Leaf Curing Practices Alter Gene Expression and the Chemical Constituents of Tobacco Leaves

Tobacco leaves senesce during the early curing phase, and their constituents are altered via specific enzymatic activities. Levels of the reducing sugars glucose and fructose and those of total free amino acids increase via active protease activity and active synthesis processes. We used transcriptomics to identify the genes up- and downregulated in tobacco leaves from the Burley, Virginia, and Oriental varieties after 48 h of curing. Approximately 8% of the transcripts were upregulated, including 591 genes common to all three varieties, and especially genes associated with oxidation-reduction processes, transcription factors, proteolysis, and hydrolase activity. All of these have been linked to the cellular senescence process. Approximately 12% of the transcripts were downregulated, including 1,486 genes common to all three tobacco types; the genes with the highest fold changes were associated with general metabolic processes. Gene ontology overrepresentation analysis suggested that the three main axes governing the curing of tobacco are dehydration stress, amino acid alteration, and sucrose metabolism. We identified sucrose synthase (SUS) as a likely driver of the accumulation of reducing sugars during the water-deprivation response, and aspartate aminotransferase (AAT) as a catalyst of amino acid metabolism.