Tyr-Asp treatment delays senescence in litchi fruit by enhancing redox balance and maintaining energy homeostasis

Litchi, a tropical and subtropical fruit with significant commercial value, is highly susceptible to browning and senescence after harvest. Dipeptides, the simplest form of peptides, play essential roles in various biological processes. However, their specific effects on litchi fruit browning and senescence remain unclear. This study demonstrated that Tyr-Asp treatment effectively delays browning of the litchi pericarp and maintained a higher proportion of marketable fruit. Tyr-Asp treatment inhibited the respiration rate, leading to reduced levels of hydrogen peroxide (H₂O₂) and malondialdehyde (MDA), while increasing glutathione (GSH) content. Additionally, Tyr-Asp enhanced the activities of antioxidant enzymes, such as superoxide dismutase (SOD) and catalase (CAT), while suppressing the activity of peroxidase (POD) compared to the control group. Moreover, Tyr-Asp treatment improved the NADPH/NADP⁺ ratio without affecting the NADH/NAD⁺ ratio. Energy status analysis revealed that Tyr-Asp notably increases the energy charge (EC) compared to the control group, suggesting enhanced energy homeostasis. RT-qPCR analysis showed that Tyr-Asp reduces the expression of energy metabolism-related genes (UCP1, GAPDH) and PPO, while simultaneously upregulating key genes involved in antioxidant defense, energy metabolism, and stress response, including SOD, CAT, APX1, POD2, DHAR, MDHAR, SnRK2, AtpB, and AAC1. These changes collectively contributed to the delayed browning and senescence of litchi fruit. Overall, these findings indicated that Tyr-Asp alleviates browning and delays senescence in litchi fruit by modulating key metabolic pathways, thereby enhancing redox balance and optimizing energy homeostasis.