Effects of Rhizospheric Microbes, Growth Regulators, and Biochar in Modulating Antioxidant Machinery of Plants Under Stress

Plants are exposed to various biotic and abiotic stresses in nature and the preceding climate change is the major impediment that makes plants more vulnerable to these stresses. Simultaneous imposition of biotic and abiotic stresses results in significant economic loss in productivity and quality of agricultural products, adversely affecting agricultural sustainability. Under normal conditions, the generation of reactive oxygen species (ROS) occurs as a cellular byproduct during various metabolic reactions and acts as a messenger for various signaling processes. Under various stresses, biotic and abiotic the balance between ROS production and detoxification gets disturbed and the accelerated formation of ROS induces oxidative stress. Plants have an antioxidant defense system that prevents oxidative stress. Previous research demonstrates the role of several compounds involved in the mitigation of oxidative stress. The current concept of Hormesis, i.e. stimulation in growth parameters due to low doses of toxic substances or low levels of stress is activated by signaling pathways involving phytohormones. The PGR (plant growth regulator) like salicylic acid well known for inducing systemic acquired resistance (SAR) in plants effectively regulates the ROS and signaling pathways. Sporadic evidence exists regarding the direct and indirect involvement of plant growth regulators (PGRs) applied exogenously or phytohormones produced endogenously in plants regarding the management of oxidative stress. PGPR (plant growth-promoting rhizobacteria) offers an eco-friendly approach to promote plant growth and development and also trigger the antioxidant defense pathway, regulate ROS production, and provide resistance to pests and parasites by inducing ISR (induced systemic resistance). Biochar is a heterogeneous substance rich in aromatic carbon and minerals and is mainly used as a sustainable soil amendment. Biochar serves as an ecological niche for the rhizosphere microbes and augments their effects. Some metal-based NPs such as Ag, Pt, Ce, and Zn possess antioxidant and anticancer properties as well. In addition to beneficial effects, engineered NPs are responsible for the production of reactive oxygen species, leading to oxidative stress. The present review documents the role of PGR, PGPR, and biochar alone and in concert to modulate oxidative stress in plants induced under various stresses encountered in nature and aggravated under the changing climate.