Rhizosphere competence and applications of plant growth-promoting rhizobacteria in food production - A review

Sustainable food production, among other non-intensive production systems, involves the important interactions of myriads of plant growth-promoting microorganisms, plant, soil, soil fauna, and production of utilizable carbon in the rhizosphere capable of enhancing soil health, plant growth, and protection that lead to increased crop productivity. Plant growth-promoting rhizobacteria (PGPR) including symbiotic rhizobia and free-living rhizobacteria possess traits that help to enhance plant growth due to their many modes of action that start with the ability to colonize both the intracellular and extracellular rhizosphere niche in their search for a carbon source and reduction in the free use and quantity of agrochemicals. In the past few decades, the focus on developing biosafety agro-products has shifted from agrochemical-based applications to a more sustainable system without posing negative impacts on the soil microflora or fauna. The present review focuses on the application of PGPR inoculants on soils and seeds to improve biological nitrogen fixation, solubilization of phosphate, and secretion of phytohormones required for growth, especially application in a pressured environment. We discuss how PGPR enhances nutritional regulation and hormonal balance in plants, bacterial taxa enrichment, and improvement of carbon sources utilization beneficial for plant growth. We highlight the antagonistic and synergistic interactions with microorganisms within the rhizosphere and beyond in bulk soil which indirectly boosts plant growth rate and induces resistance against phytopathogens. While soil-borne pathogens continually oppose the functions of these microorganisms, PGPR has improved diverse strategies in the form of agro-compatibility, root colonization, nutrient, iron, and space competition, systemic resistance, antibiotics synthesis, lytic acid, hydrogen cyanide, and siderophore production for advanced food production. Finally, we highlighted the roles of PGPR in phytoremediation, techniques of applying microbial inoculants to enhance plant growth and commercialization of PGPR products and the challenges developing countries have to defeat.