Induction of Glucose 6-Phosphate Transport Activity in Chloroplasts of Mesembryanthemum crystallinum by the C_3-CAM Transition

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To study possible changes in the transport metabolites between chloroplasts and cytoplasm during CAM induction of Mesembryanthemum crystallinum, we compared substrate specificity of P_i translocator(s) in isolated chloroplasts from the C_3 and CAM-induced plants. The [^<14>C]glucose 6-phosphate (G6P) transport activity was significant only in the chloroplasts of CAM-mode plants and not detectable in those of C_3-mode, while a similar high rate of [^<32>P]P_i uptake was observed with both types of chloroplasts. Kinetic analysis of G6P uptake in the CAM chloroplasts showed a high V_<max> [10.6 μmol (mg Chl)^<-1> h^<-1>] and a comparatively low K_m value (0.41 mM); the latter was similar to K_i values of P_i, 3-Phosphoglycerate and phosphoenolpyruvate, 0.30, 0.34 and 0.47 mM, respectively. On the other hand, [^<32>P]P_i uptake in the CAM chloroplasts was inhibited competitively by G6P with a K_i value (8.4 mM) 20-fold higher than the K_m value for G6P uptake, while that in C_3 chloroplasts was not inhibited at all. These results suggest that a new G6P/P_i counterexchange mechanism is induced in the chloroplast envelope of CAM-induced M. crystallinum in addition to the ordinary type of P_i translocator, that cannot transport G6P, already present in the C_3-type chloroplasts.
日本植物生理学会の論文