Canker Control by the Siderophore Pyochelin from Pseudomonas fluorescens

This work identifies a compound able to inhibit Xanthomonas citri subsp. citri (citrus canker agent) growth in vitro and in vivo. First, we isolated bacteria from citrus leaf surfaces that were able to inhibit X. citri subsp. citri in vitro. Among the selected isolates, we focused on one with remarkable activity. The strain was characterized as Pseudomonas fluorescens after sequencing its 16S rDNA and analyzing the sequence with BLASTn. The purification and chemical analysis of the active compound allowed us to assign the inhibitory activity to enantio-pyochelin (E-Pch). Since this molecule is a siderophore, we wondered if the inhibition observed was a result of iron scavenging. Surprisingly, when we supplemented media with an excess of iron, we observed practically no change in the inhibition activity. In an attempt to identify the action mechanism of E-Pch, we evaluated the ability of E-Pch to generate reactive oxygen species (ROS) within a culture of X. citri subsp. citri and its correlation with the inhibitory activity. In fact, we observed increased ROS levels when E-Pch was added. In addition, the reducing agent, ascorbic acid, lowered ROS levels and the antibiotic activity, implying that inhibition is probably caused by oxidative stress. Finally, we studied the use of E-Pch in a model of canker disease. EPch reduced canker formation on leaves of 'Eureka' and 'Lisbon' lemon cultivars. These results show E-Pch as a promising compound for citrus canker biocontrol.