SUGAR-STARVATION-INDUCED CHANGES OF CARBON METABOLISM IN EXCISED MAIZE ROOT-TIPS

Excised maize (Zea mays L.) root tips were used to study the early met abolic effects of glucose (Glc) starvation. Root tips were prelabeled with [1-C-13]Glc so that carbohydrates and metabolic intermediates wer e close to steady-state labeling, but lipids and proteins were scarcel y labeled. They were then incubated in a sugar-deprived medium for car bon starvation. Changes in the level of soluble sugars, the respirator y quotient, and the C-13 enrichment of intermediates, as measured by C -13 and H-1 nuclear magnetic resonance, were studied to detect changes in carbon fluxes through glycolysis and the tricarboxylic acid cycle. Labeling of glutamate carbons revealed two major changes in carbon in put into the tricarboxylic acid cycle: (a) the phosphoenolpyruvate car boxylase flux stopped early after the start of Glc starvation, and (b) the contribution of glycolysis as the source of acetyl-coenzyme A for respiration decreased progressively, indicating an increasing contrib ution of the catabolism of protein amino acids, fatty acids, or both. The enrichment of glutamate carbons gave no evidence for proteolysis i n the early steps of starvation, indicating that the catabolism of pro teins was delayed compared with that of fatty acids. Labeling of carbo hydrates showed that sucrose turnover continues during sugar starvatio n, but gave no indication for any significant flux through gluconeogen esis.