Microbe-Based Biostimulants: Latest Developments and Future Perspectives

The agricultural sector needs to achieve the demand of increasing population in an ecofriendly and sustainable manner. Microbe-based biostimulants are proving to be good replacement for chemical fertilizers and pesticides, which have many ill effects on the environment and human health. Microbes have the natural ability to elevate plant growth, inhibit plant pathogens, combat abiotic stresses, and improve soil quality, thus enhancing overall plant and soil health. Plant growth-promoting microorganisms (PGPMs) reside in rhizosphere, within plant tissues (endophytes), over plant surface (epiphytes), or found free living in the soil ecosystem. In the last few decades, commercialization of microbial biostimulants has increased manifold with the awareness in society about their benefits and negative impact of agrochemicals. Plant growth-promoting rhizobacteria (PGPR) belonging to Bacillus, Pseudomonas, Rhizobium, Azotobacter, and Azospirrilum species, plant growth-promoting fungi (PGPF) including genera of Trichoderma, Aspergillus, Piriformospora, Penicillium, and Phoma, and mycorrhizal species of genera Glomus, Entrophospora, Acaulospora, and Sclerocystis are most commonly available microbial inoculants dominating the market currently that are reported to boost the growth in several plant varieties. PGPMs are well known for fixing atmospheric nitrogen, solubilizing complex bound nutrients like phosphorus, potassium, zinc, and other elements to simple forms that can be taken up by plant roots, and producing metal chelating compounds like siderophores for uptake of iron. In the past few years, microbial biostimulants have emerged to be useful in soil remediation by removal of toxic heavy metals and pesticides, and removal of excess sodium ions from saline soils along with enhancing plant growth. They are being used in wastewater treatment plants and ecological restoration of forests, wetlands, and waterbodies. Hence, with all these benefits PGPMs are efficient tools for the development of cost-effective bioformulations, which help to improve the growth and yield of plants in a sustainable manner. Emerging technologies like metagenomics and metabolomics can be used to find new insights in molecular and metabolic cross talks between plants and microbes. Such insights may result in better products having biostimulatory effects on crops.