Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/16/2004
Publication Date: 12/1/2004
Citation: Temple, T.N., Stockwell, V.O., Loper, J.E., Johnson, K.B. 2004. Bioavailability of iron to pseudomonas fluorescens strain a506 on flowers of pear and apple. Phytopathology. 94:1286-1294 Interpretive Summary: The biological control agent Pseudomonas fluorescens A506 is registered for use on pear and apple trees to manage fire blight, an important disease that constrains pome fruit production throuhgout the US and many parts of the world. The biological control agent is typically sprayed on pear and apple trees during the bloom period, and 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 reporter gene, a tool from molecular biology, to determine if there is enough iron available on pear and apple blossoms to allow the biological control agent to produce the antibiotic. 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. Field studies are now in progress to find out if the biological control agent is more effective in combatting fire blight when it is sprayed on pear and apple blossoms in combination with FeEDDHA.
Technical Abstract: The addition of 0.1 mM FeCl3 to a defined culture medium induces the bacterial epiphyte Pseudomonas fluorescens strain A506 (A506) to produce an antibiotic toxic to the fire blight pathogen, Erwinia amylovora. Since A506 is registered and applied as a commercial product to suppress E. amylovora before floral infection of pear and apple, the relative availability of iron to A506 on surfaces of pear and apple flowers is of potential significance. An 'iron biosensor' construct of A506 was developed by transformation with an iron-regulated promoter (pvd) fused to a promoterless ice nucleation reporter gene (inaZ). This construct, A506 (pvd-inaZ), established high populations on pear and apple flowers, ranging from 10 thousand to 1 million colony forming units per flower. In seven trials on pear and apple trees, A506 (pvd-inaZ) expressed high ice nucleation activity (INA) on flowers, indicating limited iron bioavailability or a low-iron environment unlikely to induce antibiotic production by A506. A506 (pvd-inaZ) also colonized flowers when mixed with chemicals containing iron: FeSO4 or the iron chelates ferric ethylenediaminedi-(o-hydroxyphenylacetic) acid (FeEDDHA) and ferric diethylenetriamine pentaacetate (FeDTPA). These compounds represent an array of commercial iron formulations applied to foliage to avert iron chlorosis. Treatment of flowers with a mixture of A506 (pvd-inaZ) and 3 mM FeEDDHA or FeDTPA significantly decreased INA compared to flowers treated with A506 (pvd-inaZ) in water. Lower concentrations (0.3 mM) of FeEDDHA, however, did not consistently suppress INA. These results indicate that apple and pear flowers represent an iron-limited environment to A506 and that treatment with 3 mM FeEDDHA is needed to increase significantly the level of iron available to this bacterium.