Evaluation of Inorganic Phosphate solubilization and Organic Phosphate Mineralization in Rhizobia

Abstract

This work deals with study of inorganic phosphate solubilization and organic phosphate mineralization in rhizobia. Analysis was conducted with 3 isolates belonging to Rhizobium sp. isolated from Cajanus cajan, Trigonella foenum and Sesbania rostrata collected from Institute of science, Nirma University and other places in the Ahmedabad (open field). Identity of the isolates was established using biochemical as well as molecular characterization (ARDRA and 16S rRNA sequencing). The isolates showed good solubilizing ability when tested on tricalcium phosphate and rock phosphate supplemented in 25mM tris buffered media with 100mM glucose as well as xylose. Few isolates also showed solubilization in sucrose, galactose and maltose as sole carbon source after 6-8 days under buffering condition (pH 8.0). The organic acid produced was determined using HPLC (Reverse phase C-18 column) and the results showed that isolate SRG produced gluconate and acetate mediated inorganic P solubilization while both isolate SRI and SR1 produced gluconate. These three isolates were tested for their ability to produce two enzymes such as phytase and phosphatase which are important in mineralization of organic phosphorous, which form a major soil phosphate reserve. Highest phytase activity was obtained in SRI (6.960ng/ml), followed by SR1 (6.213ng/ml) and SRG (5.414ng/ml) respectively. Indole acetic acid production was also checked as IAA forms a major phytohormone influencing plant growth. 􀀫􀁌􀁊􀁋􀁈􀁖􀁗􀀃􀀬􀀤􀀤􀀃􀁓􀁕􀁒􀁇􀁘􀁆􀁗􀁌􀁒􀁑􀀃􀁚􀁄􀁖􀀃􀁒􀁅􀁖􀁈􀁕􀁙􀁈􀁇􀀃􀁌􀁑􀀃􀀶􀀵􀀪􀀃􀀔􀀛􀀚􀈝g/ml. In this study, we have isolated and characterized three rhizobial isolates for their phosphate solubilization , phytase and phosphatase activity and production of IAA. This is one of the first reports which show rock phosphate solubilization and phytase production in rhizobia. Previous reports show Gluconic acid production in rhizobia only when externally supplemented with PQQ (Pyrroloquinoline quinine) which is a cofactor of glucose dehydrogenase enzyme whereas our isolates need not be supplemented with PQQ for gluconic acid production. These isolates are clearly better than the rhizobia reported for inorganic P solubilization and organic P mineralization so far and hence may prove to be a better prospective biofertilizer compared to P non solubilizing isolates. Also, P solubilizing isolates have been shown to be better PGPR than those who cannot solubilize P which is also evident from the plant growth experiments.