Abstract
Rhizobia bacteria engage in nitrogen-fixing root nodule symbiosis, a mutualistic interaction with legume plants in which a bidirectional nutrient exchange takes place. Occasionally, this interaction is suboptimal resulting in the formation of ineffective nodules in which little or no atmospheric nitrogen fixation occurs. Rhizobium leguminosarum Norway induces ineffective nodules in a wide range of Lotus hosts. To investigate the basis of this phenotype, we sequenced the complete genome of Rl Norway and compared it to the genome of the closely related strain R. leguminosarum bv. viciae 3841. The genome comprises 7,788,085 bp, distributed on a circular chromosome containing 63% of the genomic information and five large circular plasmids. The functionally classified bacterial gene set is distributed evenly among all replicons. All symbiotic genes (nod, fix, nif) are located on the pRLN3 plasmid. Whole genome comparisons revealed differences in the metabolic repertoire and in protein secretion systems, but not in classical symbiotic genes.
Highlights
Legume crops are central to sustainable agricultural practices and food security [1, 2]. They have a low need for synthetic nitrogen fertilizers input, as they engage in a symbiosis with a group of diazotrophic bacteria collectively known as rhizobia
Upon recognition of specific signals, legume plants intracellularly accommodate rhizobia inside root organs called nodules, where they engage in a bidirectional nutrient exchange [3]
Genome project history Rhizobium leguminosarum (Rl) Norway was selected for sequencing, because of the striking diversity of ineffective nodulation phenotypes that it induces in Lotus, a host that belongs to a different cross-inoculation group
Summary
Legume crops are central to sustainable agricultural practices and food security [1, 2] They have a low need for synthetic nitrogen fertilizers input, as they engage in a symbiosis with a group of diazotrophic bacteria collectively known as rhizobia. Suboptimal interactions establish between the symbiotic partners These lead to the formation of ineffective nodules in which limited to no nitrogen fixation occurs. Liang et al Standards in Genomic Sciences (2018) 13:36 induced by Rl Norway, as it cannot induce nodules on L. japonicus Gifu, but induces bumps on L. japonicus Nepal, and white nodules on L. burttii and L. japonicus MG-20 This is in contrast to compatible Mesorhizobium strains that induce monomorphic phenotypes in the same plant ecotypes [12]. The 16S rRNA gene of Rl Norway shows more than 99.9% identity with its orthologs in other R. leguminosarum strains, such as
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