Rhizobium Mutant Deficient in Mineral Phosphate Solubilization Activity Shows Reduced Nodulation and Plant Growth in Green Gram

Abstract

The aim of the present study was to understand the mineral phosphate solubilization (MPS) in Rhizobium sp. MR-54 (MPS+) by using forward genetic approach and its effect on nodulation as well as nitrogen accumulation on green gram. Tn5 mutants (MPS−) were generated from the MPS+ strain by Tn5 mutagenesis. Screening 1,603 colonies of MR-54::Tn5 resulted in identification of those mutants which failed to show any solubilization zone up to 72 h after incubation on Tri-calcium phosphate medium (TCP). pH drops and Pi release were highly correlated up to 15 days after incubation in TCP broth. Gluconic acid was detected in the culture supernatants of MPS+ isolates through thin layer chromatography indicating the presence of direct oxidation pathway in MR-54 strain. Detailed characterization of mutants was carried out for related properties like colony morphology, Pi release and pH change in TCP broth, organic acid production, surface characteristics and plant growth promoting ability etc. Mutants deficient in MPS showed reduced nodulation and nitrogen accumulation in green gram under green house conditions in leonard jar apparatus over the wild strain indicating the MPS by Rhizobium being involved in nodulation, nitrogen accumulation and plant growth.