Characteristics and host range nodulation of a lysogenic Bradyrhizobium strain of pigeonpea ( Cajanus cajan L.) isolated from an Indian soil

A lysogenic strain (A025) and phage indicator (A039) were identified among the bradyrhizobia of pigeonpea (Cajanus cajan). The broth culture of A025 contained a phage (RT2) that showed lytic activity with A039 strain. To confirm their lysogeny (A025), cells were cultured in the presence of 0.01 M sodium citrate, which only killed free phage particles. The treated cell line was grown without sodium citrate, and the culture filtrate formed plaques with A039 strain, but not with A025 strain. Growth kinetics of A025 strain showed that the number of viable cells and cells harboring phage in the culture increased in a parallel manner. Lytic cycle of RT2 phage on A039 strain revealed a latent period of 3 h, with a multiplication period of 6 h and a burst size of 20 particles per cell. Plaques were homogeneously turbid, having a central hallow surrounded by several bacterial rings. Electron micrograph of phage particles showed that the phage had a hexagonal head and short tail. Restriction endonuclease digestion of phage DNA revealed its size of about 41 kbp. Sequence analysis for 16S–23S rDNA and DnaK regions of both strains were aligned with Bradyrhizobium japonicum. Both strains expressed nitrogenase activity in agar-grown culture and formed effective nodules on C. cajan, Macroptilium atropurpureum, Vigna radiata, Vigna unguiculata, and Vigna mungo plants. This is the first report on lysogeny in Bradyrhizobium of C. cajan having a broad host range of nodulation ability.