Allelic variation in the indoleacetic acid-lysine synthase gene of the bacterial pathogen Pseudomonas savastanoi and its role in auxin production.

Indole-3-acetic acid (IAA) production is a pathogenicity/virulence factor in the complex, including . pathovars (pvs.) genomes contain the gene, encoding an enzyme that catalyzes the biosynthesis of the less biologically active compound 3-indole-acetyl-ϵ-L-lysine (IAA-Lys). Previous studies have reported the identification of IAA-Lys in culture filtrates of strains isolated from oleander (pv. nerii), but the conversion of IAA into a conjugate was not detectable in olive strains (pv. savastanoi). In this paper, we show the distribution of alleles in all available genomes of strains isolated from woody hosts. Most strains encode two different paralogs, except for those isolated from broom (pv. retacarpa), which contain a single allele. In addition to the three previously reported alleles ( , and ), we identified , an exclusive allele of strains isolated from ash (pv. fraxini). We also found that the production of IAA-Lys in pv. savastanoi and pv. nerii depends on a functional allele, whereas in pv. fraxini depends on . The production of IAA-Lys was detected in cultures of an olive strain heterologously expressing IaaL, IaaL and IaaL, but not when expressing IaaL. In addition, seedlings treated with the strains overproducing the conjugate, and thus reducing the free IAA content, alleviated the root elongation inhibitory effect of IAA. IAA-Lys synthase activity assays with purified allozymes confirmed the functionality and specificity of lysine as a substrate of IaaL and IaaL, with IaaL showing the highest catalytic efficiency for both substrates. The IAA-Lys synthase activity of IaaL was abolished by the insertion of two additional tyrosine residues encoded in the inactive allozyme IaaL. These results highlight the relevance of allelic variation in a phytohormone-related gene for the modulation of auxin production in a bacterial phytopathogen.