Previous research has mainly focused on breeding, timber supply, and physiological-biochemical characteristics of Paulownia fortunei. However, there has been limited attention given to its endophytic rhizobia and its potential benefits for plant growth due to a lack of knowledge about the existence of root nodules. In this study, we extracted 9 bacterial strains from the root nodules after occasionally discovering the nodular-like structure in uncultivated 3-year P. fortunei roots and then sequenced the 16S rDNA and nifA sequencing to conduct the bacterial strain identifications. We then determined the carbon use capability and nitrogenase activity of the bacterial strains. The result of 16S rDNA sequencing indicated that the endophytic rhizobia belongs to the genus Agrobacterium, Rhizobium, Herbaspirillum, and Burkholderia. The results of nifA sequencing indicated that the nifA gene was sequenced in strain PG-3, PG-5, PG-6, PG-7, PG-8, rather than in strain PG-1, PG-2, and PG-4. Besides, the nitrogenase activity of strain PG-9 was significantly higher than other bacterial strains. RDA results indicated that fructose, glucose, and sucrose were significantly related to the plant height, diameter, and biomass rather than other carbon sources and nitrogenase activity. Our research revealed that the majority of bacterial strains isolated from P. fortunei exhibited a broad carbon utilization pathway and have the potential to promote plant growth after re-Inoculation. This study aims to investigate the phylogeny of endophytic rhizobia in P. fortunei, and expand the range of non-legume hosts studied for future research on biological N fixation.

We used a vernier caliper, band tape, and length rod to measure the plant height, stem diameter and dry biomass three months after planting to investigate the effects of bacterial strain applications on Paulownia seedling growth. The nitrogenase activity of 9 endophytic rhizobia strains was measured colorimetrically at 450 nm (N = 3) using the nitrogenase ELISA kit (Jining, Tongwei, Shanghai, China) according to the manufacturer’s instructions.