Cellular approach to understand bitter pit development in apple fruit

Bitter pit (BP), a Ca2+ deficiency disorder of apple fruit (Malus domestica), is a complex process that involves not only the total input of Ca2+ into the fruit, but also a proper Ca2+ homeostasis at the cellular level. The objective of this study was to test the hypothesis that Ca2+ accumulation into storage organelles and binding to the cell wall is associated with BP development in apple fruit. The experiment was carried out on ‘Granny Smith’ apples stored at 0 °C for 60 d. After storage, fruit were segregated into two lots for analysis, apples with the water-soaked initial visual symptoms of BP and those not showing this symptom. Cytochemical and ultrastructural observations showed an accumulation of Ca2+ in the vacuole of individual outer cortical cells of pitted fruit. We also observed an increase in the expression of genes encoding four pectin methylesterases, a greater degree of pectin deesterification and therefore more Ca2+ binding sites in the cell wall, and a higher fraction of the total cortical tissue Ca2+ content that was bound to the cell wall in pitted fruit compared with non-pitted fruit. Cells of the outer cortical tissue of pitted fruit consistently had higher membrane permeability than outer cortical cells of non-pitted fruit. The results provide evidence that Ca2+ accumulation into storage organelles and Ca2+ binding to the cell wall represent important contributors to BP development in apple fruit.