A comparative study of melting and non-melting flesh peach cultivars reveals that during fruit ripening endo-polygalacturonase (endo-PG) is mainly involved in pericarp textural changes, not in firmness reduction

Abstract

Peach softening is usually attributed to the dismantling of the cell wall in which endo-polygalacturonase (endo-PG)-catalysed depolymerization of pectins plays a central role. In this study, the hypothesis that the function of endo-PG is critical for achieving a melting flesh fruit texture but not for reducing fruit firmness was tested by comparing pericarp morphology and endo-PG expression and localization in melting (MF) and non-melting flesh (NMF) fruit at successive stages of ripening. MF Bolero, Springbelle, and Springcrest, and NMF Oro-A and Jonia cultivars were analysed. Both MF and NMF fruit were left to ripen on the tree and reached a firmness of <10 Newtons (N). The image analysis of pericarp tissues revealed that during softening the loss of cell turgidity was a process common to mesocarp cells of all MF and NMF fruit and was clearly visible in peaches with a firmness of less than ∼20 N. In contrast, the loss of cell adhesion was a feature exclusively observed in ripe MF fruit pericarp. In this ripe fruit, large numbers of endo-PG isoforms were highly expressed and the enzyme localization corresponded to the middle lamella. As a consequence, wide apoplastic spaces characterized the pericarp of ripe MF peaches. In contrast, no loss of cell adhesion was observed in any NMF fruit or in unripe MF peaches. Accordingly, no endo-PG was detected in unripe NMF fruit, whereas few and poorly expressed enzyme isoforms were revealed in ripe NMF and in unripe MF peaches. In this fruit, the poorly expressed endo-PG localized mainly in vesicles within the cytoplasm and inner primary cell wall. On the whole the results suggested that endo-PG function was needed to achieve melting flesh texture, which was characterized by wide apoplastic spaces and partially deflated mesocarp cells. Conversely, endo-PG activity had no critical influence on the reduction of fruit firmness given the capacity of NMF peaches to soften, reaching values of 5-10 N. As in tomato, the change of symplast/apoplast water status seems to be the main process through which peach fruit regulates its firmness.

Heterogeneity in the entire genome for three genotypes of peach [Prunus persica (L.) Batsch] as distinguished from sequence analysis of genomic variants

“…It exhibits melting flesh (losing of firmness and structure, for an accurate description see [89]), a high acid/sugar ratio, and is prone to flesh mealiness and significant browning. This cultivar is a progeny from an open pollination of a tree of the cultivar 'Chinese Cling’; however, other studies suggest the cultivar 'Late Crawford’ is the male parent [88].…”

Identification and Expression Analysis of Polygalacturonase Family Members during Peach Fruit Softening

“…However, Smith et al thought PG was not essential for tomato fruit softening [21]. Interestingly, peach fruit melting and freestone are also controlled by Endo-PGs [22,23,24,25]. In addition, PG genes also participate in the organ abscission of lychee, oilseed rape, Arabidopsis and tomato [26,27,28].…”

Identification of volatile and softening-related genes using digital gene expression profiles in melting peach

“…An array of enzymes involved in these processes, such as polygalacturonase (PG), pectin methylesterase (PME), β-galactosidase (β-Gal), α-Larabinofuranosidase (α-ARF), β-xylosidase (XYL), endo-(1, 4)-β-D-glucanase, expansin (EXP), and xyloglucan endotransglycosylase (Tomassen et al 2007), and especially endo-PG, is considered to play a critical role in the softening process of melting and nonmelting peaches (Ghiani et al 2011;Kao et al 2012). Accumulation of endo-PG, XYL, and PpExp3 transcripts has been showed to be higher in the melting flesh cultivar "Akatsuki" than in the stony hard flesh cultivar "Manami" (Hayama et al 2006).…”