UNDERSTANDING THE DIVERSITY OF PHOSPHATE SOLUBILIZING NITROGEN FIXING BACTERIA IN RHIZOSPHERIC AND BULK SOIL SAMPLES OF Oryza sativa AND Avena sativa

Abstract

Nitrogen (N) and Phosphorus (P) are essential macronutrients for plant growth. N is an essential component of biomolecules like ATP, nucleic acids, proteins etc. Plants require nitrogen in fixed form which is a limiting nutrient in soil. P although abundant in soil, in both organic and inorganic forms, is restricted in its availability as it occurs in insoluble forms. P content in average soil measures about 0.5% (w/w) out of which only 0.1% (w/w) is available for plant uptake. This is due to poor solubility and its fixation in soil. An adequate supply of phosphorus during early stages of plant development is mandatory for laying down the primordia of plant reproductive parts. To satisfy these nutritional requirements, P is usually added to soil as chemical P fertilizer. However, these are seen to have long- term impacts on the environment which have led to the search for a sustainable way of P nutrition of crops. In this regard, phosphate-solubilizing nitrogen fixing bacteria have been seen as best eco-friendly means for P and N nutrition for crop. Thus, maintaining high levels of available N and P remains a challenge. Major research on biofertilizer have focused on understanding and improving nitrogen fixation. It is known that every aspect of the process of nodule formation is restricted by the availability of P, hence the widespread use of chemical fertilizers is practiced, but most of the supplemented P become unavailable when it reacts with other metal ions in the soil. Many soil microorganisms belonging to the group of PGPR (Plant growth promoting rhizobacteria) are able to solubilize this unavailable P through their metabolic activities by exuding organic acids, which directly dissolves the complex rock phosphate, or by chelation of calcium ions that release P. PGPR were therefore isolated from soil samples (rhizospheric and bulk) of Rice (Oryza sativa) and Oats (Avena sativa) obtained from Jaspur and Choriwad village of Gujarat and screened for P solubilizers followed by N2 fixers. 14 potential isolates which exhibited both these properties were selected and were subjected to biochemical and molecular characterization. Futhermore, they were checked for various PGP properties like IAA production, HCN production, Siderophore production and Pi release by Ames method. On the basis of diversity indices more evenness and abundance of bacterial species was observed in rice rhizospheric soil as compared to rice bulk soil whereas oats rhizospheric soil was more even and rich in bacterial species as compared to oats bulk soil. In case of diversity of P solubilizing N2 fixing bacteria, rice rhizospheric soil was more abundant and rich compared to rice bulk soil whereas oats rhizospheric soil showed more richness and evenness as no P solubilizers were obtained from oats bulk soil. Three isolates obtained from Sanger sequencing were Mesorhizobium loti Rb2, Rhizobium leguminosarum Rr5 and Sinorhizobium meliloti Rr6. These were found to be good P solubilizers and also possessed other vital PGP properties. Their IAA production values are 159.4±0.01 μg/ml, 165.9±0.05 μg/ml and 120.4±0.01 μg/ml respectively. HCN production was almost equal in Rr5 and Rr6 while Rb2 showed least HCN production. Siderophore production was absent in Rb2 while Rr5 and Rr6 showed 9.55% and 26.13% siderophore production individually. Inorganic P solubilization or Pi release was given the maximum by Rb2 with a value of 266.1±0.41 μg/ml while Rr5 and Rr6 showed similar values i.e. 226.8±0.18 μg/ml. Hence, these isolates can be further exploited for formulating a customized biofertilizer.