Spatial Regulation of Pectic Polysaccharides in Relation to Pit Fields in Cell Walls of Tomato Fruit Pericarp

Scanning electron microscopic examination of intact tomato (Lycopersicon esculentum) pericarp and isolated pericarp cell walls revealed pit fields and associated radiating ridges on the inner face of cell walls. In regions of the cell wall away from pit fields, equivalent ridges occurred in parallel arrays. Treatment of isolated cell walls with a calcium chelator resulted in the loss of these ridges, indicating that they contain homogalacturonan-rich pectic polysaccharides. Immunolabeling procedures confirmed that pit fields and associated radiating ridges contained homogalacturonan. Epitopes of the side chains of pectic polysaccharides were not located in the same regions as homogalacturonan and were spatially regulated in relation to pit fields. A (1→4)-β-galactan epitope was absent from cell walls in regions of pit fields. A (1→5)-α-arabinan epitope occurred most abundantly at the inner face of cell walls in regions surrounding the pit fields.