Differential pulp cell wall structures lead to diverse fruit textures in apple (Malus domestica)

In this study, we aimed to elucidate the effect of pulp cell wall structure on fruit hardness and crispness in apples. To this end, we studied the cell wall polysaccharides in two apple varieties, "Hanfu" and "Honeycrisp," during fruit development. Compared with Hanfu, the crispness of Honeycrisp was higher, whereas its harness was lower. The intensity and distribution of immunofluorescence signals indicated that galactose and arabinose contributed to the higher hardness of Hanfu, whereas arabinose, egg-box structure, and fucosylated xyloglucans, distributed in the corners of tricellular junctions, enhanced the cell-cell adhesion and improved the crispness of Honeycrisp. Besides, fucosylated xyloglucan played an important role in promoting the formation and maintaining the strength of the cell wall skeleton and, consequently, retaining the fruit crispness. The esterification state of pectin had little effect on the fruit hardness and crispness in both varieties. Collectively, our findings provided information on the underlying mechanism of fruit texture formation in apples.