Browning mechanism of tobacco leaves during flue-curing process: Proteomics and metabolomics analysis reveals the changes in materials

Abstract

Tobacco leaves, as raw materials, undergo complex processing before the final product is obtained. During the flue-curing process, inadequate control of environmental factors such as temperature, humidity or water loss rate, may lead to enzymatic browning occur caused by starvation metabolism of tobacco leaves, and therefore large amount of internal chemical substances consumed and changed, leading to excessive darker color and affecting the quality of products. This study aimed to investigate the mechanisms and material basis of browning, as well as the relationship between browning and internal substances changes. The microstructural changes of tobacco leaves under transmission electron microscopy were observed, quantitative proteomic and untargeted metabonomic methods were conducted in browning and normal tobacco leaves. In the results, 106 differential proteins and 576 differential metabolites were identified, with differential proteins mainly involved in biosynthesis and metabolism of polysaccharides, lipid, as well as the regulation of chlorophyll degradation and polyphenol oxidation related proteins. Differential metabolites were mainly enriched in purine metabolism, histidine metabolism and isoflavone biosynthesis pathway, with more secondary metabolites biosynthesis pathway enrichment. Moreover, tyrosine metabolism and phenylpropane biosynthesis metabolism were identified as the mainly differential proteins and metabolites pathways. Additionally, two metabolites, resveratrol and iridin, that exhibited strong free radicals scavenging activity and good antioxidant properties were uncovered. Taken together, these findings demonstrated changes of chemical substances and differences of proteins and metabolites between browning and normal tobacco leaves under specific conditions, and provided a theoretical reference for exploring optimized conditions and controlling material changes during the processing of tobacco to inhibit browning.