Azomycin produced by Pseudomonas has both phytotoxic and anti-oomycete activity

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