The Pseudomonas fluorescens Complex: Molecular Mechanisms of Plant-Beneficial Traits and Biotechnological Applications in Crop Production

Abstract

The Pseudomonas fluorescens complex represents one of the most ecologically versatile and functionally diverse groups of plant-associated rhizobacteria, offering substantial promise for sustainable crop production. Members of this bacterial group exhibit a broad array of plant-beneficial effects, including growth promotion, biocontrol against phytopathogens and insect herbivores, and mitigation of abiotic stresses such as drought, salinity, and heavy metal toxicity. Despite their potential, the application of P. fluorescens complex strains in commercial inoculants remains limited, and their field performance is variable across different agricultural environments. To advance the development of effective microbial products, a clear understanding of the mechanisms underlying their plant growth promotion activities and stress mitigation is essential. In this review, we synthesize current knowledge on the taxonomy, ecological roles, and molecular mechanisms of beneficial activity within the P. fluorescens complex, and highlight key traits contributing to plant growth promotion, biocontrol, and abiotic stress tolerance, with discussion of challenges and future directions for their application in sustainable agriculture.

1. Introduction

Sustainable agricultural production faces escalating challenges from the confluence of climate change, land degradation, population growth, and rising global food demand [1, 2]. Conventional agricultural practices dependent on chemical fertilizers and pesticides have led to environmental harm, biodiversity loss, and diminished soil health, prompting the need for more environmentally sustainable alternatives [3, 4]. This situation has intensified the search for biologically based solutions, particularly those involving beneficial soil microorganisms that can inherently promote plant growth and enhance resilience to environmental stresses [5].

Among soil microorganisms, plant growth-promoting rhizobacteria (PGPR) have emerged as essential components of sustainable crop management strategies. PGPR colonize the rhizosphere and root surfaces, where they enhance nutrient uptake, modulate plant hormonal balance, induce systemic resistance, and suppress soil-borne pathogens [6]. Their multifaceted functional capabilities make them attractive candidates for integration into modern agricultural systems [7]. Beyond increasing crop productivity, PGPR-based approaches offer the potential to reduce chemical inputs, enhance soil microbial diversity, and strengthen long-term agroecosystem stability, aligning with global objectives for climate-smart and low-input agriculture [8].

Within the diverse array of beneficial rhizobacteria, the genus Pseudomonas has garnered substantial attention due to its remarkable ecological adaptability, robust rhizosphere colonization capacity, and extensive repertoire of plant-beneficial traits [9]. Among pseudomonads, the Pseudomonas fluorescens complex represents a particularly important group of fluorescent pseudomonads characterized by exceptional genomic and functional diversity [10]. This complex encompasses numerous species and genomovars that share common phenotypic characteristics but exhibit considerable variation in their metabolic capabilities and ecological functions [11]. Members of the P. fluorescens complex are ubiquitous in diverse soil environments and have been extensively documented for their plant growth-promoting activities, biocontrol efficacy against plant pathogens, and capacity to enhance plant tolerance to environmental stresses [12, 13, 14].

Recent advances in microbiome research, comparative genomics, and high-throughput sequencing technologies have considerably expanded our understanding of the mechanisms by which P. fluorescens complex members interact with plants and influence their physiology [15, 16, 17]. These technologies have revealed the genetic basis of key traits that contribute to the beneficial effects of the P. fluorescens complex on plant development and health. Furthermore, comparative genomic analyses have illuminated the intraspecific variation within the complex, demonstrating that beneficial phenotypes are shaped by both core genetic capabilities and accessory genes acquired through horizontal gene transfer [18, 19]. Understanding this genomic diversity is essential for identifying robust isolates capable of performing consistently under varied environmental conditions, a critical requirement for their effective deployment in agricultural settings [20].

Despite the wealth of knowledge accumulated on individual traits and mechanisms, comprehensive syntheses that integrate the diverse beneficial functions of the P. fluorescens complex are limited. Given the growing interest in harnessing these bacteria for sustainable agriculture, there is a pressing need to consolidate current understanding of their plant-beneficial attributes and stress tolerance mechanisms. Therefore, this review aims to provide a comprehensive synthesis of the beneficial traits of the P. fluorescens complex, with particular emphasis on: (i) mechanisms of plant growth promotion; (ii) strategies for enhancing plant tolerance to biotic stresses, including plant pathogens and herbivorous insects; and (iii) contributions to abiotic stress tolerance, particularly drought, salinity, and heavy metal stress. By integrating recent molecular, genomic, and ecological insights, this review seeks to advance our understanding of how P. fluorescens complex members can be effectively utilized to develop resilient and sustainable agricultural systems.

2. Taxonomy and ecological niche of the Pseudomonas fluorescens complex

The P. fluorescens complex is one of the most taxonomically diverse and environmentally adaptable groups in the genus Pseudomonas. It was historically categorized by phenotypic characteristics such as fluorescent pigment synthesis and metabolic adaptability. However, advancements in molecular techniques, including whole-genome sequencing and phylogenomics, have markedly improved its taxonomy [10, 21]. The P. fluorescens complex is currently believed to comprise more than 50 identified species and numerous genomovars that share core genomic characteristics, yet exhibit considerable variation in accessory genes, secondary metabolite biosynthetic clusters, and ecological activities [10, 22]. Members of this complex are ubiquitous in the environment, particularly in soil and the rhizosphere of plants, and include beneficial and a few plant-pathogenic species [23].

Species within the P. fluorescens complex are distributed across multiple phylogenomic subgroups, such as the P. fluorescens subgroup, P. gessardii subgroup, P. fragi subgroup, P. chlororaphis subgroup, P. koreensis subgroup, P. jessenii subgroup, P. corrugata subgroup, P. mandelii subgroup, P. protegens subgroup, P. asplenii subgroup, and P. kielensis subgroup, each with distinct genome content, ecological preferences, and biosynthetic capabilities [10, 16, 24]. Comparative genomic studies show that the P. fluorescens complex has a vast, open pangenome, indicating substantial horizontal gene transfer rates and niche specialization [25]. These genetic characteristics explain the group’s adaptability and wide functional repertoire, which includes nutrition uptake, antimicrobial compound synthesis, and the ability to withstand a variety of environmental stress [26]. Ecologically, the P. fluorescens complex is closely linked to the rhizosphere, which is the dynamic zone of soil impacted by plant root exudates. Chemotaxis toward root exudates, biofilm formation, flagellar motility, and competitive resource acquisition strategies are among its members’ characteristics that make them efficient root colonizers [27, 28]. Additionally, their capacity to metabolize a variety of organic acids, amino acids, and aromatic compounds enables them to thrive in diverse soil conditions [11, 29]. Furthermore, they possess a highly competitive exclusion capacity, frequently outperforming soil-borne diseases through siderophore-mediated iron sequestration, niche saturation, and the production of inhibitory metabolites [23, 30].

Members of the P. fluorescens complex live in a wide range of ecological niches beyond the rhizosphere, including bulk soils, aquatic systems, plant endospheres, and even extreme environments such as arctic soils and high-altitude ecosystem [15, 31, 32]. Some isolates have demonstrated resistance to oxidative stress, osmotic stress, desiccation, and temperature fluctuations, all of which are crucial for persistence in agricultural soils [12, 33, 34, 35]. The taxonomic diversity and ecological breadth of the P. fluorescens complex together demonstrate its functional significance in plant-microbe interactions. These bacteria serve critical roles in nutrient cycling, pathogen control, and enhancing plant stress tolerance by occupying crucial niches within the rhizosphere and possessing a wide range of metabolic and stress-adaptive characteristics. Understanding the taxonomy and ecological dynamics of the P. fluorescens complex lays the groundwork for identifying elite strains with consistent performance in biocontrol, plant growth promotion, and climate-resilient agriculture.

3. Mechanisms of plant growth promotion (PGP)

Members of the P. fluorescens complex exhibit various plant growth-promoting traits that enhance plant performance in the rhizosphere (Fig. 1). The driving factors behind these traits are the complex’s metabolic versatility and genomic diversity [36]. These traits include nutrient acquisition enhancement, phytohormone modulation, volatile-mediated growth stimulation, and stress alleviation, positioning them as one of the most important functional groups within plant-associated microbiomes.

Figure 1: Direct and indirect plant growth-promoting mechanisms of the Pseudomonas fluorescens complex

3.1 Direct mechanisms: nutrient acquisition enhancement

Beneficial P. fluorescens complex isolates promote plant growth through several direct mechanisms, including biological nitrogen fixation, phosphate solubilization via organic acid secretion, potassium mobilization, and siderophore-mediated iron acquisition, collectively enhancing nutrient availability in the rhizosphere [37, 38, 39, 40]. Although members of the P. fluorescens complex are classically considered non-diazotrophic, several isolates harbor nitrogen fixation islands encoding nitrogenase structural and accessory genes that support biological nitrogen fixation in association with plant roots [41, 42]. Under low-nitrogen conditions, these diazotrophic strains can reduce atmospheric nitrogen to ammonium, thereby partially meeting plant nitrogen demand and decreasing reliance on synthetic N fertilizers [43].

Phosphate solubilization is another critical mechanism mediated by many P. fluorescens complex isolates. Phosphate-solubilizing isolates of the P. fluorescens complex release organic acids such as gluconic, 2-ketogluconic, oxalic, and lactic acids, which acidify the rhizosphere and facilitate the conversion of insoluble mineral phosphates, such as tricalcium phosphate and rock phosphate, into orthophosphate that plants can readily absorb [44, 45, 46, 47]. The quantitative importance of this process has been demonstrated with phosphate-solubilizing P. trivialis and P. poae isolates that concurrently secrete multiple organic acids, leading to enhanced maize biomass and nutrient accumulation relative to conventional single superphosphate treatments [45]. Some isolates can also solubilize phosphate through the enzymatic cleavage of inorganic phosphate using phosphatase [48, 49].

Furthermore, the P. fluorescens complex produces low-molecular-weight iron-chelating compounds called siderophores. One example is pyoverdine, a fluorescent pigment typically produced by this bacterial group [50]. These compounds mediate iron acquisition in iron-limited soils, supporting plant iron nutrition while restricting phytopathogen access to essential iron [51].

3.2 Phytohormone modulation

Phytohormone modulation represents a central mechanism through which the P. fluorescens complex enhances plant growth and development, with strains synthesizing auxins (particularly indole-3-acetic acid, IAA), cytokinins, gibberellins, and the enzyme 1-aminocyclopropane-1-carboxylate (ACC) deaminase that collectively regulate hormonal homeostasis in planta [52]. P. fluorescens complex isolates typically produce IAA, with levels varying according to genotype and tryptophan availability, and this phytohormone is pivotal for stimulating root system architecture and improving plant access to soil resources [29, 53]. Moreover, cytokinins influence cell division, delay leaf senescence, and promote shoot growth. Cytokinin-producing strains, such as P. fluorescens G20-18, have been shown to enhance biomass accumulation and also have exhibited biocontrol effect through hormone-mediated priming of plant defenses [54, 55, 56]. Gibberellin-producing P. fluorescens complex isolates further promote stem and root elongation, seed germination, and overall vigor, thereby reinforcing the growth gains derived from improved nutrient acquisition and auxin-mediated root system development [57].

In addition, ACC deaminase cleaves the ethylene precursor ACC into α-ketobutyrate and ammonia, thereby mitigating stress-induced ethylene accumulation that would otherwise inhibit root elongation and accelerate senescence [58]. This enzymatic regulation of ethylene is particularly critical under abiotic stress conditions, where elevated ethylene levels become inhibitory to plant growth and where P. fluorescens-mediated ethylene balancing contributes to improved stress tolerance and biomass maintenance [59]. Collectively, auxins, cytokinins, gibberellins, and ACC deaminase activity act in concert to reprogram plant hormone balance, integrate nutritional and stress cues, and translate the presence of P. fluorescens complex in the rhizosphere into sustained growth promotion and enhanced stress resilience of plants (Table 1).

Table 1: Examples of Pseudomonas fluorescens complex strains that enhance plant growth and stress tolerance through phytohormone modulating effects

Subgroup	Species	Production of phytohormones	Target plant	Effect on target plant	Reference
P. fluorescens subgroup	P. fluorescens	Auxin	Thale cress (Arabidopsis thaliana)	Enhancement of lateral root growth	Ortiz‑Castro et al. [60]
	P. fluorescens SLU99	Auxin, Cytokinin	Tomato (Solanum lycopersicum), Potato (Solanum tuberosum)	Enhancement of root growth, shoot height	Hanifah et al. [61]
	P. fluorescens Z1B4	Auxin	Pea (Pisum sativum), Corn (Zea mays)	Enhancement of root growth and biomass	Vyas and Kaur [62]
	P. fluorescens	1-aminocyclopropane-1-carboxylate deaminase (ACC) deaminase	Barley (Hordeum vulgare)	Salinity tolerance	Azadikhah et al. [63]
	P. fluorescens REN1	1-aminocyclopropane-1-carboxylate deaminase (ACC) deaminase	Rice (Oryza sativa)	Reduce ethylene production during stress	Etesami et al. [64]
	P. fluorescens YsS6	1-aminocyclopropane-1-carboxylate deaminase (ACC) deaminase	Green bean (Phaseolus vulgaris)	Promoting rhizobial nodulation	Nascimento et al. [65]
P. corrugate subgroup	P. corrugate DR3	1-aminocyclopropane-1-carboxylate deaminase (ACC) deaminase	Grapevine (Vitis vinifera)	Drought tolerance	Duan et al. [66]
P. kielensis subgroup	P. purpurea DGS26, P. helvetica DGS28	Auxin	Bread wheat (Triticum aestivum)	Enhancement of root growth and biomass	Poli et al. [15]
P. protegens subgroup	P. protegens DA1.2	Abscisic acid (ABA), indole-3-acetic acid (IAA)	Bread wheat (Triticum aestivum)	Drought tolerance	Bakaeva et al. [67]

3.3 Enhancement of root system architecture and soil-plant interactions

Building on the phytohormone-driven modulation of root development, the P. fluorescens complex further augments plant growth by enhancing root system architecture and soil-plant interactions through multiple complementary mechanisms. The enhancement of root system architecture and soil-plant interactions is a critical growth promotion mechanism mediated by the P. fluorescens complex’s volatile organic compounds (VOCs), biofilm formation, and exopolysaccharide (EPS) production [68, 69]. Bacterial VOCs such as 2,3-butanediol and acetoin trigger changes in plant root morphology and hormone signaling pathways independent of direct bacterial contact [23], while exopolysaccharides improve soil aggregation, water retention, and create protective microenvironments around roots that enhance nutrient diffusion and bacterial persistence in the rhizosphere [60]. Additionally, the ability of P. fluorescens complex members to form biofilms on root surfaces and produce extracellular polymeric substances facilitates stable colonization and enables more sustained delivery of growth-promoting metabolites to plant tissues [70].

4. Strategies for enhancing plant tolerance to biotic stresses

4.1 Biocontrol mechanisms against phytopathogens

The P. fluorescens complex employs diverse biocontrol mechanisms that collectively establish highly effective pathogen suppression in both soil and foliar environments. These mechanisms include antibiotic production, siderophore-mediated iron competition, synthesis of lytic enzymes (chitinases, proteases, glucanases), and production of hydrogen cyanide and antimicrobial VOCs that suppress soilborne and foliar phytopathogens [23, 71, 72, 73, 74, 75].

Among the antibiotics produced, 2,4-diacetylphloroglucinol (DAPG) is one of the most extensively studied compounds. DAPG, produced by certain species of the P. fluorescens complex (e.g., P. protegens and P. kilonensis), inhibits a wide range of fungal and oomycete pathogens, including Gaeumannomyces graminis, Rhizoctonia solani, Fusarium spp., and Pythium ultimum [76, 77]. Some species of the P. fluorescens complex, especially P. protegens, produce multiple types of antimicrobial compounds, such as pyoluteorin and pyrrolnitrin. These compounds have distinct antimicrobial spectra and operate synergistically with DAPG, enhancing overall biocontrol efficacy [76, 78, 79, 80]. Additionally, the production of lytic enzymes, particularly chitinases and proteases, directly degrades the cell walls and cellular proteins of pathogenic fungi, providing a complementary mechanism to antibiotic action [72, 81, 82].

Siderophore-mediated iron competition represents another crucial biocontrol strategy, whereby the high-affinity iron chelation by bacterial siderophores deprives pathogens of this essential nutrient, thereby limiting their growth and virulence [83].

4.2 Protection against insect herbivores

Beyond pathogen suppression, the P. fluorescens complex also provides protection against insect herbivores through multiple mechanisms. These bacteria induce the accumulation of plant defensive metabolites like glucosinolates, which deter insect feeding and increase herbivore mortality [84]. Some P. fluorescens strains colonize the rhizosphere or even the insect gut, contributing to insect death through toxins such as Fit (P. fluorescens insecticidal toxin) and rhizoxin [26, 85]. Furthermore, comparative genome analysis has revealed that the Fit gene cluster associated with insect pathogenicity is conserved in both P. chlororaphis and P. protegens subgroups. Pronounced lethality observed against the tobacco hornworm Manduca sexta following inoculation with P. protegens strain Pf-5 [86]. These biocontrol activities, together with enhanced plant vigor and immune priming, make P. fluorescens complex a sustainable tool for integrated crop protection [9, 87].

4.3 Induced systemic resistance (ISR)

Induced systemic resistance (ISR) triggered by P. fluorescens complex colonization primes plant defense responses through jasmonate (JA) and ethylene (ET)-signaling pathways without the fitness costs associated with constitutive defense activation [88]. This mechanism is mediated by P. fluorescens complex lipopolysaccharides, flagella, and siderophores, which serve as microbe-associated molecular patterns recognized by plant pattern recognition receptors, initiating molecular cascades that upregulate key defense genes such as PR1, PDF1.2, and LOX [89, 90]. Microbial cytokinin production, a recently identified biocontrol mechanism, enhances plant tissue integrity and maintains biomass yield during pathogen attack through cytokinin-dependent signaling that requires functional plant cytokinin perception and salicylic acid-dependent defense pathway [91, 92, 93, 94]. This systemic priming enables plants to respond more rapidly and effectively to subsequent pathogen challenges, providing broad-spectrum protection against bacteria, fungi, and even certain insect herbivores while simultaneously maintaining normal plant development and productivity.

4.4 Synergistic interactions with other beneficial microorganisms

Synergistic interactions between the P. fluorescens complex and other beneficial microorganisms enhance disease suppression efficacy beyond additive effects observed with single inoculants. For example, co-inoculation of P. fluorescens complex strain with Trichoderma spp. or arbuscular mycorrhizal fungi (AMF) results in complementary biocontrol activities. P. fluorescens complex antibiotics suppress bacterial and oomycete pathogens while fungal partners control ascomycete and basidiomycete diseases, collectively providing more comprehensive plant protection than either microbial group alone [95, 96, 97, 98]. These synergistic interactions operate through multiple mechanisms, including enhanced pathogen suppression through combined metabolite production, improved nutrient mobilization that strengthens plant immunity, and differential niche occupation that amplifies competitive exclusion of pathogens [99, 100, 101, 102].

5. Contributions to abiotic stress tolerance

5.1 Drought stress mitigation

The P. fluorescens complex mitigates drought stress in plants by coordinating the production of EPS, ACC deaminase, and osmolytes. These processes collectively enhance cellular osmoprotection and water availability [103, 104]. Specifically, EPS production by drought-tolerant strains improves soil water retention and creates protective microenvironments around roots [105], while ACC deaminase activity counters drought-induced ethylene accumulation in plants, thereby helping to maintain root growth and delay senescence during water deficit [104]. Furthermore, drought-tolerant P. fluorescens complex isolates from water-limited environments demonstrate tolerance to polyethylene glycol-induced osmotic stress and promote plant growth under water deficit conditions helping plants maintaining relative water content, accumulate proline, and upregulate antioxidant enzyme activities that mitigate oxidative damage [104, 106, 107]. For example, inoculation with P. fluorescens increased shoot fresh weight by up to 89% in maize under moderate water deficit [108], and by 29% in tomato under field conditions [109], while root dry weight increases by up to 62% in tomato [109] and 17–58% in finger millet [110]. Additionally, inoculation with P. fluorescens complex isolates leads to higher chlorophyll content under drought, with increases of 19–60% reported in maize, finger millet, and other species, supporting improved photosynthetic efficiency and stress tolerance [55, 110, 111].

5.2 Salinity stress alleviation

Halotolerant members of the P. fluorescens complex alleviate salinity stress in plants through several well-documented mechanisms: (1) including ACC deaminase-mediated reduction of salt-induced ethylene, which otherwise inhibit root growth and stress tolerance, (2) EPS production that sequesters sodium ions in the rhizosphere, forming a protective rhizosheath that limits sodium entry into roots and enhances soil aggregation and water retention, (3) modulation of plant ion homeostasis, favoring potassium uptake over sodium accumulation, (4) and enhancement of antioxidant defense systems [8, 112, 113, 114, 115]. Inoculation of salt-stressed plants with the P. fluorescens complex consistently leads to reduced shoot Na⁺ content (typically by 20–35%) and increased K⁺ accumulation (by 15–30%), resulting in a significantly improved K⁺/Na⁺ ratio—an essential indicator of cellular function under salinity stress. For example, in soybean, Pseudomonas inoculation reduced shoot Na⁺ by 22% and increased K⁺ by 61% under salt stress [116]. Similar trends were observed in wheat, maize, and pea, where P. fluorescens and related PGPRs decrease Na⁺ uptake and enhance K⁺ content, restoring ion balance and supporting plant growth [113, 117, 118, 119]. Moreover, halotolerant P. fluorescens complex strains also modulate plant aquaporins and ion transporters, enhancing selective nutrient acquisition and enabling plants to maintain photosynthetic efficiency and biomass accumulation even under high salinity [120, 121, 122]. Following inoculation, salt-stressed plants treated with the P. fluorescens complex consistently show substantial improvements in physiological parameters, including increases in relative water content, marked reductions in electrolyte leakage, and notable enhancements in the K⁺/Na⁺ ratio [121, 123, 124]. Collectively, these changes underpin improved metabolic performance and stress resilience in plants experiencing salt stress.

5.3 Heavy metal stress tolerance

In addition to mitigating drought and salinity stress, the P. fluorescens complex confers plant tolerance against heavy metal toxicity through a coordinated repertoire of stress response mechanisms. Heavy metal stress tolerance is enhanced by P. fluorescens through siderophore and organic acid production that chelate toxic metals (e.g., cadmium, lead, chromium), ACC deaminase activity that reduces metal-induced ET stress, and EPS biosynthesis that immobilizes metals in the rhizosphere, preventing root uptake [125, 126]. Metal-tolerant P. fluorescens strains can survive in soils contaminated with cadmium or zinc while promoting plant growth by reducing metal translocation to shoots and promoting the accumulation of metal-binding phytochelatins and metallothioneins that compartmentalize metals in root vacuoles away from metabolically active tissues [127, 128]. Additionally, P. fluorescens-mediated heavy metal tolerance operates through detoxification of metal-induced reactive oxygen species via enhanced catalase and peroxidase activities, coupled with osmolyte production that stabilizes cellular membranes and maintains metabolic integrity under metal stress [122, 129, 130]. The studies on P. fluorescens complex species under abiotic stress conditions are summarized in Table 2.

Table 2: The beneficial effects of the Pseudomonas fluorescens complex on enhancing plant tolerance to abiotic stresses

Effect	Plants	Strain	Reference
Drought tolerance	Corn (Zea mays)	P. fluorescens S3X	Pereira et al. [108]
	Pea (Pisum sativum), Green bean (Phaseolus vulgaris)	P. fluorescens DR397	Nishu et al. [104]
	Finger millet (Eleusine coracana)	P. palleroniana DPB13, P. fluorescens DPB15, P. palleroniana DPB16	Chandra et al. [110]
	Tomato (Solanum lycopersicum)	P. fluorescens G20-18	Mekureyaw et al. [55]
	Corn (Zea mays)	P. fluorescens	Khaledi et al. [111]
Salinity tolerance	Corn (Zea mays)	P. fluorescens	Kareem and Al-Maliki [131]
	Bread wheat (Triticum aestivum)	P. fluorescens NBRC 14160	Fathalla and Abd El-Mageed [132]
	Soybean (Glycine max)	P. fluorescens	Abulfaraj and Jalal [133]
	Mung bean (Vigna radiata)	P. fluorescens LSMR-29	Kumawat et al. [134]
Heavy metal tolerance	Thale cress (Arabidopsis thaliana)	P. fluorescens KACC10327	Reddy et al. [135]
	Tomato (Solanum lycopersicum)	P. fluorescens So_08	Zhang et al. [128]
	Brown mustard (Brassica juncea)	P. fluorescens Pf 27	Fuloria et al. [136]

6. Biotechnological applications and field implications

6.1 P. fluorescens-based biofertilizers

Due to their ability to promote plant growth, protect against harmful pests, and enhance environmental stress tolerance, members of the P. fluorescens complex hold significant potential in sustainable agriculture [52, 137, 138, 139]. Their diverse metabolic capabilities, including nutrient mobilization, production of antimicrobial and insecticidal compounds, and induction of ISR, make them valuable components of biofertilizers and biopesticides.

P. fluorescens complex-based biofertilizers primarily enhance plant nutrition through phosphate solubilization, siderophore-mediated iron mobilization, and — in some cases — support of nitrogen acquisition [140, 141, 142, 143]. Field studies show improved root growth and yield in cereals, legumes, and vegetables inoculated with pseudomonad formulations [144, 145, 146]. These growth-promoting effects are particularly pronounced under abiotic stress conditions such as drought and salinity, where strains producing IAA and ACC deaminase enhance root system architecture and mitigate stress-ethylene responses [147].

However, translating laboratory efficacy to consistent field-scale performance requires optimized formulations and careful ecological assessment. Challenges include maintaining bacterial viability during storage and ensuring survival after soil application [34, 148]. Recent advances such as microencapsulation, polymer coatings, and biochar carriers have significantly improved stability and field persistence [149, 150, 151].

6.2 P. fluorescens-based biopesticides

Several P. fluorescens strains are commercialized as biofungicides due to their ability to suppress soil-borne pathogens via the production of DAPG, phenazines, pyoluteorin, and lipopeptides. DAPG-producing strains in particular show consistent field efficacy against the diseases caused by Fusarium spp., Pythium spp., and R. solani due to their broad-spectrum and multi-target activity [140]. Some formulations suppress disease by activating JA/ET-dependent ISR [102]. Field performance, however, varies across soil types and climates. Success depends on root colonization efficiency, compatibility with native microbiota, and local environmental conditions. The use of biofungicides that contain the P. fluorescens complex strain and other complementary microbial strains can ensure consistent field performance under different environmental conditions [95].

6.3 Environmental safety and climate-smart agriculture

Environmental safety evaluations reveal that the majority of the P. fluorescens complex are non-pathogenic and ecologically safe [21, 152], thereby highlighting their potential as alternatives to synthetic agrochemicals. Nevertheless, prolonged monitoring is essential to ensure that native microbial communities are not disrupted. Since these bacteria promote nutrient-use efficiency and stress tolerance, they contribute directly to climate-smart agriculture by reducing fertilizer requirements and augmenting crop resilience under drought, salinity, and temperature stress. In general, P. fluorescens complex strains represent versatile tools for developing sustainable, low-input agricultural systems. Their integration into commercial inoculants is growing, but requires rigorous strain selection, optimized formulations, and field validation to ensure stable performance across diverse agroecosystems.

7. Challenges, future prospects, and conclusions

Despite its broad functional potential, the performance of the P. fluorescens complex in the field remains inconsistent due to soil heterogeneity, environmental variability, and competition with indigenous microbiota. These limitations underscore the need for more in-depth genomic, metabolomic, and ecological studies to identify robust strains that can colonize stably and predictably act as biocontrol agents in diverse agroecosystems. Further developments in formulation technology, multi-strain consortium design, and precision microbiome engineering may improve the reliability and scalability of P. fluorescens complex-based inoculants. Integrating these bacteria into climate-smart agricultural practices could reduce dependency on chemical inputs and improve crop resilience in the face of increasing abiotic stressors. Overall, the P. fluorescens complex is a potent and versatile group of beneficial bacteria with significant potential to contribute to sustainable crop production, provided that strategic strain selection, formulation optimization, and field assessment are effectively implemented.

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