Biochemical and genomic comparison of inorganic phosphate solubilization in Pseudomonas species

Mobilization of insoluble soil inorganic phosphate by plant beneficial rhizobacteria is a trait of key impor-tance 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 pro-duction 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 organiza-tion. 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 solubili-zation 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.