Induction of light dependent pyruvate transport into chloroplasts of Mesembryanthemum crystallinum by salt stress

Mode of photosynthesis in Mesembryanthemum crystallinum changes from C3 to Crassulacean acid metabolism (CAM) when the plants were stressed with high salinity. [14C]Pyruvate uptake for 30s into intact chloroplasts isolated from leaves of the CAM mode of M. crystallinum was enhanced more than 5-fold in the light compared with that in the dark. The stromal concentration of pyruvate in the light reached to more than 2.5 times of the medium. In contrast, little or no pyruvate uptake occurred in chloroplasts from C3 leaves in either light or dark condition. The initial uptake rate (10s incubation at 4 degrees C) into the CAM chloroplasts in the light was about 3-fold higher than the rate in the dark. Km and Vmax of the initial uptake in the light were 0.54 mM and 8.5 mu-mol (mg Chl)(-1)h(-1), respectively. These suggest that pyruvate was actively incorporated into the CAM chloroplasts against its concentration gradient across the envelope in the light. When hydroponically grown M. crystallinum were stressed by 350 mM NaCl, the capacity of chloroplasts for pyruvate uptake was induced in 6 d corresponding to the induction of the activities of PEP-carboxylase and (NAD(P)(+)-malic enzymes in response to salt stress