Swedish red clover-associated Rhizobium leguminosarum bv. trifolii share high genetic similarity in the 16S gene but divergence in nod genes

Sammanfattning: Rhizobium leguminosarum biovar trifolii (Rlt) is a root-associated soil bacterium (rhizobia) capable of entering a symbiotic relationship with various species of clover (Trifolium sp.) and fix nitrogen within root nodules. Many rhizobial genes required for host infection and nitrogen fixation are located on symbiotic plasmids, which are highly mobile between strains. In this study the genetic diversity of one chromosomal phylogenetic marker gene (16S rRNA) and four symbiotically associated plasmidlocated nodulation genes (nodA, nodB, nodD1, and nodD2) of thirteen isolates were analysed. The isolated Rlt were derived from red clover root nodules collected in various locations in Sweden. It was determined that Swedish Rlt isolates were highly similar regarding the chromosomal marker 16S gene sequences but displayed sequence variations regarding the symbiotic nod genes nodA, nodB, and nodD1. The gene nodD2 was not detected. The variations among these genes could be partly geographically grouped in the north and south of Sweden. Similarities in the marker gene confirms a close genetic relationship, whereas the divergence in nod genes suggests variations in competitiveness between the strains. Considering the differences seen in the nod genes, comparison to international sequences showed that the Swedish Rlt strains are closely related. The high genetic similarities among the isolates suggest that the Rlt strains in Sweden may share a common origin. Further genetic and phenotypic analyses must be conducted to determine whether the nod gene variants result in a difference in competitiveness.