The production of organic acids is considered as the principal mechanism for mineral phosphate solubilization in bacteria, this assumption has been corroborated by the cloning of two genes involved in gluconic acid production viz., pqq and gab Y. Gluconic acid is the principal organic acid produced by Pseudomonas spp., Erwinia herbicola, Bacillus spp., Burkholderia spp., Rhizobium spp. Other organic acids such as lactic, isovaleric, isobutyric, acitic, glycolic, oxalic, malonic and succinic acids are also generated by the different phosphate solubilizing bacteria. Researchers has demonstrated that an efficient mineral phosphate solubilizing phenotype in Gram-negative bacteria resulted from extracellular oxidation of glucose to gluconic acid via. Quinoprotein glucose dehydrogenase equipped with pyrroloquinoline quinone (PQQ) as a cofactor. A pqq gene cluster producing PQQ was detected in E.intermedium and this sequence conferred phosphate-solubilizing activity to E.coli DH5α. The 6,783 bp PQQ sequence had six open reading frames (pqq A, B, C, D, E&F) showed 50-95% homology to pqq gene of other bacteria. The production of organic acids by phosphate solubilizing bacteria has been well documented. Gluconic acid seems to be the most frequent agent of mineral phosphate solubilization, along with 2-ketogluconic acid as another organic acid identified in strains with phosphate solubilizing ability.

Organic acid like Gluconic acid and 2-Ketogluconic acid have been reported to be produce by some potent bacterial strains like Pseudomonas sp., Erwinia herbicola, Pseudomonas cepacia,
Burkholderia cepacia, Rhizobium leguminosarum, Rhizobium meliloti, Bacillus firmus.
Strains of Bacillus and Pseudomonads were found to produce mixtures of lactic, isovaleric, isobutyric and acetic acids. Other organic acids, such as glycolic, oxalic, malonic, and succinic acid, have also been identified among phosphate solubilizers. Strains from the genera Pseudomonas, Bacillus and Rhizobium are among the most powerful phosphate solubilizers. Chelating substances and inorganic acids such as sulphideric, nitric, and carbonic acid are considered as other mechanisms for phosphate solubilization. However the effectiveness and their contribution to P release in soils seems to be less than organic acid production.

About Author / Additional Info:
PIYUSH JOSHI, VPKAS, ALMORA, UTTARAKHAND,INDIA
Research work on cold tolerant phosphate solubilization.