Phosphate & potassium solubilizing bacteria as PGPR

Introduction

The term PGPR (Plant growth-promoting rhizobacteria) was first used by Kloepper and Schroth to describe soil naturally occurring rhizobacteria. These bacteria settle on plant’s rhizospheric regions and are an essential source of additional nutrients and growth. The associated factors help to increase the plant growth and its productivity.

The use of PGPR also aims to significantly increase the plant nutrients. Plant roots secrete different types of exudates, mucigels, secondary metabolites, alkaloids, etc. It acts as a source of carbon and energy fuelling the microbes. Soil also contains different types of microflorae which include PGPR, actinomycetes, fungi, protozoa and algae.

Emphasing on PGPR, it extracts the necessary nutrients from this area, and also promotes the growth of plants by associating itself with the plants, which helps plants absorb nutrients from the soil and obtain food and shelter, so that plants can thrive in any adverse situation. During the stress and adverse conditions, PGPRs have increased the soil fertility in many ways.

The predominant bacterial species found in the rhizosphere, accounts for approximately 2-5% of rhizobacteria are PGPR which includes Pseudomonas, Azospirillum, Azotobacter, Klebsiella, Enterobacter, Alcaligenes, Bacillus and Serratia that actively promote plant growth and productivity.

Types of PGPRs

1. Extracellular PGPR (ePGPR): It is located in the rhizosphere, rhizoplane, or intercellular space of the cortex.

Examples: Agrobacterium, Arthrobacter, Azotobacter, Azospirillum, Bacillus, Burkholderia, Caulobacter, Chromobacterium, Erwinia, Flavobacterium, Micrococcous, Pseudomonas and Serratia.

1. Intracellular PGPR (iPGPR) disease: A generalized finding within a specific nodular pattern of cellular processes. The family Rhizobiaceae includes Allorhizobium, Mesorhizobium and Rhizobium, endophytes and Frankia species.

Direct mechanism

• Biological nitrogen fixation: Nitrogen (N) is an essential nutrient for plant growth and production. N₂ in the atmosphere is converted into energy by plants biological Nitrogen fixation (BNF) nitrogen conversion Complex-using ammonia- and nitrogen-fixing microorganism’s enzyme systems are known as nitrogenases.
• Phosphate solubilization: Phosphorus (P), the second most important element that inhibits growth nutrients,after, the most abundant nutrient in soil organic compounds.
• Phytohormone production:
  • Indole acetic acid: Microbial synthesis of the phytohormone auxin (indole-3-acetic acid/indoleacetic acid/IAA) has been known since ancient times. Rhizomes of different plants have the ability to synthesize and release auxins as secondary metabolites. IAA plays an important role in the activity of rhizobacteria and plants. IAA affects cell division, proliferation, and differentiation; sowing and germination; increases the rate of xylem and root development; regulate plant growth processes initiates lateral and adventitious root formation combines light, weight and fluorescence results; affects photosynthesis, pigment formation, Biosynthesis of different metabolites and resistance to stress times.
  • Cytokinin’s: Cytokinin’s are a class of phytohormones known to promote division, cell elongation, and cell expansion in some plants. Cytokinin’s play a greater or lesser role in development, from seed germination to leaf and plant senescence and changes in physiological functions important for plant life; photosynthesis and respiration. The growth of many plants is caused by rhizobacteria Azotobacter sp. produces. Rhizobium sp., Pantoea agglomerans, Rhodospirillum rubrum, Pseudomonas fluorescens, Bacillus subtilis and Paenibacillus polymyxa can produces cytokines and other growth-promoting factors.
  • Gibberellins: It is often associated with changes in plant morphology and cell expansion, especially in organs. These are higher plants, fungi and bacteria. The developmental processes that occur in many plants include cell division and germination, bud formation, flowering, late flowering, flowering and senescence, etc., which are included in many articles on different plant species.

Indirect mechanism

• Biocontrol agents: Biocontrol is the most prominent indirect mechanism that performed by PGPR to produce various types of antibiotics to control the spread of phytopathogens. The two biocontrol agents of PGPR are Pseudomonas and bacillus.
• Antibiotic production: One of the main mechanisms used by PGPR to prevent the spread of phytopathogens is antibiotic synthesis. Antibiotics consist of a heterogeneous group of organic compounds with growth factors of low molecular weight or metabolic activity other microbes.
• Siderophore production: Iron is an essential nutrient for life. Except for some lactobacilli, all organisms known to date have high iron requirements. In the air in the environment, iron is found mainly as Fe3+ and forms an insoluble hydroxide and oxyhydroxide and thus reacts is generally impervious to plants and microbes. Normally bacteria take up iron in the release of small molecules of iron chelators was reported. From siderophores have a continuous relationship complexing iron. Most siderophores are water soluble and can be divided into extracellular siderophores and intracellular siderophores.
• HCN (Hydrogen cyanide) production: HCN synthesis is essential for the activity of plant growth promoting compounds. Hydrogen in agriculture, cyanide is mostly used as biological control agent, production strategy based on negative toxicity to plant pathogens, metal ion chelation and indirectly related to phosphate availability. Many researchers have reported that PGPR produces HCN and its use as a biofertilizer to stimulate growth, increase tomato yields, and control diseases. Several bacterial genera have been identified as HCN species, including Aeromonas, Bacillus, Pseudomonas, and Enterobacter. Released by the tomato rhizosphere.
• Production of lytic enzymes: The growth and activity of pathogens can be suppressed by the secretion of lytic enzymes. Cell wall degrading enzymes such as glucanase, proteases, chitinases and lipases etc. are secreted by PGPR biocontrol stress involved in fungal wall lysis. These enzymes digest the spores or inactivate the cell wall components of fungal infections. Hydrolytic enzymes play a direct role in the degradation of phytopathogens and in the recovery of plants from biotic stress.

PGPR’s Characteristics

1. The strain must be highly developed for the rhizosphere and should be environmentally friendly.
2. Should promote plant growth.
3. It must be able to grow and live in the rhizosphere less time from fewer injections.
4. It should be able to perform a broad-spectrum action in the soil.
5. Each strain/bacterium should be compatible and good to other rhizosphere strains/bacteria.
6. Can tolerate abiotic stresses and physicochemical factors such as heat, drying, radiation and oxygen.
7. Severe rhizosphere lesions should be dealt with vigorously.

Phosphate solubilizing bacteria

Phosphorus is the most important macronutrient for plants after nitrogen and plays a direct role in nucleic acid synthesis, cell division and growth of new cells, and is also required in various cellular processes such as photosynthesis, carbon metabolism, energy production, redox homeostasis and signalling in plants. Phosphorus is one of the most abundant minerals in Earth’s soils and is found in both inorganic (35–70%) and organic forms (30–65%).

Phosphorus is generally found in inert form in soils is designed as an insoluble phosphorus fertilizer that is difficult for plants to directly extract and use. Inorganic phosphorus easily reacts with ions such as Fe3+, Al3+ and Ca2+ in the soil to produce insoluble phosphate. Phosphorus is part of the structure of many coenzymes, phosphoproteins and phospholipids, and is also part of the genetic memory “DNA” of all organisms. It plays a role in the transfer and storage of energy used for growth and reproduction.

Phosphorus plays a particularly important role Carbon metabolism and membrane formation in photosynthesis also play a role in root elongation, proliferation and phosphorus deficiency, which affects root architecture. In plants, phosphorus strengthens the grass, promotes flowering and fruit formation, promotes rooting and is important for seed formation. In plants, phosphorus is essential for maintaining the existence of grasses, producing flowers and fruits, developing roots, and fruit formation.

Different types of organisms, such as bacteria, fungi, actinomycetes, and specific algae, can dissolve phosphate. Certain soil bacteria are known for phosphate production, including Pseudomonas spp. , Bacillus spp. , and Agrobacterium spp. There are primarily two categories of bacteria that promote plant development: Proteobacteria and Firmicutes. The Bacillus spp. falls under the Firmicutes category.

Among the most significant phosphate-solubilizing bacteria strains are B. circulans, B. subtilis, and Enterobacter. Other phosphate-solubilizing bacteria include Azotobacter, Enterobacter, Rhizobium, Rhodococcus, Serratia, Salmonella, and Thiobacillus. Various fungi are also capable of dissolving phosphate, such as Alternaria, Aspergillus, Cephalosporium, Cladosporium, Fusarium, Micromonospora, Pichia fermentans, Pythium, Rhizopus, Saccharomyces, Sclerotium, and Trichoderma. In soil, fungi play a more significant role than bacteria in the conversion of phosphate, producing a variety of acids like citric, tartaric, acetic, gluconic, lactic, and 2-ketogluconic acids.

Distribution and Species of PSB

PSB in soil can alter the form of phosphorus that cannot be converted into usable phosphorus these bacteria are usually Gram negative. There are many types of PSB, which can be divided into two main groups according to different active substances: organic PSB, which can destroy organic phosphorus, and inorganic PSB, which can convert insoluble phosphorus into phosphorus; Many PSBs can also dissolve inorganic phosphorus and reduce organic phosphorus at same time.

Phosphorus-solubilizing microorganisms consist of phosphorus-solubilizing bacteria, phosphorus-solubilizing actinomycetes, and phosphorus-solubilizing fungi; among them, PSB is widely distributed and accounts for 1-50% of phosphorus-forming microorganisms, phosphorus-45% and phosphorus-solubilizing microorganisms in the soil. Molds 0.1% to 0.5%. Soil conditions also affect the distribution of PSB; different soils cause differences in the distribution of PSB through different activities.

Mechanism of Phosphate Solubilization

The ways in which soil microorganisms make phosphorus soluble involve several processes:

1. The generation of complexes, including organic acid anions, siderophores, protons, hydroxyl ions, and carbon dioxide.
2. The production of specific enzymes that contribute to the mineralization of biochemical phosphorus.
3. The release of phosphorus when substrates are broken down. Thus, organic matter plays a vital role in the phosphorus cycle within the soil, affecting reactions like precipitation, sorption-desorption, and mineralization.

Insoluble phosphate solubilisation: Microorganisms play a critical role in increasing phosphorus solubility by generating organic acids. A drop in pH indicates that microorganisms responsible for solubilizing phosphorus are discharging organic acids through a direct oxidation process at the surface of their cytoplasm. When phosphorus enters the soil, it interacts with elements like iron, aluminum, and calcium ions, leading to the formation of compounds like ferrous phosphate, aluminum phosphate, and calcium phosphate, which render phosphorus inaccessible to plants. The primary acids that phosphorus-solubilizing microorganisms excrete to help dissolve insoluble phosphorus are citric acid, lactic acid, tartaric acid, and aspartic acid.

Organic phosphate solubilisation: 

Phosphorus can be extracted from organic soil matter by three different types of enzymes:

1. Inorganic phosphatase, which results in the phosphorylation of phosphorus ester or phosphoanhydride compounds.
2. Phytase, which mainly releases phosphorus as phytic acid.
3. Phosphonase and C-P lyase, which are enzymes that break down phospho compounds by catalyzing the cleavage of C-P bonds. The availability of organic phosphate compounds to plants can be problematic since highly reactive phosphorus binds with other minerals in the rhiosphere soil, thus making them unreachable for plants, potentially limiting plant growth and overall productivity.

Use of PSB as Bio-Fertilizers

Bio fertilizer refers to a type of fertilizer that includes specific microorganisms known to supply essential nutrients needed for plant development or to boost agricultural yield. These microbes enhance nutrient provision through different processes, including nitrogen fixation and the solubilization and fixation of phosphates.

Notable examples of P-solubilizing bacteria include Azotobacter and Azospirillum, frequently utilized in composting practices. Combining Azotobacter with a controlled ratio of NPK fertilizers and mixing Azospirillum with a regulated amount of the NPK family while using organic fertilizers can significantly enhance crop health and increase seed production.

Numerous research findings indicate that the most efficient phosphating agents include Rhizobium, Bacillus, Penicillium, Pseudomonas, and Aspergillus. Organic fertilizers contain harsh acids like hydrochloric acid (HCl) and sulfuric acid (H2SO4), which can elevate soil acidity and harm beneficial microbial populations, ultimately hindering plant growth. In this context, PSB presents a viable alternative to traditional synthetic fertilizers.

Additionally, PSB benefits agriculture in two ways: first, by promoting plant growth, solubilizing phosphorus, producing phytohormones, and enhancing nutrient accessibility. Secondly, PSB can diminish the number of harmful microbes by releasing antibiotics or siderophores.

Potassium solubilizing bacteria

One of the essential nutrients for higher and longer-lasting yields of crops is potassium (K). It is the third crucial nutrient for plants following nitrogen (N) and phosphorus (P). Potassium is necessary for the activation of essential enzymes involved in metabolic functions, particularly the synthesis of sugars and proteins.

Additionally, it is necessary for the opening and closing of stomatal guard cells and daily changes in leaf arrangement. The development and expansion of agriculture, driven by small landholdings and the introduction of high-yielding crop varieties and hybrids during the Green Revolution, have caused rapid soil depletion of macronutrients like potassium.

Potassium deficiency in soil hampers root and shoot development reduces branching, slows growth, and leads to smaller seeds and lower yields by disrupting essential physiological processes crucial for plant growth and productivity. Potassium, a key component of plant cell structure, not only initiates essential enzyme reactions, protein synthesis, and carbohydrate metabolism but also supports various plant adaptive responses to stress.

Many arid and semi-arid soils are becoming increasingly deficient in potassium fertility, primarily due to intensive farming practices that apply little or no potassium. The availability of potassium varies with soil type and is determined by the nature of clay minerals and the level of organic matter.

Microorganisms are essential for the weathering of soil as they help dissolve nutrients from minerals that are not soluble. Various types of microbes, such as fungi, bacteria, and actinomycetes, break down potassium (K) minerals by releasing organic acids. Certain potassium-solubilizing bacteria (KSB) like Pantoea agglomerans, Rahnella aquatilis, and Pseudomonas orientalis have shown their ability to extract K from feldspar and aluminosilicate minerals using methods like acidolysis, chelation, exchange reactions, complexation, and the decomposition of organic matter.

KSB found in the rhizosphere of different crops, including Enterobacter from rubber tree soil and Mesorhizobium sp. , Paenibacillus sp. , and Arthrobacter sp. from rape rhizosphere soil, show significant ability to solubilize potassium minerals. Potassium (K) is vital for the growth and development of plants and crops. It plays an important part in cell synthesis, enzyme activity, protein creation, and in the production of starch, cellulose, and vitamins.

In addition to this, K enhances the absorption and transport of nutrients while also helping plants resist both biotic and abiotic stresses, which improves crop yield and quality. Plants take up potassium more than any other mineral, and synthetic fertilizers are often employed to boost the availability of K in the soil. Soil microorganisms play a key role in balancing potassium levels by breaking down complex potassium compounds into simpler forms that plants can use. Certain bacteria such as Bacillus, Thiobacillus, Pseudomonas, and Acidothiobacillus are recognized for their ability to solubilize potassium from soil minerals.

One of the main causes of potassium (K) depletion in the soil is the contemporary agricultural practice of not including crop residues, which leads to reduced growth and yield of crops. Potassium-solubilizing bacteria (KSB) are helpful microorganisms in the soil that can change insoluble potassium sources into soluble ones through several methods.

These methods encompass the release of both organic and inorganic acids, polysaccharides, acidolysis, complexolysis, chelation, and exchange reactions. Research indicates that KSB represents a viable solution to making K accessible for plants. It is important to identify effective bacterial strains that can quickly solubilize K minerals to conserve existing resources and prevent environmental damage caused by the overuse of K fertilizers.

Action Mechanisms of Potassium Solubilizing Microbes (KSM)

1. Decrease of pH by Organic Acids: KSM lowers the pH of its environment by generating organic acids. Acids such as citric, gluconic, and oxalic play a role in breaking down potassium-rich minerals. The reduced pH allows potassium ions to be released, making them accessible for plants to absorb. This process is similar to how phosphates are solubilized, with organic acids being crucial in changing insoluble phosphates to soluble ones.
2. Formation of Complexes and Chelation: KSM improves the solubilization of potassium by forming complexes and chelation. The organic acids from these microorganisms attach to metal ions like Fe2+, Al3+, and Ca2+, creating stable complexes. This chelation action aids in keeping potassium soluble, preventing those metal ions from re-precipitating, so plants can easily take in potassium.
3. Acidolysis: Another important method used by KSM is acidolysis. The microbes release acids that dissolve potassium-containing minerals directly. This process breaks down the structure of minerals, allowing potassium ions to be released into the soil. Acidolysis works particularly well in soils with higher pH, where other solubilizing methods may not be as effective.
4. Ion Exchange Reactions: KSM also supports potassium solubilization through ion exchange reactions. In this method, the acids produced by microorganisms lead to an exchange of hydrogen ions for potassium ions that are attached to soil particles. This exchange makes more potassium available in the soil solution for plants to absorb.
5. Production of Polysaccharides and Oxidation: KSM releases potassium from minerals through the production of capsular polysaccharides and microbial oxidation. Polysaccharides help by binding to mineral surfaces, loosening and solubilizing potassium. Additionally, microbial oxidation reactions assist in the release of potassium, making it more available for plants. These mechanisms underline the vital function of KSM in transforming insoluble potassium forms into soluble ones that plants can access, thereby enhancing soil fertility and promoting plant growth.

Reference and Sources:

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