Re-evaluation of the nodulation capacity of <i>Sphingomonas sediminicola</i> DSM 18106 ^T indicates that this strain is not capable of inducing root nodule formation on <i>Pisum sativum</i> (pea)

Rhizobia are soil-dwelling proteobacteria that can enter into symbiotic nitrogen-fixing relationships with compatible leguminous plants. Taxonomically, rhizobia are divided into alpha-rhizobia, which belong to the class <i>Alpharoteobacteria</i>, and beta-rhizobia, which belong to the class <i>Betaproteobacteria</i>. To date, all bona fide alpha-rhizobia belong to the order <i>Hyphomicrobiales</i>. However, a recent study suggested that <i>Sphingomonas sediminicola</i> DSM 18106^T is also a rhizobium and is capable of nodulating pea plants (<i>Pisum sativum</i>), which would expand the known taxonomic distribution of alpha-rhizobia to include the order <i>Sphingomonadales</i>. Here, we attempted to replicate the results of that previous study. Resequencing and computational analysis of the genome of <i>S. sediminicola</i> DSM 18106^T failed to identify genes encoding proteins involved in legume nodulation or nitrogen fixation. In addition, experimental plant assays indicated that <i>S. sediminicola</i> DSM 18106^T is unable to nodulate the two cultivars of pea tested in our study, unlike the rhizobium <i>Rhizobium johnstonii</i> 3841^T. Taken together, and in contrast to the previous study, these results suggest that <i>S. sediminicola</i> DSM 18106^T is not capable of inducing root nodule formation on pea, meaning that the taxonomic distribution of all known alpha-rhizobia remains limited to the class <i>Hyphomicrobiales</i>.