Solubilization of Soil Insoluble Phosphate by Bacteria: Molecular Mechanism and Phosphorus Use Efficiency in Crop Plant

Among all the nutrients required for better plant growth, phosphorus (P) is the second most important nutrient after nitrogen. However, it is insufficiently present in soil in the biologically available form. Normally, the average P content in soil is reported to be ~0.05% (w/w) and only 0.1% of this is present in an available form for plants. The low availability of P in the soil limits plant uptake and thus affects plant growth adversely. In addition to this, the rate of P diffusion in soil is very slow (10−12 to 10−15 m2 s−1). To meet the plant P requirement and to achieve higher crop production, farmers resort to the application of chemical fertilizers, which is unfortunately linked with adverse effects on groundwater resources and soil fertility. However, to overcome this situation, plant growth-promoting bacteria (PGPB) having P-solubilizing ability are effective and eco-friendly. These bacteria can help replace or reduce the use of chemical P fertilizers without compromising the yield of crop plants. The PGPB not only increase P availability and its uptake by the host plant but also increase P use efficiency (PUE) of the plant resulting in better plant growth and higher productivity. Several genes have been identified so far, which have the potential role in P solubilization, mobilization, acquisition, and starvation. The phosphate regulatory system (Pho regulon) and Pqq genes are most important to regulate P solubilization/mineralization along with P starvation and transport depending on the concentration of available P in soil. Knowledge concerning the potential of soil bacteria for increasing the P availability in the soil, their effects on root architecture, and the basic molecular mechanism of P solubilization/mineralization involving the role of important genes are reviewed and discussed.