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. © 1991.