Mineral Phosphate Solubilization by Rhozibium meliloti

The majority of phosphate in soil is complexed with Ca2+, Fe2+ or other minerals. Such mineral phosphate forms are poorly soluble in water and the resulting concentration of soluble phosphate which is available for plant growth is generally very low (1–2 μM or less). Hence, plant growth is often phosphate limited (Bieleski, 1973). Many soil bacteria, such as Rhizobium meliloti, can solubilize mineral phosphates. To gain insight into the process of mineral phosphate solubilization, we are genetically and biochemically characterizing the mineral phosphate solubilizing (Mps) activity of R. meliloti. Following random Tn5 mutagenesis of wild type RCR2011, two Mps− mutants (Mps-7 and Mps-26) were isolated by screening NmR transconjugants on dicalcium phosphate plates. Both mutants showed reduced Mps activity in liquid cultures (0.2% hydroxylapatite as sole source of phosphate). These initial experiments revealed that the pH of hydroxylapatite cultures of the wild type RCR2011 dropped from pH 6.6 to pH 4.5 over a 12 day incubation period. Over the same period, the pH of the Mps mutant cultures remained above 5.5, suggesting that the drop in pH resulted in solubilization of the hydroxylapatite. This view was reinforced through experiments in which the wild type cultures were buffered to pH 6.5 with 40 mM MOPS buffer. Under these conditions, no solubilization of the hydroxylapatite was observed. This corresponds well with the general belief that Mps activity is correlated with the production of organic acids. Glucose dehydrogenase (GDH) activity was assayed in the membrane fractions prepared from the wild type and mutant strains, and the results revealed an absence of activity in Mps-7 and Mps-26.