Plant physiological and molecular responses triggered by humic based biostimulants - A way forward to sustainable agriculture

Background The global population is increasing at a pace that food security has become a major concern. The 20th-century Green Revolution saved billions of people from starvation, but the continuous widespread utilization of pesticides and synthetic fertilizers to boost yields has negatively impacted arable lands, water resources, and the environment. Moreover, the production and use of chemical inputs contribute to global climate change. But this impact could be reduced by replacing synthetic chemical inputs with sustainable resources. A promising and environmentally friendly approach to reduce synthetic chemicals is to incorporate biostimulants from sustainable resources. Humic substances (HSs) are composed of humic, fulvic, and ulmic acids and are the most abundant organic matter on earth. They are well known for their beneficial effects on plant growth and development. Scope This review encompasses the most recent findings related to the bio-stimulatory effects of HSs in modulating phytohormone biosynthesis, nutrient uptake and assimilation, primary and secondary metabolism, and tolerance to biotic and abiotic stresses. Conclusion Existing evidence shows that HSs have multifaceted actions that are attributed to different functional groups and other bioactive compounds enclosed in their macrostructure. Studies have shown that HSs possess auxin-like properties that alter plant metabolism which results in beneficial effects on plant growth and productivity, such as improved nutrient use efficiency and increased abiotic and biotic stress tolerance. In future studies, delineating the mechanisms that can pave the way to further refine these products and increase their efficacy to amplify beneficial effects is required to develop novel products.