Cellulase (8-1 ,4-glucan-glucanohydrolase EC 3.2.1.4) activity in the abscission zone of red kidney bean (Phaseolus vulgaris) was previously shown to exist in at least two different molecular forms. The form of the enzyme which has an isoelectric point of 4.5 is present in both abscising and nonabscising tissue and requires grinding for extraction. Another form of the enzyme which has an isoelectric point of 9.5 is present only in tissue in which the abscission process has been induced. Further, much of this form of cellulase can be removed from the tissue by vacuum infiltration with buffer. Time course studies indicate that while the increase in measureable cellulase activity in tissue which is actively undergoing abscission was due primarily to the appearance of cellulase 9.5, this form of the enzyme cannot be removed by vacuum infiltration until after the breakstrength of the abscission zone has decreased nearly to zero. The intracellular localization of these two forms of cellulase is discussed.The role of cellulase in mediating the process of abscission has been difficult to determine even though several laboratories have been able to show a correlation between the activity of the enzyme and the event of abscission (1,5,9,11,13). This problem has been compounded by the recent report that cellulase exists in more than one molecular form (10,11). Lewis and Varner (11) were able to demonstrate a correlation between cellulase activity and decrease in breakstrength of abscission zones of Phaseolus vulgaris, and further that the increase in activity as abscission proceeded was due to the de novo synthesis of a form of the enzyme which was more soluble in a high salt buffer than in the same buffer without added salt. Lewis et al. (10) have reported that, in freshly harvested abscission zones, only an acidic form of cellulase could be extracted and that this cellulase was relatively unstable. After induction of abscission, the acidic form could still be detected as well as at least one other form of the enzyme which was a basic protein, as determined by isoelectric focusing. Further characterization of these two cellulases indicated differences in sensitivity to p-chloromercuribenzoate, mol wt, and stability at 50 C.

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