Identification of an endophytic Rhizobium in stems of Populus

We have discovered a Rhizobium species that lives within the stems of poplar (cottonwood; Populus). Poplar trees are deciduous, non-leguminous trees that are able to grow in nutrient-poor soil or gravel alongside rivers. Sequencing of 16S and 23S ribosomal RNA genes of the poplar endophyte revealed that the bacterium is Rhizobium tropici, with 99% identity to the type strain and 100% of the species-specific bases. This diazotrophic (nitrogen-fixing) species is known for its ability to nodulate an exceptionally wide range of legumes; however, its endophytic nature in non-legumes has not been described. In this paper, we describe the association of this bacterium with a variety of Populus trichocarpa x P. deltoides hybrids from both our laboratory and outdoors, and compare this unique isolate to the type strain of Rhizobium tropici, CIAT899.