Plant beneficial rhizospheric microorganism (PBRM) strategies to improve nutrients use efficiency: A review

Plant beneficial rhizospheric microorganisms (PBRMs) are rhizospheric microbes that are able to colonize rhizosphere and to improve plant growth, development and nutrient use efficiency (NUE) by means of a wide variety of mechanisms like organic matter mineralization, biological control against soil-borne pathogens, biological nitrogen (N) fixation, potassium (K), phosphorous (P) and zinc (Zn) solubilization and root growth promotion. Improved grain production to meet the food demand of an increasing population has been highly dependent on chemical fertilizer input based on the traditionally assumed notion of ‘high input, high output’, which results in overuse of fertilizers but ignores the biological potential of roots or rhizosphere for efficient mobilization and acquisition of soil nutrients. A very interesting feature of PBRMs is their ability of enhancing nutrient bioavailability or NUE by ∼ 20-40% with various nutrients. Although chemical fertilization is a quick method, it is not recommendable economically and environmentally, especially if overused. Several PBRMs species have been characterized as biological N-fixer, P, K-solubilizing microorganisms while other species have been shown to increase the solubility of micronutrients, like those that produce siderophores for iron (Fe) chelation. The enhanced amount of soluble macro- and micronutrients in the close proximity of the soil-root interface has indeed a positive effect on plant nutrition. There is a growing body of evidence that demonstrates the potential of various microbes to enhance plant productivity and yield in cropping systems. However, the molecular mechanisms underlying these phenomena, the signals involved as well as the potential applications in a sustainable agriculture approach, and the biotechnological aspects for possible rhizosphere engineering are still matters of discussion.