The use of plant growth-promoting bacteria as agricultural inoculants of plants should be encouraged because of their prominent role in biological nitrogen fixation, the increase of nutrient uptake by roots, abiotic stress mitigation, and disease control. The complex mechanisms underlying the association between plant and beneficial bacteria have been increasingly studied, and proteomic tools can expand our perception regarding the fundamental molecular processes modulated by the interaction. In this study, we investigated the changes in protein expression in maize roots in response to treatment with the endophytic diazotrophic Herbaspirillum seropedicae and the activities of enzymes related to nitrogen metabolism. To identify maize proteins whose expression levels were altered in the presence of bacteria, a label-free quantitative proteomic approach was employed. Using this approach, we identified 123 differentially expressed proteins, of which 34 were upregulated enzymes, in maize roots cultivated with H. seropedicae. The maize root colonization of H. seropedicae modulated the differential expression of enzymes involved in the stress response, such as peroxidases, phenylalanine ammonia-lyase, and glutathione transferase. The differential protein profile obtained in the inoculated roots reflects the effect of colonization on plant growth and development compared with control plants. K E Y W O R D S biostimulation, endophytes, label-free proteomics, N 2 -fixation 1 INTRODUCTION The enhanced use of fertilizers for supporting agriculture has led to deterioration of the environment [1]. Nitrogen fertilization of nonleguminous crops is one of the most expensive inputs to increase agri-cultural yields [2]. The use of plant growth-promoting bacteria (PGPB)as agricultural plant inoculants should be encouraged because of their prominent role in biological nitrogen fixation (BNF) and the increased root nutrient absorption, which mitigate abiotic stress and help in disease control. Herbaspirillum seropedicae has been studied because of its