Photosynthetic bradyrhizobia from Aeschynomene spp. are specific to stem-nodulated species and form a separate 16S ribosomal DNA restriction fragmentlength polymorphism group

We obtained nine bacterial isolates from root or collar nodules of the non- stem-nodulated Aeschynomene species A. elaphroxylon, A. uniflora, or A. sch imperi and 69 root or stem nodule isolates from the stem-nodulated Aeschyno mene species A. afraspera, A. ciliata, A. indica, A. nilotica, A. sensitiva , and A. tambacoundensis from various places in Senegal. These isolates, to gether with 45 previous isolates from various Aeschynomene species, were st udied for host-specific nodulation within the genus Aeschynomene, also revi siting cross-inoculation groups described previously by D. Alazard (Appl. E nviron. Microbiol. 50:732-734, 1985). The whole collection of Aeschynomene nodule isolates was screened for synthesis of photosynthetic pigments by sp ectrometry, high-pressure liquid chromatography, and thin-layer chromatogra phy analyses. The presence of prtf genes in photosynthetic Aeschynomene iso lates was evidenced both by Southern hybridization with a Rhodobacter capsu latus photosynthetic gene probe and by DNA amplification with primers defin ed from photosynthetic genes. In addition, amplified 16S ribosomal DNA rest riction analysis was performed on 45 Aeschynomene isolates, including strai n BTAi1, and 19 reference strains from Bradyrhizobium japonicum, Bradyrhizo bium elkanii, and other Bradyrhizobium sp. strains of uncertain taxonomic p ositions. The 16S rRNA gene sequence of the photosynthetic strain ORS278 (L MG 12187) was determined and compared to sequences from databases. Our main conclusion is that photosynthetic Aeschynomene nodule isolates share the a bility to nodulate particular stem-nodulated species and form a separate su bbranch on the Bradyrhizobium rRNA lineage, distinct from B. japonicum and B. elkanii.