Transcriptome analysis reveals key metabolic pathways and gene expression involving in cell wall polysaccharides-disassembling and postharvest fruit softening in custard apple (Annona squamosa L.)

This study aimed to investigate the effect of custard apple cell wall polysaccharides-disassembling on postharvest fruit softening and to explore its key metabolic pathways and gene expression. Custard apple fruit was stored at 15 ± 0.5 °C for 12 days, it was found that the decreased significantly in fruit firmness, contents of Na2CO3-soluble pectin, hemicellulose and cellulose, and the increased significantly in water-soluble pectin and CDTA-soluble pectin. The activities of cell wall-degrading relevant enzymes in fruit were improved significantly during storage, including cellulase, polygalacturonase, pectin methyl esterase, neutral xylanase, β-galactosidase, and β-D-glucosidase. The RNA sequencing results revealed 41,545 nonredundant unigenes and 7571 differentially expressed genes (DEGs) in custard apple fruit samples. Functional annotation and DEGs data revealed cell wall degradation potentially involved in starch and sucrose metabolism, amino sugar and nucleotide sugar metabolism, galactose metabolism, pentose and glucuronate interconversions. Specifically, two EG and six β-Glc genes controlled the cellulose decomposition, and one β-xyl and one GATU genes involved in the degradation of hemicellulose, and two PME, one Pel, and four PG genes were the major regulators of pectin disassembling. These results provide a molecular foundation for explaining fruit softening and extending shelf life of custard apple.