Pyoverdines Are Essential for the Antibacterial Activity of Pseudomonas chlororaphis YL-1 under Low-Iron Conditions.

YL-1 has extensive antimicrobial activities against phytopathogens, and its genome harbors a pyoverdine (PVD) biosynthesis gene cluster. The alternative sigma factor PvdS in PAO1 acts as a critical regulator in response to iron starvation. The assembly of the PVD backbone starts with peptide synthetase enzyme PvdL. PvdF catalyzes formylation of l-OH-Orn to produce l--hydroxyornithine. Here, we describe the characterization of PVD production in YL-1 and its antimicrobial activity in comparison with that of its PVD-deficient , , and mutants, which were obtained using a -based site-specific mutagenesis strategy. Using methods, we examined the effect of exogenous iron under low-iron conditions and an iron-chelating agent under iron-sufficient conditions on PVD production, antibacterial activity, and the relative expression of the PVD transcription factor gene in YL-1. We found that strain YL-1, the mutant, and the (pUCP26-) complemented strain produced visible PVDs and demonstrated a wide range of inhibitory effects against Gram-negative and Gram-positive bacteria under low-iron conditions and that with the increase of iron, its PVD production and antibacterial activity were reduced. The antibacterial compounds produced by strain YL-1 under low-iron conditions were PVDs based on liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis. Moreover, the antibacterial activity observed was correlated with control efficacies of strain YL-1 against rice bacterial leaf blight (BLB) disease caused by pv. oryzae. Collectively, PVDs are responsible for the antibacterial activities of strain YL-1 under both natural and induced low-iron conditions. The results demonstrated that PVDs are essential for the broad-spectrum antibacterial activities of strain YL-1 against both Gram-positive and Gram-negative bacteria under low-iron conditions. Our findings also highlight the effect of exogenous iron on the production of PVD and the importance of this bacterial product in bacterial interactions. As a biocontrol agent, PVDs can directly inhibit the proliferation of the tested bacteria in addition to participating in iron competition.