Phosphate Translocator of Mesophyll and Bundle Sheath Chloroplasts of a C₄ Plant, Panicum miliaceum L.

Abstract

The phosphate translocator was identified in the envelope membranes of both mesophyll and bundle sheath chloroplasts of Panicum miliaceum L. by labeling with [1,2-3H]1,2-(2,2'-disulfo-4,4'-diisothiocyano)diphenylethane ([3H]H2DIDS) and by using SDS-PAGE. Assay of 32Pi uptake by the chloroplasts showed that the phosphate translocators of both types of chloroplasts have a higher affinity for phosphoenolpyruvate than the C3 counterpart and can be regarded as C4 types.C4 plants possess two types of photosynthetic cells, namely, mesophyll and bundle sheath cells. Prerequisite for the operation of C4 photosynthesis is extensive flow of C4 dicarboxylic and C3 acids and triose-P not only intercellularly but also intracellularly (6, 14). Thus, metabolite flow across the envelope of C4 chloroplasts should be different from that of C3 chloroplasts for example, import of pyruvate and export of PEP3 in MC, and export of 3-PGA and import of triose-P in BC, especially in NADP+-malic enzyme type plants such as maize due to the limited availability of reducing power in BC.Experimental studies in MC support some of the expected differentiations in transport systems: existence of a novel translocator in the envelope membrane catalyzing light-dependent pyruvate uptake (12,20) and transport of PEP by PT (4,17,22,23). The PT in the envelope of C3 chloroplasts catalyzing a one to one exchange of Pi, triose-P, and 3-PGA, is now well characterized (7, 10, 11), and its primary structure has been recently determined (8). As stated above, the PT of C4 MC has an additional capacity for PEP and thus can be regarded as a C4 type. ' Supported by grants from the Deutsche Forschungsgemeinschaft.

The role of inorganic phosphate in the regulation of C4 photosynthesis

“…In contrast to the Pi translocator of C 3 chloroplasts, the Pi translocator from C4 mesophyll chloroplasts exhibits a high capacity to transport PEP (in addition to Pi, DHAP and 3-PGA) and may thus be classified as a specific C 4 type (Heldt et al 1991). In C 4 plants the mesophyll Pi translocator has a higher affinity for Pi than that of the bundle sheath (Ohnishi et al 1989). Moreover, light increases the Wma x for Pi transport and decreases the affinity for 3-PGA and D H A P (affinity for PEP is not affected) in the mesophyllic but not in the bundle sheath translocator (Ohnishi et al 1989).…”

“…In C 4 plants the mesophyll Pi translocator has a higher affinity for Pi than that of the bundle sheath (Ohnishi et al 1989). Moreover, light increases the Wma x for Pi transport and decreases the affinity for 3-PGA and D H A P (affinity for PEP is not affected) in the mesophyllic but not in the bundle sheath translocator (Ohnishi et al 1989). These properties allow a PEP/Pi exchange in the illuminated mesophyU cells, despite the high 3-PGA and D H A P concentrations that exist under such conditions (Edwards and Walker 1983).…”

“…Pi could also have a competitive effect on the transport of triose-phosphate and 3-PGA. The relatively low affinity for Pi exhibited by the Pi translocator of bundle sheath chloroplasts (Ohnishi et al 1989) suggests that in these cells the inhibitory effect could be of importance only at high Pi concentrations.…”

Involvement of Na+ in Active Uptake of Pyruvate in Mesophyll Chloroplasts of Some C4 Plants

“…miliaceum and some other species. leaves of C4 plants, Panicum miliaceum (NAD-malic enzyme type), Zea mays, Sorghum bicolor (NADP-malic enzyme type), Panicum maximum and Urochloa panicoides (PEP carboxykinase type), and intact chloroplasts obtained therefrom were purified by centrifugation through a Percoll layer (16,17). The chloroplasts were suspended in 0.35 M sorbitol, 50 mM Hepes-KOH (pH 7.8) and 5 mM EDTA (free base or K salt).…”

“…Two types of silicone oil layer filtering centrifugation methods were employed: one is the ordinary single-layer system (9,16,17) 100 ,uL of uptake layer of suspending medium (containing radioisotopes and half of its sorbitol being replaced by sucrose), 30 ,uL of low density (p = 0.96) silicone oil, and 100 ,uL of chloroplast layer. The composition of the uptake layer was such that the layer is heavier than the chloroplast layer but still lighter than the lower silicone oil layer.…”