Improved N2 fixation in 2,4-D treated wheat roots associated with Azospirillum lipoferum: Studies of colonization using reporter genes

Among several Azospirillum strains tested for N2 fixation in association with wheat in a model system, 2 A. lipoferum strains (SpBr17 and 596) were found to be more efficient than the 8 other A. lipoferum and A. brasilense strains tested. Plants treated with low concentrations (0.7 μg ml−1) of 2,4-dichlorophenoxyacetic acid (2,4-D) followed by inoculation supported larger acetylene reduction activity than plants treated only with bacteria. Three broad host range plasmids carrying lacZ, as a reporter gene, controlled by nifA, nodG and ntrC promoters were transferred to A. lipoferum SpBr17 and strain 596 by conjugation. All the transconjugants showed high β-galactosidase activity in the free-living condition. Studies on colonization of wheat roots using the SpBr17 nodG-lacZ gene fusion, which expressed the lacZ gene at the highest level, indicated the localization of bacteria on the young parts of roots as a thick coat. Counts of azospirilla on plant roots showed increased association of bacteria in 2,4-D treated plants.