Induction of glucose 6-phosphate transport activity in chloroplasts of Mesembryanthemum crystallinum by the C3-CAM transition

Abstract

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 C3 and CAM-induced plants. The [14C]glucose 6-phosphate (G6P) transport activity was significant only in the chloroplasts of CAM-mode plants and not detectable in those of C3-mode, while a similar high rate of [32P](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, [32P]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 C3 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 C3-type chloroplasts.