Melatonin pretreatment promotes the growth and physiological metabolism of tobacco seedling roots under low light

To investigate whether exogenous melatonin mitigates the detrimental effects of low irradiance on subterranean plant organs, a tobacco cultivar cv. ZY100 was cultivated under 1000 μmol m − ² s − ¹ (control) and 150 μmol m − ² s − ¹ (low light stress), with or without supplemental melatonin at 200 μM. Results demonstrated that reduced irradiance significantly decreased root length, surface area, and volume, whereas melatonin treatment effectively counteracted these morphological impairments by 32.5%, 24.3%, and 29.5%, respectively. Low light upregulated the transcript levels of NtSOD , NtPOD , and NtCAT and elevated ascorbate and glutathione levels in roots, theoretically favoring antioxidative defense, however, the actual accumulation of superoxide anions (O 2 ·- ) and hydrogen peroxide (H 2 O 2 ) remained 219.4% and 304.4% higher, respectively, which contributed to the inhibited root traits. Notably, melatonin application further enhanced the expression of these antioxidant genes under low irradiance, ultimately leading to diminished O 2 ·- and H 2 O 2 levels. Concurrently, low light stress reduced the levels of sucrose, glucose, fructose, and starch by 48.4%, 71.9%, 53.0%, and 54.6%, respectively, in tobacco roots, thereby depriving root growth of adequate carbon resources. The decline in sucrose was attributable to upregulated NtSuSy ( sucrose synthase ) and NtCWINV ( cell wall invertase ), while the reduction in starch was associated with downregulated NtAGP ( ADP-glucose pyrophosphorylase ) and increased α-amylase expression. Melatonin supplementation further stimulated NtSuSy and NtCWINV expression, accelerating sucrose cleavage and consequently raising glucose and fructose concentrations in low light-stressed roots. Moreover, melatonin upregulated NtGBSS expression under low light, boosting starch biosynthesis and resulting in higher starch reserves. The resultant enrichment of these carbohydrates supplied ample substrates for energy metabolism, which, together with the elevated expression of the glycolytic genes NtHXK ( hexokinase ) and NtPK ( pyruvate kinase ), led to a 28.5% higher ATP level observed in melatonin-treated versus untreated low-light-exposed roots. Collectively, our findings indicate that exogenous melatonin alleviates low-light-induced growth inhibition in tobacco roots through coordinated modulation of antioxidant systems, carbohydrate and energy metabolism.