Pseudomonas fluorescens as a plant growth-promoting rhizobacterium: mechanisms, applications, and prospects for sustainable agriculture

Introduction

Meeting the food demands of a global population projected to reach 9.38 billion by 2050 requires a massive scale-up in agricultural production. However, this necessity is severely constrained by widespread soil degradation, which currently affects roughly 38% of the world's 1.5 billion hectares of cropland. To counteract this, conventional farming has historically relied on intensive chemical fertilizers and synthetic pesticides. While these agrochemicals initially boosted productivity, their prolonged and excessive use has resulted in severe biodiversity loss, the elimination of beneficial soil biota, and health risks from toxic residues on crops. Consequently, there is an urgent global mandate to transition toward eco-friendly, sustainable agricultural alternatives. Fungicides illustrate this dilemma well. As chemical pesticides used to prevent or suppress fungal diseases, they account for more than 35% of the global pesticide market, and their use is projected to keep increasing (Duarte Hospital et al., 2023; Godfray & Garnett, 2014). Despite their role in protecting crop yield and quality, continuous and indiscriminate fungicide use has raised concerns about adverse effects on soil health and microbial diversity. Several studies have shown that fungicides can negatively influence beneficial soil microorganisms and inhibit key soil enzymes involved in nutrient cycling, including dehydrogenase, phosphatases, urease, and β-glucosidase (Muñoz-Leoz et al., 2011), thereby impairing soil fertility and long-term agricultural sustainability. Plant Growth-Promoting Microbes (PGPM), which include Plant Growth-Promoting Rhizobacteria (PGPR) such as Bacillus and Pseudomonas (Ex: Multiplex Bio Jodi) and Plant Growth-Promoting Fungi (PGPF) such as Arbuscular Mycorrhizal Fungi (AMF) (Ex: Multiplex Trishul) offer a promising biological alternative. These organisms aggressively colonise the plant rhizosphere, establishing robust densities of 10 5 -10 7 CFU per gram of root. PGPM enhance soil structure through exopolysaccharide production, facilitate nutrient acquisition through biological nitrogen fixation and mineral solubilization, suppress phytopathogens via direct antagonism and induced systemic resistance, and alleviate climate-driven abiotic stresses such as drought, salinity, and flooding through mechanisms including Osmoprotectant accumulation and ACC deaminase activity. PGPR promote plant health through several additional mechanisms, including competition with pathogens for nutrients and ecological niches, production of antimicrobial metabolites, secretion of pathogen-cell-wall-degrading enzymes, induction of systemic resistance, and degradation of environmental pollutants (Oleńska et al., 2020). Among these beneficial microorganisms, Pseudomonas fluorescens (Ex: Multiplex Sparsha) is one of the most extensively studied rhizobacteria because of its plant growth-promoting and biocontrol capabilities. It efficiently colonizes the rhizosphere and enhances plant growth through direct mechanisms, such as the production of phytohormones such as indole-3-acetic acid (IAA), phosphate solubilization, and siderophore-mediated iron acquisition and indirect mechanisms, including synthesis of hydrolytic enzymes and antimicrobial compounds and induction of systemic resistance against plant pathogens (David et al., 2018). Several strains also degrade a wide range of pesticides and other xenobiotic compounds, making them valuable for both sustainable crop production and environmental re-

Taxonomy and general characteristics

The genus Pseudomonas, part of the Proteobacteria, includes over 300 Gram-negative, rod-shaped species primarily found in plants, soil, and aquatic environments. [1] , Pseudomonas species are widely distributed across various environments. Advancements in phylogenomic technologies have led to significant revisions in its taxonomy, increasing the recognized species from about 250 in 2021 to over 300 by 2024. Of these, 43 were identified using whole-genome sequencing. This diversity highlights the genus' adaptability and beneficial effects on plants.

The genus Pseudomonas comprises one of the most diverse and taxonomically complex groups of Gram-negative bacteria, rod-shaped, motile, with more than 220-300 validly described species and new species continually being identified through advances in molecular taxonomy Table 1

2,4-Diacetylphloroglucinol (DAPG)

DAPG is a polyketide antibiotic primarily produced by P. protegens and P. corrugata, showing broad-spectrum activity against bacteria, fungi, oomycetes, and nematodes. It helps suppress diseases like takeall of wheat and black root rot of tobacco. Typically isolated from soils suppressive to take-all, DAPG disrupts cell membranes, induces ROS, and inhibits mitochondrial ATP synthesis. The conserved DAPG gene cluster includes phlD, a type III polyketide synthase responsible for phloroglucinol biosynthesis. Additionally, DAPG induces systemic resistance in Arabidopsis against various pathogens and stimulates root branching in tomato via auxin signaling.

Pyoluteorin

Pyoluteorin is a hybrid polyketide-nonribosomal-peptide antibiotic, comprising a resorcinol ring attached to a dichlorinated pyrrole moiety, produced mainly by P. protegens and some P. [2] , and its 17-gene biosynthetic cluster (structural genes pltABCDEFGLM; efflux genes pltIJ-KNOP; regulatory genes pltZ and pltR) is present only in strains that also carry phlD (

Pyrrolnitrin

Pyrrolnitrin is a chlorinated phenylpyrrole antibiotic produced by Pseudomonas spp. with broad-spectrum antifungal activity against important phytopathogens, including Rhizoctonia solani, Fusarium spp., Botrytis cinerea, Pythium aphanidermatum, and Sclerotium rolfsii. It suppresses fungal growth by disrupting the electron transport chain and osmotic signalling, effectively controlling damping-off diseases in crops such as cotton and cucumber. Pyrrolnitrin also served as the lead compound for the development of the commercial fungi-

Hydrogen cyanide (HCN)

Hydrogen cyanide (HCN) is a volatile secondary metabolite produced by several Pseudomonas spp. that inhibits cytochrome c oxidase, thereby suppressing the growth of fungal pathogens, nematodes, and certain insect pests. HCN contributes to the biological control of tobacco black root rot caused by Thielaviopsis basicola and has also been implicated in the suppression of root-knot nematodes, aphids, and termites (Blumer and Haas, 2000; Voisard et al., 1989 [3] ;

Siderophores

Siderophores are iron-chelating compounds produced by Pseudomonas spp. that contribute to biological control by competing with pathogens for iron, directly inhibiting pathogen growth, and inducing systemic resistance (ISR) in plants. The major siderophore produced is pyoverdine, while several strains also synthesize pyochelin, achromobactin, pseudomonine, corrugatin, and (thio)quinolobactin, enhancing their competitiveness and plant-beneficial effects (Meyer, 2000; Cornelis, 2010; Schalk et al., 2020).

Strain-specific bioactive molecules and toxins

Strain-specific metabolites: Certain Pseudomonas strains produce unique bioactive metabolites, including rhizoxins, sessilin, promysalin, and L-furanomycin, which exhibit antifungal, antibacterial, and antioomycete activities against a range of plant pathogens

Regulation of secondary metabolite production

Plant growth-promoting mechanisms

Direct mechanisms

Pseudomonas fluorescens promote plant growth through several direct mechanisms that improve nutrient availability, stimulate root development, and enhance productivity:

ACC deaminase activity, which lowers stress-induced ethylene levels and promotes root development; and

Siderophores These are high-affinity iron-chelating compounds produced by Pseudomonas spp. that enhance plant iron upake under iron-limited conditions while restricting iron availability to pathogens, thereby suppressing disease development (Neilands, 1995;Haas and Défago, 2005). A landmark study by Kloepper et al. (1980aKloepper et al. ( , 1980b) [1] demonstrated that Pseudomonas strain B10 controlled take-all and Fusarium wilt through the production of the siderophore pseudobactin. By sequestering ferric iron (Fe 3+

), pseudobactin deprived pathogens such as Fusarium oxysporum f. sp. lini and Gaeumannomyces graminis var. tritici of the iron required for growth, converting disease-conducive soils into disease-suppressive soils. This study established iron competition as a major mechanism of biological control by fluorescent Pseudomonas and laid the foundation for the use of PGPR in sustainable disease management (Kloepper et al., 1980a(Kloepper et al., , 1980b) [1] .

Collectively, these traits make P. fluorescens an efficient PGPR with the potential to improve nutrient-use efficiency, reduce fertilizer inputs, and support sustainable agricultural production (Mehmood et al., 2023).

Indirect mechanisms

Beyond direct growth promotion, P. fluorescens protects plants against biotic and abiotic stress through several indirect mechanisms, primarily mediated by: Production of antibiotics such as DAPG, phenazines, pyoluteorin, pyrrolnitrin, and HCN;

Secretion of lytic enzymes chitinase, β-1,3-glucanase, protease, cellulase, lipase, and amylase that degrade fungal cell walls;

Production of volatile organic compounds (VOCs) and biosurfactants (e.g., rhamnolipids);

Rhizosphere competence

The success of Pseudomonas spp. as PGPR depends on rapid colonisation of roots and internal plant tissues, efficient utilisation of root exudates, strong competitiveness within the rhizosphere, production of antibiotics, siderophores, and volatile compounds, and high adaptability to environmental stress (Mehmood et al., 2023).

ISR as a broad-spectrum defence mechanism

Pseudomonas species are well-known plant growth-promoting rhizobacteria (PGPR) that colonize the rhizosphere and stimulate plant growth in various ways, including improved seed germination, enhanced seedling vigour, increased biomass, and higher crop productivity across multiple cropping systems (Udaya Shankar et al., 2009).

Pseudomonas fluorescens can induce systemic resistance (ISR), which activates the plant's natural defences against a wide range of pathogens, including fungi, bacteria, viruses, and nematodes. Unlike direct antimicrobial activity, ISR prepares the plant for a quicker and more robust defense response when faced with pathogen attacks.

Although it shares similar characteristics with systemic acquired resistance (SAR), ISR is triggered by beneficial rhizobacteria instead of pathogen infections (Udaya Shankar et al., 2009).

Role in viral disease management

Multiple, complementary mechanisms of disease suppression

Taken together, the literature indicates that Pseudomonas suppresses plant pathogens through several complementary mechanisms operating simultaneously: siderophore-mediated competition for iron; synthesis of antibiotics that inhibit pathogenic microorganisms; secretion of small antimicrobial metabolites; and production of hydrolytic enzymes that interfere with pathogen development. These combined mechanisms contribute to improved plant health and enhanced disease resistance, positioning Pseudomonas spp.

Conclusion

Pseudomonas fluorescens is among the most extensively characterized plant growth-promoting rhizobacteria, combining direct growthpromoting mechanisms-phytohormone production, nutrient solubilization, and siderophore-mediated iron acquisition-with a broad indirect biocontrol repertoire built on antibiotics, lytic enzymes, competitive rhizosphere colonization, and induction of systemic resistance. Its documented efficacy spans a wide range of crops and pathogens, and several strains additionally confer tolerance to salinity, drought, and other abiotic stresses, positioning P. fluorescens as a strong candidate for reducing dependence on chemical fertilizers and fungicides. Realizing this potential at scale will require closing the gaps identified above-particularly around field validation, formulation stability, and quality standardization-alongside continued molecular and omics-based research to refine strain selection and deployment strategies for next-generation sustainable agriculture.

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