Metabolic profiling reveals a coordinated response of isolated lamb's (Valerianella locusta, L.) lettuce cells to sugar starvation and low oxygen stress

Sugar starvation is a common phenomenon occurring in most leafy vegetables after harvest and storage. Additionally, leafy vegetables are subjected to low O2 stress when stored in modified atmosphere conditions. In this study, the metabolism of isolated lamb's lettuce cells was studied upon sugar starvation under O2 stress conditions, using 13C labelled glucose. Fast depletions of the soluble sugars were observed, being more pronounced under aerobic conditions than under low O2 stress conditions. Sugar starvation under aerobic conditions resulted in increased levels and decreased 13C label incorporation of TCA cycle intermediates and amino and fatty acids originating from glycolytic and TCA cycle pathways, compared to starving cells incubated under low O2 stress. On incubation under low O2 stress a switch in metabolism from aerobic to fermentation metabolism was observed. Under low O2 stress conditions, increased levels and 13C label incorporated in hexose phosphates, pyruvate, lactate, GABA, alanine, together with increased levels of acetaldehyde, ethanol and ethyl acetate was observed indicating fermentative metabolism was triggered.