Re-evaluation of the nodulation capacity of Sphingomonas sediminicola DSM 18106T indicates that this strain is not capable of inducing root nodule formation on Pisum sativum (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 Alpharoteobacteria, and beta-rhizobia, which belong to the class Betaproteobacteria. To date, all bona fide alpha-rhizobia belong to the order Hyphomicrobiales. However, a recent study suggested that Sphingomonas sediminicola DSM 18106^T is also a rhizobium and is capable of nodulating pea plants (Pisum sativum), which would expand the known taxonomic distribution of alpha-rhizobia to include the order Sphingomonadales. Here, we attempted to replicate the results of that previous study. Resequencing and computational analysis of the genome of S. sediminicola DSM 18106^T failed to identify genes encoding proteins involved in legume nodulation or nitrogen fixation. In addition, experimental plant assays indicated that S. sediminicola DSM 18106^T is unable to nodulate the two cultivars of pea tested in our study, unlike the rhizobium Rhizobium johnstonii 3841^T. Taken together, and in contrast to the previous study, these results suggest that S. sediminicola 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 Hyphomicrobiales.