Multiple effects of glycerol on plant cell metabolism. Phosphorus-31 nuclear magnetic resonance studies

The effects of glycerol on plant cell metabolism were studied with sycamore (Acer pseudoplatanus L.) cells using 31P nuclear magnetic resonance spectroscopy. After a long period of sucrose starvation, the addition of 50 mM glycerol to the medium did not restore the original glucose-6-P pool and led to a rapid accumulation of sn-glycerol-3-P in the cytoplasmic compartment. The synthesis of sn-glycerol-3-P was rapid and occurred first at the expense of cytoplasmic P(i). Accumulated sn-glycerol-3-P competitively inhibited glucose-6-phosphate isomerase activity when fructose-6-P was the varied substrate. Such a situation prevented the rapid recycling of triose phosphates back to hexose phosphates and led to an arrest of the functioning of the cytosolic and plastidial pentose phosphate pathways. Under these conditions, the flow of carbon to drive cell respiration derived almost exclusively from glycerol, and this polyalcohol was not used as a source of carbon skeletons for biosynthesis. Glycerol also induced the accumulation of O-phosphohomoserine in the cytoplasmic compartment as long as the cell culture medium contained sucrose. Finally glycerol added to sucrose-starved cells stopped the accumulation of phosphocholine (Roby, C., Martin J.-B., Bligny, R., and Douce, R. (1987) J. Biol. Chem. 262, 5000-5007) and prevented a further decline in the uncoupled rate of O2 consumption by the cells (Journet, E. P., Bligny, R., and Douce, R. (1986) J. Biol. Chem. 261, 3193-3199). These last observations strongly suggest that glycerol prevented the triggering of autophagy induced by sucrose starvation in sycamore cells.