Comparison of benefit to sugarcane plant growth and N-15(2) incorporation following inoculation of sterile plants with Acetobacter diazotrophicus wild-type and Nif(-) mutant strains

The ability of the nitrogen-fixing bacterial endophyte Acetobacter diazotro phicus strain PA15 to enhance the growth of sugarcane SP70-1143 was evaluat ed in the growth chamber, greenhouse, and field by comparing plants inocula ted with wild-type and Nif(-) mutant MAd3A in two independent experiments. The wild-type and Nif(-) mutant strains colonized sugarcane plants equally and persisted in mature plants, In N-deficient conditions, sugarcane plants inoculated with A. diazotrophicus PA15 generally grew better and had a hig her total N content 60 days after planting than did plants inoculated with mutant MAd3A or uninoculated plants, These results indicate that the transf er of fixed N from A. diazotrophicus to sugarcane might be a significant me chanism for plant growth promotion in this association. When N was not limi ting, growth enhancement was observed in plants inoculated with either wild -type or Nif(-) mutants, suggesting the additional effect of a plant growth promoting factor provided by A, diazotrophicus, A(15)N(2) incorporation ex periment demonstrated that A. diazotrophicus wild-type strains actively fix ed N-2 inside sugarcane plants, whereas the Nif(-) mutants did not.