Plant growth promoting rhizobacteria stimulate plant growth through both direct and indirect mechanisms (Fig. 1). They are pivotal for enhancing nutrient uptake, promoting root architecture, increasing stress tolerance and acting as biocontrol agents against plant pathogens. The use of PGPR in agriculture is a sustainable approach to increase crop yield while reducing the need for chemical inputs such as fertilizers and pesticides. Consequently, in the direct mechanism, rhizobacteria stimulate crop production by enhancing the soil nutrients for e.g., nitrogen fixation, solubilization of minerals such as phosphorous, potassium, and iron, and production of enzymes, siderophores, and different phytohormones. Indirect mechanisms involve plant growth promotion by producing antagonistic substances such as diacetylphloroglucinol, hydrogen cyanide (HCN), phenazine, prevention of the deleterious effects of phytopathogens, heavy metal detoxification and biofilm formation. Understanding the mechanisms and types of PGPR actions is essential for their utilization in agriculture to promote sustainable and eco-friendly practices. They play crucial roles in enhancing plant health, nutrient uptake and overall crop productivity.
In addition to the aforementioned mechanisms, plant growth promoting rhizobacteria produce various lytic enzymes such as glucanase and chitinase which degrade the cell wall of fungal pathogens and induce systemic resistance throughout the plant system. Moreover, PGPR can influence plant growth both directly and indirectly with these effects often being strain-specific or varying among species. Studies have suggested that PGPR mediated plant growth promotion takes place through the modification of the whole microbial group found in the rhizospheric niche by the production of different substances. Usually, PGPR enhance plant growth through a direct mechanism by facilitating various resource acquisitions such as phosphorous, nitrogen, and vital minerals via biological nitrogen fixation; iron sequestration through siderophore production; phosphate mineral solubilization; and modulation of phytohormones as auxins, cytokinins (CKs), gibberellins (GAs) and nitric oxide (NO). They also contribute significantly through indirect mechanisms in the rhizosphere such as induced systemic resistance (ISR) and the biosynthesis of stress alleviating phytohormones like ethylene regulation via the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase and the production of jasmonic acid (JA) which collectively enhance plant resilience under stress conditions. The mechanisms by which PGPR promote plant growth are multifaceted and involve direct benefits such as nutrient mobilization and phytohormone production as well as indirect benefits through soil improvement and stress mitigation. Understanding these mechanisms can aid in the development of sustainable agricultural practices that harness the potential of PGPR to enhance crop productivity.
Direct effects of PGPR
Plant growth promoting rhizobacteria directly affect plant growth through several mechanisms. These interactions enhance nutrient uptake, promote root development, and enhance overall plant health. In addition, the direct mechanism of PGPR provides mechanical support and facilitates water and mineral uptake. This direct mechanism includes microbial activities like nutrient solubilization, production of phytohormones, root development, induction of systemic resistance and production of bioactive compounds and phytostimulants. Understanding these mechanisms is essential for leveraging PGPR in sustainable agriculture and improving crop yields. PGPR also enhances the crop yield by the production of plant growth regulators. At a very low concentration these regulators enhance the plant development by influencing various physiological and morphological processes. Numerous microbes can produce several plant growth regulators such as auxins, gibberellins, abscisic acid (ABA), cytokinins, and ethylene (ET). Various rhizobacteria have been reported to produce auxins (for example, Agrobacterium, Pseudomonas, Azospirillum, and Erwinia). Nutrient mobilization is one of the primary mechanisms through which PGPR enhance plant growth. These beneficial microorganisms play a crucial role in increasing the availability of essential nutrients in the rhizosphere, thereby promoting healthy plant growth.
Plant growth promoting rhizobacteria are major components of biofertilizers because of their ability to improve plant health and productivity through various biological mechanisms. The use of PGPR as biofertilizers is becoming an increasingly sustainable alternative to chemical fertilizers which maintain soil fertility, reduce environmental pollution, and promote sustainable agriculture. The application of PGPR as biofertilizers is not only environmentally friendly but also economically beneficial to farmers contributing to both improved crop yields and reduced costs. Someone reported that PGPR improves the uptake of mineral nutrients in crop plants which is required for optimum growth and development. PGPR can function as biofertilizers through several direct and indirect mechanisms. These include enhancing nutrient availability (such as nitrogen, phosphorus, and other micronutrients), producing plant hormones, and protecting plants from stress and diseases. Furthermore, the use of PGPR based biofertilizers reduces reliance on synthetic fertilizers leading to lower input costs for farmers and minimizing environmental pollution caused by fertilizer runoff. PGPR assist plants in tolerating abiotic stresses such as drought, salinity, and heavy metal toxicity, thereby enhancing plant health and productivity under suboptimal growth conditions. PGPR contribute to the reduction of greenhouse gas emissions (such as nitrous oxide) from synthetic nitrogen fertilizers, and reduce soil degradation and promote long-term agricultural sustainability. PGPR based biofertilizers can be categorized into several types based on the microbial species and their specific functions. Nitrogen biofertilizers contain nitrogen-fixing bacteria such as Rhizobium, Azospirillum, Azotobacter which convert nitrogen into forms available to plants. Phosphate-solubilizing biofertilizers enhance the availability of phosphorus by solubilizing insoluble phosphate compounds in the soil with notable examples including Pseudomonas fluorescens and Bacillus subtilis. Potassium-solubilizing biofertilizers harbor bacteria that solubilize potassium, improving potassium availability in the soil such as Bacillus mucilaginosus, Frateuria aurantia. Multifunctional biofertilizers combine diverse plant growth promoting rhizobacteria, capable of performing multiple beneficial functions such as nitrogen fixation, phosphate solubilization, and phytohormone production. For example, biofertilizers formulated with a synergistic blend of Azospirillum, Pseudomonas, and Bacillus sp. have been found to enhance nutrient availability, promote root development and support overall plant growth more effectively.
Nitrogen fixation
Nitrogen (N) is an essential macronutrient for plant growth and plays a crucial role in the synthesis of proteins, nucleic acids, and chlorophyll. Despite its abundance in the atmosphere (approximately 78%), nitrogen exists predominantly as inert nitrogen gas (N₂) which plants cannot directly utilize. To overcome this, certain bacteria known as diazotrophs possess the ability to fix atmospheric nitrogen into ammonia (NH₃) a form that plants can assimilate. This process known as biological nitrogen fixation (BNF) is a key mechanism in sustainable agriculture that reduces the need for synthetic nitrogen fertilizers. Certain PGPR convert atmospheric nitrogen into a usable form that plants can utilize. This enhances nitrogen availability for plants. These bacteria form mutualistic relationships with plants particularly legumes, where they colonize plant roots and form specialized structures called nodules. Inside these nodules, the bacteria fix nitrogen, supplying the host plant with ammonia in exchange for carbon compounds and a protective environment. Additionally, nitrogen fixing bacteria known as “Diazotrophs” are classified as symbiotic (e.g., Frankia and Rhizobium), free living endophytes or root endophyte microbes (e.g., Azotobacter and Azospirillum). Nitrogen-fixing PGPR can significantly enhance soil nitrogen levels, particularly in agricultural settings where synthetic fertilizers may be limited or undesirable. This can improve plant growth and yield considerably.
Phosphate solubilization
Phosphorus (P) is an essential macronutrient for plant growth and plays a critical role in energy transfer, photosynthesis and nutrient transport. However, a significant portion of phosphorus in soils exists in insoluble forms making it unavailable to plants. Plant growth promoting rhizobacteria contribute to the solubilization of insoluble phosphates by producing organic acids, thereby enhancing phosphorus availability in the rhizosphere and promoting plant growth. This improves the nutrient status of the soil and enhance plant nutrient uptake. Certain PGPR such as Bacillus, Pseudomonas, Rhizobium sp. and Mycobacterium are particularly effective in this process and can solubilize insoluble phosphate through the secretion of organic acids such as citric acid, gluconic acid, oxalic acid, acetic acid, and lactic acid. These organic acids can chelate metal cations and lower the pH of the soil or rhizosphere converting insoluble phosphate compounds like tricalcium phosphate, dicalcium phosphate, and hydroxyapatite into soluble forms (e.g., H₂PO₄⁻), which can be absorbed by plant roots. Some PGPR can release protons (H⁺ ions) into the rhizosphere leading to soil acidification. This decrease in pH enhances the solubility of phosphate minerals converting insoluble forms into soluble forms. PGPR improves phosphorus availability in the soil, facilitating better root uptake and overall plant growth. Phosphatase enzymes produced by PGPR play a crucial role in hydrolyzing the organic phosphates present in the soil. PGPR may also participate in ion exchange processes. The combined action of these mechanisms contributes to the solubilization of insoluble phosphates in the soil ultimately availability of P to plants. This process is beneficial for plant growth, especially in phosphorus-deficient soils and can be harnessed in agriculture to improve the nutrient uptake efficiency and crop productivity. Phosphate-solubilizing PGPR play a vital role in sustainable agriculture by reducing the need for chemical fertilizers. By enhancing phosphorus bioavailability in soils with low phosphorus levels or high phosphate fixation, these bacteria contribute to improved plant growth and yield. Phosphate-solubilizing PGPR also improve soil health by promoting beneficial microbial communities, enhancing nutrient cycling, and contributing to overall plant resilience against abiotic stresses.
Plants only absorb P in its soluble form (monobasic (H2PO4) and dibasic (HPO4 ) ions). However, the use of phosphate fertilizers is both costly and environmentally unsustainable. In this context, phosphate-solubilizing microorganisms present in the rhizosphere commonly referred to as phosphate-solubilizing rhizobacteria (PSRB) or phosphate-solubilizing bacteria (PSB) play a vital role in converting insoluble phosphates into plant available forms, thereby meeting the phosphorus requirements of plants in an eco-friendly and efficient manner(Fig. 2).
Potassium mobilization
Potassium (K) is a key macronutrient critical for maintaining water balance, enzyme activation, and the regulation of various physiological processes in plants. Although potassium is abundant in many soils, it is often found in insoluble mineral forms such as mica, feldspar, and illite which are not directly available for plant uptake. Plant growth promoting rhizobacteria are well known for their ability to enhance plant nutrient uptake by mobilizing essential nutrients including potassium, which is often present in insoluble forms in the soil. Potassium is crucial for enzyme activation, osmoregulation, and photosynthesis. PGPR solubilizes potassium from insoluble mineral sources primarily through the production of organic acids and enzymes. These acids such as gluconic acid, citric acid, and oxalic acid, lower the pH of the rhizosphere which enhances the dissolution of potassium-containing minerals and releases K⁺ ions into the soil solution making them available for plant uptake. These organic acids chelate cations such as calcium, magnesium, and iron from potassium bearing minerals, weakening the mineral structure and releasing potassium ions. Bacillus mucilaginosus is a well-known potassium-solubilizing bacterium that releases organic acids to mobilize potassium from insoluble minerals. The ability of PGPR to solubilize these nutrients plays a significant role in sustainable agriculture by reducing the need for chemical fertilizers and enhancing plant growth. Certain PGPR also secrete enzymes that degrade potassium containing minerals. These enzymes act synergistically with organic acids to increase the dissolution of potassium from its mineral form. Several genera of PGPR are known for their ability to solubilize potassium. Bacillus species, particularly B. mucilaginosus and Bacillus edaphicus have been extensively studied for their potassium-solubilizing abilities. These bacteria are capable of producing a wide range of organic acids that release potassium from insoluble mineral forms, thereby improving potassium availability to plants. Pseudomonas strains have also been found to release potassium from soil minerals through acid production. They enhance plant root surface area and nutrient uptake by promoting root growth which further improves potassium uptake from the rhizosphere. Potassium plays a vital role in several physiological processes including osmoregulation and water balance (potassium helps to regulate stomatal opening and closing, thus controlling water loss through transpiration and maintaining turgor pressure in plant cells), photosynthesis, enzyme activation (potassium is an activator of enzymes involved in photosynthesis and is essential for ATP synthesis, protein synthesis, and carbohydrate metabolism), and stress resistance (adequate potassium nutrition improves plant tolerance to various biotic and abiotic stresses, including drought, salinity, and pathogen attacks). Inoculation with K-solubilizing bacteria has been found to increase biomass, root vigor, and K content in leaves and stems of tobacco, wheat and tomatoes.
In an experiment with wheat, the application of B. mucilaginosus significantly enhanced K uptake leading to increased plant height, root length, and biomass production under K deficient conditions. By using PGPR to mobilize potassium from insoluble soil reserves, farmers can reduce their dependency on chemical fertilizers leading to more eco-friendly and cost-effective agricultural practices. Additionally, these PGPR enhance soil health by promoting microbial diversity and improving soil structure. In a field study with wheat and maize, the use of potassium-solubilizing bacteria (Bacillus sp. and Pseudomonas sp.) reduced the need for chemical potassium fertilizers and resulted in a substantial increase in crop yield and potassium use efficiency.
Zinc solubilization
Zinc (Zn) is an essential micronutrient that plays a critical role in the synthesis of proteins, nucleic acids, and in the regulation of plant hormone activities. Zn deficiency in soils can lead to chlorosis, reduced photosynthetic activity, impaired plant growth, reduced yields, and lower nutritional quality. However, zinc is often present in insoluble forms such as zinc oxide (ZnO), zinc carbonate (ZnCO₃), and zinc sulfide (ZnS) which are not readily available to plants. Zinc-solubilizing PGPR can convert insoluble zinc compounds into bioavailable forms through secretion of organic acids, siderophores, and other chelating agents. These organic acids such as gluconic acid and citric acid assist in dissolving zinc from mineral complexes by lowering the soil pH and forming soluble zinc-organic acid complexes. P. fluorescens releases organic acids and siderophores which solubilize zinc from zinc oxide making it available for plant uptake. Zinc-solubilizing PGPR not only improve zinc uptake in plants but also enhance overall plant health by promoting root elongation, increasing chlorophyll synthesis, and enhancing resistance to environmental stresses. Zinc is also a cofactor for numerous enzymes and is involved in the synthesis of auxins that regulate plant growth. Application of zinc-solubilizing bacteria such as Bacillus aryabhattai and Pseudomonas aeruginosa has been shown to enhance zinc uptake in crops such as wheat and maize leading to better growth and higher grain yield. Siderophores are high affinity iron chelating compounds secreted by many PGPR. While their primary function is to sequester iron from the environment, they can also enhance zinc availability by forming soluble zinc-siderophore complexes. These zinc-siderophore complexes are more easily absorbed by plants than insoluble zinc minerals. P. putida secretes siderophores that chelate zinc and increase its bioavailability, thereby enhancing zinc uptake by cucumber plants. PGPR can also solubilize zinc by producing chelating agents and other metabolites that bind to it and enhance its solubility. These compounds destabilize the zinc complexes in the soil allowing the release of zinc ions into the soil solution for plant uptake. Bacillus aryabhattai strains have been shown to solubilize zinc via the secretion of various chelating agents and organic acids. Furthermore, zinc is required for the activity of numerous enzymes involved in nitrogen metabolism, protein synthesis, and carbonic anhydrase function all of which are crucial for optimal plant growth and development. Solubilizing zinc from insoluble forms ensures a consistent supply of this essential nutrient supporting robust plant growth. In maize and wheat, application of zinc-solubilizing bacteria like P. aeruginosa resulted in improved enzyme activity and enhanced protein content in grains. The application of zinc-solubilizing bacteria such as B. aryabhattai increases auxin levels in wheat plants, promoting better root development and higher zinc uptake. PGPR that solubilize zinc maintain the optimal zinc levels, enhances chlorophyll production, and improves photosynthetic efficiency. For instance, in cucumber plants, P. fluorescens improves chlorophyll content and overall plant health by enhancing zinc solubilization and uptake.
Several genera of PGPR are capable of solubilizing zinc including Pseudomonas sp., which are effective zinc-solubilizers. Their use has been widely studied in various crops such as wheat, maize, and cucumber. Bacillus strains are known for their ability to solubilize zinc via the production of chelating agents and organic acids. They are highly effective in zinc-deficient soils and have shown significant results in promoting zinc uptake and plant growth in crops such as wheat and maize. The use of zinc-solubilizing PGPR in agriculture offers a sustainable approach to improve zinc nutrition in crops, reducing the need for chemical zinc fertilizers. This biological method not only enhances plant growth but also contributes to soil health by promoting microbial diversity and improving nutrient cycling. By utilizing PGPR for zinc solubilization, farmers can reduce the application of zinc-based fertilizers which are often costly and contribute to environmental pollution. The natural solubilization of Zn by PGPR ensures a continuous supply of nutrients to plants without the need for synthetic inputs making this approach more environmentally sustainable. In an experiment with wheat, the application of potassium and zinc-solubilizing bacteria improved nutrient uptake and resulted in a significant increase in yield compared with untreated controls.
Stimulation and production of phytohormones
Plant growth promoting rhizobacteria enhance plant development not only through nutrient mobilization but also by producing and modulating phytohormones which are key regulators of plant growth and stress responses. The interaction between PGPR and phytohormones such as auxins, gibberellins, cytokinins, and ethylene significantly impacts root architecture, shoot development and the overall ability of plants to withstand environmental stressors. These hormones play crucial role in plant development including cell division, elongation and, differentiation. At very low concentrations, these phytohormones affect the plants both physiologically and morphologically. Phytohormones can alter plant growth patterns and results in better branched roots with a wider surface area. Because of this, plants receive a greater area and are able to access extra minerals and nutrients from the soil. In addition, phytohormones can play a pivotal role in the symbiotic development of legume-rhizobia through Nod factor signalling.
Auxin
Auxin is one of the most essential hormone that influences plant growth and development through various processes such as tropic responses and organogenesis. Auxins also participate in cellular responses such as cell expansion, cell division, and cell differentiation along with gene regulation. Auxins, particularly Indole-3-Acetic Acid (IAA) are among the most widely studied phytohormones produced by PGPR. PGPR produce a variety of auxins (such as, IAA, IBA (indole-3-butyric acid), IPA (indole-3-pyruvic acid), tryptophol (TOL) and ILA (indole lactic acid). Of these, IAA is the most important auxin produced in presence of L-tryptophan as a precursor by Azospirillum, Pantoea, Pseudomonas, Bacillus, Rhizobium, Alcaligenes, and Enterobacter. IAA production by PGPR typically occurs through a tryptophan-dependent pathway. The presence of tryptophan in root exudates stimulates PGPR to produce IAA which then influences the plant-root system leading to enhanced lateral root formation and root hair development. In leguminous crops, the nodules have a higher concentration of IAA than the roots. The accumulation of auxin in the nodules of legumes can be attributed to rhizobia. In plant-microbe interactions, the role of IAA varies from phytostimulation to pathogenesis and it also takes part in the degradation of aromatic amino acids. Species such as Azospirillum, Pseudomonas, Erwinia and Agrobacterium have been found to produce IAA and enhance growth parameters and development in crops (such as seedling, root length, root hairs, branching, and root surface area. Earlier, studies had demonstrated that Rhizobium strains associated with legumes produce IAA and show their potential for lateral growth development along with enhanced nodulation and delayed senescence in nodules.
Gibberellin
Gibberellins (GAs) are a group of plant hormones that regulate various developmental processes including seed germination, stem elongation, flowering, and fruit development. The production of gibberellins by rhizobacteria enhances hormonal balance in plants often resulting in better growth, increased stress tolerance, and improved nutrient uptake. PGPR-induced gibberellin production has been documented in various bacterial genera including Acinetobacter, Azospirillum, Bacillus, Pseudomonas, and Rhizobium. PGPR synthesize gibberellins through biochemical pathways similar to those in plants. For example, the mevalonate and methylerythritol phosphate pathways are involved in isoprenoid biosynthesis leading to gibberellin formation. These microbial gibberellins are excreted into the rhizosphere where they influence the hormonal signaling. The interaction between PGPR and plant roots enables the bacteria to modulate the GA levels in the plants. The synthesis of gibberellins by PGPR enhances stem elongation and leaf expansion allowing plants to achieve greater photosynthetic efficiency. This increased efficiency results in enhanced biomass accumulation and improved overall plant growth. Several species of Azospirillum have been found to produce gibberellins, contributing to improved root and shoot growth in plants such as wheat and maize. Azospirillum lipoferum and A. brasilense are well-documented producers of gibberellins which play a role in the enhanced development of lateral roots and overall biomass accumulation. Bacillus pumilus and Bacillus licheniformis have been documented to produce high amount of gibberellins which promote seed germination and shoot elongation in plants. The genus Bacillus is another important group of gibberellin-producing PGPR. Studies have shown that Bacillus megaterium and B. pumilus can increase the length and biomass of various plants through the secretion of gibberellins. Several other strains of Pseudomonas have demonstrated the ability to synthesize gibberellins.
Abscisic acid (ABA)
Abscisic acid (ABA) is a plant hormone that plays a critical role in regulating abiotic stress responses, seed dormancy, stomatal closure, and other physiological processes in plants. Although traditionally considered an endogenous plant hormone, recent research indicates that certain PGPR can produce ABA contributing to plant stress tolerance and growth promotion. While ABA is commonly associated with stress responses including drought and salinity, some PGPR are capable of producing or inducing ABA synthesis in plants. ABA synthesis in plants typically involves the carotenoid pathway and similar pathways are present in some PGPR. These bacteria can either produce ABA or modulate ABA production within plants through various signaling mechanisms. Studies have demonstrated that ABA produced by PGPR contributes to the regulation of key plant processes such as stomatal behavior, root growth, and stress tolerance under drought and salinity stress. ABA produced by PGPR operates by interacting with endogenous hormone signaling pathways in plants. When plants are exposed to stress conditions, microbial ABA can enhance or synergize with the own ABA level of plants leading to quicker and more effective physiological responses such as stomatal closure and osmotic balance regulation. The increase in ABA production help plants to conserve water by closing their stomata, thereby reducing water loss during drought conditions. PGPR such as Bacillus sp. have been reported to modulate ABA levels in plants that help to improve drought tolerance in wheat by inducing higher ABA levels and reducing stomatal conductance.
Several strains of Azospirillum produce ABA which help plants to cope with water stress. For instance, A. brasilense has been shown to increase ABA levels in plants such as maize which subsequently enhances stomatal closure, reduces transpiration, and improves water-use efficiency. Certain species of Bacillus have been found to produce ABA or enhance the ABA content in plants under stressful conditions. Bacillus amyloliquefaciens has been reported to increase ABA content in wheat plants under drought conditions which is associated with improved stress tolerance and growth performance. P. fluorescens is another rhizobacterium capable of ABA production. This bacterium has been linked to improved drought tolerance in various crops through its ability to modulate ABA signaling in plants. Under drought stress, plants colonized by Pseudomonas showed increased ABA levels contributing to improved water retention and growth stability. Symbiotic bacteria such as Rhizobium are not only involved in nitrogen fixation but can also influence ABA production in leguminous plants. These bacteria enhance plant ABA levels particularly under stress conditions such as drought and salinity, thereby contributing to better plant-water relations and stress adaptation.
The production of ABA by PGPR is particularly valuable in agriculture where crops frequently encounter environmental stresses such as drought and salinity. PGPR that produce or modulate ABA provide an eco-friendly approach to enhance plant resilience without the need for chemical interventions. This hormonal interaction help plants to maintain osmotic balance, reduce water loss, and sustain growth even under suboptimal conditions. Moreover, the use of ABA-producing PGPR aligns well with sustainable agricultural practices by reducing the dependency on synthetic agrochemicals and enhancing plant-soil-microbe interactions that promote natural stress tolerance mechanisms.
Cytokinin
Cytokinins are phytohormones that help plants in root and shoot cell division. It promotes cell growth and differentiation in crop plants. They also affect apical dominance in plants; therefore, they are used by farmers to increase the overall production of crops. Cytokinins produced by PGPR promote shoot proliferation, enhance leaf formation, and influence nutrient mobilization by stimulating the translocation of nutrients from the roots to shoots. In many PGPR, cytokonins are apparently expressed; therefore, their application and potential to growing crops helps in the modification of crop plant phytohormone composition. Several PGPR strains have been found to be capable of producing cytokinins including Azotobacter sp., Rhizobium sp., Pantoea agglomerans, Rhodospirillum rubrum, B.subtilis and, P.fluorescens. Cytokinins are derivatives of purines and are classified based on their potential to enhance cell division (i.e., cytokinesis), cell enlargement and tissue development in plants. PGPR produced cytokonins affect the auxin to cytokinin ratio and hence regulates the crop root architecture. Cytokinins promote plant growth in many crops such as rapeseed and soybean. These may play a pivotal role in rhizobial infection and nodule differentiation in legumes.
Modulation of ethylene via ACC deaminase activity
Ethylene is a pivotal phytohormone that aids plants in their physiological responses at low concentrations. Ethylene is a stress-related phytohormone that is involved in the regulation of plant responses to biotic and abiotic stresses. Under stressful conditions, plants often produce excessive amounts of ethylene which can inhibit root and shoot growth. Many PGPR contain the enzyme ACC deaminase which breaks down ACC, the precursor of ethylene into α-ketobutyrate and ammonia, thereby lowering ethylene levels in plants (Fig. 3). PGPR mitigates stress responses in plants leading to enhanced root growth and improved tolerance to stresses such as drought, salinity, and pathogen attacks. Morgan and Drew found that increased and accumulated ethylene concentrations in plants cause stress responses; therefore, to eliminate this, plants increase their tolerance. It also intensifies senescence and stress response symptoms. When ethylene level is elevated, it has been seen that it gradually suppresses the growth of plant roots and shoots with the formation of infection thread. Several PGPR are known to secrete ACC deaminase which reduces elevated ethylene production in crop plants. Different studies have shown that inoculating PGPR strains that produce ACC deaminase directly mitigates environmental stress. The significant areas of cultivated soil are under biotic and abiotic environmental stresses such as phytopathogenic infection and high salt availability. ACC decreases the level of “stress ethylene,” which inhibits plant growth and development. Repeated inoculation of highly effective rhizobia and co-inoculation with other efficient PGPR have improved the growth and yield of plants.
A variety of PGPR contain the vital enzyme ACC deaminase which regulates ethylene production by metabolizing ACC (an intermediate precursor of ethylene biosynthesis in higher plants) into α-ketobutyrate and ammonia. ACC deaminase-producing bacteria also alleviate the need for plants to actively defend themselves against different environmental stressors. High temperatures can induce ethylene production which leads to stress responses. ACC deaminase helps to maintain optimal ethylene levels and prevent the detrimental effects of heat stress on plant growth and development. ACC deaminase-producing PGPR help to regulate ethylene levels preventing excessive activation of defense responses and minimizing damage during pathogen attacks, and can also induce ethylene production in plants. The use of ACC deaminase-producing PGPR in agriculture can enhance the resilience of plants to a wide range of stresses, contributing to sustainable and improved crop production in challenging environments.
Siderophore production
Siderophores are low molecular weight iron bound compounds that chelate ferric iron (Fe3+) in the environment. Iron (Fe) is the fourth essential micronutrient element in the earth’s crust. However, it is not available for crop plants as it is present in the insoluble form (Fe3+). To overcome with this problem, PGPR strains secrete iron scavengers (siderophores). These siderophores chelate iron and hence are further transported to the cells of microorganisms. Siderophores producing PGPR enhance plant growth production very efficiently and also control the several plant diseases relieving the pathogen of iron nutrition. These siderophore characteristics were found to be more effective and hence useful for the proper growth and development of crop plants. In addition, siderophores form stable complexes with several other metals that are of environmental concern (such as Cd, Al, Cu, Ga, Pb, Zn, and In).
Indirect effects of PGPR
The indirect mechanism of PGPR includes inhibitory effects against various phytopathogens by secreting different antagonistic compounds that have a defensive nature (i.e., hydrogen cyanide, bacteriocins, and hydrolytic enzymes). Several bacterial genera (such as Bacillus, Burkholderia, Staphylococcus, Serratia, Pseudomonas, Enterobacter, Herbaspirillum etc. are known to have potential against phytopathogens. Van loon et al., revealed that PGPR can inhibit the growth of other microorganisms by efficiently mineralizing nutrients, thereby gaining a competitive advantage in the rhizosphere. In addition, PGPR induced systemic resistance (ISR) decreases the activity of pathogens. Many studies have revealed the applications of several PGPR that help to control pathogenic diseases through biological control. The use of biocontrol agents for plant growth promotion by rhizobacteria is a major indirect mechanism. PGPR include different modes of activities such as niche exclusion and induced systemic resistance and play a vital role in the production of several antifungal metabolites. Many studies have found that numerous PGPRs have the potential to produce antifungal metabolic compounds (such as phenazines, HCN, pyrrolnitrin, 2,4-diacetylphloroglucinol, pyoluteorin, tensin, and viscosinamide.
Induced systemic resistance (ISR)
PGPR can trigger a plant immune response through ISR. By stimulating the production of defense-related compounds such as phenolics, flavonoids, and pathogenesis-related (PR) proteins, PGPR make plants more resistant to a wide range of pathogens. This reduction in disease pressure leads to healthier plants that can devote more energy to growth and production. Certain PGPR including Pseudomonas and Bacillus species can trigger systemic resistance in plants. ISR is a defense mechanism that primes plants to respond more effectively to pathogen attacks. PGPR stimulate the production of signaling molecules such as jasmonic acid and ethylene which activate defense-related genes in plants. ISR enhances the ability of plants to resist infections from a wide range of pathogens, reduces disease outbreaks and improves growth and productivity.
Antibiotic production and pathogen suppression
Many PGPR produce natural antibiotics, enzymes, and secondary metabolites that directly inhibit the growth of pathogenic microorganisms, such as fungi, bacteria, and nematodes. PGPR indirectly enhance plant health and productivity by suppressing diseases in the rhizosphere thereby reducing the need for chemical pesticides. Some PGPR produce antibiotics or antimicrobial substances that inhibit the growth of plant pathogens. For example, P. fluorescens and B. subtilis produce antibiotics such as phenazines, pyoluteorin, and bacillomycin. These substances suppress soil-borne pathogens such as Fusarium, Pythium, and Rhizoctonia which cause root diseases. By controlling pathogenic microbes in the rhizosphere, PGPR reduce the incidence of diseases leading to healthier plants and higher crop yields.
PGPR can outcompete harmful pathogens for essential nutrients and colonize spaces in the rhizosphere. This mechanism known as “competitive exclusion,” limits the resources available for pathogen growth, thereby suppressing the growth of harmful microbes. Bacillus and Pseudomonas species effectively colonize the root surface and form a protective barrier. PGPR indirectly reduce disease pressure and enhance plant health by preventing colonization by pathogenic organisms. Some PGPR produce enzymes such as chitinases, cellulases, and glucanases which break down the cell walls of pathogenic fungi and bacteria. Bacillus and Serratia species are known to produce these lytic enzymes which degrade the structural components of pathogens and render them inactive. By lysing pathogenic cells, PGPR provides biocontrol against a range of fungal and bacterial diseases. Some PGPR also produce volatile organic compounds (VOCs) that can directly inhibit pathogen growth or trigger plant defense mechanisms. VOCs such as HCN, acetoin, and 2,3-butanediol are known to suppress soil-borne pathogens and enhance plant growth by modulating plant hormonal pathways contributing to both pathogen suppression and enhanced plant vigor, improving overall plant health and productivity.
Future perspectives
The significance of PGPR has been widely seen all over the world for their capability in improvement and sustainability of crops and soil fertility. These rhizobacterial strains exert numerous beneficial effects via both direct and indirect mechanisms. Therefore, PGPR have several potential applications for the enhancement and sustainability of crops including phytohormone production, phosphate solubilization, iron sequestration by siderophores (iron chelators), and production of various vitamins and antibiotics. In addition, production of cell wall degrading enzymes, systemic resistance and mitigation of stress-induced ethylene by ACC-deaminase activity are some other activities. Different types of cropping such as mixed cropping, intercropping, and rotation of leguminous crops with non-leguminous crops have been engaged in biological nitrogen fixation. Besides these PGPRs are known for their multiple activities under normal as well as in stress conditions, their proficient role in sustainable agriculture have been noticed on a large scale. Moreover, an in-depth study is required to envisage colonization potential, establishment, and plant response under different field conditions.
The future of PGPR holds promise for addressing the challenges in agriculture and promoting more sustainable and resilient food production systems. Continued research, innovation, and collaboration will be essential in unlocking the full potential of PGPR for the benefit of global agriculture. Future perspectives will involve educating farmers, agronomists, and stakeholders about the benefits of PGPR and how to integrate them into existing agricultural practices. Meanwhile, increased awareness can drive the adoption and contribute to sustainable agricultural development.
Microbial products are defined as naturally active products or microbial inoculators, including bacteria, algae, fungi, or biological compounds that can benefit soils and plants. These products are different from chemical fertilizers and are environmentally friendly. They are used on plant disease control, soil conditioner, and biological pest control.
Dora Agri is the leading biocontrol agents company in China, we concentrate on using bio solutions to prevent plant diseases and soil problems.