Abscission and the Recognition of Zone Specific Target Cells. The Role of Ethylene

Abstract

Abscission in plants is a process of cell separation that occurs at precisely defined positions and leads to the shedding of organs distal to the site of separation [1,2]. In dicotyledons, we know that abscission is preceded by the enlargement of cells at the separation zone and by the localized synthesis of novel proteins, some of which are secreted through the plasmamembrane to the cell wall. Certain of these proteins are presumed to play an essential role in the enzymatic loosening of the polysaccharide and protein matrix of the wall (thereby decreasing the yield stress component in wall extensibility) and in the dissolution of the middle lamellae (thereby reducing the cohesive force that holds the cells in these regions together). One of the earliest developmental effects of ethylene to be recorded noted that abscission at these precise positions could be induced at will by exposing plants to low concentrations of the gas for several hours or more and all the present evidence points to ethylene as the natural hormonal inducer of the abscission process [3]. Normally, abscission occurs when tissues distal to abscission zones increase their ethylene production during ripening or senescence or when plants are subjected to environmental conditions that are sufficiently stressful to cause their basal levels of ethylene production to rise above a critical level.