Abstract: To investigate the pattern of softening of fresh wolfberry fruits during postharvest low-temperature storage, identify relevant regulatory genes, and clarify the underlying mechanism of softening, two large-fruited cultivars of Lycium barbarum L., ‘Ningqi 5’ and ‘Ningqi 7’, were used as experimental materials. Fruit firmness, ethylene production rate, cell wall component contents, and activities of cell wall metabolism-related enzymes were determined during storage at 4 or 13 ℃. Transcriptome sequencing was further performed on fruits under different storage conditions. Results showed that fruit firmness, cellulose, and hemicellulose contents were higher, and ethylene release rate and soluble pectin content were lower in fruits stored at 4 ℃ than at 13 ℃. During storage at 4 ℃, the activities of pectinesterase (PME), polygalacturonase (PG), cellulose, and pectolyase (PL) were all inhibited. Stronger correlations between physiological indices were observed at 4 ℃ than at 13 ℃: fruit firmness was significantly correlated with ethylene production rate, cellulose, hemicellulose and soluble pectin contents, PME and PL activities (P < 0.05), while ethylene production rate showed significant correlations with cellulose, hemicellulose and soluble pectin contents, and PG activity (P < 0.05). Principal component analysis (PCA) indicated that the first principal component was dominated by cell wall components (e.g., cellulose), and the second principal component was primarily characterized by cell wall-related enzymes (e.g., PG). Transcriptome sequencing identified a total of 6 770 differentially expressed genes (DEGs), including 3 116 up-regulated genes and 3 654 down-regulated genes. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses revealed that these DEGs were mainly enriched in biological processes such as ethylene synthesis and metabolism, lignin synthesis, and biological metabolism and regulation. Weighted gene co-expression network analysis (WGCNA) identified four key candidate genes (LbACO3, LbPME2.1, LbACS5, and LbWRKY), possibly playing a key regulatory role in the postharvest softening of wolfberry fruits. LbACO3 and LbACS5 were involved in the regulation of ethylene biosynthesis and release, LbPME2.1 regulated PME activity, and LbWRKY participated in the regulation of cell wall metabolism. The results of this study provide theoretical insights into the mechanism of postharvest softening in wolfberry fruits.

Key words: Lycium barbarum; low-temperature storage; fruit softening; cell wall degradation; ethylene metabolism; key genes; transcriptome sequencing