Mobilization of insoluble soil inorganic phosphate by plant beneficial rhizobacteria is a trait of key importance to the development of microbial biofertilizers. In this study, the ability of several Pseudomonas spp. to solubilize Ca3(PO4)2 was compared. While all Pseudomonas spp. were found to facilitate a decrease in pH and solubilize inorganic phosphate by the production of extracellular organic acids, strains varied by producing either gluconic or 2-ketogluconic acid. Furthermore, comparison between the Pseudomonas spp. of the genes involved in oxidative glucose metabolism revealed variations in genomic organization. To further investigate the genetic mechanisms involved in inorganic phosphate solubilization by Pseudomonas spp., a transposon mutant library of P. fluorescens F113 was screened for mutants with reduced Ca3(PO4)2 solubilization ability. Mutations in the gcd and pqqE genes greatly reduced the solubilization ability, whereas mutations in the pqqB gene only moderately reduced this ability. The combination of biochemical analysis and genomic comparisons revealed that alterations in the pqq biosynthetic genes, and the presence/absence of the gluconate dehydrogenase (gad) gene, fundamentally affect phosphate solublization in strains of P. fluorescens.
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