Indole-3-acetic acid biosynthetic pathways in Erwinia herbicola in relation to pathogenicity on Gypsophila paniculata

Pathogenic strains of Erwinia herbicola incite crown and root galls in the flowering ornamental gypsophila. Both pathogenic and non-pathogenic strains of the bacterium readily produce indole-3-acetic acid in culture. Two pathways for biosynthesis of indole-3-acetic acid were identified in E. herbicola: (1) the indole-3-acetamide route occurs via the following reactions: l-tryptophan → indole-3-acetamide → indole-3-acetic acid, and (2) the indole-3-pyruvate route involves the following reactions: l-tryptophan → indole-3-pyruvate → indole-3-acetaldehyde → indole-3-acetic acid. Production of indole-3-ethanol was also linked to the latter pathway. Evidence for the existence of the two pathways was based on: (a) chemical identification of the respective indole intermediates by thin layer chromatography, high performance liquid chromatography and gas chromatography-mass spectroscopy; (b) production of indole-3-acetic acid by bacterial cells treated with the various indole intermediates; and (c) incorporation of 3-¹⁴C-l-tryptophan into the indole intermediates of the two pathways. In contrast to the indole-3-pyruvate pathway which was detected in all the pathogenic and non-pathogenic strains examined, the indole-3-acetamide pathway was detected only in the pathogenic strains of E. herbicola. The possible relationship between the indole-3-acetamide pathway and gall formation by E. herbicola is discussed.