Comparison of Paenibacillus polymyxa wild-type and Nif− mutant in colonization, plant-growth promotion and nitrogen fixation contribution

Aims This study aimed to compare the effect on colonization, plant-growth promotion and nitrogen fixation contribution by inoculation with Paenibacillus polymyxa wild-type and Nif−mutant. Methods Paenibacillus polymyxa wild-type and Nif− mutant was labeled with GFP and then the GFP-labeled bacteria were used to inoculate cucumber. The colonization patterns of P. polymyxa WLY78 in these plants were observed under the confocal laser scanning microscope. The effects of plant-growth promotion were investigated by greenhouse experiments. The nitrogen fixation contribution was estimated by 15N isotope dilution experiments. Results Observation by laser confocal microscopy revealed that both P. polymyxa WLY78 and ΔnifB-V mutant can effectively colonize cucumber root, stem and leaf tissues. Greenhouse experiments showed that inoculation with P. polymyxa WLY78 can significantly enhance the lengths and fresh wights of cucumber roots and shoots, but inoculation with ΔnifB-V mutant can 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 can not provide nitrogen for plant. Conclusions This present study demonstrates that nitrogen fixation plays an import role in promoting plant growth.