BIOAVAILABILITY OF IRON TO PSEUDOMONAS FLUORESCENS STRAIN A506 ON FLOWERS OF PEAR

Abstract

Pseudomonas fluorescens strain A506 (A506) produces an antibiotic toxic to Erwinia amylovora in defined culture media containing at least 0.1 mM FeCl3. To estimate the relative availability of iron on flowers, A506 was transformed with an 'iron biosensor', which consisted of an iron-regulated promoter (pvd) fused to an ice nucleation reporter gene (inaZ). A506 (pvd-inaZ) established high populations on blossoms, ranging from 105 to 107 colony forming units on pear. In two trials on screenhouse grown pear trees, A506 (pvd-inaZ) expressed high ice nucleation activity on blossoms indicating limited iron bioavailability, or a low-iron environment unlikely to induce antibiosis by A506. A506 (pvd-inaZ) also colonized blossoms when mixed with the iron chelate, ferric ethylenediaminedi-(o-hydroxyphenylacetic) acid (FeEDDHA). Co-treatment of flowers with a mixture of A506 (pvd-inaZ) and 3 mM FeEDDHA significantly decreased ice nucleation activity compared to flowers treated with A506 (pvd-inaZ) in water. Lower concentrations (i.e. 0.3 mM FeEDDHA) did not consistently increase iron available to A506 on flowers sufficient to suppress expression of the iron regulated reporter gene construct. These results indicate that pear flowers represent an iron-limited environment to A506 and that co-treatment with at least 3 mM FeEDDHA significantly increases the level of iron biologically available to this bacterium.