Phytosulfokine treatment delays browning of litchi pericarps during storage at room temperature

Litchi, a tropical and subtropical fruit with substantial commercial value, is prone to browning after harvest. Phytosulfokine (PSK), a plant peptide hormone, plays a significant role in postharvest fruit preservation by affecting various biological processes. However, its specific effects on litchi browning have not been fully elucidated. This study demonstrates that PSK application effectively delays litchi pericarp browning, maintains a higher TSS/TA ratio, and reduces electrical conductivity. PSK treatment inhibits the degradation of anthocyanins, flavonoids, and phenolics, while also decreasing the accumulation of hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and superoxide anions (O₂⁻). Additionally, PSK enhances the activities of superoxide dismutase (SOD), catalase (CAT), and glutathione reductase (GR), while reducing the activities of ascorbate peroxidase (APX), polyphenol oxidase (PPO), and peroxidase (POD) compared to the control. Transcriptome analysis reveals that PSK upregulates genes involved in plant hormone signaling and biosynthesis of anthocyanins and flavonoids, while downregulating genes associated with glutathione metabolism and autophagy. RT-qPCR confirms that PSK treatment decreases the expression levels of APX, LAC7, PPO, POD1, POD5, POD51, and PODX, while increasing the expression of GPX4, contributing to the delayed browning. These findings suggest that PSK mitigates litchi browning by modulating oxidative enzymatic reactions and suppressing the expression of browning-related genes.