Loss of highly branched arabinans and debranching of rhamnogalacturonan I accompany loss of firm texture and cell separation during prolonged storage of apple

Abstract

Growth and maturation of the edible cortical cells of apples (Malus domestica Borkh) are accompanied by a selective loss of pectin-associated (1-->4)-beta-D-galactan from the cell walls, whereas a selective loss of highly branched (1-->5)-alpha-L-arabinans occurs after ripening and in advance of the loss of firm texture. The selective loss of highly branched arabinans occurs during the overripening of apples of four cultivars (Gala, Red Delicious, Firm Gold, and Gold Rush) that varied markedly in storage life, but, in all instances, the loss prestages the loss of firm texture, measured by both breaking strength and compression resistance. The unbranched (1-->5)-linked arabinans remain associated with the major pectic polymer, rhamnogalacturonan I, and their content remains essentially unchanged during overripening. However, the degree of rhamnogalacturonan I branching at the rhamnosyl residues also decreases, but only after extensive loss of the highly branched arabinans. In contrast to the decrease in arabinan content, the loss of the rhamnogalacturonan I branching is tightly correlated with loss of firm texture in all cultivars, regardless of storage time. In vitro cell separation assays show that structural proteins, perhaps via their phenolic residues, and homogalacturonans also contribute to cell adhesion. Implications of these cell wall modifications in the mechanisms of apple cortex textural changes and cell separation are discussed.

Figure 1.

Growth characteristics of Red Delicious apple fruit at different stages of development and ripening. A, Representative examples of apple fruit during the growth season and in the overripened state (27 weeks). Values are weeks after flowering. B, Weight and volume of apples harvested at each of these stages. Values are the mean ± sd of 10 representative samples.

Figure 2.

Linkage distribution of selected arabinosyl and galactosyl residues in the cell walls of Red Delicious apples during the different stages of development and ripening. ♦, t-Ara; ⋄, 3,5-Ara; ▴, 5-Ara; ▪, t-Gal; □, 4-Gal; •, 3,4-Gal. Bars represent variance of two samples. Variance did not extend beyond the symbol in values without bars. Absolute mol% of monosaccharides was determined by quantitative analysis of alditol acetate derivatives.

Figure 3.

Changes in the force necessary to compress (left frames) or break (right frames) cylinders of cortex taken of Gala, Red Delicious, Firm Gold, and Gold Rush apples during the storage at 25°C (○) and 4°C (•).

Figure 4.

Alterations in linkage structure and degree of RG I branching in cell walls from Gala, Red Delicious, Firm Gold, and Gold Rush apples during the storage at 25°C (open symbols) and 4°C (closed symbols). Bars represent variance of two samples. Variance did not extend beyond the symbol in values without bars. Absolute mol% of monosaccharides was determined by quantitative analysis of alditol acetate derivatives.

Figure 5.

Cell separation degree in tissue from Gold Rush apples after treatment with different solutions: Water, CDTA, Proteinase K, Control NaClO₂, NaClO₂, and PGase. All scale bars = 50 μm.

Figure 6.

Differences in the force necessary to compress the cell walls from the four cultivars of apple after chemical and enzymatic treatments. A, Treatment with (from left to right) water, proteinase K, CDTA, pectinase; hot water control NaClO₂; NaClO₂. B, After NaClO₂ treatment compared to ambient and hot water controls (from left to right); standard incubation, previously treated with proteinase K, standard incubation plus CaCl₂.