Identification of Leaf Abscission Zones as a Specific Class of Target Cells for Ethylene
The anatomy of leaf abscission zones from many species has been well described. In many plants, the fully differentiated abscission zone can be discerned as a single layer, or several layers of cells which are morphologically distinct from neighbouring tissue (Addicott, 1982). The stimulus for shedding in a large number of plants is ethylene. For example, in the initiation of cell separation at the distal pulvinus:petiole leaf abscission zone of Phaseolus vulgaris, ethylene is produced just distally to the zone as part of a programmed series of events associated with leaf senescence (Jackson and Osborne, 1970). The cell separation event is preceded by the enlargement of the one or two rows of the cells that comprise the zone and by an increase in the activity in these cells of a carboxymethy 1-/5-1:4-glucanase. These responses are not observed in neighbouring tissue (Wright and Osborne, 1974). In another plant, Sambucus nigra, an abscission zone traverses the rachis of the compound leaf and is comprised of cells which are also distinct morphologically from their neighbours. On treatment with ethylene, these zone cells undergo major changes in ultrastructure, suggestive of active protein synthesis and secretion, while neighbouring cells remain comparatively quiescent (Osborne and Sargent, 1976).
KeywordsCarbohydrate Bromide Electrophoresis Fractionation Polypeptide
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