Cellular approach to understand bitter pit development in apple fruit
2010
de Freitas, Sergio T. | Amarante, Cassandro V.T do | Labavitch, John M. | Mitcham, Elizabeth J.
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 60d. 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.
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