Analysis of Microstructure, Cell Wall Polysaccharides, and Candidate Cell Wall-related Genes in Clingstone-type and Freestone-type Stony Hard Peaches during Ripening

Abstract

Stony hard (SH) peach (Prunus persica) fruits produce no ethylene and clingstone-type SH peaches have a crispy flesh texture; however, freestone-type SH peach fruits ripen to a soft, mealy state. During this study, we compared and analyzed changes in the microstructure, cell wall polysaccharides, and candidate cell wall-related genes of freestone-type SH ‘Zhongtao 14’ (‘CP14’), ‘Zhongtao White Jade 2’ (‘CPWJ2’), clingstone-type SH ‘Zhongtao 13’ (‘CP13’), and ‘Zhongtao 9’ (‘CP9’) during fruit ripening. The parenchyma cells of mealy freestone-type SH peaches became detached, were single, dried, and irregularly arranged, and remained intact in comparison with the nonmealy clingstone-type SH peaches. Methyl-esterified homogalacturonan was strongly immunolabeled in the cell wall of clingstone SH peaches; however, nonmethylated homogalacturonan was weakly immunolabeled in freestone SH peaches. A transcriptome analysis was performed to investigate the molecular mechanism of the mealiness process. A principal component analysis indicated that ‘CP14’ S4 III (mealy) could be distinguished from the samples of ‘CP13’ (S4 I, S4 II, S4 III) and ‘CP14’ (S4 I, S4 II). The highly coexpressed gene modules linked with firmness were found using a weighted gene coexpression network analysis; 189 upregulated genes and 817 downregulated genes were identified. Six upregulated cell wall-related genes (PpPG1, PpPG2, PpAGP1, PpAGP2, PpEXT1, and PpEXP1) and one downregulated cell wall-related gene (PpXET2) were involved in the mealiness process during freestone-type SH fruit ripening. These findings will improve our understanding of the relationship between clingstone, freestone, and stony hard fruits and lay the foundation for further exploration of the mechanisms underlying the softening of peach fruits.