ETHYLENE-MEDIATED PHOSPHOLIPID CATABOLIC PATHWAY IN GLUCOSE-STARVED CARROT SUSPENSION CELLS

Glucose (Glc) starvation of suspension-cultured carrot (Daucus carota L.) cells resulted in sequential activation of phospholipid catabolic enzymes. Among the assayed enzymes involved in the degradation, phosph olipase D (PLD) and lipolytic acyl hydrolase were activated at the ear ly part of starvation, and these activities were followed by beta-oxid ation and the glyoxylate cycle enzymes in order. The activity of PLD a nd lipolytic acyl hydrolase was further confirmed by in vivo-labeling experiments. It was demonstrated that Glc added to a medium containing starving cells inhibited the phospholipid catabolic activities, indic ating that phospholipid catabolism is negatively regulated by Glc. The re was a burst of ethylene production 6 h after starvation. Ethylene a dded exogeneously to a Glc-sufficient medium activated PLD, indicating that ethylene acts as an element in the signal transduction pathway l eading from Glc depletion to PLD activation. Activation of lipid perox idation, suggestive of cell death, occurred immediately after the decr ease of the phospholipid degradation, suggesting that the observed pho spholipid catabolic pathway is part of the metabolic strategies by whi ch cells effectively survive under Glc starvation.