Leaf abscission in Impatiens (Balsaminaceae) is due to loss of highly de-esterified homogalacturonans in the middle lamellae.

PREMISE OF STUDY Abscission zones (AZ) are sites where leaves and other organs are shed. Investigating the AZ by classical biochemical techniques is difficult due to its small size and because the surrounding tissue is not involved in abscission. The goals of this study were to determine whether AZ cell walls are chemically unique from the other cells of the petiole, perhaps making them more susceptible to enzymatic degradation during abscission and to identify which cell wall polysaccharides are degraded during abscission. METHODS A battery of antibodies that recognize a large number of cell wall polysaccharide and glycoprotein epitopes was used to probe sections of the Impatiens leaf AZ at several time points in the abscission process. KEY RESULTS Prior to abscission, the walls of the AZ cells were found to be similar in composition to the walls of the cells both proximal and distal to the AZ. Of all the epitopes monitored, only the highly de-esterified homogalacturonans (HG) of the middle lamellae were found to be reduced post-abscission and only at the plane of separation. More highly esterified homogalacturonans, as well as other pectin and xyloglucan epitopes were not affected. Furthermore, cellulose, as detected by an endoglucanase-gold probe and cellulose-binding module staining, was unaffected, even on the walls of the cells facing the separation site. CONCLUSIONS In the leaf abscission zone of Impatiens, wall alterations during abscission are strictly limited to the plane of separation and involve only the loss of highly de-esterified pectins from the middle lamellae.