Evaluation of alternative IAA biosynthetic pathways in the bacteria from the pseudomonas syringae complex

Abstract

Indole-3-acetic acid (IAA) is a phytohormone belonging to the auxin group which production is widely distributed among plant-associated bacteria. In phytopathogenic bacteria, several IAA biosynthetic pathways have been described. The best characterized is the indole-3-acetamide (IAM) pathway, where tryptophan is initially converted into IAM by a monooxygenase (iaaM gene), and later transformed to IAA in a reaction catalysed by a hydrolase (iaaH gene). Pseudomonas savastanoi pv. savastanoi NCPPB 3335 (Psv), which synthesizes IAA through IAM, encodes two paralogs of these two genes organized in two operons (iaaMH-1 and iaaMH-2). Previously, we have demonstrated that a Psv mutant in the iaaMH-1 operon produces an amount of IAA significantly lower than that synthesized by the wild type strain. This strain, shows a reduced virulence in olive plants. In contrast, a mutant in the iaaMH-2 operon (which encodes a iaaM-2 pseudogene), produces IAA levels similar to those of the wild type strain and is not affected in virulence. Unexpectedly, the iaaMH-1 mutant and the double mutant iaaMH- 1/iaaMH-2 synthesize a residual amount of IAA, suggesting the existence of an alternative route for the production of this compound in Psv.