Geographical Research Bulletin Volume 4 (February 20, 2025) pp.

158-174
Online ISSN: 2758-1446 https://doi.org/10.50908/grb.4.0_158

[Review Paper]
The functional mechanisms, screening methods, and agricultural application
potential of plant growth-promoting bacteria
Yifan Zhou1), Yinshuang Bai1)

Abstract
Plant growth-promoting bacteria (PGPB) are microorganisms capable of enhancing plant growth through
various mechanisms, including rhizosphere bacteria, endophytic bacteria, and certain fungal organisms. This
paper systematically summarizes the main functions and mechanisms of PGPB, encompassing the synthesis
of plant growth regulators (such as auxins, cytokinins, and gibberellins), nitrogen fixation, activation of
nutrients like phosphorus and potassium, mitigation of abiotic stresses (e.g., drought and salinity), and
defense against biotic stresses (e.g., pathogenic infections). Additionally, it highlights the critical roles of
PGPB in regulating plant root system architecture and shaping the rhizosphere microbial community. On the
application side, this paper reviews the screening and preparation technologies of PGPB, including strain
selection, carrier development, and preservation techniques, along with diverse inoculation methods such as
seed coating, soil application, foliar spraying, and root drenching. The multifunctionality and potential of
PGPB in microbial inoculants, agricultural biopesticides, and biofertilizers are discussed through typical
application cases, emphasizing their pivotal role in promoting sustainable agriculture. This study provides
theoretical support for understanding the functional mechanisms of PGPB and their applications in
addressing climate change, improving agricultural productivity, and enhancing resource use efficiency, while
also outlining future research and practical directions in this field.
Keywords
plant growth-promoting bacteria, sustainable agriculture, microbial inoculants, abiotic stress tolerance,
biofertilizer applications

1. Introduction
In the context of rapid global population growth and intensifying climate change, agricultural production
is facing severe challenges. The excessive reliance on chemical fertilizers and pesticides in traditional
agriculture has not only led to ecological issues such as soil degradation and environmental pollution but
also posed threats to food safety and agricultural sustainability [1]. Consequently, exploring technological
pathways that reduce agricultural inputs while improving crop yield and quality has become a core topic in
current agricultural research. Plant growth-promoting bacteria (PGPB) are considered a key technology for

1) School of Environment, Zhaoqing University

This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).
https://creativecommons.org/licenses/by/4.0/legalcode

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advancing green agricultural transformation due to their potential to enhance plant growth through various
biological mechanisms [2]. By facilitating nutrient uptake, alleviating environmental stress, and regulating
root growth, PGPB functions in the rhizosphere or within plants to provide natural support for crops,
significantly reducing the use of fertilizers and pesticides while improving resource use efficiency.
In recent years, significant progress has been made in PGPB research. On the fundamental research front,
studies ranging from genomics and metabolomics to microbial ecology have revealed the molecular
mechanisms of PGPB-plant interactions and demonstrated the diversity and complexity of PGPB
functions[3,4]. In applied research, PGPB, as microbial inoculants, biofertilizers, and biopesticides, have
shown promising results in enhancing crop yield and improving agricultural practices in certain production
systems[5]. However, the practical application of PGPB still faces multiple challenges, including the
complexity of environmental conditions, dynamic changes in microbial communities, difficulties in strain
selection, and inconsistent application effectiveness. Additionally, significant differences in the requirements
for PGPB across regions, crops, and management practices impose higher demands on their
commercialization and large-scale deployment.
This paper systematically summarizes the functions and mechanisms of PGPB, providing a
comprehensive review of recent research progress in promoting plant growth, regulating rhizosphere ecology,
and alleviating both biotic and abiotic stresses. Based on this foundation, it explores the preparation
techniques for PGPB inoculants, inoculation methods, and their application potential in agricultural
production. Addressing the key issues in current research and practical application, this paper envisions the
role and value of PGPB in sustainable agricultural development, offering theoretical support and technical
reference for related studies and applications.

2. Definition and types of plant growth-promoting bacteria

2.1. Definition of plant growth-promoting bacteria
PGPB is a group of microorganisms capable of promoting plant growth through direct or indirect
mechanisms. Direct mechanisms include the production of plant hormones, nitrogen fixation, and the
activation of insoluble soil nutrients (e.g., phosphorus and potassium), thereby providing a more effective
nutrient supply for plants. Indirect mechanisms primarily involve controlling pathogenic microorganisms,
competing for ecological niches, and inducing systemic resistance in plants, which enhances the plants’ stress
tolerance. These microorganisms are widely distributed in the rhizosphere, inside roots, or on aerial plant
parts, establishing symbiotic, synergistic, or mutualistic relationships with plants, making them a key
component of agricultural ecosystems [6,7].
Based on their modes of action and functional diversity, PGPB can be categorized into multifunctional
and specific functional types. For instance, multifunctional PGPBs are capable of synthesizing various plant
hormones and dissolving phosphorus and potassium simultaneously, while specific functional types may

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primarily antagonize pathogens. As research advances, the definition of PGPB has gradually expanded to
include not only traditional rhizosphere bacteria but also certain endophytic bacteria and mycorrhizal fungi,
all of which provide vital support for plant growth.

2.2. Main types of plant growth-promoting bacteria

2.2.1. Rhizobacteria
Rhizobacteria are the most common type of PGPB, widely distributed in the rhizosphere soil of plants.
They utilize plant exudates, such as organic acids, sugars, and amino acids, as their primary carbon sources,
maintaining a close interaction with plant roots. These bacteria promote plant growth through various
mechanisms, including nitrogen fixation, solubilization of insoluble phosphorus and potassium in the soil,
synthesis of plant hormones (e.g., auxins and cytokinins), and inhibition of pathogenic microorganisms by
secreting antibiotics and siderophores. Common rhizobacteria include Pseudomonas spp., Rhizobium spp.,
and Bacillus spp. Due to their wide-ranging effects and ease of cultivation, rhizobacteria have been
extensively studied and applied in agricultural production.

2.2.2. Endophytes
Endophytes are microorganisms capable of colonizing the interior of plant tissues (typically roots, stems,
leaves, or seeds) without causing obvious harm to the host plant. These microorganisms directly exchange
substances with their host within plant tissues, promoting nutrient absorption, regulating hormonal balance,
and aiding the host plant in resisting environmental stresses such as drought, salinity, and pathogen infection.
Compared to rhizobacteria, endophytes exhibit greater stability within the host and establish closer
mutualistic relationships, making them highly promising candidates for the development of efficient
bioinoculants. Common endophytes include Burkholderia spp., Bacillus cereus, and certain actinomycetes.

2.2.3. Fungal plant growth-promoting microorganisms

Fungal plant growth-promoting microorganisms mainly include arbuscular mycorrhizal fungi (AMF)
and certain rhizosphere fungi. These fungi form symbiotic relationships with plants, particularly AMF, which
expand the absorptive range of plant roots through their hyphal networks, thereby enhancing the uptake of
water and nutrients such as phosphorus and zinc. Additionally, some fungi suppress pathogenic
microorganisms by secreting secondary metabolites, producing antimicrobial compounds, and inducing
systemic resistance in plants. Fungal plant growth-promoting microorganisms offer unique advantages in
enhancing plant stress tolerance and stabilizing rhizosphere ecosystems. Common examples include Glomus
spp. AMF, Trichoderma spp., and Paecilomyces spp.
The three types of PGPB described above possess distinct functional characteristics that complement
each other in different ecological niches and application contexts. Their use in agricultural systems not only

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enhances crop nutrient absorption and stress tolerance but also significantly reduces reliance on chemical
fertilizers and pesticides, providing strong support for sustainable agricultural development.

3. Functions and mechanisms of plant growth-promoting bacteria

3.1. Synthesis of plant growth regulators
PGPB directly promotes plant growth by synthesizing various plant hormones or hormone analogs.
These hormones include auxins, cytokinins, and gibberellins. Additionally, PGPB can indirectly improve
plant growth by degrading growth inhibitors (e.g., ethylene) and regulating abscisic acid levels [8,9]. This
capability plays a significant role in enhancing nutrient utilization, improving stress resistance, and
promoting root development in plants.

3.1.1. Synthesis and mechanism of auxins

Auxins are one of the most essential hormones in plant growth, playing a critical role in cell division,
elongation, and organ differentiation. PGPB synthesizes indole-3-acetic acid (IAA), the most common form
of auxins, through the metabolism of tryptophan. IAA stimulates root cell division and elongation, promotes
lateral and adventitious root development, and enhances the root system’s ability to absorb nutrients. Typical
PGPBs capable of synthesizing IAA include Bacillus spp. and Pseudomonas spp.

3.1.2. Synthesis and mechanism of cytokinins

Cytokinins are plant hormones that promote cell division, delay leaf senescence, and regulate nutrient
transport. By producing cytokinins, PGPB enhances the nutrient coordination between plant roots and shoots,
enabling higher metabolic activity in plants. Moreover, cytokinins can mitigate stress conditions in plants,
helping them adapt to adverse environments. For example, Rhizobium spp. Secrete cytokinins to promote
the development of root nodules and improve nutrient uptake in host plants.

3.1.3. Synthesis and function of gibberellins

Gibberellins are essential plant hormones that promote seed germination, stem elongation, and fruit
development. Certain PGPBs, such as Bacillus amyloliquefaciens, secrete gibberellins to help plants
overcome seed dormancy and accelerate shoot growth. In agricultural applications, gibberellins produced by
PGPB can shorten crop growth cycles, improve seed germination rates, and enhance early plant vigor,
particularly under low-temperature or low-light conditions.

3.1.4. Degradation of ethylene via ACC deaminase synthesis

Ethylene acts as a double-edged sword in plant growth, regulating physiological processes such as fruit
ripening and leaf abscission under normal conditions but inhibiting root development under stress. PGPB

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synthesizes 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase to degrade ACC, the precursor of

ethylene, thereby reducing ethylene accumulation in plants. This alleviates growth inhibition caused by
drought, salt stress, or heavy metal toxicity. This mechanism is widely recognized as a critical pathway
through which PGPB enhances plant stress tolerance, with representative strains including Pantoea spp. and
Enterobacter spp.

3.1.5. Regulation of abscisic acid

Abscisic acid (ABA) is a key hormone in plant responses to environmental stress. PGPB can regulate
ABA levels in plants to enhance stress tolerance. For instance, under water-deficient conditions, certain
PGPBs induce host plants to synthesize more ABA, leading to stomatal closure, reduced transpiration, and
improved drought resistance. Additionally, ABA interacts with other hormones to coordinate plant adaptation
to complex stress environments. This regulatory feature has significant agricultural applications in drought
and saline-alkaline conditions.
The synthesis of plant hormones and their regulatory mechanisms are among the key functions of PGPB
in promoting plant growth. By directly or indirectly modulating endogenous hormone levels, PGPB
significantly enhances the development of both roots and shoots while improving plant stress resistance,
providing strong technical support for green agricultural development.

3.2. Nitrogen fixation and nutrient element activation

Plant growth requires large amounts of essential nutrients such as nitrogen (N), phosphorus (P), and
potassium (K). However, these elements often exist in forms that are not readily absorbable by plants under
natural conditions. PGPB facilitates nitrogen fixation and dissolves or activates insoluble forms of
phosphorus and potassium in the soil, providing plants with accessible nutrients and significantly improving
soil nutrient utilization efficiency [10,11].

3.2.1. Nitrogen fixation

Nitrogen is one of the core nutrients required for plant growth, but atmospheric nitrogen (N₂) cannot be
directly utilized by plants. Certain PGPB, particularly nitrogen-fixing bacteria, convert atmospheric nitrogen
into ammonia or other plant-available forms using nitrogenase. For example, Rhizobium spp. Forms
symbiotic relationships with leguminous plants to fix nitrogen, directly supplying them with usable nitrogen.
Additionally, free-living nitrogen-fixing bacteria such as Azospirillum spp. and Beijerinckia spp. can
perform nitrogen fixation in non-symbiotic conditions, providing extra nitrogen for crops like maize and
wheat. The nitrogen-fixing ability of these bacteria varies depending on the strain and environmental
conditions, and harnessing this potential is an important pathway for achieving sustainable agricultural
productivity.

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3.2.2. Phosphorus solubilization and activation

Phosphorus is another essential nutrient for plant growth, playing a vital role in energy metabolism and
the synthesis of genetic materials. However, a significant portion of soil phosphorus exists in insoluble
phosphate forms, limiting its availability to plants. PGPB enhances phosphorus availability by secreting
organic acids (e.g., citric acid, oxalic acid, and lactic acid) and phosphatases, which dissolve inorganic and
organic phosphorus compounds in the soil. Typical phosphate-solubilizing bacteria include Pseudomonas
fluorescens, Bacillus subtilis, and certain Pseudomonas species. These bacteria also improve the rhizosphere
microenvironment, facilitating phosphorus uptake by plant roots and providing critical support for crop yield
improvement.

3.2.3. Potassium solubilization and activation

Potassium plays a crucial role in plant metabolism, including photosynthesis, enzyme activation, and
water balance regulation. However, a large proportion of soil potassium exists as silicate minerals, which
are inaccessible to plants. Certain PGPBs possess the ability to dissolve potassium-bearing minerals by
secreting organic acids or inorganic acids (e.g., oxalic acid and sulfuric acid), thereby releasing potassium
from silicates and increasing its availability to plants. Common potassium-solubilizing bacteria include
Bacillus amyloliquefaciens and Bacillus subtilis. Furthermore, the synergistic interaction between
potassium-solubilizing bacteria and other microorganisms enhances potassium cycling and utilization in the
soil, improving the efficiency of potassium uptake by crops.
Through nitrogen fixation, phosphorus solubilization, and potassium activation, PGPB significantly
enhances the availability of nitrogen, phosphorus, and potassium in the soil, providing plants with efficient
and stable nutrient sources. These mechanisms not only improve crop yield and quality but also reduce the
reliance on chemical fertilizers, offering critical support for sustainable agricultural development.

3.3. Alleviation of abiotic stress

Plants often face abiotic stresses such as drought and salinity during their natural growth, which severely
limit crop development and yield formation. PGPB mitigates abiotic stress through various mechanisms,
helping plants adapt to adverse environments and enhancing their stress tolerance and productivity [12,13].

3.3.1. Mechanisms for alleviating drought stress

Drought stress is one of the most common abiotic stresses in agricultural production, leading to water
deficiency, stomatal closure, reduced photosynthesis, and growth stagnation. PGPB alleviates drought stress
effects on plants through several mechanisms:
First, PGPB regulates plant hormone levels. For instance, they synthesize ABA to induce stomatal
closure, thereby reducing water loss through transpiration. Additionally, some PGPBs synthesize and

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modulate cytokinins and auxins to promote root elongation and lateral root development, enhancing the
plant’s ability to extract water from deeper soil layers.
Second, PGPB secrete drought-resistant metabolites such as polysaccharides and glycerol, which
improve soil water retention in the rhizosphere. They also enhance plant osmotic adjustment by regulating
the accumulation of osmolytes (e.g., proline and soluble sugars), thereby reducing cellular water loss. Typical
drought-resistant PGPB includes Bacillus amyloliquefaciens and Pantoea spp.

3.3.2. Mechanisms of alleviating salt stress

Salt stress primarily damages plant growth through ion toxicity and osmotic stress, manifesting as
nutrient uptake inhibition, reduced photosynthesis, and exacerbated oxidative stress. PGPB helps plants
resist salt stress through a variety of mechanisms:
First, PGPB regulates the balance of endogenous plant hormones. For example, they synthesize ACC
deaminase to break down the ethylene precursor, reducing excessive ethylene accumulation under stress
conditions and mitigating the toxic effects of salinity on roots.
Second, PGPB induces the accumulation of osmolytes in plants, such as proline, betaine, and
polysaccharides, lowering cellular osmotic potential and enhancing the plant’s ability to maintain water
balance under high-salinity conditions. Additionally, some PGPBs enhance the activity of antioxidant
enzymes (e.g., superoxide dismutase and peroxidase), reducing the accumulation of reactive oxygen species
caused by salt stress and alleviating oxidative damage.
Third, certain PGPBs, such as halotolerant Bacillus subtilis and Bacillus amyloliquefaciens, secrete
efflux pumps or utilize membrane transport proteins to reduce excessive sodium ion accumulation in root
cells, thereby mitigating ion toxicity.
Through mechanisms involving hormone regulation, osmotic adjustment, antioxidant protection, and ion
homeostasis, PGPB provides comprehensive support for alleviating drought and salt stress. These
mechanisms significantly enhance plant stress tolerance and offer theoretical and practical pathways for
developing environmentally friendly agricultural technologies.

3.4. Prevention or elimination of biotic stress

Biotic stresses (e.g., pathogen infections and pest infestations) are major threats to agricultural
production, leading to reduced crop yield and quality degradation. PGPB effectively prevents or eliminates
biotic stress through various direct and indirect mechanisms, ensuring healthy crop growth [14,15].

3.4.1. Production and functions of siderophores

Iron is an essential element for the growth of plants and microorganisms, but it is often present in the
soil in insoluble oxide forms, with limited bioavailability. PGPB secrete siderophores, which chelate iron

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ions from the environment to form siderophore-iron complexes. These complexes not only provide iron to
the bacteria themselves but also suppress pathogens’ ability to compete for iron, indirectly protecting plants
from diseases. For example, siderophores secreted by Pseudomonas spp. and Bacillus subtilis have been
shown to inhibit a range of plant pathogens. This mechanism enhances plant iron absorption while
weakening the infectivity of pathogens.

3.4.2. Synthesis and functions of antibiotics

PGPB can synthesize various antibiotics or antimicrobial metabolites, directly inhibiting or killing
pathogenic organisms. For instance, Pseudomonas spp. Produce antibiotics such as phenazine and
pyrrolnitrin, which disrupt pathogen cell membrane functions or metabolic processes. Similarly,
Streptomyces spp. Secrete actinomycin and other substances that inhibit DNA or protein synthesis in
pathogens. These antibiotics form a protective barrier in the rhizosphere’s microbial ecosystem, reducing the
risk of pathogen spread and invasion, and significantly improving plant disease resistance.

3.4.3. Nutritional competition and ecological niche competition

PGPB suppresses the colonization and growth of pathogens through competition for nutrients and
ecological niches. For example, PGPB preferentially utilizes limited resources such as carbon and nitrogen
in the rhizosphere, restricting pathogens’ nutrient acquisition. Additionally, PGPB rapidly colonizes root
surfaces, forming biofilms that occupy ecological niches and prevent pathogens from accessing plant surface
spaces. This competition mechanism, although indirect, is an effective antimicrobial strategy and synergizes
with siderophore activity. Common competitive PGPBs include various Pseudomonas and Bacillus species.

3.4.4. Mechanisms of induced systemic resistance

PGPB can enhance plants’ defense against biotic stress by inducing systemic resistance. Induced
systemic resistance primarily operates by activating plant signaling pathways, such as the salicylic acid
pathway or the jasmonic acid/ethylene pathway, thereby boosting plants’ disease resistance. For example,
Pseudomonas spp. Secrete surfactin or volatile metabolites that activate systemic resistance in plants,
enabling faster responses to pathogen infections. Systemic resistance is often accompanied by increased
activity of plant defense enzymes (e.g., peroxidase and chitinase) and upregulated expression of defense-
related genes. This mechanism provides broad-spectrum protection against various plant diseases and pests.
Through siderophore secretion, antibiotic synthesis, competition for nutrients and niches, and systemic
resistance induction, PGPB plays a critical role in preventing and eliminating biotic stress. These functions
provide a theoretical foundation for the development of biocontrol technologies and offer important support
for replacing chemical pesticides and promoting green agricultural development.

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3.5. Regulation of plant root architecture

Plant roots are the primary organs for water and nutrient absorption, and their architecture plays a
decisive role in plant growth and stress tolerance. PGPB regulates root architecture through various
mechanisms, optimizing root structure to improve water and nutrient use efficiency, thereby enhancing plant
adaptability and environmental tolerance [16,17].

3.5.1. Regulatory effects of plant growth regulators

PGPB synthesizes and secrete plant hormones such as auxins, cytokinins, and gibberellins, significantly
influencing root growth patterns. For instance, auxin-like substances promote primary root elongation and
lateral root meristem development, forming deeper and wider root networks to enhance water and mineral
uptake from the soil. Gibberellins regulate cell elongation, accelerating root growth. Additionally, the
secretion of ACC deaminase reduces excessive ethylene accumulation under stress conditions, alleviating
ethylene-induced inhibition of root development.

3.5.2. Adaptability of root architecture to abiotic stresses

PGPB improves plant adaptability to abiotic stresses (e.g., drought and salt stress) by regulating root
architecture. Under drought conditions, PGPB induces root growth into deeper soil layers, enhancing water
absorption. In saline environments, PGPB regulates root branching and root hair density, increasing the root
surface area to mitigate osmotic stress in the soil solution. Furthermore, metabolites such as polysaccharides
secreted by PGPB stabilize the rhizosphere microenvironment, effectively protecting roots from direct
environmental stress.

3.5.3. Effects of rhizosphere microbial symbiosis

PGPB influences root architecture through close symbiotic relationships with plant roots. Certain
bacterial strains secrete specific signaling molecules that alter root exudates, affecting root branching
patterns and root hair development. PGPBs such as Pseudomonas spp. and Bacillus subtilis play key roles
in optimizing nutrient utilization and maintaining microbial community balance in the rhizosphere. This
synergistic effect not only promotes more developed root systems but also significantly enhances plants’
competitive abilities and overall health.
By regulating plant hormone levels, optimizing root developmental patterns, and strengthening
rhizosphere symbiosis, PGPB effectively modulates plant root architecture. These functions improve soil
resource utilization, enhance stress resistance, and increase productivity, providing valuable applications for
crop breeding and agricultural management.

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3.6. Regulation of rhizosphere microbial community structure

The rhizosphere is the critical interface between plants and soil, hosting diverse microbial communities.
The composition, abundance, and functions of rhizosphere microbes directly affect plant growth, health, and
environmental adaptability. PGPB regulates rhizosphere microbial community structure through various
mechanisms, optimizing plant-microbe interactions, improving resource use efficiency, and enhancing stress
tolerance [18,19].

3.6.1. Suppressing harmful microbes through resource competition

PGPB suppresses the growth of pathogenic microbes by competing for limited rhizosphere resources
such as carbon, nitrogen, and iron. For example, PGPB secrete siderophores to efficiently chelate iron ions
from the soil, making it difficult for pathogens to obtain sufficient iron and thereby weakening their growth.
Additionally, PGPB rapidly colonizes rhizosphere surfaces to form biofilms, occupying ecological niches
and preventing pathogen invasion and colonization. This competitive effect directly shapes the composition
of rhizosphere microbial communities, reducing pathogen abundance and enhancing rhizosphere health.

3.6.2. Regulating microbial communities through secreted substances

PGPB influences the structure and function of rhizosphere microbial communities by secreting various
metabolites. For instance, antibiotics such as pyrrolnitrin and amides secreted by PGPB can directly kill or
inhibit certain pathogens, while volatile organic compounds (e.g., ketones and alcohols) modulate the growth
of multiple microbes. These secreted substances not only restrict pathogenic microbes but also promote the
proliferation of beneficial microbes such as nitrogen-fixing bacteria and phosphate-solubilizing bacteria,
thereby optimizing community functionality.

3.6.3. Activating plant-microbe interaction networks

PGPB enhances plant secretion of rhizosphere substances, further regulating microbial community
structure. Certain PGPB secrete signaling molecules (e.g., quorum-sensing molecules) that induce plant
roots to release more carbon sources, amino acids, or organic acids. These substances provide nutrients for
beneficial microbes and alter community composition to strengthen the competitiveness of beneficial species.
Moreover, this plant-microbe interaction can form positive feedback mechanisms that stabilize the health
and diversity of rhizosphere microbial communities.

3.6.4. Promoting the formation of functional microbial communities

PGPB not only suppresses pathogens but also promotes the proliferation of functional microbes. For
instance, phosphate-solubilizing bacteria and potassium-releasing bacteria become more active under PGPB
stimulation, enhancing soil nutrient availability. Similarly, the increased abundance of methanotrophic

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bacteria helps reduce greenhouse gas emissions from soil. The formation of these functional microbial
communities improves the ecological services of rhizosphere microbes and creates a better overall growth
environment for plants.
By competing for resources, secreting regulatory substances, activating plant-microbe interaction
networks, and promoting the formation of functional microbial communities, PGPB plays a multifaceted
role in regulating rhizosphere microbial community structure. This regulatory capability enhances plant-
microbe synergism, providing sustainable solutions to improve plant productivity and soil health.

4. Selection and application of PGPB

4.1. Preparation of PGPB inoculants
The selection and preparation of PGPB are key steps for their application in agricultural practices. The
efficient preparation of PGPB inoculants requires a combination of scientific strain selection strategies,
appropriate carrier materials, and stable preservation technologies to ensure functionality, adaptability, and
economic feasibility [20,21].

4.1.1. Strain selection strategies

Strain selection is the primary step in the preparation of efficient PGPB inoculants, with a critical focus
on identifying strains with excellent growth-promoting functions. Target strains are primarily sourced from
the rhizosphere soil of healthy crops, plant endophytes, and microbial samples from unique environments
such as saline-alkaline soils or arid regions, where microorganisms often exhibit high growth-promoting
potential and environmental adaptability. During the selection process, functional tests are conducted to
evaluate strains for nitrogen fixation, phosphate solubilization, abiotic stress resistance, and growth-
regulating hormone synthesis, thereby identifying candidate strains with significant growth-promoting
effects.
In addition, to broaden the application range of inoculants, it is necessary to assess the stress resistance
of strains under specific adverse conditions (e.g., high salinity, high temperatures, low pH). Molecular
biology techniques, including genetic sequencing and functional gene identification, are used to validate the
taxonomy, functional expression, and safety of the selected strains. This rigorous selection process provides
scientific assurance for the efficiency and stability of PGPB inoculants.

4.1.2. Carrier selection and optimization

Carriers are essential materials for converting PGPB strains into inoculants, and their quality directly
affects the preservation, transportation, and application of the inoculants. Common carriers include peat,
bentonite, plant fibers, starch-based materials, and synthetic polymers, which must possess good adsorption
capacity, biocompatibility, and environmental friendliness. To maintain strain viability, carriers are often

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supplemented with nutrients (e.g., carbon and nitrogen sources) and protectants (e.g., glycerol, algal
polysaccharides). For specific strains, additional components such as antioxidants or UV protectants are
incorporated to enhance survival. Moreover, techniques such as micronization or granulation improve the
uniformity and operability of the inoculants, reduce waste during application, and enhance practical
effectiveness.

4.1.3. Techniques for extending inoculant shelf life

Extending the shelf life of inoculants is crucial for their large-scale application. Low-temperature drying
technologies, such as freeze-drying or spray-drying, remove moisture from the culture medium while
preserving bacterial viability. Encapsulation technologies, such as chitosan or liposome coating, effectively
reduce the impact of environmental factors like oxidation or humidity fluctuations on bacterial cells.
Controlled-release techniques use carrier materials to enable the gradual release of bacteria into the soil,
preventing acute damage from environmental stressors.
Optimizing storage conditions, such as controlling temperature, humidity, and light exposure, as well as
adopting vacuum packaging or nitrogen-filled storage, can further reduce the risk of cell inactivation and
significantly prolong the shelf life of inoculants.
The precise selection of strains, scientific choice of carriers, and advancements in preservation
technologies are the key aspects of preparing efficient PGPB inoculants. Continuous optimization of these
techniques not only enhances the application efficiency of PGPB in agriculture but also promotes the
extensive development of microbial technologies in sustainable agriculture.

4.2. Methods of PGPB inoculation

The method of inoculating PGPB is a key factor in determining its application effectiveness. Scientific
and rational inoculation techniques can enhance the efficiency of contact between strains and crops,
optimizing their growth-promoting and stress-resistance functions. Based on application requirements and
practical agricultural conditions, the main methods of PGPB inoculation include seed coating, direct soil (or
substrate) application, foliar spraying, and soil drenching [22,23].

4.2.1. Seed coating

Seed coating involves uniformly applying PGPB inoculants to the seed surface, allowing the strains to
directly contact crop roots, promote rhizosphere colonization, and establish mycorrhizal symbiosis. This
improves nutrient uptake efficiency and stress tolerance in plants. Coating materials are usually mixtures
containing adhesives, such as sugars, starches, or polymers, to ensure uniform distribution and stable
adhesion of the inoculants. This method is simple, reduces inoculant usage, minimizes environmental
pollution, and protects the strains through the coating material, extending their viability. Seed coating is

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widely used for crops with larger seeds, such as cereals, legumes, and oilseed crops.

4.2.2. Direct soil application

Direct soil application involves applying PGPB inoculants in granular or liquid form to the soil or
growing substrates in planting areas, enabling rapid colonization in the rhizosphere and interaction with
plant roots. Application methods include broadcasting, mixing with soil through tillage, and spot application.
These techniques ensure even distribution of the inoculants in the soil, enhancing rhizosphere microbial
activity and nutrient utilization efficiency. This method is suitable for a variety of plants, including field
crops, greenhouse crops, and fruit trees. However, it requires a relatively large amount of inoculant and its
effectiveness is influenced by soil pH, temperature, and moisture conditions, necessitating optimized
strategies.

4.2.3. Foliar spraying

Foliar spraying involves dissolving PGPB inoculants in a solution and directly applying it to the plant
leaf surface. PGPB functions through adhesion to leaf stomata or epidermis, directly promoting
photosynthesis, resistance to diseases and stresses, and metabolic regulation. This method uses small
amounts of inoculant and produces quick results, making it particularly suitable for alleviating nutrient
deficiencies or physiological stress in plants. Spraying should be conducted under suitable weather
conditions (e.g., early morning or evening) to avoid high temperatures or rainfall, which could affect strain
viability. Foliar spraying is commonly used for high-value crops, such as fruits, vegetables, leafy greens, and
flowers.

4.2.4. Soil drenching

Soil drenching involves dissolving PGPB inoculants in irrigation water and uniformly applying them to
the rhizosphere of crops. By using irrigation systems, the strains are efficiently delivered to the vicinity of
plant roots, enhancing colonization and interaction efficiency. This method is often combined with modern
agricultural technologies, such as drip irrigation or subsurface irrigation. It has the advantages of high
precision and suitability for large-scale applications, performing especially well in arid or saline regions.
Additionally, drenching can be combined with fertilizers or pesticides to improve field operation efficiency.
However, its application requires ensuring irrigation water quality and compatibility with equipment to avoid
damage to strain viability. This method is ideal for fruit trees, horticultural crops, and economic crops in
protected agriculture.
The choice of PGPB inoculation method should consider crop type, production conditions, and desired
outcomes. Seed coating is suitable for precise root contact, direct soil application is ideal for large-scale
crops, foliar spraying delivers quick results for economic crops, and soil drenching is an efficient and

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resource-saving approach in modern agriculture. These methods complement each other, providing diverse
technological support for the application of PGPB in agriculture.

4.3. Practical applications of PGPB

The practical applications of PGPB encompass microbial inoculants, microbial pesticides, and microbial
fertilizers, playing a crucial role in sustainable agricultural development due to their unique ecological
functions and high efficiency. Different application forms provide solutions to specific problems, resulting
in extensive effects on yield improvement, stress resistance, and environmental friendliness [24,25].

4.3.1. Application effects and case studies of microbial inoculants

Microbial inoculants are the primary form of PGPB application. By improving the rhizosphere
environment and enhancing nutrient utilization efficiency, they significantly boost crop yield and quality.
These inoculants promote nutrient uptake, enhance photosynthesis, and increase resistance to diseases and
stresses. For instance, inoculants containing nitrogen-fixing and phosphate-solubilizing bacteria have
improved nitrogen self-supply and phosphorus utilization rates in legume cultivation. In wheat, inoculants
with gibberellin-synthesizing bacteria increased spike numbers and grain weight; in vegetables, cytokinin-
containing inoculants enhanced leaf thickness and disease resistance; and in drought conditions, maize
treated with inoculants showed yield increases of over 20%. However, the effectiveness of these applications
is influenced by soil type, climate conditions, and application methods, requiring optimized strategies
tailored to regional characteristics and crop demands to achieve the best results.

4.3.2. Practical applications of microbial pesticides

PGPB serves as a microbial pesticide with significant advantages in controlling plant diseases and
reducing chemical pesticide use. It suppresses pathogens and pests through mechanisms such as antibiotic
secretion, competition for nutrient sites, and induction of systemic resistance in plants. For example, the
rhizosphere Bacillus subtilis secretes antibiotics that effectively control wilt and root rot diseases. Studies
have shown that PGPB-based biopesticides can reduce the incidence of rice blasts by 50% without
compromising environmental safety. In vineyards, microbial pesticides with siderophore-producing strains
effectively inhibited downy mildew and improved fruit quality. However, compared to chemical pesticides,
microbial pesticides act more slowly and have limited application scopes. Nevertheless, with the
advancement of genetic engineering, their efficacy and potential applications are expected to improve further.

4.3.3. Applications and efficacy of microbial fertilizers

Microbial fertilizers, with PGPB as their core component, have greatly enhanced agricultural
productivity when combined with traditional fertilizers. By fixing nitrogen, solubilizing phosphorus, and

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potassium, and secreting growth hormones, they improve plant nutrient acquisition, reduce chemical
fertilizer use, and mitigate agricultural non-point source pollution. For example, in peanut production,
applying microbial fertilizers containing phosphate-solubilizing bacteria reduced phosphate fertilizer usage
by 20% and increased yield by 15%. In rice cultivation, combining PGPB fertilizers with conventional
nitrogen fertilizers improved nitrogen use efficiency and reduced nitrogen loss. Additionally, integrating
microbial fertilizers with organic fertilizers enhances soil fertility and structure, while combining them with
slow-release fertilizers enables prolonged nutrient release, providing vital support for sustainable agriculture.
The practical applications of PGPB now span multiple aspects of agricultural production. Their use as
microbial inoculants, microbial pesticides, and microbial fertilizers demonstrates significant economic,
ecological, and social benefits. In the future, with technological advancements and greater adoption, the
potential of PGPB in agriculture will be more fully realized, driving the rapid development of green
agriculture.

5. Conclusions and prospects

As a critical component of agricultural biotechnology, PGPB promotes plant growth through various
mechanisms, enhances nutrient use efficiency, strengthens resistance to biotic and abiotic stresses, and
improves soil health. This paper systematically summarizes the roles and mechanisms of PGPB in
synthesizing plant growth regulators, nitrogen fixation, nutrient activation, alleviating abiotic and biotic
stresses, and regulating root architecture and rhizosphere microbial communities. It also outlines the key
technologies and challenges in PGPB strain selection, formulation preparation, and application. Studies
show that the effective utilization of PGPB not only significantly increases crop yield and quality but also
reduces the use of chemical fertilizers and pesticides, making it essential for achieving sustainable
agricultural development.
Future research should focus on the application of multi-omics technologies to analyze PGPB functional
genes and their metabolic networks, thereby uncovering their mechanisms of action in greater depth. This
will provide a theoretical basis for discovering strains with specific functions and optimizing their biological
performance. Developing function prediction models based on omics data could guide strain screening and
functional modification, enhancing their application efficiency across different agricultural scenarios.
Despite the promising potential of PGPB inoculants in agriculture, their commercialization faces several
technical bottlenecks, such as insufficient strain stability, short shelf life, and high production costs. Future
efforts should focus on optimizing inoculant formulations and production processes. This can be achieved
by developing novel carrier materials, improving precision fermentation techniques, and introducing
advanced storage technologies to enhance the activity and environmental adaptability of PGPB. Moreover,
establishing standardized quality control systems will be a critical step to ensure the large-scale production
and promotion of PGPB inoculants.

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Under the backdrop of global climate change, agriculture faces extreme environmental challenges such
as drought and salinization, where PGPB demonstrates significant advantages in mitigating abiotic stresses.
Future studies should emphasize the adaptability and mechanisms of PGPB under varying climatic
conditions and screen functional strains suitable for specific ecological environments. Additionally,
integrating PGPB technologies with smart agriculture could further enhance their application efficiency,
providing scientific and practical support for building modern agricultural systems capable of adapting to
climate change.

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