Multilocus Phylogenetic Characterization of Mesorhizobium-like Nodule-associated Isolates from Root Nodules of Genista acanthoclada

The root nodule symbiosis between legumes and rhizobia plays a fundamental role in biological nitrogen fixation and ecosystem sustainability. Genista acanthoclada, a perennial woody legume native to arid and rocky slopes of Western Türkiye, has not previously been investigated regarding to its symbiotic microsymbionts. In this study, five bacterial isolates were obtained from root nodules of G. acanthoclada collected from Yamanlar Mountain (İzmir, Türkiye) on Yeast Extract Mannitol Agar (YEMA) and rhizobial strains (GA.7.1 and GA.9) were rapidly selected by colony PCR targeting the nodC gene. Both isolates exhibited typical rhizobial phenotypic characteristics, including Gram-negative rod-shaped morphology, positive catalase and oxidase activities, and weak Congo red absorption on YEMA medium. Taxonomic analyses based on 16S rRNA, recA, and atpD gene sequences revealed that both isolates belong to the genus Mesorhizobium. The incongruence between the phylogenetic analysis based on nodC amino acid sequences and the core gene phylogenies suggests that horizontal gene transfer may play a role in the evolution of nodulation-related genes. In contrast, the nifH-based phylogeny was largely congruent with housekeeping gene phylogenies, indicating a more conserved evolutionary history of nitrogen fixation genes. These findings provide the first data on the rhizobia associated with the nodules of G. acanthoclada and contribute to a better understanding of the evolutionary dynamics of legume–rhizobium interactions.