Abstract: The subject invention concerns materials and methods for providing or enhancing nitrogen fixation in plants. The invention provides for the use of nitrogen fixing bacteria that are isolated from nitrogen efficient plants. Plants for which enhanced nitrogen fixation is desired are inoculated with an effective amount of nitrogen firing bacteria of the invention. In an exemplified embodiment, the bacteria is Klebsiella Kp342. The subject invention also concerns means to increase the number of free-living nitrogen-fixing bacteria in plants. Mutants of beneficial endophytic bacteria that are resistant to plant defense responses can be used to colonize a plant in numbers higher than a wild type or a non-mutated bacteria can colonize a plant. The higher number of bacteria colonizing the plant provide for more nitrogen fixation for the plant.