Rhizobium sp. RM and SE were isolated from root nodule of Vigna radiata and Fenugreek respectively which able to solubilize tricalcium phosphate and rock phosphate under 50 mM Tris-Cl buffering conditions. Rhizobium sp. RM and SE were unique as they could produce two different organic acids, gluconic acid and oxalic acid using glucose and arabinose as sole C source respectively. After glucose, arabinose is the most prominent sugar present in the rhizospheric soil. However, P solubilization in these isolates was repressed in the presence of succinate resembling the phenomenon of catabolite repression.
Succinate-mediated catabolite repression (SMCR) is a phenomenon which has been reported in rhizobacteria like Pseudomonas but not in Acinetobacter. Since Acinetobacter pittii SK2 have MPS ability along with other PGP traits could mean possibility of having PGPR strains serving multi-functions to promote plant growth. However, such strains when inoculated in fields are most likely to encounter succinate-mediated repression of MPS phenotype as succinate is one of the common organic acids found in rhizosphere. Catabolite repression may be the cause of failure of several PSMs in field conditions where catabolite repression due to availability of multiple carbon sources may govern expression of several functional genes and phenotype including MPS. Identification mediators of MPS and knockout of regulators of CCR may abolish repression and the strategy can be employed in other PSMs for engineering constitutive P solubilization.
The isolate Rhizobium sp. RM has been studied in this project for elucidating the hierarchical utilization of carbon sources and its effect on the MPS phenotype. During this project the growth profile of the isolate when grown in single and multiple carbon sources was studied, with a thought that this would help to elucidate the preference of the isolate RM. As for pioneering this study, the carbon sources hexose- glucose, fructose and pentose- arabinose, xylose were selected. When grown in combinations of two carbon sources - two pentose and two hexose combinations, the isolate yielded monoauxic and diauxic growth profiles respectively