Plant Growth-Promoting Rhizobacteria (PGPR): A Potential Alternative Tool for Sustainable Agriculture

Abstract

Soil is an important natural resource that nurtures living microbial communities and improves plant productivity, thus ensuring food security. The chemical fertilizers used during the last few decades though improved plant productivity so rapidly; however, it is indiscriminate use results in poor soil health and less agricultural productivity, affecting food security and human health worldwide. There is an urgent need of biological agents, such as plant growth-promoting rhizobacteria (PGPR), which may serve as better alternative to solve this problem. PGPR plays an important role to increase soil fertility, plant growth promotion, and suppression of phytopathogens for the development of eco-friendly sustainable agriculture. The present study provides a critical overview on PGPR, its mechanism and function, and significance as a potential alternative tool for sustainable agriculture. An attempt has been made to propose an eco-friendly model integrating PGPR with various sectors, such as human health, agriculture, and food industry for its effective commercialization. The study might be helpful to identify the prospects and challenges of PGPR to fully integrate them into sustainable agriculture practices.

1. Introduction

The agricultural sector significantly contributes to a substantial portion of a country’s wealth, particularly in developing nations. The agriculture sector not only generates employment opportunities but also ensures food security. The increased use of chemical fertilizers though enhanced agricultural productivity; however, their excessive application has negatively impacted the soil health. This has led to a decline in agricultural output, posing a significant threat to both human well-being and global food security. Presently, there is a growing emphasis on maximizing agricultural output through the efficient utilization of limited resources while adopting a holistic approach to minimize adverse environmental effects. This approach not only balances environmental health and productivity but also enhances resilience in the face of changing climatic conditions, safeguarding the well-being of both present and future generations [1]. PGPR represents an alternative tool for sustainable agriculture, offering several benefits. It is a group of beneficial bacteria, which forms a symbiotic relationship with the plants and mostly found in the rhizosphere and the soil region around plant roots. PGPR not only fosters a more balanced and resilient ecosystem within the soil but also improves nutrient availability, enhances stress tolerance in plants, and increases crop productivity [2]. It contributes to the growth, development, and overall health of plants through various mechanisms such as nitrogen fixation, nutrient solubilization, and disease suppression [3]. Some of the PGPRs such as Azobacter, Azosprillum, Bacillus, Enterobacter, Klebsiella, Pseudomonas, Variovorax, and Serratiaare play important roles in the agricultural field [4].

Of late, the role of PGPR in agriculture sector has gained wide popularity since promotion of such biological agent not only align well with the principles of sustainable agriculture but also uphold ecological balance while fulfilling the food requirements of an expanding global population. However, there are various challenges involve in effective utilization of this PGPR in agriculture. These challenges might involve the careful screening of PGPR strains, evaluating their complex interactions with native soil microbes, assessing their adaptability to diverse environmental conditions, and understanding potential long-term impacts, etc. On the other hand, the process of bringing PGPR to the commercial market involves obtaining necessary regulatory approvals, ensuring the ability to scale up production processes, developing efficient methods of application, and ensuring the sustained viability of PGPR in field conditions. The present study provides a critical overview on PGPR, its mechanism and function, and significance as a potential alternative tool for sustainable agriculture. An attempt has been made to propose an eco-friendly model integrating PGPR with various sectors such as human health, agriculture, and food industry for its effective commercialization. The study might be helpful to identify the prospects and challenges of PGPR to fully integrate them into sustainable agriculture practices.

2. PGPR and its classification

The rhizosphere, the region surrounding plant roots, is characterized by elevated microbial activity, fostering a limited but crucial reservoir of macro- and micronutrients. PGPR establishes colonies within this zone, contributing to the plant’s growth and development [5]. PGPR can be classified in many ways (Figure 1). For instance, PGPR based on their interaction with plants can be divided into symbiotic bacteria and free-living rhizobacteria. Their classification is also influenced by their residing locations; intracellular PGPR (iPGPR), exemplified by Rhizobia sp. and Frankia sp., inhabit plant cells, form nodules, and are localized within specialized structures. On the contrary, extracellular PGPR (ePGPR) inhabits the external environment of plant cells, lacking nodules but still actively promoting plant growth. Furthermore, PGPRs are categorized based on their functional roles into four types such as biofertilizers (enhancing nutrient availability for plants), phytostimulators (facilitating plant growth, often through phytohormone production), rhizoremediators (involved in the breakdown of organic pollutants), and biopesticides (aiding in disease control primarily through the synthesis of antibiotics and antifungal metabolites) [6]. PGPR encompasses a variety of genera, including but not limited to Arthrobacter, Azotobacter, Azospirillum, Pseudomonas, Acetobacter, Micrococcus, Burkholderia, Bacillus, Paenibacillus, Agrobacterium, Caulobacter, Chromobacterium, Erwinia, Flavobacterium, Serratia, Rhizobium, and some belonging to the Enterobacteriaceae family [7].

Figure 1.

Classification of PGPR.

3. PGPR-assisted zone: the rhizosphere

The term “"rhizosphere” was coined by German plant biologist and agronomist Lorenz Hiltner in 1904 to describe the interface between plants and roots. The term is derived partially from the Greek word “rhiza,” meaning root [8]. Hiltner’s definition characterizes the rhizosphere as the region surrounding a plant root, hosting a distinct community of microorganisms influenced by substances released by plant roots. The rhizosphere is composed of three main zones: the endorhizosphere, rhizoplane, and ectorhizosphere (Figure 2). The endorhizosphere refers to the root tissue, including the endodermis and cortical layers. The rhizoplane is the root surface where bacteria and soil particles attach, encompassing layers such as the mucilaginous polysaccharide layer, cortex, epidermis, and the ectorhizosphere, which adheres to the soil around the root [9].

Figure 2.

PGPR-assisted zone: the rhizosphere.

The rhizosphere serves as a habitat where plants, soil, microbes, and soil microfauna engage in extensive interactions. These interactions are pivotal for biochemical exchanges and the sharing of signal molecules between plants and rhizobacteria. These interactions profoundly impact plant growth and production. Rhizobacteria, known as rhizosphere-competent bacteria, actively colonize plant roots throughout various plant growth phases, indicating the presence of rhizobacteria. The rhizobacterial populations particularly under abiotic stress associated with roots play a crucial role in maintaining plant health. Plant-microbe interactions occur within the rhizosphere, where both beneficial and potentially harmful microorganisms coexist. The composition of the rhizobacterial community in the rhizosphere varies with changes in soil properties [10]. Naturally, interactions among rhizobacteria in the rhizosphere significantly influence soil health and enhance its nutritional condition, both of which are vital for improved plant growth [11]. The exchange of resources between shoots and roots relies on the growth and proliferation of roots, facilitated by abundant interactions between roots, the rhizosphere, and rhizobial bacteria. A robust rhizosphere-rhizobacterial interaction shields root exudates, which comprise various chemical substances attracting microorganisms to the root [12, 13]. This interaction mediated by root exudation plays a pivotal role in plant-microbe interactions by facilitating root colonization and stimulating root growth.

The rhizosphere is enriched with utilizable carbon sources due to rhizodeposition, a process involving the release of organic substances by plant roots, including amino acids, fatty acids, sterols, growth factors, organic acids, and sugars. The rhizosphere hosts an intricate microbial community comprising saprophytes, endophytes, epiphytes, pathogens, and beneficial microorganisms such as bacteria, fungi, nematodes, protozoa, algae, and more [14]. According to Yadav et al., 1,200,106 bacteria/g dry soil are found in the rhizosphere, which is significantly higher than fungi (12,105 fungi/g dry soil), algae (5105 algae/g dry soil), and actinomycetes (46,106 actinomycetes/g dry soil) [15]. Root exudation, secretion, and deposition contribute various organic compounds, making the rhizosphere richer in nutrients than the bulk soil. This richness fosters an active and enhanced microbial community in the root zone, leading to the phenomenon known as the rhizosphere effect. The rhizosphere effect is quantified as the R:S ratio, where R represents the total number of microorganisms in the rhizosphere, and S represents the corresponding amount in the bulk soil. This ratio serves as a measure of microbial activity, with a higher R:S ratio indicating increased activity in the rhizosphere [6, 9].

4. Mechanism of PGPR and functions

PGPR employs both direct and indirect mechanisms to stimulate plant growth (Figure 3). These mechanisms are vital for enhancing nutrient availability, promoting hormonal balance, and fortifying plants against various stresses. By utilizing these direct and indirect mechanisms, PGPR contributes to a holistic approach in promoting plant growth and health, making them valuable allies in sustainable agriculture.

Figure 3.

Mechanism of action of PGPR.

4.1 Direct method

In case of direct method, PGPR generally enhances plant growth either by aiding in the acquisition of crucial resources such as nitrogen, phosphorus, and essential minerals or by modulating the levels of plant hormones. The specific processes involved in these mechanisms are detailed below:

4.1.1 Nitrogen fixation

Organisms capable of fixing nitrogen can be classified into two main groups namely, symbiotic nitrogen-fixing bacteria, exemplified by members of the Rhizobiaceae family, which establish symbiotic relationships with leguminous plants (e.g., rhizobia with legumes) and nonleguminous trees (e.g., Frankia) and nonsymbiotic nitrogen-fixing bacteria, including free-living, associative, and endophytic types, such as cyanobacteria. However, nonsymbiotic nitrogen-fixing bacteria only moderately fulfill the nitrogen requirements of their host plants [16]. In the case of leguminous plants, the roots form a symbiotic partnership with nitrogen-fixing rhizobia from the Rhizobiaceae family, which are α-proteobacteria. This symbiosis leads to the development of nodules, where rhizobia colonizes as intracellular symbionts, resulting from a complex interplay between the host plant and the symbiotic bacteria. Diazotrophs, a subset of plant growth-promoting rhizobacteria, fix atmospheric nitrogen (N₂) in nonleguminous plants, forming a non-obligate relationship with the host plants. The process of nitrogen fixation involves a complex enzyme, comprising the iron protein dinitrogenase reductase and dinitrogenase with a metal cofactor. Dinitrogenase reductase generates electrons with high reducing power, facilitating the conversion of N₂ to NH₃ by dinitrogenase. Three distinct nitrogen-fixing systems have been identified based on the metal cofactor viz., Mo-nitrogenase, V-nitrogenase, and Fe-nitrogenase. The structural composition of the nitrogen-fixing system varies among different bacterial genera. The predominant biological nitrogen fixation is carried out by the molybdenum nitrogenase, which is present in all diazotrophs.

Nitrogen fixation genes, referred to as nif genes, are present in both symbiotic and free-living systems. These genes, encoding nitrogenase (nif) enzymes, include structural components, genes related to Fe protein activation, iron-molybdenum cofactor biosynthesis, electron donation, and regulatory genes essential for enzyme synthesis and function. Typically organized in a 20–24 kb cluster with seven operons expressing 20 distinct proteins in diazotrophs, nif genes play a crucial role in nitrogen fixation [16]. Within this gene cluster, the nifDK and nifH genes encode two-component proteins of the molybdenum nitrogenase enzyme complex. NifDK, a heterotetrameric (α2β2) protein, consists of two ab dimers linked by twofold symmetry. Each α-subunit (NifD) within the active site of NifDK contains an iron-molybdenum cofactor (FeMo-co) [17]. In Rhizobium and other diazotrophs, the symbiotic activation of nif genes is contingent upon low oxygen levels, regulated by another set of genes known as fix genes, found in both symbiotic and free-living nitrogen fixation systems. Significantly, nitrogen fixation demands a substantial amount of energy, requiring at least 16 mol of ATP for each molecule of fixed nitrogen. Phosphate solubilization is a crucial process as phosphorus is the second most important plant growth-limiting nutrient after nitrogen. Despite the abundance of phosphorus in soils, its accessibility to plants is often limited. Key phosphate-solubilizing bacteria include Azotobacter, Bacillus, Beijerinckia, Burkholderia, Enterobacter, Erwinia, Flavobacterium, Microbacterium, Pseudomonas, Rhizobium, and Serratia [7]. Inorganic phosphorus is typically solubilized through the action of low molecular weight organic acids produced by various soil microorganisms. Conversely, organic phosphorus is mineralized through the activity of phosphatases, which catalyze the hydrolysis of phosphoric esters. It is noteworthy that certain bacterial strains can exhibit both phosphate solubilization and mineralization capabilities. Apart from providing phosphorus to plants, phosphate-solubilizing bacteria enhance plant growth by improving the efficiency of biological nitrogen fixation (BNF) and increasing the availability of other trace elements through the synthesis of essential plant growth-promoting substances [5, 18].

4.1.2 Siderophore production

Iron is an essential element for virtually all forms of life, with the exception of certain lactobacilli; hence, all known microorganisms require iron for their survival. In aerobic environments, iron is predominantly present as Fe³⁺, and it tends to form insoluble hydroxides and oxyhydroxides, making it inaccessible to both plants and microbes [19]. Bacteria typically acquire iron through the production of low-molecular-mass iron chelators known as siderophores, which exhibit high association constants for complexing iron. Most siderophores are water-soluble and can be categorized as extracellular or intracellular.

In general, rhizobacteria vary in their ability to utilize siderophores produced by others of the same genus (homologous siderophores), while some can use siderophores from different genera of rhizobacteria (heterologous siderophores) [18]. In the presence of iron limitation, the Fe³⁺⁻siderophore complex on the bacterial membrane undergoes reduction to Fe²⁺, which is then released into the cell through a gating mechanism connecting the inner and outer membranes in both Gram-negative and Gram-positive rhizobacteria. The siderophore may be degraded or recycled during this reduction process [19]. Siderophores, in the context of iron deficiency, act as solubilizing agents for iron sourced from minerals or organic substances [20]. Beyond their role in iron complexation, siderophores can also form stable complexes with other heavy metals such as Al, Cd, Cu, Ga, In, Pb, and Zn, as well as radionuclides, such as U and Np. The binding of a siderophore to a metal increases the concentration of soluble metal [19]. Consequently, bacterial siderophores play a crucial role in alleviating stresses on plants caused by elevated levels of heavy metals in the soil.

4.1.3 Phytohormone production

The microbial synthesis of the phytohormone auxin, specifically indole-3-acetic acid (IAA), has long been recognized. Generally, IAA released by rhizobacteria can impact various plant developmental processes as the plant’s endogenous pool of IAA may be influenced by the externally acquired IAA secreted by soil bacteria [16, 21]. Notably, IAA serves as a reciprocal signaling molecule, influencing gene expression in diverse bacteria, underscoring its critical role in rhizobacteria-plant interactions [22]. Moreover, the down-regulation of IAA as a signaling molecule is associated with plant defense mechanisms against various phytopathogenic bacteria. This is evidenced by the heightened susceptibility of plants to bacterial pathogens upon the exogenous application of IAA or exposure to IAA produced by the pathogen [22]. IAA has been implicated in nearly every facet of plant growth, development, and defense responses. The intricate complexity of IAA biosynthesis, transport, and signaling pathways reflects its diverse functional roles [23].

In general, IAA influences plant processes such as cell division, extension, and differentiation, stimulates seed and tuber germination, accelerates xylem and root development, regulates vegetative growth, initiates lateral and adventitious root formation, mediates responses to light, gravity, and fluorescence, and influences photosynthesis, pigment formation, biosynthesis of various metabolites, and stress resistance. Rhizobacteria-produced IAA is expected to impact these physiological processes by altering the plant auxin pool. Furthermore, bacterial IAA enhances root surface area and length, facilitating the plant’s access to soil nutrients. Additionally, rhizobacterial IAA loosens plant cell walls, promoting increased root exudation, which, in turn, provides additional nutrients to support the proliferation of rhizosphere bacteria [16]. Consequently, rhizobacterial IAA is recognized as a pivotal effector molecule in plant–microbe interactions, playing roles in both pathogenesis and phyto-stimulation [22].

4.1.4 1-aminocyclopropane-1-carboxylate (ACC) deaminase

Ethylene, a crucial plant growth hormone, is essential for normal plant growth and development [23]. This hormone is produced internally by nearly all plants and is also generated by various biotic and abiotic processes in soils, playing a vital role in inducing diverse physiological changes in plants. Aside from its role as a plant growth regulator, ethylene is recognized as a stress hormone [24]. Under stressful conditions such as salinity, drought, waterlogging, heavy metals, and pathogenicity, the endogenous production of ethylene significantly increases, leading to severe impacts on overall plant growth. Elevated concentrations of ethylene can cause defoliation and other cellular processes that may result in decreased crop performance [7, 24]. PGPR that produces the enzyme ACC deaminase contributes to plant growth and development by reducing ethylene levels. This reduction in ethylene levels is associated with increased salt tolerance and the mitigation of drought stress [25, 26]. Currently, bacterial strains displaying ACC deaminase activity have been identified across various genera, including Acinetobacter, Achromobacter, Agrobacterium, Alcaligenes, Azospirillum, Bacillus, Burkholderia, Enterobacter, Pseudomonas, Ralstonia, Serratia, and Rhizobium, among others. These rhizobacteria take up the ethylene precursor ACC and convert it into 2-oxobutanoate and NH₃ through the action of ACC deaminase.

ACC deaminase producers play a significant role in alleviating stress induced by various phytopathogenic microorganisms (such as viruses, bacteria, and fungi), as well as stressors such as polyaromatic hydrocarbons, heavy metals, radiation, wounding, insect predation, high salt concentration, drought, extremes of temperature, high light intensity, and flooding [16]. Consequently, the notable effects observed upon seed/root inoculation with ACC deaminase-producing rhizobacteria include enhanced plant root elongation, promotion of shoot growth, and improvements in rhizobial nodulation, as well as increased uptake of nitrogen (N), phosphorus (P), and potassium (K). Additionally, there is an observed enhancement in mycorrhizal colonization in various crops [16, 25, 27, 28].

4.2 Indirect mechanisms

The indirect mechanisms of PGPR involve functions aimed at alleviating inhibitory effects on plant growth and development caused by various pathogens. These functions often include the use of biocontrol agents, which have specific roles in suppressing or managing pathogenic organisms that can negatively impact plants. The details of these indirect mechanisms may vary, encompassing processes such as antagonism, competition for resources, and the induction of systemic resistance in plants. Overall, the goal is to enhance plant health and productivity by mitigating the harmful effects of pathogens indirectly through the actions of PGPR described below:

4.2.1 Biofilm formation

Recent investigation has revealed that biofilm development in the rhizosphere plays a significant role in rhizobacteria’s modes of action against root diseases. The presence of high numbers of bacterial cells in biofilms causes the release of different compounds such as toxins and antibiotics in their periphery, which inhibits phytopathogens in the soil [29].

4.2.2 Catabolic enzyme secretion

Various microbial species can release catabolic enzymes (proteases, -1,3-glucanase, and chitinases) and small compounds, which can help to control soil-borne plant diseases. Electron microscopy studies provide specifics of the antagonist effect on fusarium hyphae, revealing the clear anomaly of mycelial growth, which can be related to the influence of cell wall-degrading enzymes produced by rhizobacteria, such as chitinases [30].

4.2.3 Antibiotic production

Antibiotics and other chemicals harmful to phytopathogens have been isolated from Bacillus strain metabolites. This has been demonstrated in research on Bacillus megaterium, which can colonize roots and reduce Rhizoctoniasolani [31].

4.2.4 Production of siderophores

Rhizobacteria produce siderophores as secondary implications. These molecules have the ability to bind Fe³⁺ ions, which are required for metabolism and cell proliferation. In this manner, bacteria that colonize plant roots might compete for available iron in the soil and may impede the growth of other rhizosphere microbes. Siderophore-producing rhizobacteria can inhibit the growth of harmful microorganisms in the root zone [32].

4.2.5 Acquired systemic resistance and induced systemic resistance

Plants have a natural basal defense system against phytopathogens, but other systems can be triggered or generated to boost plant resistance [33]. Acquired systemic resistance (ASR) and induced systemic resistance (ISR) are the two most commonly researched types of resistance induction. When plants are exposed to an inducer agent (such as a pathogenic organism), defense mechanisms are activated at the induction site, which exhibits alterations (necrosis), as well as other distant sites, resulting in the plant being systemically protected against subsequent infections caused by a broad spectrum of pathogens [34]. ASR is accompanied by a rise in salicylic acid content and the accumulation of proteins associated with pathogenesis (PRPs), which are plant defense mechanisms. ISR can be initiated by nonpathogenic microorganisms in the rhizosphere and does not involve the salicylic acid signaling pathway or the generation of PRPs; rather, this type of resistance is activated by the jasmonic acid and ethylene resistance-signaling pathway.

5. PGPR: a novel approach to sustainable agriculture

Global food production increases in the twentieth century were essentially based on two broad areas of progress: chemical inputs (commercial fertilizers and pesticides) and genetic alterations via targeted breeding and gene manipulation. However, the continued use of chemicals, fertilizers, and pesticides, as well as the resultant negative impacts on the environment, has shifted public opinion. Scientists are experimenting with several strategies to boost crop output in a sustainable manner, including the use of phytomicrobiome members, which is now being recognized as a “fresh” green revolution [35]. The use of beneficial microorganisms on food crops has been extensively researched, but their application in the field is quite limited. The introduction of phytomicrobiome members in agricultural systems as a sustainable solution for disease management and nutrient supplementation could mitigate the negative effects associated with the overuse of chemical inputs (fertilizers and pesticides) [36]. Furthermore, members of the phytmicrobiome have been used as an effective technique to alleviate specific biotic and abiotic challenges that may impair crop growth and productivity [37].

5.1 PGPR in abiotic stress management

Any unfavorable environmental conditions that may affect the functional diversity of microbes and also the physicochemical properties of soil can dictate abiotic stress. Numerous drastic conditions, including heavy metal toxicity, salinity, drought, and flooding affecting the plant microbiome and the surrounding ecology, are abiotic stress [38].

5.2 PGPR in biotic stress management

Living organisms, particularly bacteria, viruses, fungi, insects, and nematodes, cause biotic stress in plants. Such stress directly interacts with host nutrition, resulting in plant death. Biotic stress causes both pre- and postharvest loss. Although few microbes participate in pathogen biological control, PGPR is known to provide protection from a variety of diseases via mechanisms such as bacteriocin, antibiosis, volatile organic compound (VOC) production, and lysis via the extracellular enzyme [39]. Microbial stimulants have been shown to be efficient in suppressing a range of plant pathogens, resulting in sound harvest growth.

5.3 Co-metabolism of PGPR

Rhizospheric microbial metabolites are thought to be critical to ecological success. Based on their substrate consumption patterns, many rhizomicrobes that share this environment play essential ecological roles [40]. If two strains have comparable substrate uptake characteristics, the fittest will survive, resulting in the competitive exclusion of the less fit strain [41]. A rhizobacteria strain frequently acts in such a way that it excretes a unique compound that was not present in the native root. This results in the creation of a novel niche that cross-feeding strains may occupy [42].

5.4 PGPR as biofertilizer

Biofertilizers are live formulations of beneficial bacteria that aid in nutrient availability through biological activity and so improve soil health and consequently soil microflora. Plant growth-promoting microorganisms (PGPM) are the key component of this biofertilizer. This PGPM can be divided into three key groups such as arbuscularmycorrhizal organisms (AMF), plant development advancing rhizobacteria (PGPR), and nitrogen-fixing rhizobia [43], all of these are beneficial to plant development and sustenance. Nonetheless, it has been reported that PGPR has been used as a biofertilizer all over the world, contributing to increased yields and soil quality. As a result of the expected PGPR commitment, it may prompt sustainable agribusiness. Such biofertilizers are available in both solid and liquid formulations. There are three forms of liquid formulations: root inoculation, seed inoculation, and soil inoculation [44].

6. Prospects and challenges

PGPR offers promising avenues for sustainable agriculture and ecosystem health, yet they come with inherent challenges. They hold the potential to enhance plant growth, nutrient uptake, and disease resistance through mechanisms such as nitrogen fixation, phosphate solubilization, and production of growth-promoting substances. Harnessing these benefits could lead to reduced reliance on chemical fertilizers and pesticides, fostering environmentally friendly agricultural practices. However, successful implementation faces hurdles such as variability in PGPR effectiveness across different plant species and environments, as well as competition with native soil microbes. Ensuring consistent and reliable results requires a deep understanding of the intricate interactions between PGPR, plants, and soil. Furthermore, developing cost-effective and scalable production methods for PGPR inoculants, ensuring their compatibility with other agricultural practices, and navigating regulatory approvals pose significant challenges. Bridging the gap between research findings and on-field application necessitates collaboration among microbiologists, agronomists, and farmers. Overcoming these challenges could unlock the full potential of PGPR, driving sustainable agriculture while mitigating concerns about food security and environmental impact.

This section describes an environmentally friendly approach using PGPR as an alternative tool to increase crop productivity ensuring sustainable agriculture. In this context, an eco-friendly model integrating PGPR with various sectors, such as human health, agriculture, and food industry, is proposed for its effective commercialization (Figure 4). At its core, PGPR acts as a linchpin, establishing beneficial connections between these sectors. In agriculture, PGPR-enhanced crops not only yield more but also require fewer synthetic agrochemicals, reducing environmental impact. The model extends to human health as well, where PGPR-associated crops can contribute to more nutritious diets, potentially lowering the incidence of diet-related illnesses. Moreover, the use of PGPR in soil enrichment aligns with sustainable land management practices, mitigating soil degradation and erosion. In the food industry, the utilization of PGPR-enriched crops can foster a supply of quality raw materials while reducing the ecological footprint of food production. The model via synergistically linking these sectors fosters circular economies where agricultural waste can be utilized as biofertilizer, reducing the need for external inputs, while PGPR-influenced agriculture generates higher-quality produce for both human consumption and industrial processing. Collaboration across sectors fuels innovation, allowing for the exploration of novel applications, such as PGPR-based probiotics for both humans and animals. However, realizing this model demands addressing challenges, such as scaling up PGPR production, ensuring regulatory compliance for human consumption, and implementing effective knowledge transfer across sectors. This integrated model through strategic partnerships, interdisciplinary research, and informed policymaking can potentially capitalize the symbiotic relationships facilitated by PGPR to drive sustainable practices, benefiting ecosystems, human health, and the global economy.

Figure 4.

An eco-friendly model integrating PGPR with other sectors, such as human health, agriculture, and food industry, is proposed for its effective commercialization.

7. Conclusion

The use of bacterial fertilizers has resulted in considerable improvements in plant growth, health, and yield. The PGPR stimulation mechanism might be direct or indirect. PGPR, showcasing a range of activities directed toward promoting plant growth, also exhibits bioremediation capabilities by detoxifying pollutants, such as heavy metals and pesticides. Additionally, they play a role in controlling various phytopathogens, functioning as biopesticides. In various crop studies, PGPR has demonstrated remarkable results. Biological control of plant diseases by microbial-based products has enough potential for a global market share of about 15–20% with an annual growth rate of around 15%. The use of microbial-based products is an environmentally friendly approach and is the best way to reduce the use of chemical fertilizers. The productive efficacy of a particular PGPR can be further improved by optimizing and acclimating to the soil conditions. In the future, it is anticipated that PGPR will replace chemical fertilizers, pesticides, and synthetic growth regulators, given their numerous detrimental effects on sustainable agriculture. Further, research and elucidation of mechanisms for PGPR-mediated phyto-stimulation will open the door to discover more effective rhizobacteria-technological strains that may work in a variety of agri-ecological environments. Since, its discovery, PGPR has shown great promise as a major contributor to sustainable agriculture development. However, much remains to be done in terms of both explorations and implementation. Explorations, which involve understanding the mechanism at the same time as implementation, require a great deal of optimization in field application. PGPR should be encouraged and prioritized as a bioremediation tool for biocontrol. PGPR has all the potential to act as biofertilizer, which could work in a better ecosystem with increased productivity. However, further understanding of the PGPR process could aid in the identification of more particular strains for successful implementation at the industrial level, thus could reduce the application of chemical fertilizers safeguarding the ecosystem.

Conflict of interest

The authors declare no conflict of interest.