Activity of different antibacterial peptides on Erwinia amylovora growth, and evaluation of the phytotoxicity and stability of cecropins

Antibacterial peptides of plant or non-plant origin are attractive candidates for pear (Pyrus communis L,) genetic engineering to increase resistance to fire blight caused by Erwinia amylovora. The antibacterial activity of several peptides isolated from cereals (purothionins and puroindolines), insects (cecropins), animals then egg white (HEW) lysozyme) and microorganisms (T-4 lysozyme) was tested against E. amylovora. Cecropins had the higher bactericidal efficiency? followed by T-4 lysozyme. A synergistic effect between Cecropin B and HEW lysozyme was also demonstrated. No toxicity of cecropins towards pear cell suspension cultures was observed at the bactericidal concentrations. However, the incubation of cecropins with pear leaf extracellular fluids (ECF) caused a loss of their antibacterial activity and a degradation of ECF-treated cecropins was observed. We conclude that the expression of a cecropin gene in transgenic pear will be efficient only if the peptide breakdown in pear tissue is counterbalanced by its continuous secretion into the intercellular spaces to maintain a bactericidal concentration at the point of infection. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.