CHARACTERIZATION OF THE CELL-WALL MICRODOMAIN SURROUNDING PLASMODESMATA IN APPLE FRUIT

In fleshy fruits ripening is generally associated with a loss in tissu e firmness resulting from depolymerization of wall components and sepa ration of adjacent cells. In the regions of the wall that contain plas modesmata, the usual sequences of ripening events, i.e. depolymerizati on of the middle lamellae and splitting of the walls, are not observed . In the present study we attempted to characterize in apple (Malus do mestica Borkh.) fruit the structural microdomain of the cell wall that surrounds the plasmodesmata by in mure visualization of the cell wall components. Anionic sites of galacturonic acids were labeled with cat ionic gold. Low-esterified homogalacturonans were labeled with the mon oclonal antibody JIM 5. In addition, a polyclonal antibody directed to ward beta(1 --> 3)-glucopyranose was used to target callose in situ. T he results indicated that the plasmodesmata-wall complexes were surrou nded by a pectic microdomain. This domain was composed of low-esterifi ed homogalacturonans that were not involved in calcium cross-bridging but were probably surrounded by a cationic environment. These structur al features may result in the prevention of normal cell wall separatio n in regions containing plasmodesmata. However, observations by low-te mperature scanning electron microscopy suggested that splitting of the se walls ruptured the plasmodesmata and ultimately resulted in the spa tial separation of adjacent cells.