Members of Ensifer and Rhizobium genera are new bacterial endosymbionts nodulating Pisum sativum (L.)

Pisum sativum is nodulated novel putative Rhizobium chromosomal lineage, and interestingly we described for the first time the presence of the Ensifer medicae and E. meliloti both with the symbiovar meliloti in the nodules of P. sativum. A total of 84 Pisum sativum legume nodulating bacteria (LNB) were isolated from seven geographical sites from southern Tunisia. Phylogenetic analyses based on partial sequences of 16S rRNA gene and the housekeeping genes glnII, and recA grouped strains into six clusters, four of which belonged to the genus Rhizobium and two to the Ensifer genus. Among Rhizobium clusters, 41 strains were affiliated to Rhizobium leguminosarum, two strains to R. pisi, two strains to R. etli, and interestingly two strains belonged to previously undescribed Rhizobium species. The remaining two strains were closely related to Ensifer medicae (two strains) and Ensifer meliloti (two strains). A symbiotic nodC gene-based phylogeny and host specificity test showed that all Rhizobium strains nodulating pea belonged to the symbiovar viciae, whereas the Ensifer strains were associated with the symbiovar meliloti never described to date. All strains under investigation differed in the number of induced root nodules and the effectiveness of atmospheric nitrogen fixation. The R. leguminosarum PsZA23, R. leguminosarum PsGBL42, and E. medicae PsTA22a, forming the most effective symbiosis with the plant host, are potential candidates for inoculation programs.

Automated summary

A total of 84 Pisum sativum legume nodulating bacteria (LNB) were isolated from southern Tunisia, belonging to the genera Rhizobium and Ensifer. All Rhizobium strains nodulating pea belonged to the symbiovar viciae, while the Ensifer strains were associated with a previously undescribed symbiovar meliloti. Among the strains tested, R. leguminosarum PsZA23, R. leguminosarum PsGBL42, and E. medicae PsTA22a showed the most effective symbiosis with the plant host.