Loss of Highly Branched Arabinans and Debranching of Rhamnogalacturonan I Accompany Loss of Firm Texture and Cell Separation during Prolonged Storage of Apple

Growth and maturation of the edible cortical cells of apples (Malus domestica Borkh) are accompanied by a selective loss of pectin-associated (1(right arrow)4)-(beta)-D-galactan from the cell walls, whereas a selective loss of highly branched (1(right arrow)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(right arrow)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.