Comprehensive study on softness mechanisms and the metabolic pattern of key cell wall components in tobacco leaves during the curing stage.

BACKGROUND: Curing is an essential process for transforming tobacco leaves into an economic product. Optimizing curing parameters to regulate the orderly senescence and apoptosis of tobacco cells serves as a direct approach to enhancing the softness of leaves and a critical guarantee of quality. Therefore, understanding the intrinsic mechanisms underlying leaf softening during curing is vital for curing conditions and improving tobacco usability. RESULTS: To elucidate the degradation patterns of key cell wall components and dynamic changes in related enzyme activities during the softening of tobacco leaves during curing stage, we systematically analyzed the softening progression, cell wall structure, and dynamic transformation of key metabolites in the flue-cured variety Yunyan 87. The results demonstrated that leaf softness peaked during the yellowing stage (minimal softness value: 11.83mN). The cell wall structure progressively disintegrated throughout curing. Pectin methylesterase (PME), polygalacturonase (PG), and pectin lyase (PL) exhibited higher activities in fresh leaves and the yellowing stage, with corresponding values of 8.33 and 20.56 U/g for PME, 1.20 and 1.33 U/g for PG, and 2.39 and 4.93 U/g for PL. The cell wall matrix content decreased significantly during curing, reaching 412.84 mg/g in the drying stage.LC-MS/MS analysis identified 1,220 metabolites with significant alterations during curing, 54 of which were closely associated with cell wall metabolism. We further delineated metabolic pathways for cellulose, pectin, and lignin. CONCLUSIONS: This study comprehensively investigated the metabolic basis of tobacco leaf softening during curing, identifying the fresh leaves and yellowing stage as critical regulatory nodes. These findings provide valuable references for optimizing tobacco curing parameters.