Variety of biotic and abiotic factors influence the plant growth. The rhizosphere, a thin layer of soil immediately around the roots, is vital for root activity. The term "rhizosphere" was first used by Hiltner, who defined it as the little area of soil that surrounds roots and is where root activity stimulates the growth of microorganisms.

The soil rhizosphere is inhabited by a variety of microorganisms, including bacteria, fungi, protozoa, and algae, with bacteria being the most prevalent (Sivasakti et al., 2014). Soil rhizosphere is also known as storehouse of microbes as it contains different microorganisms and important for microbial activity (Kundan et al., 2015). In addition to providing habitat and resource variability for soil organisms, the rhizosphere is the location of organic deposition (Bajeli et al., 2023). 

Sustainable agriculture, or organic farming, avoids many of the social and environmental problems that plague conventional farming (Wani and Kumar 2024). 

Plant-associated bacteria can be either plant growth-promoting rhizobacteria (PGPR) or plant pathogenic bacteria. Plant growth promoting rhizobacteria (PGPR) are bacteria that live in roots and promote plant growth. By assembling growth-promoting compounds, they increase the host plant's availability of micronutrients, which has a major effect on growth and development. They are also widely known for their contribution to better root growth patterns. Exudates from the roots contain a vast array of organic molecules that serve as a signal to draw in soil bacteria, which are a rich source of carbon in the soil (Drogue et al., 2013).

Free-living soil-borne rhizobacteria that promote plant growth have the ability to accelerate plant growth. Azotobacter, Azospirillum, Pseudomonas, Acetobacter, Burkholderia, Bacillus, Paenibacillus, and a few members of the Enterobacteriaceae family are among the genera that are included in PGPR. They can reduce soil-borne diseases at the rhizosphere, quickly colonize the rhizosphere, and promote plant growth, can have direct effect on a plant's metabolism by controlling the breakdown or production of different phytohormones (Bloemberg and Lugtenberg 2001; Rangarajan et al., 2003; Bajeli et al., 2023). Plant growth-promoting rhizobacteria (PGPR) offer a promising strategy for enhancing sustainable agriculture despite environmental stress (Parnate et al., 2024).

A variety of secondary metabolites, including siderophores, phytohormones, antibiotics, and volatile chemicals, can be produced by PGPRs. Of these, the creation of indole-3-acetic acid, siderophores, and antibiotics are the primary substances that contribute to the bacteria's ability to promote plant growth. Also, PGPRs have the ability to decompose organic matter present in soil which have crucial role in plant production (Mohamed et al., 2019).

Commercialized as pesticide substitutes, microbial bio-inoculants, and bio-fertilizers, PGPRs that were isolated and screened from rhizospheric soils have been utilized as agricultural inputs to boost plant development and yields by reducing plant illnesses (biological control) (Nazneen et al., 2022). Different mechanisms exerted by PGPR helps in enhancing the plant production and combating different plant diseases which eventually helps in increased yield and sustainable agriculture.

- Major role of these PGPR includes (Kundan et al., 2015):

(a) Provision of nutrients to the plants

(b)  Increasing plant growth by generating various phytohormones

(c) Control or inhibition of activity of phytopathogens

(d) Bio-accumulation or microbial leaching of inorganic compounds

PGPR are classified into two groups according to their relationship with plants:

(a) Free-living rhizobacteria

 (b) Symbiotic bacteria

PGPR exerts direct or indirect mechanism which influence the plant growth.

- Mechanism of PGPR

Fig. 1. Direct and Indirect mechanisms of PGPRs.

Direct mechanism

(a) Production of phytohormones:

- Production of indole acetic acid (IAA): The most extensively researched auxin, IAA, is responsible for cell division, elongation, differentiation, and extension. According to Saharan and Nehra (2011), bacteria employ IAA to interact with plants during colonization, including phytostimulation and evading the plant's defence mechanisms. Sweet potato cultivar inoculated with IAA producing isolates significantly improves plant growth by absorbing N, P, K, Ca and Mg (Farzana and Radizah 2005). Studies reported role of IAA production in Lysinibacillus spp. strains which showed that proliferation of lateral roots was significantly impacted by PGP activity. Also, all strains produced IAA between 20-70 µg/mL range (Pantoj-Guerra et al., 2023). In the presence of 10000µg/ml L-tryptophan Bacillus aerius produces 7µg/ml auxin while in the case of pot experiment inoculation with strains Z-54 and Z-16 produced maximum value of shoot length and fresh weight of Vigna radiata (Saboor et al., 2024). 24 PGPR strains isolated from the roots and rhizosphere of Ceanothus velutinus, a native plant in the intermountain west region of the USA. Of them, 17 isolates generated IAA levels greater than 10 µg/mL. In Arabidopsis thaliana, strains CK-6, CK-22, CK-41, CK-44, CK-47, CK-50, CK-53, and CK-55 notably enhanced shoot biomass. One of these isolates (CK-55) was recognized as Sphingobium, whereas the others were classified as Pseudomonas (Ganesh et al., 2024).

- Production of cytokinins and gibberellins: Cytokinins is responsible for cell growth and differentiation as well as apical dominance is also promoted by them. PGPR can produce them in soil and also in pure culture. Gibberellins are produced naturally by plants which influence developmental process of plants like germination, flowering, stem elongation, fruit senescence (Kundan et al., 2015). Certain PGPR strains are known to produce cytokinins, thereby stimulating plant growth. For instance, Azospirillum brasilense has been reported to produce cytokinins, contributing to enhanced plant growth and activation of antioxidant and physiological systems (Zaheer et al., 2022). Research has indicated that certain PGPR strains, including Bacillus subtilis and Bacillus toyonensis, produce cytokinins contributing to plant growth promotion (Biswas et al., 2024).

(b) Direct nitrogen fixation: Nitrogen fixation is the process of converting atmospheric nitrogen into usable nitrogen that transforms into ammonia. At moderate temperatures, nitrogen-fixing bacteria carry out biological nitrogen fixation. The rhizosphere of plants is known to be colonized by a variety of bacterial species belonging to the genera Azospirillum, Alcaligenes, Arthrobacter, Acinetobacter, Bacillus, Burkholderia, Enterobacter, Erwinia, Flavobacterium, Pseudomonas, Rhizobium, and Serratia. By having a variety of advantageous impacts, these bacteria greatly aid in the growth of plants by directly fixing the nitrogen (Kundan et al., 2015). 

Table 1: Nitrogen fixing bacteria and their relationship with the host plants (Kundan et al., 2015).

Pseudomonas spp., a genus of PGPR, differ from other PGPR genera in several ways, including nitrogen fixation and root colonization. These bacteria not only fix atmospheric nitrogen but also produce bioactive metabolites and siderophores that combat plant pathogens, further supporting plant health and growth (Singh et al., 2024). 

(c) Phosphate solubilization: Phosphate solubilizing bacteria converts the inorganic form of phosphorus into the available form which is required for plant growth. Bacillus, Rhizobium, and Pseudomonas are the most effective genera of bacteria that solubilize phosphate. In general, bacteria solubilize phosphate in two ways: (1) by releasing organic acid and altering phosphorus mobility through ionic interactions, and (2) by using phosphatases, which aid in the unbinding of phosphate groups from organic matter (Kundan et al., 2015). Pantoea isolates HCF6 and HCF9 isolated from the closed flowers of Hedychium coronarium demonstrated efficient solubilization of calcium phosphate (Ca₃(PO₄)₂) and aluminum phosphate (AlPO₄). Inoculation of wheat plants with these isolates under phosphorus-deficient conditions resulted in increased root and shoot lengths, enhanced plant dry mass, and higher phosphorus content in plant tissues (Prasad et al., 2022).

Table 2: Phosphate solubilizing bacteria with their host plants (Kundan et al., 2015).

Fig. 2. Flow chart showing phosphorus solubilization by the PGPRs (Yadav et al., 2022).

Indirect mechanism: Indirect mechanism involves the ability of PGPR to reduce the harmful effects of plant pathogens on the growth (Kundan et al., 2015).

(d) HCN production: Rhizobacteria are biocontrol agents that have the ability to colonize plant root surfaces and inhibit plant growth. Many microorganisms, including bacteria, algae, fungus, and plants, produce the poisonous chemical cyanide as a strategy of surviving by outcompeting their counterparts. Inoculation with cyanide-producing bacterial strains usually has no adverse effects on the host plants. The majority of HCN is produced by Bacillus and Pseudomonas species. Cell death is thought to result from HCN's inhibition of the electron transport chain and the cell's energy source. Additionally, it appears that PGPR inhibits the reversible process of inhibition of natural receptors and enzymes, and it is also known to limit the action of cytochrome oxidase. PGPR strains isolated from the sorghum rhizosphere and used to assess their plant growth-promoting traits which showed HCN production under controlled condition. The study found that certain isolates, such as Bacillus subtilis (PG-152), exhibited multiple beneficial traits, including HCN production, which contributed to enhanced plant growth and biocontrol of pathogens (Chiranjeevi et al., 2024).

(e) Competition: Sometimes, PGPR competes for nutrients with hazardous microorganisms that are present in trace amounts, which can limit the disease-causing agent. This is evident when the soil contains a large number of non-pathogenic microorganisms that quickly colonize plant surfaces, consume up available nutrients, and so prevent the growth of pathogenic germs. When the pepper plant infected with the Alternaria alternata was inoculated with Bacillus amyloliquefaciens RaSh1 PGPR strain it showed 40 percent disease incidence while plants with infection of A. alternata alone recorded 80 percent disease incidence (Soliman et al., 2023).

These mechanisms are critical yet challenging to study within the system. Among them, nutrient competition between PGPR and pathogens stands out as the most significant interaction. This competition indirectly promotes plant growth by suppressing pathogen development.

Table 3: PGPR having the ability to biocontrol several diseases in the host plant (Kundan et al., 2015).

(f) Induced systemic resistance (ISR): When a plant pathogen attacks, ISR is triggered. It is a process that results in enhanced resistance at certain plant locations after induction. It aids in the plant's disease control but is not pathogen-specific. It involves the plants receiving signals from jasmonate and ethylene, which are hormones that boost the host plant's defence mechanisms against a variety of diseases (Kundan et al., 2015). Systemic resistance in tomato was induced by PGPR strain Bacillus cereus AR156 by modulating the WRKY8 transcription factor. This modulation enhances the plant's defence against pathogens and contributes to improved development under stressful circumstances (Li et al., 2023). 

The future scope of Plant Growth Promoting Rhizobacteria (PGPR) in sustainable agriculture is vast, driven by the need for eco-friendly solutions to enhance crop productivity and soil health. Advances in biotechnology and precision agriculture are paving the way for customized PGPR formulations tailored to specific crops and environmental conditions. The integration of PGPR with organic and regenerative farming practices can significantly reduce dependency on chemical fertilizers and pesticides, promoting long-term soil fertility and biodiversity. Furthermore, research on stress-tolerant PGPR strains can help mitigate the effects of climate change, including drought, salinity, and heavy metal contamination. The application of omics technologies, such as metagenomics and transcriptomics, is expected to unveil new PGPR strains with enhanced growth-promoting capabilities. Additionally, innovations in microbial delivery systems, including nano-formulations and seed coatings, can improve the efficacy and stability of PGPR in agricultural systems. With increasing global awareness and policy support for sustainable farming, the commercialization of PGPR-based biofertilizers and biopesticides is expected to expand, offering cost-effective and environmentally friendly alternatives for farmers worldwide.