Phosphate-solubilizing microbes: Diversity and phosphates solubilization mechanism

Abstract

Phosphorus (P) is an essential plant nutrient second after nitrogen. Soil phosphorus, especially in soils of high P-fixing capacity, remains unavailable to plants. Soil microorganisms belonging to diverse genera having ability to transform insoluble P into soluble and plant accessible forms are collectively referred as phosphate-solubilizing microorganisms (PSM). The strains of Pseudomonas, Bacillus, Aspergillus, Pénicillium, etc. are some known phosphate solubilizers. These microorganisms in addition to supplying soluble P to plants also facilitate the growth of plants by several other mechanisms, for instance, improving the uptake of other nutrients and stimulating the production of some phytohormones and protecting the plant from biotic and abiotic stresses. Even though several bacterial, fungal, and actinomycetal strains have been identified as PSM, the mechanism through which they make P available to plants is poorly understood. This chapter focuses on the biochemical and molecular mechanisms exhibited by PSM for phosphate solubilization from inorganic and organic P sources. A short overview of the phosphate- solubilizing microbes and their effect on P uptake and crop growth is also presented herein. Phosphorus dynamics in soils and its availability to plants, metabolic pathways effecting the release of organic acids by PSM, are covered. The aspects of immobilization of PSM for the ease of application and the role of VAM in P mobilization in soil plant system have been also explored.