Azomycin produced by <i>Pseudomonas</i> has both phytotoxic and anti-oomycete activity

Members of the genus <i>Pseudomonas</i> synthesize diverse natural products that contribute to their versatility in free-living and host-associated lifestyles. Here, we characterized the <i>in vivo</i> functions of the nitroimidazole antibiotic azomycin in the genus <i>Pseudomonas</i>. We found that genes with similarity to azomycin biosynthesis genes <i>rohPQRST</i> are prevalent within the <i>Pseudomonas syringae</i> species complex and rarely present in the <i>Pseudomonas fluorescens</i> species complex. Azomycin production was detectable in culture by biocontrol strains <i>Pseudomonas</i> spp. DF41 and CMR5a. <i>Pseudomonas</i> sp. DF41 exhibited anti-oomycete activity that was lost in a <i>∆rohPQRST</i> mutant. Purified azomycin was sufficient to kill the oomycete pathogens <i>Aphanomyces</i> and <i>Phytophthora</i>. Although DF41 has been studied for its role in biocontrol of plant pathogens, we found that azomycin exhibited phytotoxicity against <i>Pisum sativum</i> (pea) plants at similar concentrations to those that inhibited oomycetes. However, consistent with its use as a biocontrol agent, <i>Pseudomonas</i> sp. DF41 only produced azomycin <i>in planta</i> when pea plants were infected with the oomycete pathogen <i>Aphanomyces euteiches</i>. Our findings suggest dual roles for azomycin in <i>Pseudomonas,</i> functioning both as a biocontrol agent of oomycete pathogens, as well as a phytotoxic molecule with a potential role in plant virulence.IMPORTANCEWhile many natural products are studied for their roles in the treatment of plant or human disease, the ecological functions of natural products are understudied. We found that an antibiotic, azomycin, is produced by <i>Pseudomonas</i> species and has toxicity against both plants and oomycete pathogens. Our findings suggest a complex ecological role of azomycin production by <i>Pseudomonas</i> in both the amelioration and exacerbation of plant disease.