Diversity among rhizobiophages from rhizospheres of legumes inhabiting three ecogeographical regions of India

Abstract

Diversity in rhizobiophages from rhizosphere soils of different legumes inhabiting eight localities in the Delhi, Jaisalmer Plateau and Great Nicobar ecogeographical regions of India was assessed using 14 rhizobial indicator strains that nodulate three species of Sesbania (S. sesban, S. aegyptica and S. rostrata). Phage activity was detected in 47% of the soil samples tested. Most of the positive responses were observed in soils of localities inhabited by host legumes of indicator bacterial strains and their close relatives. The results suggest that rhizobiophages are maintained in the vicinity of their target host populations and are spread through the man-made introduction of host legumes and their rhizobia. Diversity among phage isolates was assessed by studying the plaque traits on their indicator bacterial strains, and variation in morphology using transmission electron microscopy. Except for plaque size, the variation in the plaque traits was rather narrow. Based on the variability pattern, the 64 phages were grouped into 11 plaque types. In contrast to plaque characteristics, the diversity in morphological characters was high and phage isolates differ from the rhizobiophages reported from Indian soils; the Indian phages of Azorhizobium caulinodans strain ORS571 are different from the Senegal isolates. Based on the morphological diversity, the phages are grouped into Bradley's basic four phage groups (A1, B1, B2 and C1) and three families, Myoviridae, Siphoviridae and Podoviridae. This pattern of grouping is reflected in cluster formation based on the variability in plaque and morphological characters. The host range of the phage isolates is rather narrow and restricted to their indicator strains and their close relatives due to marked genetic divergence among rhizobia due to legume host specialization and ecological conditions. The phages can be used for typifying closely related stem-nodulating rhizobia and other rhizobia, which are promiscuous within sesbanias.