Canker Control by the Siderophore Pyochelin from Pseudomonas fluorescens

Abstract

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 fromcitrus leaf surfaces that were able to inhibit X. citri subsp. citri in vitro. Among theselected isolates, we focused on one with remarkable activity. The strain wascharacterized as Pseudomonas fluorescens after sequencing its 16S rDNA andanalyzing the sequence with BLASTn. The purification and chemical analysis of theactive compound allowed us to assign the inhibitory activity to enantio-pyochelin (EPch).Since this molecule is a siderophore, we wondered if the inhibition observedwas a result of iron scavenging. Surprisingly, when we supplemented media with anexcess of iron, we observed practically no change in the inhibition activity. In anattempt to identify the action mechanism of E-Pch, we evaluated the ability of E-Pchto generate reactive oxygen species (ROS) within a culture of X. citri subsp. citri andits correlation with the inhibitory activity. In fact, we observed increased ROS levelswhen E-Pch was added. In addition, the reducing agent, ascorbic acid, lowered ROSlevels and the antibiotic activity, implying that inhibition is probably caused byoxidative stress. Finally, we studied the use of E-Pch in a model of canker disease. EPchreduced canker formation on leaves of ?Eureka? and ?Lisbon? lemon cultivars.These results show E-Pch as a promising compound for citrus canker biocontrol.