A MUTATION IN THE INDOLE-3-ACETIC-ACID BIOSYNTHESIS PATHWAY OF PSEUDOMONAS-SYRINGAE PV SYRINGAE AFFECTS GROWTH IN PHASEOLUS-VULGARIS AND SYRINGOMYCIN PRODUCTION

Homologs of the genes for indole-3-acetic acid (IAA) biosynthesis from Pseudomonas syringae pv. savastanoi were retrieved from a genomic library of P. syringae pv. syringae, and their nucleotide sequences were determined. Sequence relatedness between the P. syringae pv. syringae and P. syringae pv. savastanoi iaa operons is greater than 90% within the iaaM and iaaH loci but declines dramatically at a position approximately 200 bp 5' of the iaaM translation initiation codon. A third open reading frame was detected downstream of iaaH. Production of IAA was undetectable in mutant strain Y30-53.29, which was generated by transposition of Tn5 into the iaaM gene of P. syringae pv. syringae Y30. The IAA-deficient (IAA(-)) mutant retained the ability to colonize the bean phylloplane and induced disease symptoms on bean which were similar to those produced by the parental strain. However, the population dynamics of the IAA(-) strain during the parasitic phase in leaves differed from those of both the parental strain and the mutant genetically restored for IAA biosynthesis. The mutant was capable of inducing disease symptoms when established in bean tissues at a lower initial cell density than either IAA-producing strain. Syringomycin biosynthesis by the IAA(-) strain was diminished in comparison with the parental strain or the mutant genetically restored for IAA production. The results indicate that bacterially derived IAA or its biosynthesis, is involved in the regulation of in planta growth and in the expression of other factors that affect the host-pathogen interaction.