Plant growth promoting microbes: Potential link to sustainable agriculture and environment

Abstract The host-microbe interaction established through natural evolution in plants sets the basis of plant growth and productivity benefits from a single microbe or a consortium. Select algal, fungal, bacterial (particularly LABs), actinomycetes and yeast groups are potential effective microbes (EMs) in agriculture. As environmental probiotics, EMs help in crop growth and wellbeing by fixing N2, solubilising K and P, unlocking soil trace elements, secreting exopolysaccharides, transforming organic matter into usable nutrients, enhancing soil water-holding capacity and improving the overall soil health. They also release bioactive compounds like vitamins, hormones and enzymes to stimulate plant growth, secrete biocontrol agents and enhance drought tolerance. As the soil profile and its intrinsic microbial ecology along geographical regions vary, a universal ‘one-size-fits-all’ effective microbial formulation cannot be envisaged. Although minerals cycling is majorly accomplished by specialised microbes, their activity is positively or adversely impacted by the interacting biotic community (microbe-microbe, microbe-plant or microbe-animal/human) and abiotic constituents. Crops benefit by positive associations with beneficial microbes as mutualism, symbiosis, commensalism, amensalism, photocooperation, etc., and by negative association against the harmful microbes like antagonism. The plant-growth-promoting role of microbes and the scope to formulate EM are the basis on these facts. EMs are applied for crop growth in the form of consortia which usually include phototrophs, lactic acid bacteria, actinomycetes, fermentative fungi, yeasts, etc. along with an effective carrier substrate (prebiotic) as formulations. The rDNA technology in plant growth promoting microbe (PGPM) with enhanced performance is also discussed. The review provides an insight into the dynamics of ecological interactions as the guiding principles of EM formulation with respect to plant growth promoting microbes, and the beneficial roles of effective microbial formulations in crop wellbeing and productivity enhancement.