New Insight into Carboxylic Acid Metabolisms and pH Regulations During Insoluble Phosphate Solubilisation Process by Penicillium oxalicum PSF-4

The phosphate-solubilising fungi in sustainable agriculture: unleashing the potential of fungal biofertilisers for plant growth

Phosphate-solubilising fungi (PSF) are beneficial microorganisms that play a pivotal role in plant growth by increasing the availability of phosphorus (P) in soil. Although phosphorus is an essential nutrient for plants, it often becomes inaccessible as it binds into insoluble forms. PSF effectively facilitate the release of this bound phosphorus through diverse mechanisms. Numerous fungal species demonstrate the ability to solubilise various types of phosphate compounds. Among the commonly researched PSF are Penicillium, Aspergillus, Rhizopus, Fusarium, Trichoderma, and Sclerotium. Moreover, yeasts such as Saccharomyces cerevisiae can potentially be leveraged as PSF. PSF secrete organic acids that chelate phosphate ions, thereby increasing their solubility in the soil. Moreover, PSF contribute to the decomposition of organic phosphorus compounds in soil by employing enzymes such as phosphatases, phytases, and phosphonatases. Furthermore, PSF can interact with other soil microorganisms, including nitrogen-fixing bacteria and arbuscular mycorrhizal fungi (AM-fungi), fostering synergistic effects that further enhance plant growth and nutrient absorption. The utilisation of PSF as biofertilisers offers numerous advantages over chemical fertilisers, including environmental friendliness, cost-effectiveness, and enhanced fertiliser utilisation efficiency. Furthermore, PSF can prove beneficial in challenging environments characterised by high phosphate sorption. Hence, this review serves as an updated study aimed at broadening the understanding of PSF and its potential applications in P solubilisation. This review also focuses on the diversity of PSF, the mechanisms underlying solubilisation, ecological roles of PSF in soil microbiome, and the benefits of sustainable agriculture. By delving into the ecological roles of PSF and their potential as biofertilisers, this study contributes to a deeper understanding of sustainable agriculture practices and addresses challenges in phosphate-scarce environments.

Penicillium oxalicum induced phosphate precipitation enhanced cadmium (Cd) immobilization by simultaneously accelerating Cd biosorption and biomineralization

Soil cadmium (Cd) is immobilized by the progressing biomineralization process as microbial induced phosphate precipitation (MIPP), which is regulated by phosphate (P) solubilizing microorganisms and P sources. However, little attention has been paid to the implications of Cd biosorption during MIPP. In this study, the newly isolated Penicillium oxalicum could immobilize 5.4-12.6 % of Cd2+, while the presence of hydroxyapatite (HAP) considerably enhanced Cd2+ immobilization in P. oxalicum and reached over 99 % Cd2+ immobilization efficiency within 7 days. Compared to P. oxalicum mono inoculation, MIPP dramatically boosted Cd biosorption and biomineralization efficiency by 71 % and 16 % after 96 h cultivation, respectively. P. oxalicum preferred to absorbing Cd2+ and reaching maximum Cd2+ biosorption efficiency of 87.8 % in the presence of HAP. More surface groups in P. oxalicum and HAP mineral involved adsorption which resulted in the formation of Cd-apatite [Ca8Cd2(PO4)6(OH)2] via ion exchange. Intracellular S2-, secreted organic acids and soluble P via HAP solubilization complexed with Cd2+, progressively mineralized into Cd5(PO4)3OH, Cd(H2PO4)2, C4H6CdO4 and CdS. These results suggested that Cd2+ immobilization was enhanced simultaneously by the accelerated biosorption and biomineralization during P. oxalicum induced P precipitation. Our findings revealed new mechanisms of Cd immobilization in MIPP process and offered clues for remediation practices at metal contaminated sites.

Soil phosphorus transformation and plant uptake driven by phosphate-solubilizing microorganisms

Phosphorus (P) is an important nutrient for plants, and a lack of available P greatly limits plant growth and development. Phosphate-solubilizing microorganisms (PSMs) significantly enhance the ability of plants to absorb and utilize P, which is important for improving plant nutrient turnover and yield. This article summarizes and analyzes how PSMs promote the absorption and utilization of P nutrients by plants from four perspectives: the types and functions of PSMs, phosphate-solubilizing mechanisms, main functional genes, and the impact of complex inoculation of PSMs on plant P acquisition. This article reviews the physiological and molecular mechanisms of phosphorus solubilization and growth promotion by PSMs, with a focus on analyzing the impact of PSMs on soil microbial communities and its interaction with root exudates. In order to better understand the ability of PSMs and their role in soil P transformation and to provide prospects for research on PSMs promoting plant P absorption. PSMs mainly activate insoluble P through the secretion of organic acids, phosphatase production, and mycorrhizal symbiosis, mycorrhizal symbiosis indirectly activates P via carbon exchange. PSMs can secrete organic acids and produce phosphatase, which plays a crucial role in soil P cycling, and related genes are involved in regulating the P-solubilization ability. This article reviews the mechanisms by which microorganisms promote plant uptake of soil P, which is of great significance for a deeper understanding of PSM-mediated soil P cycling, plant P uptake and utilization, and for improving the efficiency of P utilization in agriculture.

The Regulation of Phosphorus Release by Penicillium chrysogenum in Different Phosphate via the TCA Cycle and Mycelial Morphology

Phosphate-solubilizing fungi (PSF) efficiently dissolve insoluble phosphates through the production of organic acids. This study investigates the mechanisms of organic acid secretion by PSF, specifically Penicillium chrysogenum, under tricalcium phosphate (Ca3(PO4)2, Ca-P) and ferric phosphate (FePO4, Fe-P) conditions. Penicillium chrysogenum exhibited higher phosphorus (P) release efficiency from Ca-P (693.6 mg/L) than from Fe-P (162.6 mg/L). However, Fe-P significantly enhanced oxalic acid (1193.7 mg/L) and citric acid (227.7 mg/L) production by Penicillium chrysogenum compared with Ca-P (905.7 and 3.5 mg/L, respectively). The presence of Fe-P upregulated the expression of genes and activity of enzymes related to the tricarboxylic acid cycle, including pyruvate dehydrogenase and citrate synthase. Additionally, Fe-P upregulated the expression of chitinase and endoglucanase genes, inducing a transformation of Penicillium chrysogenum mycelial morphology from pellet to filamentous. The filamentous morphology exhibited higher efficiency in oxalic acid secretion and P release from Fe-P and Ca-P. Compared with pellet morphology, filamentous morphology enhanced P release capacity by > 40% and > 18% in Ca-P and Fe-P, respectively. This study explored the strategies employed by PSF to improve the dissolution of different insoluble phosphates.

Promoting Electrocatalytic Hydrogenation of Oxalic Acid to Glycolic Acid via an Al3+ Ion Adsorption Strategy Coupled with Ethylene Glycol Oxidation

Electrocatalytic hydrogenation (ECH) of oxalic acid (OX) to produce glycolic acid (GA), an important building block of biodegradable polymers as well as application in various branches of chemistry, has attracted extensive attention in the industry, while it still encounters challenges of low reaction rate and selectivity. Herein, we reported a cation adsorption strategy to realize the efficient ECH of OX to GA by adsorbing Al³⁺ ions on an anatase titanium dioxide (TiO₂) nanosheet array, achieving 2-fold enhanced GA productivity (1.3 vs 0.65 mmol cm^-2 h^-1) with higher Faradaic efficiency (FE) (85 vs 69%) at -0.74 V vs RHE. We reveal that the Al³⁺ adatoms on TiO₂ both act as electrophilic adsorption sites to enhance the carbonyl (C═O) adsorption of OX and glyoxylic acid (intermediate) and also promote the generation of reactive hydrogen (H*) on TiO₂, thus promoting the reaction rate. This strategy is demonstrated effective for different carboxylic acids. Furthermore, we realized the coproduction of GA at the bipolar of a H-type cell by pairing ECH of OX (at cathode) and electrooxidation of ethylene glycol (at anode), demonstrating an economical manner with maximum electron economy.

Microbial induced phosphate precipitation accelerate lead mineralization to alleviate nucleotide metabolism inhibition and alter Penicillium oxalicum's adaptive cellular machinery

Microbial-induced phosphate (P) precipitation (MIPP) based on P-solubilizing microorganisms (PSM) is regarded as a promising approach to bioimmobilize environmental lead (Pb). Nevertheless, the underlying changes of Pb²⁺ biotoxicity in PSM during MIPP process were rarely discussed. The current study explored the Pb²⁺ immobilization and metabolic changes in PSM Penicillium oxalicum postexposure to Pb²⁺ and/or tricalcium phosphate (TCP). TCP addition significantly increased soluble P concentrations, accelerated extracellular Pb mineralization, and improved antioxidative enzyme activities in P. oxalicum during MIPP process. Secondary Pb²⁺ biomineralization products were measured as hydroxypyromorphite [Pb₁₀(PO₄)₆(OH)₂]. Using untargeted metabolomic and transcriptomics, we found that Pb²⁺ exposure stimulated the membrane integrity deterioration and nucleotide metabolism obstruction of P. oxalicum. Correspondingly, P. oxalicum could produce higher levels of gamma-aminobutyric acid (GABA) to enhance the adaptive cellular machineries under Pb²⁺ stress. While the MIPP process improved extracellular Pb²⁺ mineralization, consequently alleviating the nucleotide metabolism inhibition and membrane deterioration. Multi-omics results suggested that GABA degradation pathway was stimulated for arginine biosynthesis and TCA cycle after Pb²⁺ mineralization. These results provided new biomolecular information underlying the Pb²⁺ exposure biotoxicities to microorganisms in MIPP before the application of this approach in environmental Pb²⁺ remediation.

Colonization of Penicillium oxalicum SL2 in Pb-contaminated paddy soil and its immobilization effect on soil Pb

Penicillium oxalicum SL2 (SL2) is a previously screened Pb-tolerant fungus that can promote crops growth. The relationship between SL2 colonization and Pb immobilization was studied to provide a theoretical basis for microbial remediation of Pb-contaminated paddy soil. In this study, green fluorescent protein (GFP) labeled SL2 was inoculated into different Pb-contaminated paddy soils (S1-S6). The Pb extracted from the soil by HNO₃, EDTA and CaCl₂ were used to characterize the available Pb. The results showed that the colonization of SL2 was divided into lag phase (0-7 days), growth phase (7-30 days), and mortality phase (30-90 days). SL2 colonized well in sandy soils rich in clay and total phosphorus with initial pH of 4.5-7.0. In addition, SL2 increased soil pH and decreased soil Eh, which was beneficial to immobilize Pb. In different soils, the highest percentages of CaCl₂-Pb, EDTA-Pb, and HNO₃-Pb immobilized by SL2 were 34.34%-40.53%, 17.05%-20.11%, and 7.39%-15.62%, respectively. Pearson correlation analysis showed that the percentages of CaCl₂-Pb and EDTA-Pb immobilized by SL2 were significantly positively correlated with the number of SL2 during the growth phase. SL2 mainly immobilized Pb in the growth phase and a higher peak number of SL2 was beneficial to the immobilization of Pb.

Enhanced Lead (Pb) immobilization in red soil by phosphate solubilizing fungi associated with tricalcium phosphate influencing microbial community composition and Pb translocation in Lactuca sativa L

Phosphate (P) minerals and phosphate solubilizing fungi (PSF) play essential roles in lead (Pb) immobilization, but their roles in driving Pb bioavailability and ecological risks in red soil remains poorly understood. In this study, the inoculation of P. oxalicum and TCP successfully enhanced available P (AP) and urease concentrations in artificially Pb contaminated red soil. Combined P. oxalicum and TCP inoculation significantly reduced Pb bioavailability, bioaccessibility, leachability and mobility by increasing soil AP concentration and forming stable Pb-P compounds during the 21-day experiment. Soil AP and Pb bioavailability play an important role in shifting soil microbial communities induced by co-occurrence of P. oxalicum and TCP. Combined P. oxalicum and TCP could notably promote the relative abundances of predominant soil genus to enhance microbial resistance to soil Pb. Likewise, coexistence of P. oxalicum and TCP showed the highest biomass and better branch root development of Pb-stressed in lettuces (Lactuca sativa L.) in pot experiment, and significantly reduced up to 88.1% of Pb translocation from soil to root over control. The reductions of Pb translocation and accumulation in root in P. oxalicum + TCP treatment could enhance the oxidase activities and alleviate the oxidative damages of H2O2 and O2.- in shoot tissues. Our study provided strong evidence to use PSF associated with P materials for the stable and eco-friendly soil Pb remediation.

Combined effects of Penicillium oxalicum and tricalcium phosphate on lead immobilization: Performance, mechanisms and stabilities

Phosphorus (P) containing minerals are identified as effective Pb stabilizers in soil, while their low solubility limit the Pb immobilization efficiency. In this work, the combination of phosphate solubilizing fungi (PSF) Penicillium oxalicum and tricalcium phosphate (TCP) was constructed and applied to improve Pb immobilization stabilities in medium and soils. P. oxalicum+ TCP could significantly improve Pb²⁺ removal to above 99% under different TCP/Pb²⁺ and pH values. TCP and P. oxalicum could remarkably immobilize Pb by ion exchange, and PbC₂O₄ precipitation or surface adsorption, respectively. While the enhanced Pb immobilization in P. oxalicum+ TCP was explained by stronger Pb²⁺ interaction with tryptophan protein-like substances in extracellular polymeric substance, and the formation of the most stable Pb-phosphate compound hydroxypyromorphite (Pb₅(PO₄)₃OH). Toxicity characteristic leaching procedure test showed that only 0.91% of Pb²⁺ was leachable in P. oxalicum+ TCP treatment, significantly lower than that in P. oxalicum (2.90%) and TCP (7.52%) treatments. In addition, the lowest soil exchangeable Pb fraction (37.1%) and the highest available soil P (88.0 mg/kg) were both found in P. oxalicum+ TCP treatment. By synergistically forming stable Pb-containing products, thus the combination of PSF and P minerals could significantly improve Pb²⁺ immobilization and stability in soils.

A study of P release from Fe-P and Ca-P via the organic acids secreted by Aspergillus niger

Phosphate solubilizing fungi (PSF) have been widely applied to dissolve insoluble phosphates (IPs). However, the PSF usually demonstrates a different phosphate solubilizing capacity for various IPs. This study explored the mechanisms of Aspergillus niger for the dissolution of ferric phosphate (FePO₄, Fe-P), and tricalcium phosphate (Ca₃[PO₄]₂, Ca-P) regarding the tricarboxylic acid (TCA) cycle. Aspergillus niger has higher phosphorus (P) content released from Ca-P, reached the maximum value of 861 mg/L after seven days of incubation, compared with the 169 mg/L from Fe-P. Oxalic acid promoted the release of P from Ca-P through the formation of calcium oxalate. The presence of Fe-P can stimulate A. niger to secrete large amounts of citric acid, confirmed by the enhancement of citrate synthase (CS) activity. However, citric acid only promotes 0.5% of P released from Fe-P. Meanwhile, although oxalic acid still dominates the release of P from Fe-P, its abundance was significantly declined. In contrast, oxalic acid also shows a higher P release ratio in Ca-P than citric acid, i.e., 36% vs. 22%. This study points to the future usage of A. niger to dissolve IPs in soil required to enhance oxalic acid secretion.

Is phosphate solubilizing ability in plant growth-promoting rhizobacteria isolated from chickpea linked to their ability to produce ACC deaminase?

AIMS

Since most phosphate solubilizing bacteria (PSB) also produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase, we investigated if there was an association between these two plant growth-promoting properties under in vitro conditions.

METHODS AND RESULTS

A total of 841 bacterial isolates were obtained using selective and enrichment isolation methods. ACC deaminase was investigated using in vitro methods and by sequencing the acdS gene. The effect of ACC deaminase on P solubilization was investigated further using five efficient PSB. ACC deaminase production ability was found amongst a wide range of bacteria belonging to the genera Bacillus, Burkholderia, Pseudomonas and Variovorax. The amount of ACC deaminase produced by PSB was significantly associated with the liberation of Pi from Ca-P when ACC was the sole N source. Ca-P solubilization was associated with the degree of acidification of the medium. Additionally, the P solubilization potential of PSB with (NH₄ )₂ SO₄ was determined by the type of carboxylates produced. An in-planta experiment was conducted using Burkholderia sp. 12F on chickpea cv. Genesis-863 in sand : vermiculite (1 : 1 v/v) amended with rock phosphate and inoculation of this efficient PSB significantly increased growth, nodulation and P uptake of chickpea fertilized with rock phosphate.

CONCLUSION

ACC deaminase activity influenced the capacity of PSB to solubilize P from Ca-P when ACC was the sole N source and Burkholderia sp. 12F promoted the chickpea-Mesorhizobium symbiosis.

SIGNIFICANCE AND IMPACT OF THE STUDY

ACC deaminase activity could enhance the P solubilizing activity of rhizobacteria that improve plant growth.

Roles of Phosphate Solubilizing Microorganisms from Managing Soil Phosphorus Deficiency to Mediating Biogeochemical P Cycle

Phosphorus (P) is a vital element in biological molecules, and one of the main limiting elements for biomass production as plant-available P represents only a small fraction of total soil P. Increasing global food demand and modern agricultural consumption of P fertilizers could lead to excessive inputs of inorganic P in intensively managed croplands, consequently rising P losses and ongoing eutrophication of surface waters. Despite phosphate solubilizing microorganisms (PSMs) are widely accepted as eco-friendly P fertilizers for increasing agricultural productivity, a comprehensive and deeper understanding of the role of PSMs in P geochemical processes for managing P deficiency has received inadequate attention. In this review, we summarize the basic P forms and their geochemical and biological cycles in soil systems, how PSMs mediate soil P biogeochemical cycles, and the metabolic and enzymatic mechanisms behind these processes. We also highlight the important roles of PSMs in the biogeochemical P cycle and provide perspectives on several environmental issues to prioritize in future PSM applications.