Exogenous melatonin alleviates juice sac granulation in postharvest pomelo by coordinating sucrose metabolism, energy homeostasis, membrane integrity, and lignin biosynthesis

• Melatonin orchestrates a defense network against juice sac granulation in pomelo. • A sugar-energy-membrane-lignification cascade model elucidates granulation. • Preserving membrane integrity is central to melatonin's inhibition of lignification. • Melatonin inhibits lignin biosynthesis at both transcriptional and enzymatic levels. Juice sac granulation, an important physiological disorder, significantly reduces the commercial quality of postharvest pomelo. This study demonstrates that postharvest melatonin treatment alleviates pomelo granulation via a coordinated pathway. This pathway integrates sucrose metabolism, energy homeostasis, membrane integrity, and lignification inhibition. Melatonin sustained elevated levels of sucrose, glucose, and fructose in postharvest pomelo through coordinated regulation of key sucrose-metabolizing enzyme activities, including neutral invertase (NI), acid invertase (AI), sucrose synthase (SSs, SSc), and sucrose phosphate synthase (SPS), thereby increasing soluble sugar content. This enhancement was accompanied by marked upregulation of energy metabolism. Key indicators such as ATP content, energy charge, and the activities of H + -ATPase, Ca 2+ -ATPase, Mg 2+ -ATPase, cytochrome c oxidase (CCO), and succinate dehydrogenase (SDH) were all significantly increased. The improved energy status contributed to better membrane integrity, evidenced by reductions in LOX activity, MDA content, and electrolyte leakage. Furthermore, melatonin markedly suppressed lignin biosynthesis through down-regulation of pivotal enzymes and their corresponding genes, including PAL, C4H, 4CL, CAD, LAC and POD. Crucially, maintaining membrane integrity helps prevent phenolic precursors from leaking into the cell wall, thereby restricting the initiation of lignification. These findings demonstrate that melatonin alleviates juice sac granulation through a sequential regulatory pathway. Melatonin sustains sucrose metabolism to maintain energy homeostasis, supports membrane integrity and ion balance, and preserves membrane function to ultimately restrict cell wall lignification. This integrated network advances our understanding of juice sac granulation and presents a promising strategy for maintaining the quality of postharvest pomelo.