Phytotoxic properties of Pseudomonas syringae pv. syringae toxins

Syringomycin E, syringomycin G and syringopeptin 25A, the main components of the Pseudomonas syringae pv. syringae toxin mixture, were assayed for their phytotoxicity, determined as electrolyte leakage from carrot tissues, necrosis of tobacco leaves, and death of potato tissues, and for their antimicrobial activity on Rhodotorula pilimanae. p]In the antimicrobial assay, syringomycins were 30 times more active than syringopeptin 25A, but, in the electrolyte leakage assay, they proved to be 40 times less active than the former since a significant effect required concentrations of 16 and 0·4 μm respectively. A statistically identical effect on electrolyte leakage was obtained with syringopeptin 25A or with an unfractionated toxin mixture equimolar for syringopeptin 25A. A similar pattern of activity was observed in the tobacco leaf and potato disc assays. After a short incubation at pH 10, syringomycins completely lost their antimicrobial and phytotoxic activities, while syringopeptin 25A retained all its antimicrobial activity and most of its phytotoxicity. These findings indicate that syringopeptin 25A and syringomycins are mainly responsible for, respectively, the phytotoxic activity and the antimicrobial activity on R. pilimanae, characteristic of the unfractionated toxin mixture of P. s. pv. syringae. The high phytotoxicity of syringopeptin 25A is a new finding which prompts a careful examination of the role played by the individual metabolites in the disease caused by P. s. pv. syringae ecotypes.