Diversity of pseudomonads isolated from three different plant rhizospheres

Aims: To study the diversity of the Pseudomonas populations isolated from three different plant rhizospheres, namely pearl millet, cotton and paddy, grown in saline soils along the coastline of Southern India. Methods and Results: The Pseudomonas populations were analysed for their biochemical characters and genetic diversity using molecular tools including RAPD and PCR-RFLP. The biochemical characterization, antibiotic resistance assay and RAPD profiles revealed a largely homogenous population. Even in PCR-RFLP restriction studies, only two groups of isolates were seen. One group was predominant in all three rhizospheres, while the other minor group consisted of salt-sensitive isolates restricted to the paddy rhizosphere alone. Conclusions: It was observed that increasing salinity caused a predominant selection of salt-tolerant species, in particular Ps. pseudoalcaligenes and Ps. alcaligenes irrespective of the host rhizosphere. Significance and Impact of the Study: This study has reinstated the importance of the soil over the host plant with regard to rhizosphere populations. It has also resulted in the isolation of several salt-tolerant Pseudomonas strains, which are being screened for their biological control activity against common plant pathogens of the coastal agri-ecosystem.