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