Effect of iron on biological control of fire blight by Pseudomonas fluorescens A506

Graduation date: 2004Competitive exclusion has been the mechanism hypothesized to account for the\ud biological control of fire blight disease of pear and apple by the bacterium Pseudomonas\ud fluorescens A506 (A506). Recent laboratory assays demonstrated, however, that A506\ud produces an antibiotic that is toxic to the fire blight pathogen, Erwinia amylovora, when\ud cultured on media amended with iron (Fe⁺² or Fe⁺³). This study investigated this iron-dependent\ud antibiosis by A506 by: 1) examining bioavailability of iron to A506 on\ud blossom surfaces, 2) mutagenizing A506 to disrupt genes involved in antibiotic\ud production, and 3) evaluating suppression of fire blight by A506 when co-treated with an\ud iron chelate (FeEDDHA). Bioavailability of iron on blossoms was investigated with an\ud iron biosensor [iron-regulated promoter (pvd) fused to an ice nucleation reporter gene\ud (inaZ)] in A506. A506 (pvd-inaZ) expressed high ice nucleation activity (INA) on\ud blossoms indicating a low-iron environment unlikely to induce antibiosis by A506.\ud Spraying blossoms with FeEDDHA at concentrations ≥0.1 mM significantly suppressed\ud INA by A506 (pvd-inaZ).\ud Transposon mutagenesis was used to generate and select mutants of A506\ud exhibiting altered antibiotic production profiles. One antibiotic-deficient mutant, A506\ud Ant⁻, was recovered; this mutant showed reduced epiphytic fitness on blossoms of apple\ud and pear trees compared to the parent stain, A506. Another mutant, A506 Ant⁺, lost the\ud characteristic fluorescent phenotype and exhibited iron-independent antibiotic production\ud in defined culture media. A506 Ant⁺ established high populations on blossoms of apple\ud and pear trees, similar to populations attained by A506, and reduced incidence of fire\ud blight between 20 to 40%, levels comparable to A506 in orchard trials.\ud In orchard trials, A506 was co-treated with FeEDDHA and fire blight suppression\ud was evaluated. Bacterial strains established high populations on blossoms when co-treated\ud with 0.1 mM FeEDDHA or in water. Significantly enhanced suppression of fire\ud blight incidence by antibiotic producing strains of A506 amended with 0.1 mM\ud FeEDDHA was observed in 2 of 5 trials, providing some evidence that iron-induced\ud antibiosis can be a contributing mechanism in disease control. Lack of disease control by\ud the antibiotic deficient strain, A506 GacS, and by 0.1 mM FeEDDHA alone added\ud support to this hypothesis