Metabolomics and transcriptomics uncover the pectin hydrolysis during tobacco stem fermentation by Aspergillus niger

Tobacco stems (TSs) are an inevitable by-product during tobacco processing, and pectin acts as a primary constituent in TSs. Aspergillus niger (A. niger) is widely used for enzyme production in food industry, such as pectinase. Thus, the pectin hydrolysis, pectinase activity, extracted metabolites, and expressed genes were investigated during TS fermentation (60 g/L) by A. niger. The pectin in TSs was effectively hydrolyzed by A. niger with fermentation time, and its removal efficiency exceeded 90% at 7 d no matter whether glucose was added (0–20 g/L), while the pectinase activity was inhibited with glucose addition. Metabolomics analysis revealed that the differences of metabolites were mainly occurred at 7 d during TS fermentation after supplying glucose, and the dominant metabolites included acids, esters, glycosides, phenols, and amines. Transcriptomics analysis showed that the external glucose promoted the up-regulation of differential expression genes (DEGs) during TS fermentation, and the DEGs mainly participated in the catalytic activity. A total of 28 genes involved in pectin hydrolysis were identified during TS fermentation, and the dominating genes encoding pectate lyase occupied an important position. This study provides a basis for the further study of TS fermentation and pectinase production by A. niger.