Studies on the role of indole-3-acetic acid and cytokinins in the formation of knots on olive and oleander plants by Pseudomonas syringae pv. savastanoi

Relationships between the ability to produce indole-3-acetic acid (IAA) and cytokinins in culture and pathogenicity were studied in different strains of Pseudomonas syringae pv. savastanoi from olive and oleander and in their α-methyltryptophan resistant mutants. Wild type olive and oleander strains of pv. savastanoi produced both IAA and cytokinins in culture but the amount produced varied between strains. The α-methyltryptophan resistant mutants produced little or no IAA but about the same amount of cytokinins as their parent strains.

All wild type strains induced knots on olive whereas only the wild type strains from oleander were pathogenic on oleander. The size of the knots on olive and the time necessary for their development varied between strains. The olive strain mutants and the mutant from the oleander strain PBa219 did not usually induce symptoms on either olive or oleander, but mutants derived from oleander strains ITM519 and NCPPB640 induced atypical knots on olive. Mutants from the oleander strains induced necrotic symptoms only on oleander. The results indicate that the amount of IAA produced determines the length of the incubation period, whereas the amount of cytokinin produced determines the size of the knot.

All the wild type olive strains and their mutants exhibited the same plasmid profile whereas the mutants derived from the oleander strains lacked one plasmid. This suggests that the oleander strains do not carry the genetic determinants for cytokinin production on the plasmid which encodes for IAA production.