Biofertilizers function as key player in sustainable agriculture by improving soil fertility, plant tolerance and crop productivity

Abstract

Current soil management strategies are mainly dependent on inorganic chemical-based fertilizers, which caused a serious threat to human health and environment. The exploitation of beneficial microbes as a biofertilizer has become paramount importance in agriculture sector for their potential role in food safety and sustainable crop production. The eco-friendly approaches inspire a wide range of application of plant growth promoting rhizobacteria (PGPRs), endo- and ectomycorrhizal fungi, cyanobacteria and many other useful microscopic organisms led to improved nutrient uptake, plant growth and plant tolerance to abiotic and biotic stress. The present review highlighted biofertilizers mediated crops functional traits such as plant growth and productivity, nutrient profile, plant defense and protection with special emphasis to its function to trigger various growth- and defense-related genes in signaling network of cellular pathways to cause cellular response and thereby crop improvement. The knowledge gained from the literature appraised herein will help us to understand the physiological bases of biofertlizers towards sustainable agriculture in reducing problems associated with the use of chemicals fertilizers.

Figure 1

Potential use of soil microbes in sustainable crop production. The beneficial soil micro-organisms sustain crop production either as biofertilizers [19] or symbiont [17]. They perform nutrient solubilisation which facilitate nutrient availability and thereby uptake [20,21]. It improves the plant growth by advancing the root architecture [26]. Their activity provides several useful traits to plants such as increased root hairs, nodules and nitrate reductase activity and [36]. Efficient strains of Azotobacter, Azospirillum, Phosphobacter and Rhizobacter can provide significant amount of available nitrogen through nitrogen cycling [22]. The biofertilizers produced plant hormones, which include indole acetic acid (IAA), gibberellins (GA) and cytokinins (CK) [25,44]. Biofertilizers improve photosynthesis performance to confer plant tolerance to stress [44] and increase resistance to pathogens [45] thereby resulting in crop improvement [18].

Figure 2

Hypothetical mechanism of action of biofertilizers in the root cell. Bioactive ligands called Myc factors and Nod factors secreted by mycorrhiza and Rhizobium were perceived by host roots to trigger the signal transduction pathway [106,107], which initiates further signal transduction pathway through unknown receptors (SYMRK and NORK) [101] which trigger release of Ca²⁺ in the cytosol [108]. The whole pathway involves receptor like kinases or other kinase related proteins like DMI and SYM71 to phosphorylate their substrates [123,124]. Nuclear pore complex (NPC) and some of its proteins (NUP) play role in calcium spiking. DM1 proteins play role in maintaining periodic oscillation of calcium ions inside and outside the nucleus. Several channels proteins (Ca²⁺channel proteins) also facilitate this process with the help of various transporters [108]. CCaMK is a calcium calmodulin-dependent protein kinase, which phosphorylate the product of CYCLOPS protein thus initiating activation of various genes involving formation of structures like nodule and (PPA) pre-penetration apparatus [124].