Submitted to: Acta Horticulturae
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/1/2005
Publication Date: 2/1/2006
Citation: Temple, T.N., Stockwell, V.O., Johnson, K.B., Loper, J.E. 2006. Bioavailability of iron to pseudomonas fluorescens a506 on flowers of pear. Acta Horticulturae. 704:301-306. Interpretive Summary: The biological control agent Pseudomonas fluorescens A506 is registered in the United States for control of fire blight, an important disease of apple and pear. Fire blight is caused by a bacterium, Erwinia amylovora, which infects blossoms and moves from the blossoms throughout the plant, sometimes killing the entire tree. The biological control agent is typically sprayed on pear and apple trees during the bloom period; it colonizes blossom surfaces, thereby preventing the infection of those blossoms by the fire blight pathogen. We discovered that the biological control agent produces an antibiotic that is toxic against the pathogen that causes fire blight, but the antibiotic is produced only in the presence of iron. We used a "biosensor," constructed using a reporter gene, to determine if there is enough iron available on pear and apple blossoms to allow antibiotic production by the biological control agent. We found that iron levels were low on the blossoms, but they could be enhanced by spraying trees with an iron chelator such as FeEDDHA, which is a registered product for correcting iron deficiency of plants.
Technical 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 blossoms, 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 104 to 107 colony forming units on pear and apple. In ten trials on pear and apple 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 FeSO4 or the iron chelate, ferric ethylenediaminedi-(o-hydroxyphenylacetic) acid (FeEDDHA). Co-treatment of blossoms with a mixture of A506 (pvd-inaZ) and 3 mM FeEDDHA significantly decreased ice nucleation activity compared to blossoms treated with A506 (pvd-inaZ) in water. Lower concentrations (i.e. 0.3 mM FeEDDHA) did not measurably increase iron available to A506 on blossoms. These results indicate that apple and pear blossoms represent an iron-limited environment to A506 and that co-treatment with at least 3 mM FeEDDHA is required to significantly increase the level of iron biologically available to this bacterium.