Background and aims: Herbaspirillum seropedicae is a Plant Growth Promoting Bacterium (PGPB) that colonizes crops of economic interest, especially grasses. Here, we aimed to characterize the association of H. seropedicae with the model dicot Arabidopsis thaliana and its effect on plant growth. The molecular pathways activated during the association were monitored by comparative proteomics. Methods: Arabidopsis thaliana seedlings were inoculated with H. seropedicae and grown for 28 days. Plant growth promotion was analyzed by quantifying the total leaf area and the fresh and dry biomass. The endophytic bacterial population was quantified by counts of colony-forming units (CFU), and the bacterial location in plant tissues was determined by epifluorescence microscopy. Changes in the proteomic profile of the plants were investigated by ESI-LC-MS/MS. Results: Herbaspirillum seropedicae colonized the roots of A. thaliana, but was absent from the shoots. The association promoted plant growth significantly. The proteomic analysis revealed several protein groups regulated in the roots and shoots during the association, with an emphasis on the photosynthesis and phenylpropanoid biosynthesis. Conclusions: Herbaspirillum seropedicae colonizes A. thaliana roots and promotes its growth, leading to an increase in biomass and the modulation of specific proteomic responses. These findings open new perspectives into the study of the molecular basis of the positive interaction between H. seropedicae and host plants, both monocots and dicots. [ABSTRACT FROM AUTHOR]