Nutrient availability in soils is a critical factor for plant growth and agricultural productivity. Among the various soil microorganisms, Bacillus spp. have emerged as key contributors to improving nutrient solubilization, particularly for essential nutrients like phosphorus, potassium, and nitrogen. These bacteria possess unique metabolic pathways that enable them to release nutrients from insoluble forms, making them more accessible to plants. This mini article reviews the mechanisms employed by Bacillus species, including the production of organic acids, enzymes, and biofilms, that enhance nutrient solubilization. It also explores the synergistic interactions between Bacillus spp. and plants, as well as their role in promoting sustainable agricultural practices. By understanding these processes, the use of Bacillus spp. could be optimized to enhance soil fertility, reduce dependence on chemical fertilizers, and contribute to more sustainable farming systems.

• Bacillus spp.
• Nutrient solubilization
• Phosphorus solubilization
• Nitrogen fixation

Verma P, Chauhan P, Kumar N, Mishra A (2024) Enhancing Nutrient Availability: “The Role of Bacillus spp. in Solubilizing Essential Nutri- ents”. J Materials Applied Sci 5(2): 1012.

Global food insecurity remains a persistent challenge and is projected to worsen due to the impacts of climate change and rapid population growth. A major contributing factor is the low yields of cereal, pulse, and millet crops, which are impacted by various elements within the soil-plant-environment system. The global population, now approximately 7 billion, is expected to grow to 8.3 billion by 2025. By 2050, food production will need to increase by 70–100% to meet rising demand [1]. Nutrients are vital for plant growth and development, and their deficiency can lead to substantial yield losses while also weakening the plant’s immunity to various biotic and abiotic stresses [2]. Balanced nutrient management in the soil is a great challenge because soil is suffering due to erosion, pollution, and imbalanced nutrients worldwide. The strong demand for food necessitates and the involvement of researchers in the agricultural sector to ensure sufficient food production [3]. All those problems are contemplated on human health, with growing malnutrition issues. These issues influence the economic system of growing and evolved nations [4]. Agricultural productivity is decreasing due to intensive cultivation of high yielding varieties, monoculture systems, rice-wheat cropping systems, and imbalanced use of fertilizers.

Global agriculture today faces new challenges, with a focus on integrating ecological and molecular approaches to boost crop yields while minimizing environmental damage. Key strategies include improving nutrient availability, boosting plant growth and yield, and enhancing resistance to multiple stresses. Eco- friendly microorganisms associated with roots, soil, and plants play a crucial role in promoting plant growth through specific actions that contribute to soil sustainability. By 2050, global agriculture must double food production to meet the demands of a growing population, while also reducing its reliance on mineral or inorganic inputs. Achieving this will require an urgent focus on leveraging the beneficial interactions between soil microorganisms, plants, and the environment. These interactions play a crucial role in enhancing the sustainability of the soil– plant–environment ecosystem [5-8]. This mini review focused on Bacillus Sp. which involved in solubilization of nutrients along with have PGPR activity.

Bacillus species play a significant role in enhancing nutrient availability in soil, which is critical for plant growth and agricultural productivity. Their multifaceted mechanisms contain nitrogen fixation, phosphate solubilization, phytohormones production, and biofilm formation which enhance nutrient uptake [9-11]. Moreover, Bacillus species employ various molecular mechanisms to enhance nutrient availability for plants, which ultimately promote plant growth and development. These mechanisms include nitrogen fixation, phosphate solubilization, siderophore production, and phytohormone synthesis [11,12].

Mechanisms of Nutrient Enhancement

Nitrogen Fixation: Some strains of Bacillus, such as Bacillus subtilis and Bacillus pumilus, are capable of fixing atmospheric nitrogen into a form that plants can utilize [13] [Table 1].

Table1: Impact of Bacillus sp. on nutrient uptake, growth and yield of crop

This process is vital because nitrogen is often a limiting nutrient in soils. These bacteria convert inert nitrogen gas (N?) into ammonia (NH?), which can then be assimilated by plants for protein synthesis and other metabolic functions. However, Bacillus strains possess the nitrogenase enzyme complex, which catalyzes the conversion of atmospheric nitrogen (N?) into ammonia (NH?), a process known as biological nitrogen fixation. The genes responsible for nitrogen fixation, such as nifH, nifD, and nifK, are crucial for this process [12].

Phosphate Solubilization: Besides the nitrogen fixation, Bacillus species are also having ability to solubilize phosphate. This ability make phosphate more available for plant uptake. They achieve this through the secretion of organic acids and enzymes that break down insoluble phosphate compounds in the soil [14]. This process is crucial as phosphorus is essential for energy transfer, photosynthesis, and nutrient transport within plants. For instance, Bacillus subtilis has been shown to effectively solubilize phosphorus from mineral sources, thereby enhancing its availability in the rhizosphere [13]. Bacillus species secrete organic acids and phosphatase enzymes to solubilize insoluble phosphates in the soil. The production of these compounds is regulated by genes involved in phosphate metabolism, such as pqq (pyrroloquinoline quinone) and phytase genes [14]. The pqq genes encode enzymes that catalyze the oxidation of glucose to gluconic acid, while phytase genes encode enzymes that hydrolyze phytic acid, the primary storage form of phosphorus in many soils [15].

Production of Phytohormones: Bacillus species contribute to nutrient availability indirectly through the phytohormones synthesis such as auxins, gibberellins, and cytokinins [11]. These hormones promote root development and enhance the plant’s ability to absorb nutrients from the soil. For example, auxins stimulate root elongation and branching, which increases the surface area for nutrient uptake. Additionally, these hormones can help plants cope with abiotic stresses by regulating physiological processes. The genes involved in phytohormone biosynthesis, such as ipdC (indole-3-pyruvate decarboxylase) for auxin production and aro genes for cytokinin synthesis, are essential for the production of these plant growth regulators [16]. In addition to these direct mechanisms, Bacillus species can also indirectly facilitate nutrient availability by promoting root growth and development through the production of ACC (1-aminocyclopropane-1-carboxylate) deaminase [17]. This enzyme breaks down ACC, the immediate precursor of ethylene, thereby reducing ethylene levels and promoting root elongation and branching.

Biofilm Formation: The ability of Bacillus species to form biofilms in the rhizosphere is another important mechanism that enhances nutrient availability. Biofilms consist of microbial communities encased in a protective extracellular matrix that facilitates nutrient exchange and retention in the soil [13]. The biofilm environment supports beneficial interactions between Bacillus and plant roots, improving water retention and nutrient absorption under drought conditions [9]. Furthermore, biofilms can act as barriers against pathogens, promoting overall plant health.

Siderophore Production: Bacillus species produce siderophores—molecules that bind iron with high affinity— thereby increasing iron availability in the soil [9,12]. Iron is a crucial micronutrient necessary for various biochemical processes in plants. By chelating iron from less soluble forms in the soil, Bacillus enhances its uptake by plants, which is particularly important in alkaline or calcareous soils where iron availability is often limited. The genes responsible for siderophore biosynthesis, such as those encoding no ribosomal peptide synthetases (NRPSs), are crucial to produce these iron-scavenging molecules. Siderophores help solubilize and sequester iron from the soil, making it available for plant uptake [18].

Zinc Solubilization: Zinc is vital for several plant enzymes and proteins, but it often remains unavailable in the soil in insoluble forms like zinc oxide or carbonate. Zinc is one of the most important micronutrients required for plant growth, development, disease defence and resistance against stress [19]. Bacillus species release organic acids and chelators to convert insoluble zinc into soluble forms that plants can easily absorb. Several Bacillus species have been identified as potential zinc solubilizers from various crops such as wheat, paddy, chickpea, maize, soybean, cow dung, cotton, and stone quarry dust [20].

Alteration of root structure and biomass

Changes in root architecture are crucial as they affect a plant’s capacity to explore the soil, thereby enhancing its uptake of water and nutrients [36]. Since root elongation and the development of lateral roots—key processes in root architecture modification—are regulated by plant hormones like auxins [37], the role of rhizobacteria becomes significant. These microorganisms produce and release auxins. Notably, the well-known rhizobacterium Bacillus subtilis has the ability to synthesize and release auxins [38,39], as well as organic volatile compounds [40], which influence plant root architecture. For example, inoculation of soybean with B. subtilis resulted in notable changes in root structure, significantly increasing root surface area and length [40].

The first case study, according to Araujo et al. [41], in Brazil, soybean cultivation has expanded in areas with low fertility and poor organic matter, which can lead to potential yield losses [42]. However, inoculating plants with plant growth-promoting bacteria (PGPB) can enhance growth even in low-fertility conditions. This support can occur through direct means, such as the production of growth-stimulating compounds, or indirectly by solubilizing nutrients [43].

The second case study, according to de Sousa et al. [44], they suggested that Bacillus sp. enhanced maize yield and phosphorus (P) uptake following inoculation result from multiple mechanisms of plant growth promotion. The production of indole-3-acetic acid (IAA) increases root surface area, enabling roots to expand and explore a larger volume of soil until they reach localized areas with higher P availability [45]. This, combined with phosphorus solubilization, makes this nutrient more accessible to the plants.

Bacillus spp. play a pivotal role in enhancing nutrient availability by solubilizing essential nutrients such as phosphorus, nitrogen, which are critical for plant growth. Through mechanisms like organic acid production, enzyme secretion, and biofilm formation, these bacteria transform insoluble nutrients into bioavailable forms, significantly improving soil fertility and plant health. The use of Bacillus-based biofertilizers offers a promising and sustainable alternative to chemical fertilizers, helping reduce environmental impact while supporting agricultural productivity. Understanding and harnessing the potential of Bacillus spp. can contribute to more efficient, eco-friendly farming practices, fostering long-term sustainability in agricultural systems.

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