Plant Growth-Promoting Rhizobacteria (PGPR): A Unique Strategy for Sustainable Agriculture

Introduction

Non-pathogenic strains of soil bacteria that multiply in the rhizosphere and grow in and around the root tissues, by stimulating host plant growth by different biological mechanisms are defined as plant growth-promoting rhizobacteria (PGPR). Rhizospheric zones surrounding roots are the hotspots for all the microbial interactions due to the presence of various organic, biochemical metabolites secreted by the roots. These root secretions majorly include water-soluble sugars, vitamins, organic acids, amino acids, sugar-phosphate esters, phenolics, and amino compounds (Uren, 2000). Root exudates serve as the major source of nourishment for the microorganisms and they attract huge number of microbial populations when compared to the non-rhizospheric soils. These root secretions govern the plant-microbial interactions via the chemotaxis mediated response. Microbial interactions in response to the exudates secreted by the plant play a significant role in successful root colonization. The success of plant growth-promoting rhizobacteria depends upon the colonization of roots, as the colonization of roots is the first crucial stage during the interaction between the PGPR and the host plant. After successful root colonization PGPR facilitates host plant growth (Ahemad et al., 2014; Goswami et al., 2016), through direct or indirect mechanisms (Ortíz-Castro et al., 2009).

Direct mechanisms include nitrogen fixation, potassium, phosphorus, zinc solubilization, production of siderophores, and phytohormones. Direct mechanisms help in enhancing soil fertility. Indirect mechanisms comprised of production of antibiotics, exopolysaccharides hydrolytic enzymes, and cyanide compound. Indirect mechanisms do not affect soil fertility directly but help in maintaining soil health by repressing pathogenic soil microorganisms, mentioned in Figure 1. PGPR repress the phytopathogens by; 1) exhibiting direct antagonistic activities against the pathogens (Beneduzi et al., 2012), 2) competing for space and nutrients (Kumari and Srivastava 1999), and 3) triggering induced systemic resistance (ISR) in plants (Egamberdieva et al., 2017). Induced systemic resistance elevates the defense capacity of host plants against the phytopathogens and pests to overcome the biotic stresses.

Biotic and abiotic stress is the major cause of yield loss in agriculturally important crops. Rainfed crops mainly suffer from abiotic stresses like nutrient deficiencies and environmental factors like high temperature, drought, salinity, and pH of soils. Crop loss due to physiological modulation in plants is observed against abiotic stresses. These stresses cause a 50-82% significant decrease in agricultural productivity. Among biotic stresses pathogenic microorganisms, pests and weeds cause enormous damage to the crop. The pathogenic microflora damages the root hairs, lateral roots, release toxins and destroys the plants (Singh et al., 2014; Mishra et al., 2015). Nearly 7-15% of the crops are damaged by various soil-borne, bacteria, fungi oomycetes, and nematodes.

Figure 1.

Direct and indirect mechanisms of PGPR in promoting plant growth

Microorganisms that colonize the roots and possess the ability for salt-tolerant, nutrient uptake and produce compatible solutes play a significant role in abiotic stress management. Plants are affected by salt stress in three different ways viz., ionic toxicity, osmotic imbalance, and decrease in nutrient uptake (Selvakumar et al., 2014). Proline is vital compatible solute for both bacteria and plants to respond against the osmotic imbalance and ion toxicity. Proline can influence cell proliferation and apoptosis and regulates specific gene expression to reduce salt stress (Ahmad et al., 2016). Thus, the above facts illustrate that the rhizobacteria with plant growth-promoting abilities can be used as a suitable bio inoculant to promote plant growth and enhance productivity through different mechanisms in addition to the accumulation of proline as osmo-regulators.