Phosphate Solubilizing Capacity Research Articles

Lactic acid bacteria induce phosphate recrystallization for the in situ remediation of uranium-contaminated topsoil: Principle and application

Uranium (U) contamination often occurs in the topsoil (arable layer), and is a serious threat to crop growth. However, conventional microbial reduction methods are sensitive to oxygen and cannot be used to treat aerobic topsoils. In this study, phosphate-solubilizing microorganisms (PSM) were isolated from U-contaminated topsoil and used for soil remediation. Microbial metabolites and products were analyzed, and the pathways and mechanisms of PSM immobilization were revealed. The results showed that strain PSM8 had the highest phosphate-solubilizing capacity (dissolved P was 208 ± 5 mg/L) and the highest U removal rate (97.3 ± 0.1%). Multi-technical analyses indicated that bacterial surface functional groups adsorbed (UO2)2+ ions on the cell surface, glycolysis produced 3–10 mg/L of lactic acid (pH 4.7–6.0), and lactic acid solubilized Ca3(PO4)2 to form stable chernikovite (a type of uranyl phosphate) on the cell surface. The coupled application of Ca3(PO4)2 and strain PSM8 significantly reduced the bioavailability of soil U (62 ± 11%), converting U from the exchangeable to the residual phase and P from the steady to the available form. In addition, pot experiments showed that soil remediation promoted crop growth and significantly reduced U uptake and toxicity to photosynthetic systems. These findings demonstrate that PSM and Ca3(PO4)2 are good coupled fertilizers for U-contaminated agricultural soil.

Changes in phosphorus mobilization and community assembly of bacterial and fungal communities in rice rhizosphere under phosphate deficiency.

Rhizosphere microorganisms are closely associated with phosphorus (P) uptake in plants and are considered potential agents to mitigate P shortage. However, the mechanisms of rhizospheric microbial community assembly under P deficiency have yet to be elucidated. In this study, bacterial and fungal communities in rice rhizosphere and their P mobilization potential under high (+P) and low (−P) concentrations of P were investigated. Bacterial and fungal community structures were significantly different between −P and +P treatments. And both bacterial and fungal P-mobilizing taxa were enriched in-P treatment; however, the proportion of P-mobilizing agents in the fungal community was markedly greater than that in the bacterial community. A culture experiment confirmed that microbial phosphate solubilizing capacity was significantly higher in −P treatment compared with that in +P treatment. −P treatment lowered bacterial diversity in rice rhizosphere but increased fungal diversity. Further analysis demonstrated that the contribution of deterministic processes in governing bacterial community assembly was strengthened under P deficiency but was largely weakened in shaping the fungal community. These results highlighted that enriching P-mobilizing microbes in the rhizosphere is a vital way for rice to cope with P deficiency, and that fungi contribute considerably to P mobilization in rice rhizosphere. Findings from the study provide novel insights into the assembly of the rhizosphere microbiome under P deficiency and this will facilitate the development of rhizosphere microbial regulation strategies to increase nutrient uptake in plants.

Burkholderia ambifaria XN08: A plant growth-promoting endophytic bacterium with biocontrol potential against sharp eyespot in wheat.

Plant growth-promoting bacteria (PGPB) have been considered promising biological agents to increase crop yields for years. However, the successful application of PGPB for biocontrol of sharp eyespot in wheat has been limited, partly by the lack of knowledge of the ecological/environmental factors affecting the colonization, prevalence, and activity of beneficial bacteria on the crop. In this study, an endophytic bacterium XN08 with antagonistic activity against Rhizoctonia cerealis (wheat sharp eyespot pathogenic fungus), isolated from healthy wheat plants, was identified as Burkholderia ambifaria according to the sequence analysis of 16S rRNA. The antibiotic synthesis gene amplification and ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS) analyses were used to characterize the secondary metabolites. The results showed that the known powerful antifungal compound named pyrrolnitrin was produced by the strain XN08. In addition, B. ambifaria XN08 also showed the capacity for phosphate solubilization, indole-3-acetic acid (IAA), protease, and siderophore production in vitro. In the pot experiments, a derivate strain carrying the green fluorescent protein (GFP) gene was used to observe its colonization in wheat plants. The results showed that GFP-tagged B. ambifaria could colonize wheat tissues effectively. This significant colonization was accompanied by an enhancement of wheat plants' growth and an induction of immune resistance for wheat seedlings, which was revealed by the higher activities of polyphenol oxidase (PPO), peroxidase (POD), and phenylalanine ammonia-lyase (PAL). As far as we know, this is the first report describing the colonization traits of B. ambifaria in wheat plants. In addition, our results indicated that B. ambifaria XN08 might serve as a new effective biocontrol agent against wheat sharp eyespot disease caused by R. cerealis.

Agronomic efficiency and genome mining analysis of the wheat-biostimulant rhizospheric bacterium Pseudomonas pergaminensis sp. nov. strain 1008T.

Pseudomonas sp. strain 1008 was isolated from the rhizosphere of field grown wheat plants at the tillering stage in an agricultural plot near Pergamino city, Argentina. Based on its in vitro phosphate solubilizing capacity and the production of IAA, strain 1008 was formulated as an inoculant for bacterization of wheat seeds and subjected to multiple field assays within the period 2010–2017. Pseudomonas sp. strain 1008 showed a robust positive impact on the grain yield (+8% on average) across a number of campaigns, soil properties, seed genotypes, and with no significant influence of the simultaneous seed treatment with a fungicide, strongly supporting the use of this biostimulant bacterium as an agricultural input for promoting the yield of wheat. Full genome sequencing revealed that strain 1008 has the capacity to access a number of sources of inorganic and organic phosphorus, to compete for iron scavenging, to produce auxin, 2,3-butanediol and acetoin, and to metabolize GABA. Additionally, the genome of strain 1008 harbors several loci related to rhizosphere competitiveness, but it is devoid of biosynthetic gene clusters for production of typical secondary metabolites of biocontrol representatives of the Pseudomonas genus. Finally, the phylogenomic, phenotypic, and chemotaxonomic comparative analysis of strain 1008 with related taxa strongly suggests that this wheat rhizospheric biostimulant isolate is a representative of a novel species within the genus Pseudomonas, for which the name Pseudomonas pergaminensis sp. nov. (type strain 1008T = DSM 113453T = ATCC TSD-287T) is proposed.

Characteristics of the Phosphorus-Solubilizing Bacteria Derived from the Rhizosphere of Persimmon Tree in Beijing and their Plant Growth-Promoting Potential

In this study, fourteen active phosphorus solubilizing bacteria (PSB) were obtained by qualitative and quantitative experiments from the rhizosphere of persimmon grown in Beijing, China, with a relatively dry climate. Based on the 16S rRNA gene sequencing, the PSB isolates were clustered under two phyla with Proteobacteria (accounting for 64.28%) and Firmicutes (35.72%). The former included four genera, Pseudomonas (28.57%), Phyllobacterium (21.43%), Variovorax (7.14%), and Mesorhizobium (7.14%), while Bacillus (21.43%) and Paenibacillus (14.29%) belonged to Firmicutes. These PSB strains showed different degrees of phosphate solubilization capacity from 8.36 to 226.28 mg/L in the fermentation culture. Further, the plant growth-promoting potentials include indol-3-acetic acid (IAA), siderophores, and proteases. Among them, eleven PSB strains produced IAA, ten strains had proteolytic activity, and four can produce siderophores. Significantly, this study was the first report of the PSB derived from the rhizosphere of persimmon trees, and the new functions of some isolates were detected as follows, V. soli SHI05 and M. helmanticense SHI08 can simultaneously produce IAA, siderophores, and proteases, P. knackmussii SHI01 and P. sophorae SHI12 can produce IAA, P. frederiksbergensis SHI02, P. illinoisensis SHI09, and P. illinoisensis SHI13 can secrete proteases, P. sophorae SHI03 can produce siderophores. The effect of bacterial inoculation on the early growth of mung bean revealed that V. soil SHI05 can distinctly improve the seed vigor index, total length, fresh weight, dry weight, and root activity of mung bean, increasing by 28.65, 28.71, 174.14, 237.56, and 9.39%, respectively, compared to the positive control (Bacillus megaterium). This showed that the rhizosphere of the persimmon tree was rich in diverse and multifunctional PSB, and V. soli SHI05 may have great potential to become a high-quality candidate microbial fertilizer agent in promoting the growth of mung bean.

Co-application of spent mushroom substrate and PGPR alleviates tomato continuous cropping obstacle by regulating soil microbial properties

Continuous cropping of tomato (Solanum lycopersicum L.) in China has contributed to the decline of crop quality and yield. The aims of this study were to evaluate the integrated effect of plant growth-promoting rhizobacteria (PGPR) Bacillus sp. HW27 and spent mushroom substrate (SMS) on tomato plant growth and microbiological properties of rhizosphere soil. Bacillus sp. HW27 showed indole-3-acetic acid (IAA) and siderophore production, phosphate solubilization, and root colonization capacity in vitro assays. A pot experiment was carried out with four treatments: (1) CK: unamended control, (2) PGPR, (3) SMS, and (4) PGPR + SMS: combined application of strain HW27 and SMS. The enzyme activities, the abundances of beneficial microbes Bacillus and Pseudomonas and soil-borne pathogenic Fusarium oxysporum (using real-time PCR), the microbial functional diversity (using Biolog Ecoplates), and microbial community (using phospholipid fatty acid (PLFA) analysis) in rhizosphere soil were studied 20, 40 and 60 days after transplantation. At the end of the 60-day pot experiment, the results showed that the PGPR + SMS treatment performed best in improving tomato plants growth, which significantly (p < 0.05) increased the shoot length, root length, shoot dry weight and root dry weight by 34.4, 78.4, 93.5 and 82.4%, respectively, compared to the CK treatment. The enzyme activities of β-1,4-glucosidase, β-d-cellobiosidase, β-1,4-N-acetyl-glucosaminidase, protease and acid phosphatase were higher in the SMS and/or PGPR application treatments than in the CK treatment in the order PGPR < SMS < SMS + PGPR. The SMS + PGPR treatment showed the highest abundances of Bacillus and Pseudomonas, and the lowest abundance of Fusarium oxysporum. Furthermore, the application of SMS and/or PGPR stimulated microbial functional diversity and microbial growth (total PLFAs, bacterial, and Gram (+) and Gram (−) bacterial PLFAs), in which SMS + PGPR treatment had the greatest effect. Consequently, this study showed that co-application of PGPR and SMS expressed synergistic effect to increase tomato plant growth, enzyme activities and microbial biomass in continuous cropping greenhouse soil.

Screening and identification of a Burkholderia strain and optimization of its phosphate solubilizing capacity

In order to solve the problem that soil soluble phosphorus content in most cultivated land in China is insufficient and the plant growth is inhibited, a phosphate solubilizing microorganism (PB) was screened and identified, and its phosphate solubilizing performance was optimized. The results showed that the PB strain was belonged to Burkholderia stabilis. It had the ability of nitrogen fixation and indole-3-acetic acid (IAA) secretion, as well as a certain inhibitory effect on Escherichia coli. It could maintain high activity and phosphorus solubilizing ability at pH 8.0-10.0, indicating good alkali resistance. The results of phosphorus dissolving performance optimization showed that the phosphate solubilizing capacity of strain PB reached the best at 30℃, pH 7.0, 180 r·min-1, using glucose as carbon source, ammonium sulfate as nitrogen source, tricalcium phosphate as phosphorus source and adding 50 μmol·L-1 lysine. The amount of dissolved phosphorus was 569.33 mg·L-1, which was 1.9 times of that before optimization. The strain mainly secreted citric acid, malonic acid, and glucuronic acid during metabolism. After adding lysine, the type of organic acids secreted by the strain did not change, but the content increased significantly. Results from pot experiments showed that the application of PB bacterial fertilizer could significantly improve the growth and physiological indicators of garlic seedlings, and that the promotion effect was more obvious after adding lysine. Compared with the control, the height of seedling was increased by 18.6%, seedling diameter was increased by 16.7%, aboveground fresh and dry weight were increased by 22.1% and 15.7%, and belowground fresh and dry weight were increased by 22.0% and 28.7%, respectively in PB with lysine treatment. Soil available phosphorus content was 2.1 and 2.3 times of the control in PB and PB+lysine treatments, indicating that PB could improve soil available phosphate content. Adding lysine could strengthen such function.

Isolation, Mutagenesis, and Organic Acid Secretion of a Highly Efficient Phosphate-Solubilizing Fungus.

The highly effective phosphate-solubilizing microorganisms are significant for making full use of the potential phosphorus resources in the soil and alleviating the shortage of phosphorus resources. In this study, a phosphate-solubilizing fungus was isolated from wheat and cotton rhizosphere soils in the lower reaches of the Yellow River in China and was identified as Penicillium oxalicum by morphological and ITS sequencing analysis. In order to obtain a fungus with more efficient phosphorus solubilization ability, we tested three positive mutant strains (P1, P2, and P3) and three negative mutant strains (N1, N2, and N3) through low-energy nitrogen ion implantation mutagenesis. Compared with the parental strain, the phosphate-solubilizing capacity of P1, P2, and P3 was enhanced by 56.88%, 42.26%, and 32.15%, respectively, and that of N1, N2, and N3 was weakened by 47.53%, 35.27%, and 30.86%, respectively. Compared with the parental strain, the total amount of organic acids secreted significantly increased in the three positive mutant strains and decreased in the negative mutant strains; the pH of culture medium was significantly lower in the positive mutant strains and higher in the negative mutant strains. The capacity of phosphate-solubilizing fungus to secrete organic acids and reduce the growth-medium pH was closely related to its phosphate-solubilizing ability. The changes in the amount of organic acids secreted by mutants can alter their acidification and phosphate-solubilizing capacity. In conclusion, this study offers a theoretical basis and strain materials for the exploration and application of phosphate-solubilizing fungi.

Plant growth-promoting activities of bacterial endophytes isolated from the medicinal plant Pairs polyphylla var. yunnanensis.

Pairs polyphylla var. yunnanensis (Paris L.) is a valuable medicinal plant used in traditional Chinese medicine. The market demand for P. polyphylla has increased over time, but it has slow growth and a low natural propagation rate. Endophytic bacteria are bioactive microorganisms that form a mutualistic relationship with host plants in long-term coordinated evolution, and they can promote the growth and accumulation of effective components in host plants. The aims of this study were to identify endophytic bacteria of P. polyphylla and to characterize their properties in promoting plant growth. A total of 10 endophytic bacteria were isolated from rhizomes of P. polyphylla. The isolated endophytes exhibited a variable capacity for indole acetic acid production, phosphate solubilization and nitrogen fixation. To investigate the effects of the endophytes on plant growth, four endophyte strains, G5, J2, G20, and Y2, were selected to compare their ability to promote plant growth. The results indicated that microbial endophytes isolated from P. polyphylla rhizomes play a vital role in improving P. polyphylla plant growth and could be used as inoculants to establish a sustainable crop production system.

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

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

Effect of Bacillus spp. strains on wheat nutrient assimilation and bioformulation by new spray drying approach using natural phosphate powder

The present work is aiming to propose a new formulation of Bacillus spp. strains in combination with natural phosphate by spray drying. Bacillus sp. strain (SS0306) has proven its plant growth promoting effectiveness mainly on phosphate solubilization capacity and improved nutrient assimilation in wheat plants. Bacterial microencapsulation methods involved spray-drying assays that were conducted using 5% of rock phosphate with 2%–10% of maltodextrin and 2% of alginic acid sodium salt. The process parameters were also optimized with inlet temperature (155 °C), feed rate (9 min mL−1), and spray air rate (413.87 L h−1). Results showed a moisture content of the dried powders ranged between 2% and 8% and satisfactory viability levels of Bacillus sp. SS0306 (105–106 CFU g−1) after drying assay, with the survival rates between 61.56% and 64.18%. Optimized process reached 50.30% and 65.10% for the product yield, with 1.25 ± 0.006 mg g−1 of higher Pi content observed in rock phosphate formulations based on Alg 2%. The present study opens new possibilities to develop Bio-mineral fertilizers using the spray drying approach.

Isolation, Identification, Biocontrol Activity, and Plant Growth Promoting Capability of a Superior Streptomyces tricolor Strain HM10.

Streptomyces is a genus with known biocontrol activity, producing a broad range of biologically active substances. Our goal was to isolate local Streptomyces species, evaluate their capacity to biocontrol the selected phytopathogens, and promote the plant growth via siderophore and indole acetic acid (IAA) production and phosphate solubilization. Eleven isolates were obtained from local soil samples in Saudi Arabia via the standard serial dilution method and identified morphologically by scanning electron microscope (SEM) and 16S rRNA amplicon sequencing. The biocontrol of phytopathogens was screened against known soil-borne fungi and bacteria. Plant growth promotion capacity was evaluated based on siderophore and IAA production and phosphate solubilization capacity. From eleven isolates obtained, one showed 99.77% homology with the type strain Streptomyces tricolor AS 4.1867, and was designated S. tricolor strain HM10. It showed aerial hyphae in SEM, growth inhibition of ten known phytopathogens in in vitro experiments, and the production of plant growth promoting compounds such as siderophores, IAA, and phosphate solubilization capacity. S. tricolor strain HM10 exhibited high antagonism against the fungi tested (i.e., Colletotrichum gloeosporides with an inhibition zone exceeding 18 mm), whereas the lowest antagonistic effect was against Alternaria solani (an inhibition zone equal to 8 mm). Furthermore, the most efficient siderophore production was recorded to strain HM8, followed by strain HM10 with 64 and 22.56 h/c (halo zone area/colony area), respectively. Concerning IAA production, Streptomyces strain HM10 was the most effective producer with a value of 273.02 μg/ml. An autochthonous strain S. tricolor HM10 should be an important biological agent to control phytopathogens and promote plant growth.

Phosphate-Solubilizing Enterobacter ludwigii AFFR02 and Bacillus megaterium Mj1212 Rescues Alfalfa’s Growth under Post-Drought Stress

Drought stress is a prevalent environmental stress that adversely affects agricultural industries worldwide. In this study, bacterial isolates, AFFR02 and Mj1212, showed tolerance to polyethylene glycol-induced (PEG) drought stress (approximately 15%) and possess strong phosphate-solubilizing capacity. Moreover, we investigated the plant growth attributes, chlorophyll content, and ion uptake in alfalfa plants (Medicago sativa L) inoculated with isolates AFFR02 and Mj1212 under drought stress. We observed that drought stress drastically affects alfalfa’s growth attributes: shoot length: SL (24.88%), root length: RL (29.62%), shoot fresh weight: SFW (49.62%), root fresh weight: RFW (45.09%), stalk diameter: SD (52.84%), and chlorophyll content: CC (19.2%). However, in bacterial-inoculated alfalfa plants, the growth attributes significantly recovered were SL (12.42%), RL (21.30%), SFW (50.74%), RFW (46.42%), SD (76.72%), and CC (17.98%). In drought-stressed alfalfa plants, we observed a significant decrease in the relative water content (7.45%), whereas there was an increase in electrical conductivity (68.87%) and abscisic acid contents (164.42%). Antioxidant analysis showed a significant increase in total phenolic content (46.08%), DPPH-scavenging activity (39.66%), total flavonoid (13.68%), and superoxide dismutase (28.51%) in alfalfa treated with drought stress and bacterial isolates AFFR02 and Mj1212 simultaneously. Moreover, an increase in inductively coupled plasma (ICP) analysis of potassium (17.98%), phosphorous (11.14%), calcium (3.07%), and magnesium (6.71%) was recorded for bacteria-inoculated alfalfa plants under drought stress. In conclusion, bacterial isolates AFFR02 and Mj1212 enhance alfalfa growth under drought stress. Therefore, the isolates could be used as potential candidates in smart-climate agricultural practices in drought-stricken areas worldwide.

Peanut Endophytic Phosphate Solubilizing Bacteria Increase Growth and P Content of Soybean and Maize Plants.

Phosphorus (P) is a limiting factor of plant development due to its low availability in the soil. The use of endophytic phosphate solubilizing bacteria as a more sustainable alternative to the use of chemical phosphorus fertilizers is proposed in this study. The objectives were to analyze the effect of simple inoculations of native peanut endophytic phosphate solubilizing bacteria on plant growth promotion and P content of soybean and maize and to evaluate their survival and endophytic colonization capacity on these plants. In addition, bacterial plant cell wall degrading enzymes activities in presence or absence of root exudates was determined. Soybean, maize and peanut plants were grown on a microcosm scale and inoculated with Enterobacter sp. J49 or Serratia sp. S119. It was observed that phosphate solubilizing strains promoted the growth of maize and soybean plants and contributed significantly P to their tissues. A significant increase in the phosphate solubilizing capacity of the plant rhizosphere after the end of the assay was observed. The strains showed to survive in plant's growth substrate and in the case of Enterobacter sp. J49, it showed also to colonize endophytically maize and soybean. Root exudates of the three plants showed to produce changes in pectinase and cellulase activities of the strains. The bacterial strains analyzed in this study constitutes potential sources for the formulation of biofertilizers for their application for several crops in agricultural soils with low P content.

Enhanced Crop Productivity and Sustainability by Using Native Phosphate Solubilizing Rhizobacteria in the Agriculture of Arid Zones

The importance of phosphate solubilizing rhizobacteria (PSB) has been well-document as an option for enhancing sustainable agriculture. As a particular group of plant growth promoting rhizobacteria (PGPR), PSB play an important role in the soil phosphorus cycle, increasing the bioavailability for growth and plant development. This study analyses the plant growth promoting effects of 5 strains (BN0009, BN0013, BN0015, BN0024, and BN0035) out of 180 isolated from Jarava frigida (Phil.) F.Rojas (Poaceae), a native grass from the Andean Atacama desert from North of Chile. The five bacterial isolated (BN strains) were identified as non-pathogenic Erwinia sp. and show a high phosphate solubilization capacity for Ca(PO4) ranging from 608.9 to 781.4 mg/L. Strains IAA production varies between 23.5 and 35.9 mg/L, siderophores, phosphatase (alkaline and acid) production was also observed, but none of the five isolated presented antagonism against plant pathogens Botrytis sp. and Sclerotinia sp. All isolates enhanced seed germination in Lactuca sativa and Solanum lycopersicum (excepting BN009). Additionally, all strains stimulated the early root elongation and seedling development in lettuce and tomato. Pot experiments displayed that BN0015, BN0024, and BN0035 significantly promote plant growth regarding root and leaf area, root and leaf weight, as well as leaf number compared with non-treated plants. In a field experiment with lettuce and two fertilization treatments (50 and 100% of the recommended crop fertilization), BN0024 application improved crop productivity compared to respective control. P content in plants with bacterial inoculations increased significantly compared to control in either fertilization treatment, suggesting an improved nutrient uptake. Also, lettuce with 50% fertilization and inoculation with BN0024 equate productivity with the control 100% fertilization. Finally, we discuss these results in the context of applicability to enhance the agroecosystem productivity in arid and semiarid zones.

Isolation, identification and characterization of phosphate solubilizing bacteria in soils of coal mines landfills of Chhattisgarh

Phosphate solubilizing bacteria have the ability to solubilize the insoluble phosphates and to improve the quality of soil health and fertility. Efficacy of phosphate solubilizing microorganisms has been identified on the basis of kinetics and phosphorus accumulation. In this study, twenty eight soil samples were collected from different coal mines landfills of Chhattisgarh and occurrence of phosphate solubilizing bacteria (PSB) was isolated, purified and identified. In addition, the phosphate solubilizing capacity of bacteria based on the formation of visible or halo zone on Pikovskaya agar plates (PVK) and broth having tricalcium phosphate (TCP) and rock phosphate (RP) as a phosphate source was estimated at Dr. C.V. Raman University, Kota, Bilaspur (Chhattisgarh). The result revealed dominance of Pseudomonas syringae as major phosphate solubilizers, along with Bacillus subtillis followed by Pantoea dispersa, Bacillus circulans. Use of these PSB as bioinoculants increased the available P in soil to the extent of 194 and 246 µg/ml of P. the soluble P in case of rock phosphate was less than tricalcium phosphate. The maximum solubilization was observed after 15 days follow by a decrease in amount of soluble P.

Selected Rhizosphere Bacteria Help Tomato Plants Cope with Combined Phosphorus and Salt Stresses.

Plants are often challenged by multiple abiotic stresses simultaneously. The inoculation of beneficial bacteria is known to enhance plant growth under these stresses, such as phosphorus starvation or salt stress. Here, for the first time, we assessed the efficiency of selected beneficial bacterial strains in improving tomato plant growth to better cope with double stresses in salty and P-deficient soil conditions. Six strains of Arthrobacter and Bacillus with different reservoirs of plant growth-promoting traits were tested in vitro for their abilities to tolerate 2–16% (w/v) NaCl concentrations, and shown to retain their motility and phosphate-solubilizing capacity under salt stress conditions. Whether these selected bacteria promote tomato plant growth under combined P and salt stresses was investigated in greenhouse experiments. Bacterial isolates from Cameroonian soils mobilized P from different phosphate sources in shaking culture under both non-saline and saline conditions. They also enhanced plant growth in P-deficient and salt-affected soils by 47–115%, and their PGP effect was even increased in higher salt stress conditions. The results provide valuable information for prospective production of effective bio-fertilizers based on the combined application of local rock phosphate and halotolerant phosphate-solubilizing bacteria. This constitutes a promising strategy to improve plant growth in P-deficient and salt-affected soils.

Acid and high-temperature tolerant Bradyrhizobium spp. strains from Brazilian soils are able to promote Acacia mangium and Stizolobium aterrimum growth

In selection of rhizobia, it is important to consider tolerance of the strain to adverse conditions, as well as its symbiotic efficiency and capacity to perform other plant growth-promoting processes. In this study, we evaluated Bradyrhizobium spp. strains from Brazilian soils regarding tolerance to different pH and temperature conditions, inorganic calcium phosphate solubilization capacity, and symbiotic efficiency when inoculated on acacia (Acacia mangium) and velvet bean (Stizolobium aterrimum) in axenic conditions. In each experiment of symbiotic efficiency, 18 strains (including representatives of Bradyrhizobium sp., B. viridifuturi, B. brasilense, B. uaiense, B. stylosanthis, and B. forestalis) were compared to two uninoculated controls, one with low (5.25 mg L−1) and another with high (52.5 mg L−1) concentrations of mineral nitrogen (N), and one inoculant strain [BR 3617 (B. japonicum) in the acacia experiment and BR 2811(B. elkanii) in the velvet bean experiment]. All strains grew at the different pH values tested (4 to 10) and tolerated a wide temperature range (15 to 37 °C) and most of them solubilized inorganic calcium phosphate. Most strains did not nodulate acacia, but were able to promote growth of this species. The UFLA03–268 (Bradyrhizobium sp.) strain was the most efficient in symbiosis with acacia. In the velvet bean experiment, the UFLA03–144 (B. viridifuturi), INPA104A (Bradyrhizobium sp.), and UFLA03-321T (B. brasilense) strains outperformed the other treatments in shoot dry matter production of velvet bean, showing high potential for use as inoculants in this species. Strains representing the five Bradyrhizobium species and Bradyrhizobium sp. were similar or more efficient in shoot nitrogen accumulation (SNA) than the control with mineral N. They also led to 84 (UFLA 04–0212) to 246 (INPA 104A) % increases in SNA compared to inoculation with BR 2811. This study showed an efficient symbiotic relationship between B. viridifuturi, B. brasilense, B. uaiense, B. stylosanthis and B. forestalis and velvet bean.

Plant Growth-Promoting Effect of the Chitosanolytic Phosphate-Solubilizing Bacterium Burkholderia gladioli MEL01 After Fermentation with Chitosan and Fertilization with Rock Phosphate

Phosphate-solubilizing bacteria (PSB) are important plant growth-promoting rhizobacteria that can increase soil fertility through the solubilization of insoluble inorganic phosphate and organophosphorus. In this study, a PSB, Burkholderia gladioli MEL01, was isolated and identified from rice–wheat rotation rhizosphere soil. MEL01 had an excellent phosphate-solubilizing capacity (reaching 107.69 mg/L) toward insoluble inorganic phosphate rock phosphate. HPLC analysis revealed that the mechanism of phosphate solubilization of MEL01 was probably due to secreted oxalic acid and gluconic acid transformation of phosphate from insoluble to soluble. MEL01 also exhibited 4030 U/L specific chitosanase activity when cultured with chitosan fermentation medium. Interestingly, the chitosan hydrolysis product chitooligosaccharide could significantly enhance the MEL01 phosphate-solubilizing capacity. Pot experiments showed that MEL01 chitosan medium fermentation liquor (MCMFL) could promote improvement of soil available phosphorus and pakchoi growth when supplemented with phosphate rock phosphate as the phosphate fertilizer. In addition, pot experiments demonstrated that MCMFL could also promote the growth of wheat, which could decrease the amount of compound fertilizer used. Microbial diversity analysis showed that the genera Pseudomonas, Burkholderia, Mycoplana, and Cellvibrio were enriched, which might participate in synergetic phosphate solubilization. Therefore, after fermentation with chitosan and fertilization with rock phosphates, MEL01 has potential as a phosphate biofertilizer in ecological agricultural production.

Screening and identification of an efficient phosphate-solubilizing Burkholderia sp. and its growth-promoting effect on Pinus massoniana seedling

We isolated phosphate solubilizing bacteria (PSB) from the rhizosphere soil of P. massoniana by the standard dilution plating technique, and determined the phosphate solubilizing characteristics of PSB by the Molybdenum antimony resistance colorimetric method. We explored the mechanism of phosphorus dissolution through analysis of the relationship between the phosphate-solubilizing ability of PSB and the pH of the fermentation broth and the determination of organic acids in the fermentation broth by HPLC-MS. The effects of PSB on the growth and physiology of P. massoniana were clarified by measuring the growth, physiology, soil nutrients, and soil enzyme activity of potted P. massoniana seedlings inoculated with PSB. The results showed that 16 strains of PSB were screened from the rhizosphere of P. massoniana. Among those strains, WJ27 had the best effect of solubilizing phosphorus, with the amount of phosphate solubilized reaching 411.98 mg·L-1 after five days of liquid culture. The strain was identified as Burkholderia sp. by phenotype observation, physiological and biochemical identification and phylogenetic tree analysis. The highest phosphate solubilizing capacity in four phosphorus sources mediums was Ca3(PO4)2(220.85 mg·L-1) > AlPO4(182.33 mg·L-1) > FePO4·2H2O (129.19 mg·L-1) > CaHPO4·2H2O (115.23 mg·L-1). WJ27 could reduce the pH of fermentation broth by secreting organic acids, such as citric acid and malonic acid, which contribute to phosphorus solubilization. Results from pot experiments showed that WJ27 had positive effects on the growth, physiology, soil nutrients, and soil enzyme activities of P. massoniana seedlings. Compared with the control, seedling height, main root length, total lateral root number, shoot (stem, leaf and branch) fresh weight, shoot dry weight, root fresh weight and root dry weight of P. massoniana inoculated with WJ27 increased by 14.3%, 36.9%, 56.1%, 44.7%, 60.0%, 158.3% and 100.0%, respectively. Chlorophyll b, total chlorophyll, shoot soluble protein, shoot soluble sugar, root activity, and root soluble protein increased by 145.8%, 45.2%, 206.3%, 59.4%, 80.5% and 260.0%, respectively. The SOD and POD acti-vities of root, and CAT activities of shoot of P. massoniana seedlings increased by 71.2%, 197.5% and 36.6%, respectively. Contents of available N, available K, available P, urease, catalase and phosphatase increased by 18.1%, 17.0%, 11.9%, 34.3%, 45.5%, and 62.6%, respectively. Burkholderia sp. WJ27 could improve soil nutrient concentrations and enzyme activities and thus facilitate the growth of P. massoniana seedlings.