaria italica and Amaranthus paniculatus1

Abstract

The basic features of the C4 dicarboxylic acid pathway of photosynthesis are the fixation of carbon dioxide into C4 acids (namely oxalacetate, malate and aspartate) in the mesophyll cells and their transport into the bundle sheath cells where they are decarboxylated. Thus released carbon dioxide is refixed and further metabolized via the Calvin cycle in the sheath cells (Hatch and Slack, 1970; Black, 1973). Such two-step carboxylation is calculated to result in an energy cost of 5 ATP and 2 NADPH2 per molecule of C02 fixed, compared to that of 3 ATP and 2 NADPH2 per each C02 molecule reduced through Calvin cycle alone (Chen, Brown and Black, 1969; Hatch and Slack, 1970). Chen et al. (1969) suggested that the extra requirement of ATP in C4 plants is met with the capacities of C4 plant-chloroplasts to catalyze higher rates of cyclic photophosphorylation. Bundle sheath chloroplasts of C4 plants, producing malate as the principal C4 acid, e.g., maize and sorghum, are deficient in photosystem (PS) II compared to PS I (Anderson, Woo and Boardman, 19716; Downton, 1971 ; Mayne, Edwards and Black, 1971). Such PS II deficiency has important consequences for Calvin cycle metabolism in the bundle sheath cells of these plants, since all the carbon fixed during photosynthesis eventually has to be reduced through the Calvin cycle. In such instances, it is proposed that for normal C02 reduction, NADPH2 is provided by malic enzyme (Slack, Hatch and Goodchild, 1969; Downton, 1971) and ATP is supplied through cyclic photophosphorylation (Chen et al ., 1969 ; Slack et al ., 1969). These complications do not arise with other C4 plants which use aspartate as the major C4 acid (e.g. Amaranthus and Atriplex ) where bundle sheath chloroplasts possess an active PS II as well as PS I. Though the studies on mesophyll and bundle sheath chloroplasts showed high cyclic photophosphorylation (mediated through phenazine methosulphate) capacities of bundle sheath chloroplast fragments (Anderson, Boardman and Spencer, 1971a; Polya and Osmond, 1972), convincing evidence for endogenous cyclic photophosphorylation is still not available. Here we report the presence of an active endogenous photophosphorylation of bundle sheath chloroplasts insensitive to 3(3,4-dichlorophenyl)-l,l-dimethyl urea (DCMU), evidently mediated through PS I. Plants of Setaria italica Beauv. var. H-l and Amaranthus paniculatus L. were raised in seed pans under an approximate 12-h photoperiod (35 °C day and 20 °C night). The second to fourth leaves starting from the oldest leaf were picked from 3 to 4 week old seedlings. The leaves were chilled to 0 °C and cut into pieces of c. 1 cm2. Mesophyll and bundle sheath chloroplasts were prepared by differential grinding of leaf tissue (Raghavendra, 1975;