Phosphate Solubilizing Bacteria Strains Research Articles

Elevating Sorghum Prosperity: Unveiling Growth Trends through Phosphate-Solubilizing Bacteria and Arbuscular Mycorrhizal Fungi Inoculation in Phosphate-Enriched Substrates

This study aimed to elucidate the impact of phosphate-solubilizing bacteria (PSB) and arbuscular mycorrhizal fungi (AMF) inoculation on sorghum growth within substrates derived from phosphate solid sludge, with the overarching objective of repurposing phosphate sludge to be a viable agricultural substrate. Four PSB strains (Serratia rubidaea, Enterobacter bugandensis, Pantoea agglomerans, Pseudomonas sp.) were meticulously selected from phosphate solid sludge, along with two AMF strains (Rhizophagus intraradices and Funneliformis mosseae), constituting the experimental inocula. Phosphate solid sludge was judiciously blended with peat at varying volumetric proportions (0%, 10%, 20%, 40%, and 60%), providing the matrix for sorghum cultivation, and concomitantly subjected to inoculation with PSB and AMF. Following a meticulously monitored two-month duration, a comprehensive evaluation of diverse morphological parameters, biomass accrual, nitrogen content, total phosphorus concentration, potassium levels, calcium content, and root colonization in sorghum plants was conducted. The empirical findings underscored a discernible decline in the assessed parameters with escalating concentrations of phosphate solid sludge. Particularly noteworthy was the pronounced amelioration observed in plants inoculated with AMF in comparison to both the control and PSB-inoculated counterparts. In conclusion, the application of raw phosphate solid sludge as an agricultural substrate is deemed unsuitable, prompting the imperative need for further in-depth investigations to ascertain the nuanced intricacies underlying these outcomes.

Exploration and Characterization of High-Efficiency Phosphate-Solubilizing Bacteria Isolates from Chickpea Rhizospheric Soil

The study was conducted during April, 2021 to June, 2022 to study the isolation and identification of high effective strains of phosphate solubilizing bacteria and evaluation the ability to solubilize tricalcium phosphate in vitro. Microorganisms that live in soil are actively involved in the carbon, nitrogen, sulphur, and phosphorus cycles in nature, enabling them to maintain ecological balance. Phosphate-solubilizing microbes are used as agricultural biofertilizers and play an essential role in plant nutrition by enhancing phosphate uptake. Phosphate is one of the most important macronutrients necessary for the growth and development of plants. Numerous microorganisms present in the rhizosphere solubilize insoluble phosphorus, making it readily available to plants. Fourteen phosphate-solubilizing bacterial colonies were identified on Pikovskaya’s agar medium, containing insoluble tricalcium phosphate (TCP) from rhizospheric soil. Among them, eight colonies showing clear halo zones around the microbial growth were considered as phosphate-solubilizing. All isolates showing the highest phosphate solubilization index (PSI), ranging from 2.8 to 4.03, were selected for further qualitative and quantitative studies. Out of these eight potent isolates, two strains showed the maximum PSI: V7 (4.03) and V8 (3.85) in agar plates, along with a high production of soluble phosphate, measuring 448.03 and 441.43 mg l-1, and a greater reduction in pH in the broth culture. The morphological, biochemical, and molecular identification of V7 and V8 strains revealed them to belong the Acinetobacter and Paenibacillus genera, respectively, and were phylogenetically close to Acinetobacter baumannii and Paenibacillus lautus species.

Improving soil phosphorus availability in saline areas by marine bacterium Bacillus paramycoides.

The utilization of phosphate-solubilizing bacteria (PSB) in agriculture has long been proposed as an eco-friendly method to enhance soil phosphorus (P) availability, thereby reducing reliance on chemical P fertilizers. However, their application in saline soils is challenged by salt-induced stress on common PSB strains. In this study, we sourced bacterial strains from marine environments, aiming to identify robust PSB strains adaptable to saline conditions and assess their potential as P bio-fertilizers through a microcosm experiment. Our findings indicate that the inoculation of a selected marine PSB, Bacillus paramycoides 3-1a, increased soil available P content by 12.5% when applied alone and by 61.2% when combined with organic amendments. This enhancement results from improved inorganic P solubilization and organic P mineralization in soils. Additionally, these treatments raised soil nitrogen levels, reshaped microbial community structures, and significantly enhanced wheat (Triticum aestivum L.) growth, with P accumulation increasing by 24.2-40.9%. Our results underscore the potential of marine PSB in conjunction with organic amendments for the amelioration of saline agricultural soils.

The effects of herbicide application on two soil phosphate solubilizing bacteria: Pantoea agglomerans and Serratia rubidaea.

The application of synthetic pesticides is one of the fastest acting tools at farmers' disposal to prevent and mitigate the threats posed by plant pests in agriculture. However, the effects of these above-ground applications of pesticides are known to be detrimental to some belowground, non-target soil biota. At present, the effects many pesticides have on key functional microbial groups associated with phosphate (P) solubilization in the soil are still largely unknown. The purpose of this study was to compare the effects of two herbicides, glyphosate, and paraquat, on phosphate solubilizing bacteria (PSB) with and without pH adjustment (after herbicide addition) since pH is a major indicator of P solubilization. In our assay, two PSB strains (Pantoea agglomerans and Serratia rubidaea) were chosen to assess their ability to solubilize tricalcium phosphate (TCP) by using the vanadate-molybdate method (to measure the amount of P solubilized) in the presence of glyphosate (5.4 g/L and 10.8 g/L) or paraquat (2 g/L and 4 g/L) separately. To assess the effect of PSB treated by the herbicides, a growth experiment using PSB inoculated wheat seedlings was performed under greenhouse conditions (25 °C, light 16 h/8 h dark). After four weeks, wheat above-ground growth parameters were measured. Our results showed that even under recommended doses of glyphosate (5.4 g/L) and paraquat (2 g/L), a decrease in P solubilization activity was observed in P. agglomerans and S. rubidaea. Whilst paraquat affected TCP solubilization more than glyphosate with and without pH adjustment, there was a significant decrease (p < 0.05) in TCP solubilization, up to 39% and 93% in the presence of glyphosate and paraquat, respectively, for S. rubidaea, and up to 45% and 95% in the presence of glyphosate and paraquat, respectively, for P. agglomerans. The effect of the herbicides on the PSB had the same results as in the greenhouse test on wheat seedling growth, confirming that these herbicides have both above and belowground negative effects, despite being used at recommended doses.

Assessment of Potent Phosphate-Solubilizing Bacteria Isolated from the Olive Tree Rhizosphere Grown on Phosphate Sludge and Their Effect on Common Bean Growth

One of the most restricting macronutrients for crop yield worldwide is phosphorus, which is often less than 1% of the total amount contained in soils. To promote its bioavailability, there are microorganisms considered vital that can solubilize and mineralize their pools in soils. In the current study, we assessed the potential of phosphate-solubilizing bacteria (PSB), isolated from the olive tree rhizosphere grown on phosphate sludge, to solubilize phosphate, promote plant growth, and tolerate abiotic stresses. Based on 16S rRNA gene sequencing, twenty-four PSB strains were identified and retained for in vitro analysis. According to the results, all the strains were able to solubilize the Moroccan rock phosphate, with soluble phosphate concentrations ranged from 5.34 µg/mL to 227 µg/mL. Furthermore, the majority of the strains are thermo-tolerant and halotolerant. Nonetheless, only five strains produced indole acetic acid. Regarding biocontrol potentialities, several PSB strains were characterized by producing hydrogen cyanide and hydrolytic enzymes (cellulase, and chitinase), of which three strains identified as Pseudomonas moraviensis, Bacillus cereus, and Bacillus aryabhattai, with a remarkable multi-trait combination were selected for antagonism and co-inoculation tests. The findings revealed that these PSB strains significantly inhibited Fusarium oxysporum (17.65%–62.35%) and Verticillium dahliae (52.35%–66.87%) and promoted common bean growth. The consortium of the three strains showed the best results by significantly increasing both plant height and tap root length and dry biomass compared to individual inoculation. The PSB selected from the olive tree rhizosphere growing on phosphate sludge have the potential to be useful as biofertilizer and biocontrol agents for attaining sustainable food crop production.

Inoculations of phosphate-solubilizing bacteria alter soil microbial community and improve phosphorus bioavailability for moso bamboo (Phyllostachys edulis) growth

Moso bamboo plantations suffer from phosphorus (P) limitations. A better understanding of soil P cycling contributes to the sustainable management and development of the bamboo forests. In this study, different phosphate-solubilizing bacteria (PSB) were applied to soils to stimulate bamboo growth, and the metagenomics method was employed to investigate the changes in soil microbial communities and relative abundances of functional genes. Meanwhile, soil properties and bamboo biomass and physiological indices were also quantitatively analyzed. The PSB inoculations changed soil P fractions and significantly elevated available P (AP) content by stimulating phosphatase activity and functional genes involved in P-transformation. The PSB inoculations significantly stimulated soil acid phosphatase activities and enhanced the relative abundances of functional genes associated with inorganic P-solubilization and organic P-mineralization. Moreover, increased soil available nutrients were beneficial to the moso bamboo growth, and the bamboo biomass in the different treatments increased by from 32.43 % to 69.04 % compared with the control treatment. Significant increases in root activity, carotenoid, chlorophyll a and total chlorophyll contents after the PSB inoculations also contributed to moso bamboo growth. The PSB strain inoculations shaped soil microbial communities and increased microbial connections. Besides adjusting soil P supplies and functional genes, the PSB also promoted the moso bamboo growth by increasing soil mineral N, AK and bamboo chlorophyll contents and stimulating functional microorganisms. Our study could provide a theoretical basis to improve soil nutrient utilization for moso bamboo with the PSB inoculations.

Plant growth promoting characterization of indigenous phosphate solubilizing rhizobacteria and effects on germination of some crops in Vietnam

Inorganic Phosphate Solubilizing Bacteria (PSB) are widely present in nature and have been successfully applied in fields in many countries. However, researches on indigenous PSB are still very limited in Vietnam. The objective of this study was to isolate and evaluate the growth promoting characteristics and effects on germination of seed of indigenous PSB for fertilizer production from PSB. Thirteen isolates of indigenous PSB were collected using selective isolation medium containing Ca3(PO4)2, of which eight added the ability to dissolve AlPO4 and five isolates added the ability to dissolve FePO4. Initial qualitative tests indicated that all thirteen PSB isolates were incapable of HCN and lipase production while other growth promoting activities including amylase, caseinase, cellulase, chitinase, pectinase, indole acetic acid, K solubility, Zn solubility, and N fixation varied according to PSB isolate. The collected PSB isolates had no effect on seed germination rate, root length, and hypocotyl length of plantules of soybean, rice, maize, cucumber, tomato, and chili by plate assay. This study had shown that PSB was also common in the rhizosphere of various crops in Vietnam. Therefore, isolation to enrich the indigenous PSB collection was essential for the screening of suitable PSB strains for subsequent fertilizer production.

Cold-tolerant phosphate-solubilizing Pseudomonas strains promote wheat growth and yield by improving soil phosphorous (P) nutrition status.

It is well-known that phosphate-solubilizing bacteria (PSB) promote crop growth and yield. The information regarding characterization of PSB isolated from agroforestry systems and their impact on wheat crops under field conditions is rarely known. In the present study, we aim to develop psychrotroph-based P biofertilizers, and for that, four PSB strains (Pseudomonas sp. L3, Pseudomonas sp. P2, Streptomyces sp. T3, and Streptococcus sp. T4) previously isolated from three different agroforestry zones and already screened for wheat growth under pot trial conditions were evaluated on wheat crop under field conditions. Two field experiments were employed; set 1 includes PSB + recommended dose of fertilizers (RDF) and set 2 includes PSB - RDF. In both field experiments, the response of the PSB-treated wheat crop was significantly higher compared to the uninoculated control. In field set 1, an increase of 22% in grain yield (GY), 16% in biological yield (BY), and 10% in grain per spike (GPS) was observed in consortia (CNS, L3 + P2) treatment, followed by L3 and P2 treatments. Inoculation of PSB mitigates soil P deficiency as it positively influences soil alkaline phosphatase (AP) and soil acid phosphatase (AcP) activity which positively correlated with grain NPK %. The highest grain NPK % was reported in CNS-treated wheat with RDF (N-0.26%, P-0.18%, and K-1.66%) and without RDF (N-0.27, P-0.26, and K-1.46%), respectively. All parameters, including soil enzyme activities, plant agronomic data, and yield data were analyzed by principal component analysis (PCA), resulting in the selection of two PSB strains. The conditions for optimal P solubilization, in L3 (temperature-18.46, pH-5.2, and glucose concentration-0.8%) and P2 (temperature-17°C, pH-5.0, and glucose concentration-0.89%), were obtained through response surface methodology (RSM) modeling. The P solubilizing potential of selected strains at <20°C makes them a suitable candidate for the development of psychrotroph-based P biofertilizers. Low-temperature P solubilization of the PSB strains from agroforestry systems makes them potential biofertilizers for winter crops.

Periplasmic space is the key location for Pb(II) biomineralization by Burkholderia cepacia.

Phosphate solubilizing bacteria (PSB) induced phosphate precipitation is considered as an effective method for Pb(II) removal through the formation of stable Pb(II)-phosphate compound, but the location of end-products is still unclear. Herein, the PSB strain of Burkholderia cepacia (B. cepacia) coupled with the hydroxyapatite (HAP) was used in this study to investigate the Pb(II) removal mechanism and the biomineralization location. The dissolving phosphate of three particle sizes of HAP and Pb(II) resistant capabilities, and the effect factors such as HAP dosage, initial concentrations of Pb(II), pH, temperature, and different treatments were determined. The results indicated that the highest soluble phosphate could reach 224.85mg/L in a 200nm HAP medium and the highest removal efficiency of Pb(II) was about 96.32%. Additionally, it was interesting that Pb(II) was mainly located in the periplasmic space through the cellular distribution experiment, which was further demonstrated by scanning electron microscope (SEM) and transmission electron microscopy (TEM). Besides, the characterization results showed that the functional groups such as amide, hydroxy, carboxy and phosphate played an important role in Pb(II) biomineralization, and the free Pb(II) in aqueous solution could be transformed into pyromorphite through phosphate dissolution, extracellular adsorption/complexation, and intracellular precipitation.

Screening of phosphorus solubilizing microorganisms in cold environment and their effects on the growth of Brassica napus.

To screen out phosphorus solubilizing strains that can adapt to cold climate in Qinghai Province, Bacillus mucilaginosus, B. megaterium, B. cereus, Streptomyces violovariabilis, S. cinnamofuscus, and S. flavoagglomeratus were screened with solid plate medium as the primary and liquid medium as the secondary screening, with calcium phosphate, lecithin, and phytic acid as the single source of phosphorus. By comprehensively comparing the size of phosphate solubilizing circle in the solid plate medium and the soluble phosphorus content in the liquid medium, three strains of phosphate solubilizing bacteria with good phosphate solubilizing effects were screened, S. violovariabilis, S. cinnamofuscus, and B. mucilaginosus. The three phosphate solubilizing bacteria were made into liquid ino-culants, and the small rapeseed pot experiment was carried out with two soils with different fertilities in a cold climate in September. Compared with the control, plant height, fresh weight, root length, and root weight of rapes in high-fertility cultivated soil increased by 35.5%, 191.0%, 26.2%, and 282.7%, while plant phosphorus absorption, total phosphorus and available phosphorus contents in the rhizosphere soil increased by 968.9%, 5.1%, and 2.1%, respectively. In low-fertility soil, plant height and fresh weight was increased by 45.8% and 61.3%, root length and weight was decreased by 2.6% and 4.4%, while plant phosphorus absorption and the contents of total P and available P in rhizosphere soil were increased by 91.5 %, 29.1%, and 213.7%, respectively. The effect of the other two inoculants treatments was less significant than S. violovariabilis. Therefore, S. violovariabilis was the phosphate solubilizing strain suitable for the cold climate in Qinghai.

Effects of Phosphate-solubilizing Bacteria on Soil Phosphorus Fractions and Supply to Maize Seedlings Grown in Lateritic Red Earths and Cinnamon Soils.

Phosphorus (P) is often the limiting factor for plant growth because of its low mobility and availability in soils. Phosphate-solubilizing bacteria (PSB) have been shown to increase the availability of soil P fractions, thereby promoting plant growth. We herein investigated the effects of PSB on P availability in two important Chinese soil types: Lateritic red earths (La) and Cinnamon soils (Ci). We initially isolated 5 PSB strains and assessed their effects on soil P fractions. PSB mainly increased moderately labile P in La and labile P in Ci. We then selected the most promising PSB isolate (99% similarity with Enterobacter chuandaensis) and examined its effects on P accumulation in maize seedlings. The results obtained showed that plant P accumulation increased in response to a PSB inoculation in both soil types and the combination of the PSB inoculation and tricalcium phosphate fertilization in La significantly enhanced P accumulation in plant shoots. The present study demonstrated that the PSB isolates tested differed in their ability to mobilize P from distinct P fertilizers and that PSB isolates have potential as a valuable means of sustainably enhancing seedling growth in Chinese agricultural soils.

Effects and mechanisms of phosphate solubilizing bacteria on enhancing phytoextraction of lead from contaminated soil by Celosia cristata L.

Phosphate solubilizing bacteria (PSB)-assisted phytoextraction can enhance the removal of toxic metals by plants from contaminated soil. Here, we investigated the promotion effects of a Pb-tolerant PSB strain, Leclercia adecarboxylata L1-5, on assisting Pb phytoextraction by Celosia cristata L.. The results showed that PSB inoculation could increase the plant shoot and root biomass by 34.90% and 55.56% when the Pb concentration was 800 mg kg−1. Pb contents in the root and shoot were enhanced 3.06 and 0.72-fold by PSB inoculation, resulting in a bioaccumulation factor (BCFaboveground) of 0.2, a translocation factor (TF) of 1.92, and a maximum total Pb extraction amount of 222.94 μg kg−1. The positive effects of PSB on Pb bioaccumulation by C. cristata L. could be attributed to its abilities of increasing soil available phosphorus. PSB inoculation benefited osmotic potential balance and membrane stability in the plant leaves, as evidenced by 2.39-fold increase of the soluble protein, 71.39% decrease of malondialdehyde and 74.31% decrease of hydrogen peroxide. The main mechanism of the enhancement of Pb phytoextraction efficiency induced by PSB inoculation could be summarized as helping Pb absorption by root elongation, and increasing endurable Pb concentration by osmotic regulation and detoxification ability optimization. This work demonstrated that PSB combined with C. cristata L. was an excellent candidate for lead extraction and could be considered as an effective way for soil lead remediation in the future.

Relationships among phosphatase activities, functional genes and soil properties following amendment with the bacterium Burkholderia sp. ZP‐4

AbstractPhosphorus (P) limitation is common in subtropical and tropical regions. Although phosphate‐solubilizing bacteria (PSB) can transform soil P fractions and enhance soil P availability, the mechanism regarding the linkage between PSB abundance and soil P fractions transformation were still unknown. In this study, Burkholderia sp. ZP‐4, a PSB strain, was inoculated into subtropical bamboo forest soil at the inoculation rates of 0% (bacterial suspension: soil weight = v: w, the control), 2% (2%IR), 6% (6%IR) and 10% (10%IR). The changes in soil P fractions, bacterial community, phosphatase activities, P transforming functional genes and soil properties were also determined after the PSB inoculation. Compared with the control, soil inorganic P extracted by deionized water (H2O‐Pi) and extracted by sodium bicarbonate (NaHCO3‐Pi) in the 6%IR treatments were increased 45.35% and 16.05%, respectively. The lowest content of residual P (residual‐P) was in the 6%IR treatment among the four treatments. The 6%IR treatment contributed to stimulating and inducing soil acid phosphatase activity and P cycling genes (phoD and phoC) abundances, relative to the control. The PSB inoculation significantly decreased soil pH but increased soil available nitrogen P supplies among all treatments. Simultaneously, the PSB inoculation also significantly changed soil bacterial community. The transformation of P fractions in the 6%IR was more efficient than the other treatments, implying that 6% is the proper inoculation rate of functional Burkholderia for accelerating P cycling at subtropical bamboo forest soil. Our findings will provide a scientific guidance for the application of biofertilizer.

Phosphate-solubilizing bacteria abate cadmium absorption and restore the rhizospheric bacterial community composition of grafted watermelon plants

The grafting of watermelon plants to rootstocks is common due to the strong capacity of rootstocks to adapt to abiotic and biotic stresses. However, the effect of phosphate-solubilizing bacteria (PSB) on grafted watermelon plant growth and bacterial structures in root soil is unclear. In this study, the growth and hormone levels of grafted plants were measured, and the bacterial communities under cadmium (Cd) stress and inoculation with PSB were sequenced in three treatments (S1, control; S2, 50 μmol Cd [CdCl2]; and S3, 50 μmol Cd plus inoculation with the Cd-resistant PSB strain ‘N3′). The results showed that inoculation with PSB significantly (P < 0.05) improved the total dry weight of the grafted plants. Typically, inoculation with PSB significantly (P < 0.05) reduced Cd content in scions and roots. The level of the phytohormone jasmonic acid increased in treatment S2, but decreased in treatment S3 under inoculation with PSB. The functional annotation of prokaryotic taxa showed that Cd decreased the abundance of nitrogen respiration and chloroplast functional groups. Nevertheless, inoculation with PSB helped restore bacterial community structures. These findings provide a new understanding of the effect of PSB on the promotion of seedling growth and bacterial communities in grafted watermelon plants under Cd stress.

Improvement of phosphorus uptake, phosphorus use efficiency, and grain yield of upland rice (Oryza sativa L.) in response to phosphate-solubilizing bacteria blended with phosphorus fertilizer

Phosphorus (P) limitation in soil is a major concern for crop productivity. However, the use of chemical fertilizer is hazardous to the environment and costly. Therefore, the use of phosphate-solubilizing bacteria (PSB) is an eco-friendly approach for a sustainable agricultural system. In the present study, a field trial was conducted for two consecutive years to study the effects of three PSB strains isolated, Bacillus licheniformis, Pantoea dispersa, and Staphylococcus sp., with different P fertilizer rates on P uptake, P use efficiency (PUE), and grain yield of rice. The activities of soil enzymes were also studied in relation to PSB treatments. Comparative analysis of the yield and biochemical parameters revealed that inoculation of PSB consortium could reduce almost 50% of the recommended P dose in rice cultivation. Three PSB strains in combination with 50% P dose was most effective and showed the highest increases in P uptake and PUE as compared to the uninoculated control. Moreover, the PSB consortium combined with 50% P dose contributed to 50.58% and 35.64% yield increases compared to the uninoculated control for 2018 and 2019, respectively. Significant increases in the activities of soil dehydrogenase, alkaline phosphatase, and acid phosphatase were also recorded under PSB treatment.

Phosphate-Solubilizing Bacteria Isolated from Phosphate Solid Sludge and Their Ability to Solubilize Three Inorganic Phosphate Forms: Calcium, Iron, and Aluminum Phosphates.

Biofertilizers are a key component of organic agriculture. Bacterial biofertilizers enhance plant growth through a variety of mechanisms, including soil compound mobilization and phosphate solubilizing bacteria (PSB), which convert insoluble phosphorus to plant-available forms. This specificity of PSB allows them to be used as biofertilizers in order to increase P availability, which is an immobile element in the soil. The objective of our study is to assess the capacity of PSB strains isolated from phosphate solid sludge to solubilize three forms of inorganic phosphates: tricalcium phosphate (Ca3(PO4)2), aluminum phosphate (AlPO4), and iron phosphate (FePO4), in order to select efficient solubilization strains and use them as biofertilizers in any type of soil, either acidic or calcareous soil. Nine strains were selected and they were evaluated for their ability to dissolve phosphate in the National Botanical Research Institute’s Phosphate (NBRIP) medium with each form of phosphate (Ca3(PO4)2, AlPO4, and FePO4) as the sole source of phosphorus. The phosphate solubilizing activity was assessed by the vanadate-molybdate method. All the strains tested showed significantly (p ≤ 0.05) the ability to solubilize the three different forms of phosphates, with a variation between strains, and all strains solubilized Ca3(PO4)2 more than FePO4 and AlPO4.

Designing Synergistic Biostimulants Formulation Containing Autochthonous Phosphate-Solubilizing Bacteria for Sustainable Wheat Production.

Applying phosphate-solubilizing bacteria (PSB) as biofertilizers has enormous potential for sustainable agriculture. Despite this, there is still a lack of information regarding the expression of key genes related to phosphate-solubilization (PS) and efficient formulation strategies. In this study, we investigated rock PS by Ochrobactrum sp. SSR (DSM 109610) by relating it to bacterial gene expression and searching for an efficient formulation. The quantitative PCR (qPCR) primers were designed for PS marker genes glucose dehydrogenase (gcd), pyrroloquinoline quinone biosynthesis protein C (pqqC), and phosphatase (pho). The SSR-inoculated soil supplemented with rock phosphate (RP) showed a 6-fold higher expression of pqqC and pho compared to inoculated soil without RP. Additionally, an increase in plant phosphorous (P) (2%), available soil P (4.7%), and alkaline phosphatase (6%) activity was observed in PSB-inoculated plants supplemented with RP. The root architecture improved by SSR, with higher root length, diameter, and volume. Ochrobactrum sp. SSR was further used to design bioformulations with two well-characterized PS, Enterobacter spp. DSM 109592 and DSM 109593, using the four organic amendments, biochar, compost, filter mud (FM), and humic acid. All four carrier materials maintained adequate survival and inoculum shelf life of the bacterium, as indicated by the field emission scanning electron microscopy analysis. The FM-based bioformulation was most efficacious and enhanced not only wheat grain yield (4–9%) but also seed P (9%). Moreover, FM-based bioformulation enhanced soil available P (8.5–11%) and phosphatase activity (4–5%). Positive correlations were observed between the PSB solubilization in the presence of different insoluble P sources, and soil available P, soil phosphatase activity, seed P content, and grain yield of the field grown inoculated wheat variety Faisalabad-2008, when di-ammonium phosphate fertilizer application was reduced by 20%. This study reports for the first time the marker gene expression of an inoculated PSB strain and provides a valuable groundwork to design field scale formulations that can maintain inoculum dynamics and increase its shelf life. This may constitute a step-change in the sustainable cultivation of wheat under the P-deficient soil conditions.

Insight into soil nitrogen and phosphorus availability and agricultural sustainability by plant growth-promoting rhizobacteria.

Nitrogen and phosphorus are critical for the vegetation ecosystem and two of the most insufficient nutrients in the soil. In agriculture practice, many chemical fertilizers are being applied to soil to improve soil nutrients and yield. This farming procedure poses considerable environmental risks which affect agricultural sustainability. As robust soil microorganisms, plant growth-promoting rhizobacteria (PGPR) have emerged as an environmentally friendly way of maintaining and improving the soil's available nitrogen and phosphorus. As a special PGPR, rhizospheric diazotrophs can fix nitrogen in the rhizosphere and promote plant growth. However, the mechanisms and influences of rhizospheric nitrogen fixation (NF) are not well researched as symbiotic NF lacks summarizing. Phosphate-solubilizing bacteria (PSB) are important members of PGPR. They can dissolve both insoluble mineral and organic phosphate in soil and enhance the phosphorus uptake of plants. The application of PSB can significantly increase plant biomass and yield. Co-inoculating PSB with other PGPR shows better performance in plant growth promotion, and the mechanisms are more complicated. Here, we provide a comprehensive review of rhizospheric NF and phosphate solubilization by PGPR. Deeper genetic insights would provide a better understanding of the NF mechanisms of PGPR, and co-inoculation with rhizospheric diazotrophs and PSB strains would be a strategy in enhancing the sustainability of soil nutrients.

Are native phosphate solubilizing bacteria a relevant alternative to mineral fertilizations for crops? Part I. when rhizobacteria meet plant P requirements.

Deployment of Phosphate Solubilizing Bacteria (PSB) can enhance plant P uptake through remobilization of recalcitrant forms of P in soil. In this study, bacterial strains isolated in the rhizospheric soil of rapeseed, winter pea and faba bean, with various P requirements, were screened for their PSB potential through the characterization of several interesting traits linked to P solubilization. In parallel, other Plant Growth Promoting Rhizobacteria (PGPR) traits of interest (indole-3-acetic acid (IAA) production or 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase activity) were also determined for each isolate. An abundant proportion of PSB strains were successfully emphasized, mainly belonging to Pseudomonas and Bacillus genera. The present study also reveals for the first time the potential of phosphate solubilization of an isolate belonging to the species Sanguibacter inulinus. Interestingly, PSB isolated from rapeseed soil showed higher solubilization efficiency compared to those isolated from faba bean and winter pea rhizosphere. These results highlighted the great influence of plant to shape their associated microbiome in order to fit specific P requirement. Two distinct profiles of PSB isolates could be highlighted, deploying either direct or indirect processes. Plants seemed to preferentially enrich PSB populations presenting complementary characteristics to their own arsenal to favor P nutrition. Valuing native PSB potential, through artificial addition or through the legacy of previous crop for the further culture could represent a promising alternative to sustain plant growth in P-deprived situation.

Indigenous plant growth-promoting rhizobacterial consortia greatly reduces fertilizer need for tea nurseries: Characterization and evaluation

A study was conducted to characterize plant growthpromoting rhizobacteria (PGPR) of tea [Camellia sinensis (L.) O. Kuntze] and to investigate the potential of using them to enhance growth and nutrient uptake of tea plants, and also to reduce the use of chemical fertilizers in tea nurseries in the mid-country of Sri Lanka. Nitrogenfixing Azospirillum sp. (AZO) and phosphate solubilizing bacteria (PSB) isolated from the rhizosphere of tea grown in three main soil series i.e. Kandy, Matale and Ukuwela in Sri Lanka, were screened in an in vitro study. Soil series specific dual inoculants were formulated using the most effective strains of N2 fixer and PSB obtained from each soil series, and they were tested in tea nurseries having respective soil series. In addition, a common consortium was tested across all the three soil series. Dual inoculants were tested along with a modified T 65 fertilizer mixture composed of ½ of N and P replaced with Eppawala Rock Phosphate (ERP) in place of Di Ammonium Phosphate (DAP) in recommended T 65 fertilizer and compared with two non-inoculated controls; modified T 65 fertilizer and T 65 recommended fertilizer, each with ten replicates. Plant growth measurements were taken and N and P uptakes were measured. Application of series-specific consortium to nursery tea plants raised in soil belonging to Ukuwela soil series along with modified T 65 fertilizer mixture improved dry matter contents (12.86 g/plant) and total N and P uptake (786 mg/plant and 155 mg/plant, respectively) compared to that of recommended fertilizer treatment (dry matter contents; 13.24 g/plant, N uptake; 727 mg/plant and P uptake; 166 mg/plant). Results indicated that a 50% reduction of N and replacement of imported DAP with locally available ERP in the present recommendation for tea nursery mixture (i.e. T 65) is possible with the application of dual inoculants formulated with series-specific strains.