Phosphate-Solubilizing Bacillus sp. Modulate Soil Exoenzyme Activities and Improve Wheat Growth.

Phosphorus (P) is a vital mineral nutrient in agriculture and its deficiency results in reduced growth, yield, and grain quality in cereals. Much of the applied P in agriculture becomes fixed in soils, limiting its accessibility to plants. Thus, investigating sustainable strategies to release fixed P resources and enhance plant uptake is crucial. This study explored how plant-associated bacteria employ phosphate solubilizing mechanisms to improve P availability. The growth patterns of four bacterial strains, namely Bacillus subtilis ZE15 and ZR3, along with Bacillus megaterium ZE32 and ZR19, were examined in Pikovskaya's broth culture with and without the addition of insoluble phosphorus (P). In the absence of P amendment, most strains reached a stationary growth phase by the fourth day. However, their responses diverged when exposed to P-amended media. Particularly, ZE15 demonstrated the highest P solubilization capability, achieving up to 130 µg mL-1 solubilization in vitro. All strains produced organic acids in Pikovskaya's broth culture. A comparison of the influence of Ca3(PO4)2 revealed significantly greater organic acid quantities in the presence of insoluble P. Notably, strain ZE15 exhibited the highest phosphate esterase activity (3.65 nmol g-1 dry matter), while strain ZE32 showed the highest ß-D glucosidase activity (2.81 nmol g-1 dry matter) in the presence of insoluble P. The ability of Bacillus species to solubilize P in combination with increased exoenzyme activity in the rhizosphere could be used in future studies to support P uptake through enhanced solubilization and mineralization.

Differential Mechanisms of Microbial As(III) and Sb(III) Oxidation and Their Contribution to Tailings Reclamation.

Mine tailings are extremely oligotrophic environments frequently contaminated with elevated As and Sb, making As(III) and Sb(III) oxidation potentially important energy sources for the tailing microbiome. Although they have been proposed to share similar metabolic pathways, a systemic comparison of the As(III) and Sb(III) oxidation mechanisms and energy utilization efficiencies requires further elucidation. In this study, we employed a combination of physicochemical, molecular, and bioinformatic analyses to compare the kinetic and genetic mechanisms of As(III) and Sb(III) oxidation as well as their respective energy efficiencies for fueling the key nutrient acquisition metabolisms. Thiobacillus and Rhizobium spp. were identified as functional populations for both As(III) and Sb(III) oxidation in mine tailings by DNA-stable isotope probing. However, these microorganisms mediated As(III) and Sb(III) oxidation via different metabolic pathways, resulting in preferential oxidation of Sb(III) over As(III). Notably, both As(III) and Sb(III) oxidation can facilitate nitrogen fixation and phosphate solubilization in mine tailings, with Sb(III) oxidation being more efficient in powering these processes. Thus, this study provided novel insights into the microbial As(III) and Sb(III) oxidation mechanisms and their respective nutrient acquisition efficiencies, which may be critical for the reclamation of mine tailings.

Genomic insight of phosphate solubilization and plant growth promotion of two taxonomically distinct winter crops by Enterobacter sp. DRP3.

AIMS: Study of rhizospheric microbiome-mediated plant growth promotional attributes currently highlighted as a key tool for the development of suitable bio-inoculants for sustainable agriculture purposes. In this context, we have conducted a detailed study regarding the characterization of phosphate solubilizing potential by plant growth-promoting bacteria that have been isolated from the rhizosphere of a pteridophyte Dicranopteris sp., growing on the lateritic belt of West Bengal. METHODS AND RESULTS: We have isolated three potent bacterial strains, namely DRP1, DRP2, and DRP3 from the rhizoids-region of Dicranopteris sp. Among the isolated strains, DRP3 is found to have the highest phosphate solubilizing potentiality and is able to produce 655.89 and 627.58 µg ml-1 soluble phosphate by solubilizing tricalcium phosphate (TCP) and Jordan rock phosphate, respectively. This strain is also able to solubilize Purulia rock phosphate moderately (133.51 µg ml-1). Whole-genome sequencing and further analysis of the studied strain revealed the presence of pyrroloquinoline quinone (PQQ)-dependent glucose dehydrogenase gdh gene along with several others that were well known for their role in phosphate solubilization. Further downstream, quantitative reverse transcriptase PCR-based expression study revealed 1.59-fold upregulation of PQQ-dependent gdh gene during the solubilization of TCP. Root colonization potential of the studied strain on two taxonomically distinct winter crops viz. Cicer arietinum and Triticum aestivum has been checked by using scanning electron microscopy. Other biochemical analyses for plant growth promotion traits including indole acetic acid production (132.02 µg ml-1), potassium solubilization (3 mg l-1), biofilm formation, and exopolymeric substances productions (1.88-2.03 µg ml-1) also has been performed. CONCLUSION: This study highlighted the active involvement of PQQ-dependent gdh gene during phosphate solubilization from any Enterobacter group. Moreover, our study explored different roadmaps for sustainable farming methods and the preservation of food security without endangering soil health in the future.

Rhizobacterial diversity of Portuguese olive cultivars in the Douro valley and their potential as plant growth promoters.

AIMS: This study investigated the bacterial communities in the rhizosphere of two traditional Portuguese olive cultivars, Cobrançosa and Negrinha de Freixo, in relation to soil properties. Additionally, we aimed to isolate and identify bacteria with potential for biocontrol and other plant growth-promoting traits from these rhizosphere communities. METHODS AND RESULTS: Bacterial communities in the olive rhizosphere were investigated using a metabarcoding approach and the soil physicochemical properties of the olive groves were also analyzed. Higher bacterial richness was associated with Negrinha de Freixo growing in soil with high organic matter content and water-holding capacity. In contrast, the soils of the Cobrançosa grove presented higher pH and electric conductivity. Negrinha de Freixo rhizosphere was enriched with ASVs (Amplicon Sequence Variants) belonging to Bacillus, Gaiella, Acidothermus, Bradyrhizobium, and uncultured Xanthobacteraceae. On the other hand, the Cobrançosa rhizosphere was characterized by higher relative abundance of Streptomyces and Sphingomonas. Bacterial isolation from the rhizosphere and screening for plant growth-promoting activities were also performed. Six bacteria strains, predominantly Bacillus isolated from Negrinha de Freixo, demonstrated antagonistic activities against the olive fungal pathogen Colletotrichum gloeosporoides and other plant growth promotion (PGP) traits. CONCLUSIONS: Our findings demonstrate that the structure of rhizosphere bacterial communities associated with olive trees is shaped by both plant cultivar and soil-related factors. The higher number of bacterial species in the rhizosphere of Negrinha de Freixo was related to a higher organic matter content and a greater abundance of isolates with plant growth promotion traits, particularly Bacillus strains.

T-DNA insertion mutagenesis in Penicillium brocae results in identification of an enolase gene mutant impaired in secretion of organic acids and phosphate solubilization.

Penicillium brocae strain P6 is a phosphate-solubilizing fungus isolated from farmland in Guangdong Province, China. To gain better insights into the phosphate solubilization mechanisms of strain P6, a T-DNA insertion population containing approximately 4500 transformants was generated by Agrobacterium tumefaciens-mediated transformation. The transformation procedure was optimized by using a Hybond N membrane for co-cultivation of A. tumefaciens and P. brocae. A mutant impaired in phosphate solubilization (named MT27) was obtained from the T-DNA insertion population. Thermal asymmetric interlaced PCR was then used to identify the nucleotide sequences flanking the T-DNA insertion site. The T-DNA in MT27 was inserted into the fourth exon of an enolase gene, which shows 90.8 % nucleotide identity with enolase mRNA from Aspergillus neoniger. Amino acid sequence homology analysis indicated that the enolase is well conserved among filamentous fungi and Saccharomyces cerevisiae. Complementation tests with the MT27 mutant confirmed that the enolase gene is involved in phosphate solubilization. Analysis of organic acids in culture supernatants indicated reduced levels of oxalic acid and lactic acid for the MT27 mutant compared to the parent strain P6 or the complementation strain. In conclusion, we suggest that the identified enolase gene of P. brocae is involved in production of specific organic acids, which, when secreted, act as phosphate solubilizing agents.

An insight into the role of the organic acids produced by Enterobacter sp. strain 15S in solubilizing tricalcium phosphate: in situ study on cucumber.

BACKGROUND: The release of organic acids (OAs) is considered the main mechanism used by phosphate-solubilizing bacteria (PSB) to dissolve inorganic phosphate in soil. Nevertheless, little is known about the effect of individual OAs produced by a particular PSB in a soil-plant system. For these reasons, the present work aimed at investigating the effect of Enterobacter sp. strain 15S and the exogenous application of its OAs on (i) the solubilization of tricalcium phosphate (TCP), (ii) plant growth and (iii) P nutrition of cucumber. To this purpose two independent experiments have been performed. RESULTS: In the first experiment, carried out in vitro, the phosphate solubilizing activity of Enterobacter 15S was associated with the release of citric, fumaric, ketoglutaric, malic, and oxalic acids. In the second experiment, cucumber plants were grown in a Leonard jar system consisting of a nutrient solution supplemented with the OAs previously identified in Enterobacter 15S (jar's base) and a substrate supplemented with the insoluble TCP where cucumber plants were grown (jar's top). The use of Enterobacter 15S and its secreted OAs proved to be efficient in the in situ TCP solubilization. In particular, the enhancement of the morpho-physiological traits of P-starved cucumber plants was evident when treated with Enterobacter 15S, oxalate, or citrate. The highest accumulation of P in roots and shoots induced by such treatments further corroborated this hypothesis. CONCLUSION: In our study, the results presented suggest that organic acids released by Enterobacter 15S as well as the bacterium itself can enhance the P-acquisition by cucumber plants.

Plant growth-promoting and heavy metal-resistant Priestia and Bacillus strains associated with pioneer plants from mine tailings.

Open mine tailings dams are extreme artificial environments containing sizeable potentially toxic elements (PTEs), including heavy metals (HMs), transition metals, and metalloids. Furthermore, these tailings have nutritional deficiencies, including assimilable phosphorus sources, organic carbon, and combined nitrogen, preventing plant colonization. Bacteria, that colonize these environments, have mechanisms to tolerate the selective pressures of PTEs. In this work, several Priestia megaterium (formerly Bacillus megaterium), Bacillus mojavensis, and Bacillus subtilis strains were isolated from bulk tailings, anthills, rhizosphere, and endosphere of pioneer plants from abandoned mine tailings in Zacatecas, Mexico. Bacillus spp. tolerated moderate HMs concentrations, produced siderophores and indole-3-acetic acid (IAA), solubilized phosphates, and reduced acetylene in the presence of HMs. The strains harbored different PIB-type ATPase genes encoding for efflux pumps and Cation Diffusion Facilitator (CDF) genes. Moreover, nifH and nifD nitrogenase genes were detected in P. megaterium and B. mojavensis genomic DNA. They showed similarity with sequences of the beta-Proteobacteria species, which may represent likely horizontal transfer events. These Bacillus species precede the colonization of mine tailings by plants. Their phenotypic and genotypic features could be essential in the natural recovery of the sites by reducing the oxidative stress of HMs, fixing nitrogen, solubilizing phosphate, and accumulating organic carbon. These traits of the strains reflect the adaptations of Bacillus species to the mine tailings environment and could contribute to the success of phytoremediation efforts.

Novosphingobium kaempferiae sp. nov., a phosphate-solubilizing bacterium isolated from stem of Kaempferia marginata Carey.

A Gram-stain-negative, aerobic, rod-shaped, non-spore-forming and yellow-pigment-producing bacterium, designated as Sx8-5T, was isolated from stem tissue of Kaempferia marginata Carey in Kanchanaburi Province, Thailand. The strain exhibited tricalcium phosphate solubilizing activity. Its taxonomic position was investigated using a polyphasic approach. Sx8-5T grew at 25-37 °C (optimum 30 °C), pH 6-9 (optimum 7) and with 0 and 1% NaCl (optimum 0 %). According to the 16S rRNA gene phylogeny, Sx8-5T represents a member of genus Novosphingobium and shared the highest sequence similarities to Novosphingobium barchaimii LL02T of 99.4 % and shared sequence similarities with other species of the genus Novosphingobium of less than 99.4 %. The whole-genome size was 5.7 Mb, comprised of one contig, with a DNA G+C content of 66 %. The average nucleotide identity using BLASTn (ANIb) or MUMMER (ANIm) values for whole genome comparisons between Sx8-5T and Novosphingobium barchaimii LL02T and six closely related type strains were 72.33-82.14 % and 83.82-87.38 %, respectively, and the digital DNA-DNA hybridization (dDDH) values ranged from 21.0 to 28.6% when compared with the type strains of the members of the genus Novosphingobium. Major fatty acids were summed feature 8 (C18 : 1 ω7c and/or C18 : 1 ω6c), C16 : 0 and summed feature 3 (C16 : 1 ω7c and/or C16 : 1 ω6c), respectively. Polar lipids were diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylmonomethylethanolamine, phosphatidylglycerol, unidentified phospholipids and unidentified polar lipids. The major isoprenoid quinone was Q-10. According to results obtained using a polyphasic approach, Sx8-5T represents a novel species of the genus Novosphingobium, the name Novosphingobium kaempferiae sp. nov. is proposed. The type strain is Sx8-5T (=JCM 35076T =TBRC 15600T).

Bacterial diversity of a floating vegetation (Phumdi) of Loktak Lake and its extracellular enzymes and bacterial antagonistic property.

134 bacterial strains were isolated from phumdis of Loktak Lake. Through 16S rRNA sequencing, Bacillus sp. (23, 17.1%), Staphylococcus sp. (14, 10.4%), Pseudomonas sp. (11, 8.2%) and Acinetobacter sp. (8, 5.9%) were identified as the predominant bacterial taxa of Loktak Lake. B. pumulis (12, 8.9%), S. arlettae (4, 2.9%), P. knackmussii (6, 4.4%) are the leading species of Bacillus, Staphylococcus and Pseudomonas, respectively. Similarly, A. seifertii (2, 1.4%) and A. calcoaceticus (2, 1.4%) are the common species of Acinetobacter. 75 (55.9%) bacterial strains showed the ability to hydrolyze one or more extracellular enzymes tested. Among the extracellular enzymes produced by the bacterial isolates, the presence of elastase activity cannot be underestimated, since the enzyme is involved in the process of bacterial lung infection. Phosphate solubilizing activity could be seen in 11.1% of the bacterial isolates. 27 (20.1%) of the strains shown to have antagonistic activity against one or more tested pathogens. An isolate, MRC 52 showed antagonistic activity against eleven different pathogens including carbapenem resistant E. coli which was further subjected to extraction and identification of the biomolecule exerting the antimicrobial property. Based on GC-MS analysis, the bioactive compound was identified as phenyl ethyl alcohol.

Identification of genes involved in phosphate solubilization and drought stress tolerance in chickpea symbiont Mesorhizobium ciceri Ca181.

Chickpea plant root colonizing bacteria Mesorhizobium ciceri Ca181 promotes plant growth and development through symbiotic association with root nodules. The potentially beneficial effects on plants generated due to this bacterium are mineral nutrient solubilization, abiotic stress tolerance, and nitrogen-fixation, though the molecular mechanisms underlying these probiotic capacities are still largely unknown. Hence, this study aims to describe the molecular mechanism of M. ciceri Ca181 in drought stress tolerance and phosphorus solubilization. Here we have developed the transposon inserted mutant library of strain Ca181 and further screened it to identify the phosphorous solubilization and PEG-induced drought stress tolerance defective mutants, respectively. Resultantly, a total of four and three mutants for phosphorous solubilization and drought stress tolerance were screened and identified. Consequently, Southern blot confirmation was done for the cross verification of insertions and stability in the genome. Through the sequencing of each mutant, the interrupted gene was confirmed, and the finding revealed that the production of gluconic acid is necessary for phosphorus solubilization, while otsA, Auc, and Usp genes were involved in the mechanism of drought stress tolerance in M. ciceri Ca181.

Genetic diversity of N-fixing and plant growth-promoting bacterial community in different sugarcane genotypes, association habitat and phenological phase of the crop.

This study aimed to evaluate the genetic diversity of bacterial community associated to different sugarcane genotypes, association habitat and phenological phase of the culture, as well as to isolate, to identify and to characterize your potential for plant growth-promoting. Root and rhizospheric soil samples from RB 92579 and RB 867515 varieties were collected at 120 and 300 days after regrowth (DAR). The diversity of bacterial was evaluated through of the 16S rRNA and nifH genes. We found greater genetic diversity in the root endophytic habitat at 120 DAR. We identify the genera Burkholderia sp., Pantoea sp., Erwinia sp., Stenotrophomonas sp., Enterobacter sp. and Pseudomonas sp. The genera Bacillus sp. and Dyella sp. were only identified in the variety RB 92579. We found indices above 50% for biological nitrogen fixation, production of indole acetic acid and phosphate solubilization, showing that the use of these bacteria in biotechnological products is very promising.

Fuscibacter oryzae gen. nov., sp. nov., a phosphate-solubilizing bacterium isolated from the rhizosphere of rice plant.

An ovoid to rod shaped, white to brown pigmented, facultative anaerobic, mesophilic, non-phototrophic, Gram-staining-negative, non-motile, multiply by binary fission designated strain KVB23T, which was isolated from root of rice plant, near Ilsan, South Korea, was investigated for its taxonomic position by polyphasic approach. Optimal growth was found to occur at 30˚C, at pH 6.5 and in the absence of NaCl on R2A. Phylogenetic analysis based on the 16S rRNA gene sequence of strain KVB23T revealed that it formed a distinct lineage, as a separate deep branch within the family Rhodobacteriaceae, with < 96.5% sequence similarity to representatives of the genera Rhodobacter, Xinfangfangia, Tabrizicola, Falsirhodobacter, Haematobacter, Paenirhodobacter, Pseudorhodobacter and Pararhodobacter. Based in 16S rRNA sequences strain KVB23T was most closely related to Tabrizicola fusiformis KCTC 62105 T (96.5%) and Rhodobacter thermarum KCTC 52712 T (96.2%). The draft genome of strain KVB23T was 3.80 bp long with a DNA G + C content of 63.1%. Genome of strain KVB23T harboured gene clusters for tryptophan and cobalamin biosynthesis. The strain contained Q-10 as the sole respiratory quinone. The predominant fatty acids were found to consist of C16:0, C18:0 and summed feature 8 (comprising C18:1 ω7c and / or C18:1 ω6). The polar lipids were identified as diphosphatidylglycerol, phosphatidylethanolamine, seven unidentified phosphoglycolipids, two unidentified aminophosphoglycolipid, one unidentified glycolipid and four unidentified lipids. Phosphate-solubilizing bacteria have the ability to dissolve insoluble phosphates and enhance the soil fertility. Strain KVB23T can solubilize calcium phosphate tribasic. Phosphate solubilizing and tryptophan biosynthesis property of strain KVB23T could be a possible factor for the increase in growth of rice plant. Differential phenotypic, chemotaxonomic and genotypic properties, together with the phylogenetic distinctiveness, demonstrated that strain KVB23T was found to represent a novel genus in the Rhodobacteriaceae family, for which the name Fuscibacter oryzae gen. nov., sp. nov. is proposed, with the type strain KVB23T(= KACC 21711 T = NBRC 114716 T).

Natural occurrence of Azospirillum brasilense in petunia with capacity to improve plant growth and flowering.

To evaluate the natural occurrence of the plant growth-promoting bacterium Azospirillum brasilense and petunia plants, local strains were isolated and characterized by biochemical and molecular methods. Three strains were assessed in greenhouse conditions using Petunia × hybrida Ultra™. Treatments: Plants without bacterial inoculation or chemical fertilization; fertilized with NPK and KNO3 ; and independently inoculated with the strains 2A1, 2A2, and 2E1 by submerging their roots in a bacterial suspension (~106 CFU·ml-1 ). Root length, dry weight of roots and shoots, leaf area, leaf greenness, and nutrient content were evaluated. The number of days from transplanting to the opening of the first flower and the number of flowers per plant were also determined. As a result, five isolates were characterized as A. brasilense, showing the capacity to produce indoles and siderophores, to solubilize phosphate, nitrogenase activity, and nifH-PCR amplification. In general, all the parameters of the plant assay were improved in plants inoculated with A. brasilense, with variations among the strains, as well as the onset of flowering and the number of flowers per plant, compared with uninoculated or fertilized plants. This is the first report on the natural occurrence of A. brasilense in petunia with the capacity to improve plant growth and flowering.

Metabolic activity and bioweathering properties of yeasts isolated from different supraglacial environments of Antarctica and Himalaya.

Yeasts have been frequently isolated from cold habitats, but their contribution to essential ecological processes such as the mineralization of organic matter in these environments is less known. Here, the diversity, metabolic capability, and extracellular enzyme profiles of yeasts from snow, blue ice and cryoconite hole environments from East Antarctica and cryoconite holes from a glacier in Western Himalaya were determined. Eighty-six yeast strains isolated were affiliated to the genera Glaciozyma, Goffeauzyma, Mrakia, Phenoliferia, and Rhodotorula. Variations in the abundance, diversity, physiological properties, extracellular enzyme and carbon substrate utilization patterns of the isolated yeasts, reflect the specific environmental conditions from which they were isolated. Overall, 20-90% of the yeasts across all habitat types and geographical locations produced extracellular enzymes to degrade proteins, esters, carbohydrates, pectin, cellulose, lignin, and tannin. About 10 and 29% of the yeasts also exhibited ability to solubilize rock-minerals like phosphate and silicate, respectively. Additionally, selected isolates were able to metabolize 28-93% of the carbon substrates comprising different compound classes on Biolog YT plates. Overall, the ability of yeasts to use diverse organic compounds prevalent on the glacier surface, points to their ecological significance in the decomposition of organic matter, cycling of nutrients, and in the weathering of minerals in supraglacial environments. Moreover, their wide metabolic capabilities suggest that they can colonize new niches and environments when meltwater export during the summer that enables links with surrounding ecosystems.

Genome sequence of the endophytic strain Enterobacter sp. J49, a potential biofertilizer for peanut and maize.

Enterobacter sp. J49 is a plant growth promoting endophytic strain that promotes the growth of peanut and maize crops. This strain promotes plant growth by different mechanisms with the supply of soluble phosphorus being one of the most important. Enterobacter sp. J49 not only increases the phosphorus content in the plant but also in the soil favoring the nutrition of other plants usually used in rotation with these crops. The aims of this study were to analyze the genome sequence of Enterobacter sp. J49 in order to deepen our knowledge regarding its plant growth promoting traits and to establish its phylogenetic relationship with other species of Enterobacter genus. Genome sequence of Enterobacter sp. J49 is a valuable source of information to continuing the research of its potential industrial production as a biofertilizer of peanut, maize and other economically important crops.

Evaluation and improvement of phosphate solubilization by an isolated bacterium Pantoea agglomerans ZB.

A phosphate solubilizing bacterium ZB was isolated from the rhizosphere soil of Araucaria, which falls into the species Pantoea agglomerans. Optimization for phosphate solubilization by strain ZB was performed. At optimum culture conditions, the isolate showed great ability of solubilizing different insoluble inorganic phosphate sources viz. Ca3(PO4)2 (TCP), Hydroxyapatite (HP), CaHPO4, AlPO4, FePO4 along with rock phosphates (RPs). Inoculation with planktonic cells was found to enhance dissolved phosphorous as compared to that achieved by symplasma inoculation. Besides inoculation with different status of cells, pre-incubation could also exert a great effect on phosphate solubilization ability of P. agglomerans. When isolate ZB was cultured with glucose as carbon sources, phosphorous was more efficiently dissolved from HP and RP without pre-incubation in comparison to that obtained with pre-cultivation. Pre-cultivation, however, was more suitable for P solubilization than no pre-cultivation when bacteria were grown with xylose. A positive correlation was detected between the production of organic acids and phosphate solubilization. P. agglomerans ZB possessed many plant growth promotion traits such as N2 fixation and production of indole 3-acetic acid, phytase, alkaline phosphatase. Pot experiment showed inoculation with single isolate ZB or biofertilizer prepared from semi-solid fermentation of isolate ZB with spent mushroom substrate (SMS) compost could enhance plant growth with respect to number of leaves, plant leave area, stem diameter, root length, root dry mass, shoot dry mass and biomass when compared to the abiotic control, revealing strain ZB could be a promising environmental-friendly biofertilizer to apply for agricultural field.

Integration of molecular tools in microbial phosphate solubilization research in agriculture perspective.

Phosphorus (P) is the second most crucial nutrient for plant growth after nitrogen. However, its highly reactive nature causes formation of insoluble derivatives and limits uptake by the plant roots. The wide spread applications of P based chemical fertilizers cause detrimental effects on soil fertility, agricultural product quality and environment. In this regard, phosphate-solubilizing microorganisms (PSMs) stand out as the most remarkable and promising tools for the development of safer and sustainable technologies. As a result of this, many bacterial and fungal species with significant phosphate-solubilizing activity have been discovered by using the conventional screening methods. However, the growing need for the discovery of new strains of PSMs necessitates the replacement or support to the time-consuming conventional methods with techniques that are more sensitive, reliable, reproducible and less time consuming. In this context, molecular tools and techniques provide novel approaches for microbial phosphate solubilization research. Hence, in this review information on the molecular approaches for the PSMs research is provided and its importance explained. The review also discusses the genes related to phosphate solubilizing mechanisms and molecular tools for screening these genes.

Succinate irrepressible periplasmic glucose dehydrogenase of Rhizobium sp. Td3 and SN1 contributes to its phosphate solubilization ability.

Td3 and SN1 are phosphate-solubilizing nodule rhizobia of Cajanus cajan and Sesbania rostrata, respectively. They solubilized 423 µg/mL and 428 µg/mL phosphate from tricalcium phosphate through the secretion of 19.2 mM and 29.6 mM gluconic acid, respectively, when grown in 100 mM glucose. However, 90% and 80% reduction in phosphate solubilization coupled to the production of 40 mM (Td3) and 28.2 mM (SN1) gluconic acid was observed when the two isolates were grown in 50 mM succinate + 50 mM glucose. Our results illustrate the role of succinate irrepressible glucose dehydrogenase (gcd) in phosphate solubilization and the role of succinate: proton symport in succinate-mediated repression of phosphate solubilization in these rhizobia. Calcium ion supplementation reduced the 88% and 72% repression in P solubilization to 18% and 9% in Td3 and SN1, respectively, coupled to a reduction in media pH from 6.8 to 4.9 in Td3 and 6.3 to 4.8 in SN1. Hence, repression had no genetic basis and is purely due to the biochemical interplay of protons and other cations.

Characterization of plant growth promoting feature of a neutromesophilic, facultatively chemolithoautotrophic, sulphur oxidizing bacterium Delftia sp. strain SR4 isolated from coal mine spoil.

A new facultative chemolithoautotrophic heavy metal resistant sulfur-oxidizing bacterium was isolated from spoil sample of an open cast coal mine. FESEM demonstrated that the bacterium from Delftia genus was rod-shaped mucoid and motile. It autotrophically oxidized 20 mM thiosulfate and 1 g l-1 elemental sulfur to 220 mg l-1 and 203 mg l-1 of sulfate, respectively in 7 days under oxic condition and was also able to grow heterotrophically. The strain showed many plant growth promoting properties like production of IAA (23 ug ml-1), ammonia (6 umol ml-1), siderophore (55% siderophore unit), and HCN (30 ppm) upon 48 hours of incubation. In Pikovskaya's agar, the strain showed phosphate solubilization index of 2.0 and solubilized tri-calcium phosphate (232 ug ml-1) and lowered pH from 8.0 to 4.5 within 18 days. The strain yielded promising results on Brassica juncea growth and sulfur, phosphorus, and lead uptake. Where sulfur and phosphorous accumulation was 52 and 116% higher in whole treated plants (derived from microbe-coated seeds), lead accumulation were 81 and 50% higher in shoot and root of the treated plants than control plants (derived from untreated seeds) . These results point that this multifunctional strain can be proposed for phytorestoration of heavy metal contaminated sites.

Exploring the response of Actinobacteria to the presence of phosphorus salts sources: Metabolic and co-metabolic processes.

This study tested the solubilization of phosphorus by five actinobacterial strains in liquid media containing Ca3 PO4 ; AlPO4 or FePO4 as the sole phosphate source, and discusses the possible mechanisms involved in this process. P solubilization by different strains was accompanied by a significant drop in pH from 7.0 to 2.15-5.0 after 14 days. The efficiency of different strains depended on the P-source. Streptomyces spp. MM140 and MM141 were the most efficient in solubilizing Ca3 PO4 , MM136, and MM141 were the most efficient in solubilizing AlPO4 , while all strains were equally efficient in solubilizing FePO4 . Gluconic, oxalic, citric, malic, succinic, formic, and acetic acid were detected in the medium with Ca3 PO4 , while all except acetic acid were detected in the media with FePO4 or AlPO4 . Although we did not use an organic source of phosphorus in the media, all strains produced acid and alkaline phosphatase. It is concluded from this study that actinobacteria produced multiple organic acids followed by a decrease in the pH to solubilize phosphate salts. As well as producing phosphatase, these microorganisms were found to have different ways of making P available, suggesting an ecological advantage as they form part of soil microbiomes important for plants.