Lanthanide-Dependent Methanol Dehydrogenase XoxF Confers a Competitive Advantage to <i>Sinorhizobium meliloti</i> During Symbiosis with <i>Medicago sativa</i>

The recent discovery of the lanthanide(Ln)-dependent methanol dehydrogenase (Ln-MDH) XoxF has expanded the spectrum of bacteria recognized for methylotrophic metabolism. Many bacteria, including rhizobia, have historically escaped being categorized as methylotrophs because they exclusively produce XoxF-type Ln MDHs and entirely lack the long-studied calcium-dependent methanol dehydrogenase MxaFI. We report that the XoxF-type Ln-MDH encoded by the <i>smb20173</i> gene is the sole methanol dehydrogenase that supports methylotrophic growth of <i>Sinorhizobium meliloti</i>. The lanthanides that consistently supported growth of <i>S. meliloti</i> in minimal media with methanol included lanthanum, cerium, praseodymium, and neodymium. Based on genome, whole-transcriptome, and mutant phenotype analyses, we propose a metabolic model for Ln-dependent methylotrophy in <i>S. meliloti</i> wherein oxidation of one-carbon compounds, such as methanol, generate the reducing power needed to assimilate carbon via the Calvin-Benson-Bassham cycle. By investigating how these newfound insights about lanthanides reshape our understanding of the methylotrophic capabilities of rhizobia, we explored how methanol produced by plants has the potential to create a nutritional niche in the rhizosphere. Using a <i>Medicago sativa</i> (alfalfa) nodule occupancy assay, we found that a <i>xoxF</i> mutant strain was outcompeted by the wild-type strain only when lanthanides were available, suggesting that Ln-dependent methylotrophy promotes an efficient rhizobia-legume symbiosis.