Plant Growth-Promoting Rhizobacteria (PGPR) and Their Action Mechanisms in Availability of Nutrients to Plants

One of the main obstacles to plant growth is the lack of the availability of nutrient elements in many agricultural environments in the world, especially the tropics where soils can be extremely low in nutrients. Using different mechanisms of action, plant growth-promoting rhizobacteria (PGPR) participate in geochemical nutrition cycles and determine their access to plants and the microbial community of the soil. Use of these bacteria as bio-inoculants will increase the availability of nutrient elements in soil, help to minimize the chemical fertilizer application, reduce environmental pollution, and promote sustainable agriculture. Considering comprehensive reviews previously published on plant growth enhancement mechanisms, this review focuses on what is known about the action mechanisms underlying the increase of the availability of nutrient elements as an effect of microbial colonization especially PGPR. In this chapter, some of the most important mechanisms and processes regarding the effects of PGPR on the availability and hence uptake of nutrient elements by plant are reviewed. The awareness of such mechanisms can be important for the selection and hence production of microbial inoculums, which are appropriate for biological fertilization as substituting or decreasing the need of using chemical fertilizers in crops. In this review, special consideration is given to the role of PGPR in the availability of nitrogen (N), phosphorus (P), potassium (K), and sulfur (S) as macronutrients and iron (Fe) and manganese (Mn) as micronutrients.