Biological properties and spectrum of activity of Pseudomonas syringae pv syringae toxins

Syringomycin E (SR-E), syringotoxin (ST) and two forms (SP22-A and SP25-A) of syringopeptin (SP), the main lipodepsipeptides produced by strains of Pseudomonas syringae pv. syringae, were investigated for their biological activities. All toxins caused lysis of sheep erythrocytes, but SPs were more active than SR-E and ST. The lytic concentrations were 0.8 and 0.6 mu m for SP22-A and SP25-A and 1.4 and 5.9 mu M for SR-E and ST, respectively. The effect of toxins on the electrolyte leakage from carrot tissues and on the viability of potato tuber tissues confirmed the higher phytotoxicity of SPs in comparison to the SR-E or ST. A relevant effect on electrolyte leakage was induced by 0.4 mu M SP22-A and 10 mu M ST, respectively. The spectrum of antimicrobial activity indicated that 50 mu M toxin solutions did not inhibit the growth of Gram-negative bacteria. Grampositive bacteria were sensitive to SPs but in contrast, SR-E and ST had no relevant effect on the growth of these bacteria. In particular, the growth of Bacillus megaterium was inhibited by 1.56 mu M SP22-A and 3.12 mu M SP25-A, but was unaffected by 50 mu M SR-E or ST. The effects of toxins on fungi were variable. For example, 6.25 mu M SR-E or ST, 12.5 mu M SP22-A and 37.5 mu M SP25-A inhibited the growth of the yeast Rhodotorula pilimanae. The mycelial growth of the plant pathogenic fungus Botrytis cinerea was inhibited by 1.6 mu M SP25-A, 12.5 mu M SP22-A, 18.7 mu M SR-E and 25 mu M ST. Furthermore, SR-E was the most active toxin in inhibiting conidial germination and germ tube elongation of this fungus. (C) 1997 Academic Press Limited.