AimsThe aim of this study was to determine whether ACC (1-aminocyclopropane-1-carboxylic acid) deaminase-containing bacterial treatments could enhance the tolerance of poppy (Papaver somniferum L.) plants against biotic stress of downy mildew caused by Peronospora sp. Methods and ResultsThree different genotypes of P. somniferum, that is, Sampada, J-16 and I-14 were included in the experiment. The ACC deaminase-containing bacteria Pseudomonas putida (WPTe) reduced the downy mildew disease severity and significantly improved the growth and yield of P. somniferum plants. The chlorophyll content, photosynthetic rate, stomatal conductance and transpiration rate were modulated upon WPTe treatments in the poppy plants. We observed reduced synthesis of ethylene precursor (ACC) and abscisic acid (ABA), and enhanced production of indole acetic acid (IAA) in P. somniferum plants upon WPTe treatments. Moreover, WPTe treatment reduced proline and lipid peroxidation in plant leaves. ConclusionThese results highlight that the ACC deaminase-containing plant growth-promoting rhizobacteria (PGPR) enhance the tolerance of P. somniferum plant against downy mildew. Significance and Impact of the StudyACC deaminase-containing PGPR may be used against phytopathogens which apart from protecting the plants from the disease could also be useful in reducing ethylene-induced damages in the event of abiotic stresses.