Heterologous expression of pyrroloquinoline quinone (pqq) gene cluster confers mineral phosphate solubilization ability to Herbaspirillum seropedicae Z67

Revealing the Existence of Diverse Strategies for Phosphorus Solubilization and Acquisition in Plant-Growth Promoting Streptomyces misionensis SwB1

Phosphorus deficiency poses a significant challenge to plant growth and development, particularly in red soil. To alleviate this limitation, phosphorus-solubilizing bacteria (PSB) play a crucial role by converting insoluble phosphates present in the soil into soluble forms that are accessible to plants. Cornus wilsoniana Wangerin is a representative oil crop cultivated in red soil, holding a prominent position within China's forestry economic system. Consequently, it is essential to develop highly stable microbial phosphorus enhancement strategies to manage agricultural phosphorus in red soil regions, thereby maintaining the available phosphorus content necessary for the production of C. wilsoniana. In this study, the application of Streptomyces misionensis SwB1 bacterial suspension to the rhizosphere of C. wilsoniana significantly increased the content of various phosphorus fractions (H2O-P, NaHCO3-P, NaOH-P, HCl-P) in red soil, with NaHCO3-P content increasing by 4.97 times and NaOH-P content by 3.87 times. Additionally, the genome of S. misionensis SwB1 contains 25 phosphorus-solubilizing genes, 13 nitrogen-fixing genes, 17 siderophore production genes, and 11 indole-3-acetic acid (IAA) production genes, indicating its potential for enhancing nutrient availability. Comparative genomic analysis of 15 strains belonging to five species of Streptomyces revealed that S. misionensis SwB1 possesses an extensive genetic repertoire and complete gene clusters associated with phosphorus solubilization. Furthermore, five phosphorus solubilization pathways of S. misionensis SwB1 were summarized: the Pst system, Pit system, siderophore transport, phosphatase synthesis, and organic acid synthesis. Ultimately, the inoculation of S. misionensis SwB1 significantly enhanced the growth and biomass accumulation of C. wilsoniana at the seedling stage, evidenced by an increase in fresh weight by 81.44%, a rise in net photosynthetic rate by 18.51%, and a surge in the number of root tips by 36.24%. Taken together, our findings support a sophisticated multi-pathway bacteria phosphorus solubilization approach and identified a highly efficient phosphorus-solubilizing strain, S. misionensis SwB1, which has the potential to become a microbial fertilizer.

Two Novel Bacterial Species, Rhodanobacter lycopersici sp. nov. and Rhodanobacter geophilus sp. nov., Isolated from the Rhizosphere of Solanum lycopersicum with Plant Growth-Promoting Traits

Two novel bacterial species were isolated from the rhizosphere of Solanum lycopersicum (tomato plant), both exhibiting plant growth-promoting properties. Two isolated strains, Rhodanobacter lycopersici sp. nov. Si-cT and Rhodanobacter geophilus sp. nov. S2-gT, were classified through a polyphasic approach, confirming their novel status within the Rhodanobacter genus. The strains demonstrated a remarkable tolerance to extreme pH conditions, with R. lycopersici Si-cT surviving in pH 3.0-13.0 and R. geophilus S2-gT tolerating pH 2.0-13.0. Additionally, both strains exhibited multiple plant growth-promoting traits, including indole-3-acetic acid and ammonia production, phosphate solubilization, and siderophore formation. These characteristics suggest that the two strains may play an important role in promoting plant growth, especially in soils with variable pH levels. However, since the direct impact on plant growth was not experimentally tested, the potential of these bacteria for agricultural applications remains to be confirmed through further research. This study expands our understanding of the diversity within the Rhodanobacter genus and provides insights into the potential use of these novel species in sustainable agriculture.

Plant-microbe interactions in the rhizosphere for smarter and more sustainable crop fertilization: the case of PGPR-based biofertilizers

Biofertilizers based on plant growth promoting rhizobacteria (PGPR) are nowadays gaining increasingly attention as a modern tool for a more sustainable agriculture due to their ability in ameliorating root nutrient acquisition. For many years, most research was focused on the screening and characterization of PGPR functioning as nitrogen (N) or phosphorus (P) biofertilizers. However, with the increasing demand for food using far fewer chemical inputs, new investigations have been carried out to explore the potential use of such bacteria also as potassium (K), sulfur (S), zinc (Zn), or iron (Fe) biofertilizers. In this review, we update the use of PGPR as biofertilizers for a smarter and more sustainable crop production and deliberate the prospects of using microbiome engineering-based methods as potential tools to shed new light on the improvement of plant mineral nutrition. The current era of omics revolution has enabled the design of synthetic microbial communities (named SynComs), which are emerging as a promising tool that can allow the formulation of biofertilizers based on PGPR strains displaying multifarious and synergistic traits, thus leading to an increasingly efficient root acquisition of more than a single essential nutrient at the same time. Additionally, host-mediated microbiome engineering (HMME) leverages advanced omics techniques to reintroduce alleles coding for beneficial compounds, reinforcing positive plant-microbiome interactions and creating plants capable of producing their own biofertilizers. We also discusses the current use of PGPR-based biofertilizers and point out possible avenues of research for the future development of more efficient biofertilizers for a smarter and more precise crop fertilization. Furthermore, concerns have been raised about the effectiveness of PGPR-based biofertilizers in real field conditions, as their success in controlled experiments often contrasts with inconsistent field results. This discrepancy highlights the need for standardized protocols to ensure consistent application and reliable outcomes.

Employment of pqqE gene as molecular marker for the traceability of Gram negative phosphate solubilizing bacteria associated to plants

Insoluble phosphorous compounds solubilization by soil bacteria is of great relevance since it puts available the phosphorus to be used by plants. The production of organic acids is the main microbiological mechanism by which insoluble inorganic phosphorus compounds are solubilized. In Gram negative bacteria, gluconic acid is synthesized by the activity of the holoenzyme glucose dehydrogenase-pyrroloquinoline quinine named GDH-PQQ. The use of marker genes is a very useful tool to evaluate the persistence of the introduced bacteria and allow to follow-up the effect of biotic and abiotic factors on these beneficial microorganisms in the soil. In previous studies we detected the presence of the pqqE gene in a great percentage of both non-culturable and culturable native soil bacteria. The objective of this study was to analyze the phylogeny of the sequence of pqqE gene and its potential for the study of phosphate solubilizing bacteria from pure and mixed bacterial cultures and rhizospheric soil samples. For this, the presence of the pqqE gene in the genome of phosphate solubilizing bacteria that belong to several bacteria was determined by PCR. Also, this gene was analyzed from mixed bacterial cultures and rhizospheric soil associated to peanut plants inoculated or not with phosphate solubilizing bacteria. For this, degenerate primers designed from several bacterial genera and specific primers for the genus Pseudomonas spp., designed in this study, were used. DNA template used from simple or mixed bacterial cultures and from rhizospheric soil samples was obtained using two different DNA extraction techniques. Results indicated that pqqE gene amplification product was found in the genome of all Gram negative phosphate solubilizing bacteria analyzed. It was possible to detect this gene in the DNA obtained from mixed cultures where these bacteria grew in interaction with other microorganisms and in that obtained from rhizospheric soil samples inoculated or not with these bacteria. The phylogenetic analysis indicated that pqqE gene is a conserved gene within related genera. In conclusion, pqqE gene could be a potential marker for the study of phosphate solubilizing bacterial populations.

Polyextremophile engineering: a review of organisms that push the limits of life

Nature exhibits an enormous diversity of organisms that thrive in extreme environments. From snow algae that reproduce at sub-zero temperatures to radiotrophic fungi that thrive in nuclear radiation at Chernobyl, extreme organisms raise many questions about the limits of life. Is there any environment where life could not "find a way"? Although many individual extremophilic organisms have been identified and studied, there remain outstanding questions about the limits of life and the extent to which extreme properties can be enhanced, combined or transferred to new organisms. In this review, we compile the current knowledge on the bioengineering of extremophile microbes. We summarize what is known about the basic mechanisms of extreme adaptations, compile synthetic biology's efforts to engineer extremophile organisms beyond what is found in nature, and highlight which adaptations can be combined. The basic science of extremophiles can be applied to engineered organisms tailored to specific biomanufacturing needs, such as growth in high temperatures or in the presence of unusual solvents.

Elucidation of genes enhancing natural product biosynthesis through co-evolution analysis

Streptomyces has the largest repertoire of natural product biosynthetic gene clusters (BGCs), yet developing a universal engineering strategy for each Streptomyces species is challenging. Given that some Streptomyces species have larger BGC repertoires than others, we proposed that a set of genes co-evolved with BGCs to support biosynthetic proficiency must exist in those strains, and that their identification may provide universal strategies to improve the productivity of other strains. We show here that genes co-evolved with natural product BGCs in Streptomyces can be identified by phylogenomics analysis. Among the 597 genes that co-evolved with polyketide BGCs, 11 genes in the 'coenzyme' category have been examined, including a gene cluster encoding for the cofactor pyrroloquinoline quinone. When the pqq gene cluster was engineered into 11 Streptomyces strains, it enhanced production of 16,385 metabolites, including 36 known natural products with up to 40-fold improvement and several activated silent gene clusters. This study provides an innovative engineering strategy for improving polyketide production and finding previously unidentified BGCs.

Long-Term Organic Fertilization Strengthens the Soil Phosphorus Cycle and Phosphorus Availability by Regulating the pqqC- and phoD-Harboring Bacterial Communities

The pqqC and phoD genes encode pyrroloquinoline quinone synthase and alkaline phosphomonoesterase (ALP), respectively. These genes play a crucial role in regulating the solubilization of inorganic phosphorus (Pi) and the mineralization of organic phosphorus (Po), making them valuable markers for P-mobilizing bacterial. However, there is limited understanding of how the interplay between soil P-mobilizing bacterial communities and abiotic factors influences P transformation and availability in the context of long-term fertilization scenarios. We used real-time polymerase chain reaction and high-throughput sequencing to explore the characteristics of soil P-mobilizing bacterial communities and their relationships with key physicochemical properties and P fractions under long-term fertilization scenarios. In a 38-year fertilization experiment, six fertilization treatments were selected. These treatments were sorted into three groups: the non-P-amended group, including no fertilization and mineral NK fertilizer; the sole mineral-P-amended group, including mineral NP and NPK fertilizer; and the organically amended group, including sole organic fertilizer and organic fertilizer plus mineral NPK fertilizer. The organically amended group significantly increased soil labile P (Ca2-P and enzyme-P) and Olsen-P content and proportion but decreased non-labile P (Ca10-P) proportion compared with the sole mineral-P-amended group, indicating enhanced P availability in the soil. Meanwhile, the organically amended group significantly increased soil ALP activity and pqqC and phoD gene abundances, indicating that organic fertilization promotes the activity and abundance of microorganisms involved in P mobilization processes. Interestingly, the organically amended group dramatically reshaped the community structure of P-mobilizing bacteria and increased the relative abundance of Acidiphilium, Panacagrimonas, Hansschlegelia, and Beijerinckia. These changes had a greater positive impact on ALP activity, labile P, and Olsen-P content compared to the abundance of P-mobilizing genes alone, indicating their importance in driving P mobilization processes. Structural equation modeling indicated that soil organic carbon and Po modulated the relationship between P-mobilizing bacterial communities and labile P and Olsen-P, highlighting the influence of SOC and Po on the functioning of P-mobilizing bacteria and their impact on P availability. Overall, our study demonstrates that organic fertilization has the potential to reshape the structure of P-mobilizing bacterial communities, leading to increased P mobilization and availability in the soil. These findings contribute to our understanding of the mechanisms underlying P cycling in agricultural systems and provide valuable insights for enhancing microbial P mobilization through organic fertilization.

Roles of phosphate-solubilizing bacteria in mediating soil legacy phosphorus availability

Phosphorus (P), an essential macronutrient for all life on Earth, has been shown to be a vital limiting nutrient element for plant growth and yield. P deficiency is a common phenomenon in terrestrial ecosystems across the world. Chemical phosphate fertilizer has traditionally been employed to solve the problem of P deficiency in agricultural production, but its application has been limited by the non-renewability of raw materials and the adverse influence on the ecological health of the environment. Therefore, it is imperative to develop efficient, economical, environmentally friendly and highly stable alternative strategies to meet the plant P demand. Phosphate-solubilizing bacteria (PSB) are able to improve plant productivity by increasing P nutrition. Pathways to fully and effectively use PSB to mobilize unavailable forms of soil P for plants has become a hot research topic in the fields of plant nutrition and ecology. Here, the biogeochemical P cycling in soil systems are summarized, how to make full use of soil legacy P via PSB to alleviate the global P resource shortage are reviewed. We highlight the advances in multi-omics technologies that are helpful for exploring the dynamics of nutrient turnover and the genetic potential of PSB-centered microbial communities. Furthermore, the multiple roles of PSB inoculants in sustainable agricultural practices are analyzed. Finally, we project that new ideas and techniques will be continuously infused into fundamental and applied research to achieve a more integrated understanding of the interactive mechanisms of PSB and rhizosphere microbiota/plant to maximize the efficacy of PSB as P activators.

Temperature-induced modulation of stress-tolerant PGP genes bioprospected from Bacillus sp. IHBT-705 associated with saffron (Crocus sativus) rhizosphere: A natural -treasure trove of microbial biostimulants

There is a renewed interest in sustainable agriculture wherein novel plant growth-promoting rhizobacteria (PGPR) are being explored for developing efficient biostimulants. The key requirement of a microbe to qualify as a good candidate for developing a biostimulant is its intrinsic plant growth-promoting (PGP) characteristics. Though numerous studies have been conducted to assess the beneficial effects of PGPRs on plant growth under normal and stressed conditions but not much information is available on the characterization of intrinsic traits of PGPR under stress. Here, we focused on understanding how temperature stress impacts the functionality of key stress tolerant and PGP genes of Bacillus sp. IHBT-705 isolated from the rhizosphere of saffron (Crocus sativus). To undertake the study, Bacillus sp. IHBT-705 was grown under varied temperature regimes, their PGP traits were assessed from very low to very high-temperature range and the expression trend of targeted stress tolerant and PGP genes were analyzed. The results illustrated that Bacillus sp. IHBT-705 is a stress-tolerant PGPR as it survived and multiplied in temperatures ranging from 4°C-50°C, tolerated a wide pH range (5-11), withstood high salinity (8%) and osmolarity (10% PEG). The PGP traits varied under different temperature regimes indicating that temperature influences the functionality of PGP genes. This was further ascertained through whole genome sequencing followed by gene expression analyses wherein certain genes like cspB, cspD, hslO, grpE, rimM, trpA, trpC, trpE, fhuC, fhuD, acrB5 were found to be temperature sensitive while, cold tolerant (nhaX and cspC), heat tolerant (htpX) phosphate solubilization (pstB1), siderophore production (fhuB and fhuG), and root colonization (xerC1 and xerC2) were found to be highly versatile as they could express well both under low and high temperatures. Further, the biostimulant potential was checked through a pot study on rice (Oryza sativa), wherein the application of Bacillus sp. IHBT-705 improved the length of shoots, roots, and number of roots over control. Based on the genetic makeup, stress tolerance potential, retention of PGP traits under stress, and growth-promoting potential, Bacillus sp. IHBT-705 could be considered a good candidate for developing biostimulants.

Insights into Genomic Evolution and the Potential Genetic Basis of Klebsiella variicola subsp. variicola ZH07 Reveal Its Potential for Plant Growth Promotion and Autotoxin Degradation

The accumulation of autotoxins in soil causes continuous cropping obstacle stress in crops, and the bioremediation of autotoxins by microorganisms is an efficient process. In this study, strain ZH07 was isolated from the peanut rhizosphere and was found to be utilizing multiple autotoxins as its carbon sources. Based on its genomic characteristics and a phylogenetic analysis, ZH07 represents a member of Klebsiella variicola subsp. variicola. A comparative genomic analysis exhibited evolutionary dynamics exhibited by mobile genetic elements (MGEs), strain-specific genes, potential horizontal genes, and evolutionary constraints driven by purifying selection, which facilitated its genomic adaptation to rhizosphere soil. Genome mining revealed the potential genomic properties associated with plant growth promotion, such as nitrogen fixation, indole acetic acid synthesis, phosphonate solubilization and assimilation, siderophore production, and secondary metabolite synthesis. Moreover, abundant genes putatively responsible for the biodegradation of aromatic xenobiotics, including benzoic acid, cinnamic acid, vanillic acid, protocatechuic acid, phenylacetic acid, and p-hydroxybenzoic acid were also observed in the ZH07 genome. Compared to autotoxin stress alone, the combination of ZH07 application promoted peanut germination and seedling growth. Our analysis revealed the genetic adaptation of ZH07 to the rhizosphere environment and the potential genetic basis and effectiveness of the isolate to serve as a plant growth stimulator. IMPORTANCE Continuous cropping obstacles reduce the production and quality of agricultural products, and the application of rhizosphere beneficial microbes is an important strategy. Strain ZH07 showed autotoxin-degrading and plant growth-promoting capacities. The objectives of this study were to characterize its genomic evolution and the potential genetic basis of the autotoxin degradation and plant growth promotion. ZH07 represents a member of Klebsiella variicola subsp. variicola, based on genomic and phylogenetic analyses. Its genomic components have undergone different degrees of purifying selection, and the disparity in the evolutionary rate may be associated with its niche adaptation. A systematic analysis of the ZH07 genome identified the potential genetic basis that contributes to plant growth promotion and to aromatic xenobiotic biodegradation. This study demonstrates that plant growth-promoting rhizobacteria (PGPR) play important roles in autotoxin biodegradation and can be used as biofertilizers to enhance the growth of peanuts in response to continuous cropping obstacle stress.

Fungi, P-Solubilization, and Plant Nutrition

The application of plant beneficial microorganisms is widely accepted as an efficient alternative to chemical fertilizers and pesticides. It was shown that annually, mycorrhizal fungi and nitrogen-fixing bacteria are responsible for 5 to 80% of all nitrogen, and up to 75% of P plant acquisition. However, while bacteria are the most studied soil microorganisms and most frequently reported in the scientific literature, the role of fungi is relatively understudied, although they are the primary organic matter decomposers and govern soil carbon and other elements, including P-cycling. Many fungi can solubilize insoluble phosphates or facilitate P-acquisition by plants and, therefore, form an important part of the commercial microbial products, with Aspergillus, Penicillium and Trichoderma being the most efficient. In this paper, the role of fungi in P-solubilization and plant nutrition will be presented with a special emphasis on their production and application. Although this topic has been repeatedly reviewed, some recent views questioned the efficacy of the microbial P-solubilizers in soil. Here, we will try to summarize the proven facts but also discuss further lines of research that may clarify our doubts in this field or open new perspectives on using the microbial and particularly fungal P-solubilizing potential in accordance with the principles of the sustainability and circular economy.

Diverse responses of pqqC- and phoD-harbouring bacterial communities to variation in soil properties of Moso bamboo forests

Phosphate‐mobilizing bacteria (PMB) play a critical role in the regulation of phosphorus availability in the soil. The microbial genes pqqC and phoD encode pyrroloquinoline quinone synthase and bacterial alkaline phosphatase, respectively, which regulate inorganic and organic phosphorus mobilization, and are therefore used as PMB markers. We examined the effects of soil properties in three Moso bamboo forest sites on the PMB communities that were profiled using high‐throughput sequencing. We observed differentiated responses of pqqC‐ and phoD‐harbouring PMB communities to various soil conditions. There was significant variation among the sites in the diversity and structure of the phoD‐harbouring community, which correlated with variation in phosphorus levels and non‐capillary porosity; soil organic carbon and soil water content also affected the structure of the phoD‐harbouring community. However, no significant difference in the diversity of pqqC‐harbouring community was observed among different sites, while the structure of the pqqC‐harbouring bacteria community was affected by soil organic carbon and soil total nitrogen, but not soil phosphorus levels. Overall, changes in soil conditions affected the phoD‐harbouring community more than the pqqC‐harbouring community. These findings provide a new insight to explore the effects of soil conditions on microbial communities that solubilize inorganic phosphate and mineralize organic phosphate.

Growth promotion ability of phosphate-solubilizing bacteria from the soybean rhizosphere under maize-soybean intercropping systems

BACKGROUND

Optimum cultivation and management measures are needed to increase the phosphorus (P) absorption efficiency of crops for sustainable agricultural production. Previous studies indicated that leguminous crops can promote P absorption by neighboring gramineous crops. In this study, we isolated and screened the phosphate-solubilizing bacteria (PSB) from soybean rhizosphere under a maize-soybean intercropping system in Southwest China, and nine PSBs with high P-solubilizing ability were identified.

RESULTS

16S rDNA sequencing and phylogenetic analysis showed that these PSBs belong mainly to Bacillus and Pseudomonas. The phosphate solubility of Bacillus aryabhattai B8W22 reached 388.62 μg mL-1 . High-performance liquid chromatographic analysis showed that each strain could secrete a large quantity of organic acids, including oxalic acid, malonic acid, citric acid and succinic acid. In addition, all strains produced indole acetic acid (IAA) and siderophores that could promote plant growth. Seed germination experiments testified that PSBs isolated in this study have an innate ability to promote plant growth. The plant culture pot experiment further illustrated that soil acid phosphatase (ACP) activity and available P content, as well as plant P uptake, increased significantly with PSBs inoculation.

CONCLUSION

PSBs from the rhizosphere soil of intercropped soybean could secrete organic acids that increase the solubilization of unavailable P, improve soil ACP activity and P availability, and produce IAA and siderophores that promote maize seed germination and seedling growth. Our findings indicate the PSBs from soybean rhizosphere have significant potential to reduce the application of chemical phosphate fertilizers and to promote sustainable agricultural development. © 2021 Society of Chemical Industry.

Contrasting effects of carbon source recalcitrance on soil phosphorus availability and communities of phosphorus solubilizing microorganisms

Carbon (C) additions to soil interact through chemical and microbiological processes to cause changes in soil phosphorus (P) availability. However, the response of soil P transformations and relevant microbial communities to C additions having different degrees of recalcitrance remains uncertain. We studied the effects of glucose, hemicellulose and lignin addition on soil P availability, P transformation processes and relevant microbial activity and communities in a P-deficient flooded soil. Lignin significantly increased soil available P concentrations, which was attributed to chemical release of inorganic P and increased alkaline phosphatase activity. Glucose and hemicellulose additions stimulated microbial metabolism of C thereby enhancing microbial demand for P, with increased soil P availability especially in the early incubation period. Glucose or hemicellulose addition changed soil microbial diversity and community composition, leading to enhanced growth and interactions of P solubilizing microorganisms such as Desulfitobacterium, Bacillus and Desulfosporosinus. Our results infer the importance of pH alteration and competitive sorption between PO₄ and functional groups of recalcitrant C (e.g., lignin) with Fe/Al (hydr) oxides in regulating soil P availability. Further, the microbial response to labile C additions led to increased P availability in the P-deficient soil. This study provides important mechanistic information to guide microbially-regulated soil P management in agricultural ecosystems.

Phosphate-Solubilizing Bacterium Acinetobacter pittii gp-1 Affects Rhizosphere Bacterial Community to Alleviate Soil Phosphorus Limitation for Growth of Soybean (Glycine max)

Phosphorus (P) availability is a major restriction to crop production, and phosphate-solubilizing bacteria (PSBs) in soils are responsible for P turnover. However, it remains unknown whether the application of PSB can facilitate both inorganic and organic P transformation and enhance function of plant rhizosphere bacteria. In this study, we applied Illumina MiSeq sequencing, plate-colony counting, quantitative PCR, and multiple ecological analyses. We found that the inoculation of PSB Acinetobacter pittii gp-1 significantly promoted the growth of soybean represented by better vegetation properties (e.g., plant height and root P) and increased activities of phosphatase (4.20–9.72 μg/g/h) and phytase (0.69–1.53 μmol/g/day) as well as content of indole acetic acid (5.80–40.35 μg/g/h). Additionally, the application of strain A. pittii gp-1 significantly increased abundances of both inorganic and organic P-cycling-related genes (i.e., phoD, bpp, gcd, and pstS). More importantly, the application of A. pittii gp-1 could increase the function represented by P-cycling-related enzymes (e.g., phosphotransferase) of rhizosphere bacterial community based on functional profiling. To our knowledge, this is the first report that the application of PSB A. pittii promotes inorganic and organic P utilization and increases the function of rhizosphere bacterial community. Therefore, the PSB A. pittii gp-1 could be a good candidate for the promotion of soybean growth.

A Combination of Genomics, Transcriptomics, and Genetics Provides Insights into the Mineral Weathering Phenotype of Pseudomonas azotoformans F77

Silicate mineral weathering (dissolution) plays important roles in soil formation and global biogeochemical cycling. In this study, a combination of genomics, transcriptomics, and genetics was used to identify the molecular basis of mineral weathering activity and acid tolerance in Pseudomonas azotoformans F77. Biotite was chosen as a silicate mineral to investigate mineral weathering. The genome of strain F77 was sequenced, and the genes significantly upregulated when grown in the presence of biotite included mineral weathering-related genes associated with gluconic acid metabolism, flagellar assembly, and pilus biosynthesis and acid tolerance-related genes associated with neutralizing component production, reducing power, and proton efflux. Then, the biotite-weathering behaviors of strain F77 and its mutants that were created by deleting the tkt, tal, gntP, potF, nuoF, and gdtO genes, which are involved in gluconic acid metabolism and acid tolerance, respectively, were determined. The Fe and Al concentrations in the strain F77-inoculated medium increased 2.2- to 13.7-fold compared to the controls. The cell numbers of strain F77 increased over time, while the pH values in the medium ranged from 3.75 to 3.90 between 20 and 36 h of incubation. The release of Al and Fe was significantly reduced in the mutants F77Δtal, F77ΔgntP, F77ΔpotF, and F77ΔnuoF. Bacterial growth was significantly reduced in the presence of biotite in the mutants F77ΔpotF and F77ΔnuoF. Our results demonstrated the acid tolerance of strain F77 and suggested that multiple genes and metabolic pathways in strain F77 are involved in biotite weathering and acid tolerance during the mineral weathering process. IMPORTANCE Acid production and tolerance play important roles in effective and persistent mineral weathering in bacteria, although the molecular mechanisms governing acid production and acid tolerance in bacteria have not been fully elucidated. In this study, the molecular mechanisms underlying biotite (as a silicate mineral) weathering (dissolution) and acid tolerance of P. azotoformans F77 were characterized using genomics, transcriptomics, and genetics analyses. Our results showed that the genes and metabolic pathways for gluconic acid metabolism, flagellar assembly, and pilus biosynthesis may play important roles in mineral weathering by strain F77. Notably, the genes associated with neutralizing component production, reducing power, and proton efflux may be related to acid tolerance in strain F77. The expression of these acid production- and acid tolerance-related genes was observed to be increased by biotite in strain F77. Our findings may help to elucidate the molecular mechanisms governing mineral weathering and, especially, acid tolerance in mineral-weathering bacteria.

Herbaspirillum seropedicae expresses non-phosphorylative pathways for D-xylose catabolism

Herbaspirillum seropedicae is a β-proteobacterium that establishes as an endophyte in various plants. These bacteria can consume diverse carbon sources, including hexoses and pentoses like D-xylose. D-xylose catabolic pathways have been described in some microorganisms, but databases of genes involved in these routes are limited. This is of special interest in biotechnology, considering that D-xylose is the second most abundant sugar in nature and some microorganisms, including H. seropedicae, are able to accumulate poly-3-hydroxybutyrate when consuming this pentose as a carbon source. In this work, we present a study of D-xylose catabolic pathways in H. seropedicae strain Z69 using RNA-seq analysis and subsequent analysis of phenotypes determined in targeted mutants in corresponding identified genes. G5B88_22805 gene, designated xylB, encodes a NAD⁺-dependent D-xylose dehydrogenase. Mutant Z69∆xylB was still able to grow on D-xylose, although at a reduced rate. This appears to be due to the expression of an L-arabinose dehydrogenase, encoded by the araB gene (G5B88_05250), that can use D-xylose as a substrate. According to our results, H. seropedicae Z69 uses non-phosphorylative pathways to catabolize D-xylose. The lower portion of metabolism involves co-expression of two routes: the Weimberg pathway that produces α-ketoglutarate and a novel pathway recently described that synthesizes pyruvate and glycolate. This novel pathway appears to contribute to D-xylose metabolism, since a mutant in the last step, Z69∆mhpD, was able to grow on this pentose only after an extended lag phase (40-50 h). KEY POINTS: • xylB gene (G5B88_22805) encodes a NAD⁺-dependent D-xylose dehydrogenase. • araB gene (G5B88_05250) encodes a L-arabinose dehydrogenase able to recognize D-xylose. • A novel route involving mhpD gene is preferred for D-xylose catabolism.

Genomic assessment of Stenotrophomonas indicatrix for improved sunflower plant

Diverse agriculturally important microbes have been studied with known potential in plant growth promotion. Providing several opportunities, Stenotrophomonas species are characterized as promising plant enhancers, inducers, and protectors against environmental stressors. The S. indicatrix BOVIS40 isolated from the sunflower root endosphere possessed unique features, as genome insights into the Stenotrophomonas species isolated from oilseed crops in Southern Africa have not been reported. Plant growth-promotion screening and genome analysis of S. indicatrix BOVIS40 were presented in this study. The genomic information reveals various genes underlining plant growth promotion and resistance to environmental stressors. The genome of S. indicatrix BOVIS40 harbors genes involved in the degradation and biotransformation of organic molecules. Also, other genes involved in biofilm production, chemotaxis, and flagellation that facilitate bacterial colonization in the root endosphere and phytohormone genes that modulate root development and stress response in plants were detected in strain BOVIS40. IAA activity of the bacterial strain may be a factor responsible for root formation. A measurable approach to the S. indicatrix BOVIS40 lifestyle can strategically provide several opportunities in their use as bioinoculants in developing environmentally friendly agriculture sustainably. The findings presented here provide insights into the genomic functions of S. indicatrix BOVIS40, which has set a foundation for future comparative studies for a better understanding of the synergism among microbes inhabiting plant endosphere. Hence, highlighting the potential of S. indicatrix BOVIS40 upon inoculation under greenhouse experiment, thus suggesting its application in enhancing plant and soil health sustainably.

Integrated Genomic and Greenhouse Assessment of a Novel Plant Growth-Promoting Rhizobacterium for Tomato Plant

Plant growth promoting rhizobacteria (PGPR) can display several plant-beneficial properties, including support to plant nutrition, regulation of plant growth, and biocontrol of pests. Mechanisms behind these effects are directly related to the presence and expression of specific genes, and different PGPR strains can be differentiated by the presence of different genes. In this study we reported a comprehensive evaluation of a novel PGPR Klebsiella variicola UC4115 from the field to the lab, and from the lab to the plant. The isolate from tomato field was screened in-vitro for different activities related to plant nutrition and growth regulation as well as for antifungal traits. We performed a functional annotation of genes contributing to plant-beneficial functions previously tested in-vitro. Furthermore, the in-vitro characterization, the whole genome sequencing and annotation of K. variicola UC4115, were compared with the well-known PGPR Azospirillum brasilense strain Sp7. This novel comparative analysis revealed different accumulation of plant-beneficial functions contributing genes, and the presence of different genes that accomplished the same functions. Greenhouse assays on tomato seedlings from BBCH 11-12 to BBCH > 14 were performed under either organic or conventional management. In each of them, three PGPR inoculations (control, K. variicola UC4115, A. brasilense Sp7) were applied at either seed-, root-, and seed plus root level. Results confirmed the PGP potential of K. variicola UC4115; in particular, its high value potential as indole-3-acetic acid producer was observed in increasing of root length density and diameter class length parameters. While, in general, A. brasilense Sp7 had a greater effect on biomass, probably due to its high ability as nitrogen-fixing bacteria. For K. variicola UC4115, the most consistent data were noticed under organic management, with application at seed level. While, A. brasilense Sp7 showed the greatest performance under conventional management. Our data highlight the necessity to tailor the selected PGPR, with the mode of inoculation and the crop-soil combination.

Use of Mineral Weathering Bacteria to Enhance Nutrient Availability in Crops: A Review

Rock powders are low-cost potential sources of most of the nutrients required by higher plants for growth and development. However, slow dissolution rates of minerals represent an obstacle to the widespread use of rock powders in agriculture. Rhizosphere processes and biological weathering may further enhance mineral dissolution since the interaction between minerals, plants, and bacteria results in the release of macro- and micronutrients into the soil solution. Plants are important agents in this process acting directly in the mineral dissolution or sustaining a wide diversity of weathering microorganisms in the root environment. Meanwhile, root microorganisms promote mineral dissolution by producing complexing ligands (siderophores and organic acids), affecting the pH (via organic or inorganic acid production), or performing redox reactions. Besides that, a wide variety of rhizosphere bacteria and fungi could also promote plant development directly, synergistically contributing to the weathering activity performed by plants. The inoculation of weathering bacteria in soil or plants, especially combined with the use of crushed rocks, can increase soil fertility and improve crop production. This approach is more sustainable than conventional fertilization practices, which may contribute to reducing climate change linked to agricultural activity. Besides, it could decrease the dependency of developing countries on imported fertilizers, thus improving local development.

Cadmium Immobilization in the Rhizosphere and Plant Cellular Detoxification: Role of Plant-Growth-Promoting Rhizobacteria as a Sustainable Solution

Food is the major cadmium (Cd)-exposure pathway from agricultural soils to humans and other living entities and must be reduced in an effective way. A plant can select beneficial microbes, like plant-growth-promoting rhizobacteria (PGPR), depending upon the nature of root exudates in the rhizosphere, for its own benefits, such as plant growth promotion as well as protection from metal toxicity. This review intends to seek out information on the rhizo-immobilization of Cd in polluted soils using the PGPR along with plant nutrient fertilizers. This review suggests that the rhizo-immobilization of Cd by a combination of PGPR and nanohybrid-based plant nutrient fertilizers would be a potential and sustainable technology for phytoavailable Cd immobilization in the rhizosphere and plant cellular detoxification, by keeping the plant nutrition flow and green dynamics of plant nutrition and boosting the plant growth and development under Cd stress.

Tools for successful proliferation: diverse strategies of nutrient acquisition by a benthic cyanobacterium

Freshwater cyanobacterial blooms have increased worldwide, channeling organic carbon into these systems, and threatening animal health through the production of cyanotoxins. Both toxic and nontoxic Microcoleus proliferations usually occur when there are moderate concentrations of dissolved inorganic nitrogen, but when phosphorus is scarce. In order to understand how Microcoleus establishes thick biofilms (or mats) on riverbeds under phosphorus-limiting conditions, we collected Microcoleus-dominated biofilms over a 19-day proliferation event for proteogenomics. A single pair of nitrogen-dependent Microcoleus species were consistently present in relatively high abundance, although each followed a unique metabolic trajectory. Neither possessed anatoxin gene clusters, and only very low concentrations of anatoxins (~2 µg kg-1) were detected, likely originating from rarer Microcoleus species also present. Proteome allocations were dominated by photosynthesizing cyanobacteria and diatoms, and data indicate biomass was actively recycled by Bacteroidetes and Myxococcales. Microcoleus likely acquired nutrients throughout the proliferation event by uptake of nitrate, urea, and inorganic and organic phosphorus. Both species also harbored genes that could be used for inorganic phosphate solubilization with pyrroloquinoline quinone cofactors produced by cohabiting Proteobacteria. Results indicate that Microcoleus are equipped with diverse mechanisms for nitrogen and phosphorus acquisition, enabling them to proliferate and out-compete others in low-phosphorus waters.

Isolation and Characterization of Phosphorus Solubilizing Bacteria With Multiple Phosphorus Sources Utilizing Capability and Their Potential for Lead Immobilization in Soil

Phosphorus solubilizing bacteria (PSB) can promote the level of plant-absorbable phosphorus (P) in agro-ecosystems. However, little attention has been paid to PSB harboring abilities in utilizing multiple phosphorus sources and their potentials for heavy metal immobilization. In this study, we applied the strategy of stepwise acclimation by using Ca₃(PO₄)₂, phytate, FePO₄, and AlPO₄ as sole P source. We gained 18 PSB possessing abilities of multiple P sources utilization, and these bacteria belonged to eight genera (Acinetobacter, Pseudomonas, Massilia, Bacillus, Arthrobacter, Stenotrophomonas, Ochrobactrum, and Cupriavidus), and clustered to two apparent parts: Gram-positive bacteria and Gram-negative bacteria. The isolate of Acinetobacter pittii gp-1 presented good performance for utilizing Ca₃(PO₄)₂, FePO₄, AlPO₄, and phytate, with corresponding P solubilizing levels were 250.77, 46.10, 81.99, and 7.91 mg/L PO₄^3–-P, respectively. The PSB A. pittii gp-1 exhibited good performance for solubilizing tricalcium phosphate in soil incubation experiments, with the highest values of water soluble P and available P were 0.80 and 1.64 mg/L, respectively. Additionally, the addition of A. pittii gp-1 could promote the immobilization of lead (Pb), and the highest Pb immobilization efficiency reached 23%. Simultaneously, we found the increases in abundances of both alkaline phosphatase gene (phoD) and β-propeller phytase gene (bpp) in strain gp-1 added soils. Besides, we observed the expression up-regulation of both pyrroloquinoline quinone gene (pqq) and polyphosphate kinases gene (ppk), with the highest relative expression levels of 18.18 and 5.23, respectively. We also found the polyphosphate particles using granule staining. To our knowledge, our findings first suggest that the solubilizing of tricalcium phosphate by phosphorus solubilizing bacterium belonging to Acinetobacter is coupled with the synthesis of polyphosphate. Taken together, A. pittii gp-1 could be a good candidate in improving soil fertility and quality.

Interactions between Biotite and the Mineral-Weathering Bacterium Pseudomonas azotoformans F77

In this study, the mineral-weathering bacterium Pseudomonas azotoformans F77, which was isolated from the soil of a debris flow area, was evaluated for its weathering activity under direct contact with biotite or without contact. Then, biotite-weathering behaviors of strain F77, mutants that had been created by deleting the gcd and adh genes (which are involved in gluconic acid metabolism and pilus formation, respectively), and the double mutant F77ΔgcdΔadh were compared. The relative gene expression levels of F77 and its mutants F77Δgcd and F77Δadh were also analyzed in the presence of biotite. Direct contact with biotite increased Fe and Al release from the mineral in the presence of F77. All strains had similar abilities to release Fe and Al from the mineral except for F77Δgcd and F77Δadh Mobilized Fe and Al concentrations were decreased by up to 72, 26, and 87% in the presence of F77Δgcd, F77Δadh, and F77ΔgcdΔadh, respectively, compared to levels observed in the presence of F77 during the mineral-weathering process. Gluconic acid production was decreased for F77Δgcd and F77ΔgcdΔadh, while decreased cell attachment on the mineral surface was observed for F77Δadh, compared to findings for F77. The F77 genes involved in pilus formation and gluconic acid metabolism showed increased expression levels in the presence of biotite. The results of this study showed important roles for the genes involved in gluconic acid metabolism and pilus formation in mineral weathering by F77 and demonstrated the distinctive effect of these genes on mineral weathering by F77.IMPORTANCE Bacteria play important roles in mineral weathering and soil formation, although the molecular mechanisms underlying the interactions between bacteria and silicate minerals are poorly understood. In this study, the interactions between biotite and the highly effective mineral-weathering bacterium P. azotoformans F77 were characterized. Our results showed that the genes involved in gluconic acid metabolism and pilus formation play important roles in mineral weathering by F77. The presence of biotite could promote the expression of these genes in F77, and a distinctive effect of these genes on mineral weathering by F77 was observed in this study. Our results provide new knowledge and promote better understanding regarding the interaction between silicate minerals and mineral-weathering bacteria, as well as the molecular mechanisms involved in these processes.

PQQ-GDH - Structure, function and application in bioelectrochemistry

This review summarizes the basic features of the PQQ-GDH enzyme as one of the sugar converting biocatalysts. Focus is on the membrane -bound and the soluble form. Furthermore, the main principles of enzymatic catalysis as well as studies on the physiological importance are reviewed. A short overview is given on developments in protein engineering. The major part, however, deals with the different fields of application in bioelectrochemistry. This includes approaches for enzyme-electrode communication such as direct electron transfer, mediator-based systems, redox polymers or conducting polymers and holoenzyme reconstitution, and covers applied areas such as biosensing, biofuel cells, recycling schemes, enzyme competition, light-directed sensing, switchable detection schemes, logical operations by enzyme electrodes and immune sensing.

Beneficial Endophytic Bacteria-Serendipita indica Interaction for Crop Enhancement and Resistance to Phytopathogens

Serendipita (=Piriformospora) indica is a fungal endophytic symbiont with the capabilities to enhance plant growth and confer resistance to different stresses. However, the application of this fungus in the field has led to inconsistent results, perhaps due to antagonism with other microbes. Here, we studied the impact of individual bacterial isolates from the endophytic bacterial community on the in vitro growth of S. indica. We further analyzed how combinations of bacteria and S. indica influence plant growth and protection against the phytopathogens Fusarium oxysporum and Rhizoctonia solani. Bacterial strains of the genera Bacillus, Enterobacter and Burkholderia negatively affected S. indica growth on plates, whereas Mycolicibacterium, Rhizobium, Paenibacillus strains and several other bacteria from different taxa stimulated fungal growth. To further explore the potential of bacteria positively interacting with S. indica, four of the most promising strains belonging to the genus Mycolicibacterium were selected for further experiments. Some dual inoculations of S. indica and Mycolicibacterium strains boosted the beneficial effects triggered by S. indica, further enhancing the growth of tomato plants, and alleviating the symptoms caused by the phytopathogens F. oxysporum and R. solani. However, some combinations of S. indica and bacteria were less effective than individual inoculations. By analyzing the genomes of the Mycolicibacterium strains, we revealed that these bacteria encode several genes predicted to be involved in the stimulation of S. indica growth, plant development and tolerance to abiotic and biotic stresses. Particularly, a high number of genes related to vitamin and nitrogen metabolism were detected. Taking into consideration multiple interactions on and inside plants, we showed in this study that some bacterial strains may induce beneficial effects on S. indica and could have an outstanding influence on the plant-fungus symbiosis.

Complete Genome Sequence of the Plant Growth-Promoting Bacterium Hartmannibacter diazotrophicus Strain E19T

Strain E19^T described as Hartmannibacter diazotrophicus gen. nov. sp. nov. was isolated from the rhizosphere of Plantago winteri from a natural salt meadow in a nature protection area. Strain E19^T is a plant growth-promoting rhizobacterium able to colonize the rhizosphere of barley and to promote its growth only under salt stress conditions. To gain insights into the genetic bases of plant growth promotion and its lifestyle at the rhizosphere under salty conditions, we determined the complete genome sequence using two complementary sequencing platforms (Ilumina MiSeq and PacBio RSII). The E19^T genome comprises one circular chromosome and one plasmid containing several genes involved in salt adaptation and genes related to plant growth-promoting traits under salt stress. Based on previous experiments, ACC deaminase activity was identified as a main mechanism of E19^T to promote plant growth under salt stress. Interestingly, no genes classically reported to encode for ACC deaminase activity are present. In general, the E19^T genome provides information to confirm, discover, and better understand many of its previously evaluated traits involved in plant growth promotion under salt stress. Furthermore, the complete E19^T genome sequence helps to define its previously reported unclear 16S rRNA gene-based phylogenetic affiliation. Hartmannibacter forms a distinct subcluster with genera Methylobrevis, Pleomorphomonas, Oharaeibacter, and Mongoliimonas subclustered with genera belonging to Rhizobiales.

Engineered Root Bacteria Release Plant-Available Phosphate from Phytate

Phosphate fertilizers are essential for high-yield agriculture yet are costly and environmentally damaging. Microbes that release soluble phosphate from naturally occurring sources in the soil are appealing, as they may reduce the need for such fertilizers. In this study, we used synthetic biology approaches to create a collection of engineered root-associated microbes with the ability to release phosphate from phytate. We demonstrate that these strains improve plant growth under phosphorus-limited conditions. This represents a first step in the development of phosphate-mining bacteria for future use in crop systems.

Microorganisms that release plant-available phosphate from natural soil phosphate stores may serve as biological alternatives to costly and environmentally damaging phosphate fertilizers. To explore this possibility, we engineered a collection of root bacteria to release plant-available orthophosphate from phytate, an abundant phosphate source in many soils. We identified 82 phylogenetically diverse phytase genes, refactored their sequences for optimal expression in Proteobacteria, and then synthesized and engineered them into the genomes of three root-colonizing bacteria. Liquid culture assays revealed 41 engineered strains with high levels of phytate hydrolysis. Among these, we identified 12 strains across three bacterial hosts that confer a growth advantage on the model plant Arabidopsis thaliana when phytate is the sole phosphate source. These data demonstrate that DNA synthesis approaches can be used to generate plant-associated strains with novel phosphate-solubilizing capabilities.

IMPORTANCE Phosphate fertilizers are essential for high-yield agriculture yet are costly and environmentally damaging. Microbes that release soluble phosphate from naturally occurring sources in the soil are appealing, as they may reduce the need for such fertilizers. In this study, we used synthetic biology approaches to create a collection of engineered root-associated microbes with the ability to release phosphate from phytate. We demonstrate that these strains improve plant growth under phosphorus-limited conditions. This represents a first step in the development of phosphate-mining bacteria for future use in crop systems.

Igneous phosphate rock solubilization by biofilm-forming mycorrhizobacteria and hyphobacteria associated with Rhizoglomus irregulare DAOM 197198

Biofilm formation on abiotic and biotic surfaces was studied with two hyphobacteria, strongly attached to the surface of the arbuscular mycorrhizal fungus (AMF) Rhizoglomus irregulare (Ri) DAOM 197198 and two mycorrhizobacteria, loosely attached to the roots of different mycorrhizal plants. When the sparingly soluble igneous phosphate rock (PR) from Quebec, or when the chemical hydroxyapatite were used as sole phosphorus (P) source, hyphobacteria Rhizobium miluonense Rm3 and Burkholderia anthina Ba8 produced significantly more biofilms than mycorrhizobacteria Rahnella sp. Rs11 and Burkholderia phenazinium Bph12, as indicated by the crystal violet assay or by quantifying biofilm exopolysaccharides. As previously observed with planktonic bacteria, biofilms mobilized P by lowering the pH and releasing gluconic acid. The high efficiency of P mobilization by the hyphobacteria Ba8 was linked to the presence of more viable cells in its biofilm as revealed by the hydrolysis of fluorescein diacetate. Scanning electron microscopy micrographs showed a high adherence of the best P-solubilizer hyphobacteria Ba8 on the surface of Quebec PR. Hydroxyapatite porous structure did not allow a good adherence of Ba8. Ba8 formed an important biofilm on the hyphae of Ri DAOM 197198 with low reactive Quebec PR while no biofilm was observed with the high reactive hydroxyapatite. Results confirm the possible presence of specificity between the Ri DAOM 197198 and the hyphobacteria and suggest that the interaction would be regulated by the availability of P.

Whole genome analysis of halotolerant and alkalotolerant plant growth-promoting rhizobacterium Klebsiella sp. D5A

This research undertook the systematic analysis of the Klebsiella sp. D5A genome and identification of genes that contribute to plant growth-promoting (PGP) traits, especially genes related to salt tolerance and wide pH adaptability. The genome sequence of isolate D5A was obtained using an Illumina HiSeq 2000 sequencing system with average coverages of 174.7× and 200.1× using the paired-end and mate-pair sequencing, respectively. Predicted and annotated gene sequences were analyzed for similarity with the Kyoto Encyclopedia of Genes and Genomes (KEGG) enzyme database followed by assignment of each gene into the KEGG pathway charts. The results show that the Klebsiella sp. D5A genome has a total of 5,540,009 bp with 57.15% G + C content. PGP conferring genes such as indole-3-acetic acid (IAA) biosynthesis, phosphate solubilization, siderophore production, acetoin and 2,3-butanediol synthesis, and N2 fixation were determined. Moreover, genes putatively responsible for resistance to high salinity including glycine-betaine synthesis, trehalose synthesis and a number of osmoregulation receptors and transport systems were also observed in the D5A genome together with numerous genes that contribute to pH homeostasis. These genes reveal the genetic adaptation of D5A to versatile environmental conditions and the effectiveness of the isolate to serve as a plant growth stimulator.

Engineering Escherichia coli for methanol conversion

Methylotrophic bacteria utilize methanol and other reduced one-carbon compounds as their sole source of carbon and energy. For this purpose, these bacteria evolved a number of specialized enzymes and pathways. Here, we used a synthetic biology approach to select and introduce a set of "methylotrophy genes" into Escherichia coli based on in silico considerations and flux balance analysis to enable methanol dissimilation and assimilation. We determined that the most promising approach allowing the utilization of methanol was the implementation of NAD-dependent methanol dehydrogenase and the establishment of the ribulose monophosphate cycle by expressing the genes for hexulose-6-phosphate synthase (Hps) and 6-phospho-3-hexuloisomerase (Phi). To test for the best-performing enzymes in the heterologous host, a number of enzyme candidates from different donor organisms were selected and systematically analyzed for their in vitro and in vivo activities in E. coli. Among these, Mdh2, Hps and Phi originating from Bacillus methanolicus were found to be the most effective. Labeling experiments using (13)C methanol with E. coli producing these enzymes showed up to 40% incorporation of methanol into central metabolites. The presence of the endogenous glutathione-dependent formaldehyde oxidation pathway of E. coli did not adversely affect the methanol conversion rate. Taken together, the results of this study represent a major advancement towards establishing synthetic methylotrophs by gene transfer.

Artificial citrate operon confers mineral phosphate solubilization ability to diverse fluorescent pseudomonads

Citric acid is a strong acid with good cation chelating ability and can be very efficient in solubilizing mineral phosphates. Only a few phosphate solubilizing bacteria and fungi are known to secrete citric acids. In this work, we incorporated artificial citrate operon containing NADH insensitive citrate synthase (gltA1) and citrate transporter (citC) genes into the genome of six-plant growth promoting P. fluorescens strains viz., PfO-1, Pf5, CHAO1, P109, ATCC13525 and Fp315 using MiniTn7 transposon gene delivery system. Comprehensive biochemical characterization of the genomic integrants and their comparison with plasmid transformants of the same operon in M9 minimal medium reveals the highest amount of ∼7.6±0.41 mM citric and 29.95±2.8 mM gluconic acid secretion along with ∼43.2±3.24 mM intracellular citrate without affecting the growth of these P. fluorescens strains. All genomic integrants showed enhanced citric and gluconic acid secretion on Tris-Cl rock phosphate (TRP) buffered medium, which was sufficient to release 200–1000 µM Pi in TRP medium. This study demonstrates that MPS ability could be achieved in natural fluorescent pseudomonads by incorporation of artificial citrate operon not only as plasmid but also by genomic integration.