EVIDENCE FOR THE BREAKDOWN OF CECROPIN-B BY PROTEINASES IN THE INTERCELLULAR FLUID OF PEACH LEAVES

Peach (Prunus persica (L.) Batsch) leaves were found to tolerate relat ively high levels of the bacteriocidal polypeptide cecropin B as deter mined by infiltration tests. Detached leaves showed no symptoms when i nfiltrated with 25 mu M cecropin B, and only infrequent necrotic sympt oms with 50 mu M. Incubation of cecropin B with intercellular fluid (I CF) extracted from peach leaves reduced its biotoxicity toward the pea ch pathogen Pseudomonas syringae pv. syringae. The reduction in toxici ty was lessened by adding proteinase inhibitors to ICF - and prevented by boiling ICF prior to incubation with cecropin B. Electrophoretic e vidence suggested that ICF constituents caused endopeptidase cleavage of cecropin during the first hour of incubation followed by complete d igestion of the cleavage products during the following 8 h. Incubation s of various levels of cecropin B and ICF indicated that as much as 90 % of the activity could be destroyed in 10 min, but that levels adequa te for lethality against P. syringae pv. syringae would remain. We con clude that peach trees transgenic for cecropin B could feasibly produc e a wide range of cecropin levels sufficient to control pathogenic bac teria without damaging the leaf tissue.