a P. de Ecolog|Ła Molecular y Microbiana, Centro de InvestigacioŁn sobre FijacioŁn de NitroŁgeno, Universidad Nacional AutoŁnoma de MeŁxico,

Acetobacter diazotrophicus is a nitrogen-fixing endophytic bacterium, originally isolated from sugarcane. Its colonizing ability was evaluated in high and low N-fertilized sugarcane plants by inoculating stem-cuts with a L-glucuronidase marked A. diazotrophicus strain. Bacterial quantification by the most probable number technique showed a severe decrease of A. diazotrophicus cells in plants fertilized with high levels of nitrogen. The inoculated strain was detected inside low N-fertilized sugarcane plants by histochemical staining of L-glucuronidase and scanning electron microscopy. A. diazotrophicus was found mainly inside cortical cells of stems and inside xylem vessels. No L-glucuronidase activity was observed in non-inoculated plants. High nitrogen fertilization of fields might be a threat to maintaining naturally occurring endophyti