The multifunctional role of Pseudomonas spp. as plant growth-promoting rhizobacteria (PGPR): A sustainable approach to biofertilization and plant health

Sustainable agriculture relies on maintaining healthy, ecologically balanced soil. The excessive use of chemical fertilizers has resulted in declining soil fertility, biodiversity loss, and structural degradation. In particular, the use of biofertilizers containing microbial inoculants that enhance plant growth and soil health offers a promising alternative by improving nutrient availability, suppressing pathogens, and restoring soil ecosystems. Studies have shown that rhizosphere microbes can enhance plant growth and control diseases by producing phytohormones and aiding nutrient uptake. Many bacterial genera can be used as biofertilizers, including nitrogen-fixers, phosphorus-solubilizers, potassium-solubilizers (K-solubilizers), and others. Among the diverse plant growth-promoting rhizobacteria (PGPR), species within the Pseudomonas genus have emerged as promising candidates due to their exceptional versatility and resilience. Pseudomonas spp. exhibits a wide array of plant growth-promoting traits, including atmospheric nitrogen fixation, solubilization of phosphorus and potassium, and the secretion of phytohormones such as indole-3-acetic acid (IAA) and gluconic acid. By systematically linking their metabolic traits to specific soil health and crop productivity outcomes, this review provides new insights into their potential application in sustainable and climate-resilient agriculture. Several Pseudomonas species have been extensively utilized as biofertilizers due to their multifunctional traits, adaptability to a wide range of soil environments, and capacity to enhance plant health under both biotic and abiotic stress conditions. The primary objective of this review is to present a comprehensive overview of biofertilizer mechanisms and the functional roles of Pseudomonas spp., including selected strains known to improve plant growth and soil fertility.

1. INTRODUCTION

1. Fundamental concept of biofertilizers

Biofertilizers constitute formulations that contain viable or dormant microbial strains, which improve plant nutrient acquisition through active colonization of the rhizosphere, thus increasing nutrient accessibility to plant root systems. Improving nutrient effectiveness in soil to stimulate plant development and enhance overall productivity represents a fundamental aspect of rhizosphere management, which stands at the core of sustainable agriculture(Singh et al., 2021). Nevertheless, successful rhizosphere management involves complex ecological challenges, especially within sustainable and climate-resilient farming approaches. Biofertilizers operate by encouraging beneficial relationships between plants and microorganisms within the rhizosphere, commonly when administered through soil or seed treatments(Prasad et al., 2019). The utilization of microbial inoculants via seed coating, root dipping, or soil amendment has been extensively practiced for decades as an approach to improve plant physiological development, enhance nutrient acquisition, and support crop health and resilience(Nosheen et al., 2021). These inoculants enable functions including symbiotic and non-symbiotic nitrogen fixation, biodegradation of xenobiotic compounds, enhancement of plant growth, and biocontrol of phytopathogens(Sammauria et al., 2020). The main microbial groups utilized in biofertilizer production encompass bacteria, fungi, and cyanobacteria (commonly known as blue-green algae). One of the earliest recorded practices involved large-scale rhizobial inoculation of leguminous crops in the early 20^thcentury to improve biological nitrogen fixation and soil fertility(Backer et al., 2018).

During recent years, commercial formulations containing genera such as Bacillus, Pseudomonas, and Glomus have been developed. Reviews have additionally documented the functions of microbial taxa, including Bacillus, Pseudomonas, and Actinobacteria(Shivlata & Satyanarayana, 2017), and Lactobacillus(Lamont et al., 2017). Other beneficial genera, such as Acetobacter, Azospirillum, Paenibacillus, Serratia, Burkholderia, Herbaspirillum, and Rhodococcus, also provide significant contributions to crop performance(Babalola, 2010). Within these, Pseudomonas spp. have surfaced as highly promising plant-associated microbes owing to their extensive range of plant growth-promoting (PGP) characteristics. These encompass improved nutrient solubilization and availability (particularly N, P, K, Zn, and S), regulation of phytohormones, suppression of plant pathogens through biocontrol activity, and alleviation of both biotic and abiotic stresses(Dorjey et al., 2017). In summary, Pseudomonas spp. possesses considerable potential in improving plant growth, crop yield, and resilience under both greenhouse and field conditions.

There exists growing evidence that urbanization, climate change, and the excessive utilization of agrochemicals are disrupting the environment and affecting food security. Agricultural soils are experiencing quality loss, deterioration of physical and chemical properties, and biological health decline due to indiscriminate agrochemical usage(Novo et al., 2018). Generating more food on limited arable land while protecting the environment has emerged as a key challenge of the millennium. The utilization of microorganisms in agriculture can progressively reduce dependence on chemical fertilizers, fostering biological sustainability(Ortíz-Castro et al., 2009). Rhizosphere management involves improving soil nutrient efficiency to enhance plant growth and productivity(Singh et al., 2022). Biofertilizers facilitate plant growth by suppressing soil-borne pathogens and improving nutrient uptake and utilization efficiency(Thomas & Singh, 2019). This process supports multiple interactions within the rhizosphere that establish pathways for improving plant health and nutrient availability. Core mechanisms encompass increased symbiotic and non-symbiotic nitrogen fixation, degradation of xenobiotic chemicals, plant development enhancement, and biological control of pathogenic bacteria(Laslo et al., 2012). The utilization of microbial-based agricultural inputs, including single strains and microbial consortia, has a lengthy history in sustainable farming. Since the early 1900s, rhizobial inoculants have been applied to leguminous crops to improve biological nitrogen fixation and soil fertility(Backer et al., 2018). Contemporary biofertilizers are developed using diverse formulation techniques, including solid and liquid forms, incorporation of cell protectants such as glycerol, lactose, and starch, selection of suitable carrier materials, and efficient delivery systems(Novo et al., 2018). Recent advances include entrapment or microencapsulation, nano-immobilization of microbial inoculants, and biofilm-based formulations, all designed to enhance microbial viability, stability, and field performance(Arora et al., 2018).

Microorganisms typically isolated from roots are termed plant growth-promoting rhizobacteria (PGPR). In recent decades, bio-based approaches have gained momentum as core strategies for sustainable agriculture. Various bacterial genera, such as Agrobacterium, Arthrobacter, Azotobacter, Azospirillum, Bacillus, Burkholderia, Caulobacter, Chromobacterium, Erwinia, Flavobacterium, Micrococcus, and notably Pseudomonas, are recognized as effective PGPR. These can be classified according to their location: extracellular PGPR (ePGPR), which inhabit the rhizosphere, rhizoplane, or intercellular spaces of the root cortex; and intracellular PGPR (iPGPR), which colonize root cells, often within specialized structures such as nodules(Figueiredo et al., 2010). PGPR establishes complex, beneficial interactions with plants, promoting health, nutrient acquisition, and stress resilience(Bhattacharyya & Jha, 2012). They produce and release secondary metabolites that suppress phytopathogens and facilitate nutrient uptake from the root environment(Singh et al., 2019). Through direct and indirect mechanisms, these materials contribute to nutrient cycling and yield improvement in agroecosystems(Prasad et al., 2019). Given their capacity to function as both biofertilizers and biopesticides, PGPR are increasingly viewed as viable alternatives to chemical agro-inputs.

PGPR-based biofertilizers have demonstrated the potential to partially substitute chemical fertilizers at a considerably lower cost, with yield increases of approximately 20%-35% compared to untreated controls(Basu et al., 2021). These beneficial microbes function as ecological engineers, contributing to sustainable soil fertility management and reducing dependence on synthetic inputs. Implementing PGPR in agriculture represents an environmentally friendly, resource-efficient approach to improving food production while conserving natural resources(Prasad et al., 2019). They

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