Use of cellulase antibodies to study leaf abscission

Abstract

The primary leaves of the bean (Phaseolus vulgaris c.v. red kidney) are shed as the result of cell wall breakdown in the abscission zones found at the base of the lamina and at the stem–petiole junction. Horton and Osborne¹ suggested that cellulase (B l : 4 glucan 4-glucanohydrolase) was involved in this wall hydrolysis because its activity increased considerably during abscission. Although this result has been repeated on a wide range of plant material², critical scrutiny of the data has led several authors^3,4 to question the direct involvement of the enzyme in wall weakening. The following objections have been raised: (1) cellulase may be present at relatively high levels before abscission is induced⁵, (2) although wall breakdown is confined to two or three layers of cells, cellulase is usually found over a wider area of the petiole, (3) the increase in cellulase which accompanies abscission may not take place until cell separation is essentially complete³, (4) cellulase may increase without concomitant breakdown of the abscission layer⁴. One possible explanation for these anomalies is that these assays failed to distinguish between the various isozymes of cellulase, of which only one is postulated to be involved in abscission⁶. Recently, the specific isozyme thought to participate in bean leaf abscission (9.5 cellulase) has been purified and monospecific rabbit antibodies raised against it⁷. These antibodies do not cross-react with the other form (s) of cellulase and thus can be used both to quantify changes in 9.5 cellulase during abscission and to determine cytochemically the distribution of the enzyme. Using these antibodies, we have now made observations consistent with the participation of cellulase in the wall hydrolysis which leads to fracture.