Evidence of nitrogen fixation and growth promotion in canola (Brassica napus L.) by an endophytic diazotroph Paenibacillus polymyxa P2b-2R

Abstract

An endophytic diazotroph, Paenibacillus polymyxa P2b-2R, previously isolated from internal tissues of lodgepole pine tree was tested for its ability to colonize, fix nitrogen, and promote growth of an important oilseed crop, canola. For this, we inoculated canola seeds with P. polymyxa P2b-2R and grew them for up to 60 days. Canola seedlings were harvested thrice, 20, 40, and 60 days after inoculation, to determine the rhizospheric and endophytic P2b-2R population (colony-forming units (cfu)) and quantify the biological nitrogen fixation (15N dilution assay) and growth promotion (height and biomass). The entire experiment was repeated to confirm the treatment effects. P2b-2R colonized canola rhizosphere with a population abundance of 1.41 × 108 cfu/g dry root in the first growth trial and 7.49 × 107 cfu/g dry root in the second growth trial. P2b-2R developed a persistent endophytic population in canola root, comprising 3.48 × 105 cfu/g fresh weight in the first growth trial and 2.36 × 103 cfu/g fresh weight in the second growth trial. In the first growth trial, P2b-2R inoculation enhanced seedling height by 25 % and biomass by 30 % after 60 days of inoculation. Similarly, in the second growth trial, P2b-2R inoculation promoted seedling height by 21 % and biomass by 24 %. P2b-2R inoculated seedlings fixed 22 and 19 % of nitrogen from the atmosphere in the first and second growth trials, respectively. These results clearly indicate that P. polymyxa P2b-2R, a lodgepole pine endophyte, is capable of fixing nitrogen and promoting growth of an important oilseed crop canola, which is an indication of its broad range host capability.