Abstract
Many methods of storage are currently used to delay the postharvest senescence and the ensuing quality losses of detached lettuce leaves. This transcriptome study explores the senescence mechanisms of packaged lettuce leaves over 7 d of cold storage in darkness. At the end of the storage, the detached lettuce leaves showed a 10% water loss and 24% cut-edge discoloration. No chlorophyll fluorescence variation was observed. A total of 1048 and 1846 genes were differentially expressed (DE) after 2 and 7 d of storage, respectively. In terms of gene expression modulation, the data showed a global shutdown of primary-metabolism but no obvious chlorophyll breakdown. Although an early stress-response clearly takes place, the cold storage under darkness appeared to delay senescence by protecting chloroplasts from degradation and enhancing an antioxidative response. Then, a progressive global expression shutdown appeared, concomitantly with the dehydration stress. Among the transcripts that showed an early high expression, the data highlighted many stress-related genes (PIN, bzip TF, Heat-shock, stress A-like), 30 redox-regulation associated transcripts, the majority of the phenylpropanoid pathway markers and ethylene and auxin-related genes. Later (day 7), many dehydration-responsive markers showed an increased expression, as did some abscisic acid related markers. Phytohormones (cytokinins) and transcriptions factors (WRKYs) appeared to play complex roles as senescence progressed. No simple link between cut-edge discoloration and PPO-related genes expression was made. The main known process of senescence induction through the phaephorbide a oxygenase and phospholipase D pathways were not highlighted in this study. To the best of our knowledge, this study is the first to explore the transcriptional response of postharvest senescence in detached lettuce leaves. Deeper analysis of several markers will help to dissect the crosstalk between wounding, dehydration and senescence mechanisms. In addition, post-transcriptional studies are needed to conclude about the described patterns.