Nitrogen fixation by Paenibacillus polymyxa WLY78 is responsible for cucumber growth promotion

To study nitrogen contribution to cucumber derived from nitrogen fixation of Paenibacillus polymyxa WLY78. The nif gene cluster deletion mutant (ΔnifB-V) of Paenibacillus polymyxa WLY78 was constructed by a homologous recombination method. The effects of plant-growth promotion were investigated by greenhouse experiments. The nitrogen fixation contribution was estimated by 15N isotope dilution method (also being called the 15N natural abundance technique). Deletion of nif gene cluster of P. polymyxa WLY78 resulted in complete loss of nitrogenase activity. Greenhouse experiments showed that inoculation with P. polymyxa WLY78 could significantly enhance the lengths and dry weights of cucumber roots and shoots, but inoculation with ΔnifB-V mutant could not. 15N isotope dilution experiments showed that cucumber plants derive 25.93% nitrogen from nitrogen fixation performed by P. polymyxa WLY78, but the ΔnifB-V mutant nearly could not provide nitrogen for plant growth. This present study demonstrated that nitrogen fixation performed by P. polymyxa WLY78 contributes to plant growth.