Arbuscular mycorrhiza (AM) is an extremely ancient, most common and widespresd terrestrial symbiont. The formation of AM symbiont involves complicated pathways of signals molecules exchange and transduction, which are stringent and coordinated processes. Our studies focused on the signals transduction pathways in AM plants. Roles of nitric oxide (NO) in the process of symbiosis between AM fungus Glomus mosseae (G.m) and tobacco (Nicotiana tabacum, cultivar CF90NF) were investigated with using pharmacological experiments combined with spectrophotometry and fluorescence microscopy in the present study. Results showed that there was some endogenous NO in tobacco lateral roots, the endogenous NO content and NO fluorescence intensity significantly enhanced in tobacco lateral root 10 days after inoculation with G.m Sodium nitroprusside (SNP), a NO donor, increased the colonization percentage of tobacco by G.m. However, a NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline- 1-oxyl-3-oxidepotassium salt (cPTIO) decreased the NO fluorescence intensity in the lateral root of tobacco seedlings and fungal hyphae, as well as the colonization percentage. Meanwhile, nitrate reductase (NR) activity and the expression of Nia-1 increased obviously in the lateral root of tobacco seedlings inoculated with G.m However, the NR inhibitor sodium tungstate (Na₂WO₄) could decrease the NO fluorescence intensity in tobacco lateral roots, but had no significant effect on that in fungal hyphae. The present study suggested that NO may participate in tobacco-AM symbiosis and NR play important role in NO production in tobacco roots, and there may be exist other sources of NO in AM fungal hyphae. If there existed other signal substances in symbiotic process between AM fungi and plants, what is the relationship of these signal substances? And how they interact? These all require for further investigation with molecular biological methods. Only by understanding the identification and symbiosis signal between each other, can they unlock the mystery of AM fungi symbiosis with plants.