Aim The use of plant growth-promoting bacteria (PGPB) to reduce requirements for mineral nitrogen fertilization in sustainable agriculture is an important issue. We studied how reduced nitrogen fertilization affects the growth-promoting capacity oí Enterobacter radicincitans, the bacteria's root colonization behavior, and the regulation of nitrogen, phosphate and hormone marker genes in tomato (Solanum lycopersicum). Method Tomato seeds inoculated with 10 7 cfixmL⁻¹ E. radicincitans or 0.05 M NaCl, were grown under two distinct nitrogen fertilization regimes. Results When treated with E. radicincitans, plant root or shoot dry weight increased by 180 %, or 150 % in high nitrogen (HN) conditions, but increased only by 120 %, or 140 % in low nitrogen (LN) conditions. HN plants were colonized by a higher number of total bacteria, including E. radicincitans, than LN plants. We found that E. radicincitans-trQatQd plants expressed higher levels of a pathogen defense-related marker gene in LN conditions than in HN conditions. Transcription of the oxylipin biosynthesis marker genes was suppressed in E. radicincitans-treatod plants under both fertilization regimes. Conclusion Our findings indicate that drastic reduction in nitrogen fertilization results in modulating plant hormone signaling, thus inhibiting the optimal effect of E. radicincitans on tomato.