Glucose dehydrogenase gene containing phosphobacteria for biofortification of Phosphorus with growth promotion of rice

Prospects of phosphate solubilizing microorganisms in sustainable agriculture

Phosphorus (P), an essential macronutrient for various plant processes, is generally a limiting soil component for crop growth and yields. Organic and inorganic types of P are copious in soils, but their phyto-availability is limited as it is present largely in insoluble forms. Although phosphate fertilizers are applied in P-deficit soils, their undue use negatively impacts soil quality and the environment. Moreover, many P fertilizers are lost because of adsorption and fixation mechanisms, further reducing fertilizer efficiencies. The application of phosphate-solubilizing microorganisms (PSMs) is an environmentally friendly, low-budget, and biologically efficient method for sustainable agriculture without causing environmental hazards. These beneficial microorganisms are widely distributed in the rhizosphere and can hydrolyze inorganic and organic insoluble P substances to soluble P forms which are directly assimilated by plants. The present review summarizes and discusses our existing understanding related to various forms and sources of P in soils, the importance and P utilization by plants and microbes,, the diversification of PSMs along with mixed consortia of diverse PSMs including endophytic PSMs, the mechanism of P solubilization, and lastly constraints being faced in terms of production and adoption of PSMs on large scale have also been discussed.

Mechanism of A. oleivorans S4 treating soluble phosphorus deficiency and hydrocarbon contamination simultaneously

Both soluble phosphorus (P) deficiency and petroleum hydrocarbon contamination represent challenges in soil environments. While phosphate-solubilizing bacteria and hydrocarbon-degrading bacteria have been identified and employed in environmental bioremediation, the bacteria co-adapted to soluble P deficiency and hydrocarbon contamination has rarely been reported. This study explored the ability of Acinetobacter oleivorans S4 (A. oleivorans S4) to solubilize phosphate using n-hexadecane (H), glucose (G), and a mixed carbon source (HG) in tricalcium phosphate (TCP) medium. A. oleivorans S4 exhibited robust growth in H-TCP, releasing 31 mg L-1 of soluble P. Conversely, A. oleivorans S4 barely grew in G-TCP, releasing 654 mg L-1 of soluble P. In HG-TCP, biomass surpassed that in H-TCP, with phosphate release comparable to that in G-TCP. HPLC analysis revealed a small amount of TCA cycle acids in H-TCP and a large amount of gluconate in G-TCP and HG-TCP. Transcriptomic analysis showed elevated expression of genes associated with alkane degradation, P starvation, N utilization, and trehalose synthesis in H-TCP, revealing the molecular co-adaptation mechanism of A. oleivorans S4. Furthermore, the addition of glucose enhanced alkane degradation, P and N utilization, and reduced trehalose synthesis. It indicated that incomplete glucose metabolism may provide energy for other reactions, and the increase in soluble P mediated by gluconate may alleviate oxidative stress. Overall, A. oleivorans S4 proves promising for remediating soluble P-deficient and hydrocarbon-contaminated environments, and glucose stimulates its transformation into a super phosphate-solubilizing bacterium.

Plant communication with rhizosphere microbes can be revealed by understanding microbial functional gene composition

Understanding rhizosphere microbial ecology is necessary to reveal the interplay between plants and associated microbial communities. The significance of rhizosphere-microbial interactions in plant growth promotion, mediated by several key processes such as auxin synthesis, enhanced nutrient uptake, stress alleviation, disease resistance, etc., is unquestionable and well reported in numerous literature. Moreover, rhizosphere research has witnessed tremendous progress due to the integration of the metagenomics approach and further shift in our viewpoint from taxonomic to functional diversity over the past decades. The microbial functional genes corresponding to the beneficial functions provide a solid foundation for the successful establishment of positive plant-microbe interactions. The microbial functional gene composition in the rhizosphere can be regulated by several factors, e.g., the nutritional requirements of plants, soil chemistry, soil nutrient status, pathogen attack, abiotic stresses, etc. Knowing the pattern of functional gene composition in the rhizosphere can shed light on the dynamics of rhizosphere microbial ecology and the strength of cooperation between plants and associated microbes. This knowledge is crucial to realizing how microbial functions respond to unprecedented challenges which are obvious in the Anthropocene. Unraveling how microbes-mediated beneficial functions will change under the influence of several challenges, requires knowledge of the pattern and composition of functional genes corresponding to beneficial functions such as biogeochemical functions (nutrient cycle), plant growth promotion, stress mitigation, etc. Here, we focus on the molecular traits of plant growth-promoting functions delivered by a set of microbial functional genes that can be useful to the emerging field of rhizosphere functional ecology.

Strigolactones shape the assembly of root-associated microbiota in response to phosphorus availability

Plants rely on strigolactones (SLs) to regulate their development and form symbiotic relationships with microbes as part of the adaptive phosphorus (P) efficiency strategies. However, the impact of SLs on root-associated microbial communities in response to P availability remains unknown. Here, root microbiota of SL biosynthesis (max3-11) and perception (d14-1) were compared to wild-type Col-0 plants under different P concentrations. Using high-throughput sequencing, the relationship between SLs, P concentrations, and the root-associated microbiota was investigated to reveal the variation in microbial diversity, composition, and interaction. Plant genotypes and P availability played important but different roles in shaping the root-associated microbial community. Importantly, SLs were found to attract Acinetobacter in low P conditions, which included an isolated CP-2 (Acinetobacter soli) that could promote plant growth in cocultivation experiments. Moreover, SLs could change the topologic structure within co-occurrence networks and increase the number of keystone taxa (e.g., Rhizobiaceae and Acidobacteriaceae) to enhance microbial community stability. This study reveals the key role of SLs in mediating root-associated microbiota interactions.IMPORTANCEStrigolactones (SLs) play a crucial role in plant development and their symbiotic relationships with microbes, particularly in adapting to phosphorus levels. Using high-throughput sequencing, we compared the root microbiota of plants with SL biosynthesis and perception mutants to wild-type plants under different phosphorus concentrations. These results found that SLs can attract beneficial microbes in low phosphorus conditions to enhance plant growth. Additionally, SLs affect microbial network structures, increasing the stability of microbial communities. This study highlights the key role of SLs in shaping root-associated microbial interactions, especially in response to phosphorus availability.

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

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

Characterization and evaluation of potential halotolerant phosphate solubilizing bacteria from Salicornia fruticosa rhizosphere

Soil salinization is a significant abiotic factor threatening agricultural production, while the low availability of phosphorus (P) in plants is another worldwide limitation. Approximately 95-99% of the P in soil is unavailable to plants. Phosphate-solubilizing bacteria (PSB) transform insoluble phosphates into soluble forms that plants can utilize. The application of PSB can replace or partially reduce the use of P fertilizers. Therefore, selecting bacteria with high solubilization capacity from extreme environments, such as saline soils, becomes crucial. This study aimed to identify twenty-nine bacterial strains from the rhizosphere of Salicornia fruticosa by sequencing the 16S rDNA gene, evaluate their development in increasing concentrations of NaCl, classify them according to their salinity response, and determine their P solubilization capability. The bacteria were cultivated in nutrient agar medium with NaCl concentrations ranging from 0.5% to 30%. The phosphate solubilization capacity of the bacteria was evaluated in angar and broth National Botanical Research Institute (NBRIP) media supplemented with calcium phosphate (CaHPO4) and aluminum phosphate (AlPO4), and increased with 3% NaCl. All bacterial strains were classified as halotolerant and identified to the genera Bacillus, Enterobacter, Halomonas, Kushneria, Oceanobacillus, Pantoea, Pseudomonas, and Staphylococcus, with only one isolate was not identified. The isolates with the highest ability to solubilize phosphorus from CaHPO4 in the liquid medium were Kushneria sp. (SS102) and Enterobacter sp. (SS186), with 989.53 and 956.37 mg·Kg-1 P content and final pH of 4.1 and 3.9, respectively. For the solubilization of AlPO4, the most effective isolates were Bacillus sp. (SS89) and Oceanobacillus sp. (SS94), which raised soluble P by 61.10 and 45.82 mg·Kg-1 and final pH of 2.9 and 3.6, respectively. These bacteria demonstrated promising results in in vitro P solubilization and can present potential for the development of bioinput. Further analyses, involving different phosphate sources and the composition of produced organic acids, will be conducted to contribute to a comprehensive understanding of their applications in sustainable agriculture.

Phosphate-Solubilizing Bacteria: Advances in Their Physiology, Molecular Mechanisms and Microbial Community Effects

Phosphorus is an essential nutrient for all life on earth and has a major impact on plant growth and crop yield. The forms of phosphorus that can be directly absorbed and utilized by plants are mainly HPO42- and H2PO4-, which are known as usable phosphorus. At present, the total phosphorus content of soils worldwide is 400-1000 mg/kg, of which only 1.00-2.50% is plant-available, which seriously affects the growth of plants and the development of agriculture, resulting in a high level of total phosphorus in soils and a scarcity of available phosphorus. Traditional methods of applying phosphorus fertilizer cannot address phosphorus deficiency problems; they harm the environment and the ore material is a nonrenewable natural resource. Therefore, it is imperative to find alternative environmentally compatible and economically viable strategies to address phosphorus scarcity. Phosphorus-solubilizing bacteria (PSB) can convert insoluble phosphorus in the soil into usable phosphorus that can be directly absorbed by plants, thus improving the uptake and utilization of phosphorus by plants. However, there is no clear and systematic report on the mechanism of action of PSB. Therefore, this paper summarizes the discovery process, species, and distribution of PSB, focusing on the physiological mechanisms outlining the processes of acidolysis, enzymolysis, chelation and complexation reactions of PSB. The related genes regulating PSB acidolysis and enzymatic action as well as genes related to phosphate transport and the molecular direction mechanism of its pathway are examined. The effects of PSB on the structure and abundance of microbial communities in soil are also described, illustrating the mechanism of how PSB interact with microorganisms in soil and indirectly increase the amount of available phosphorus in soil. And three perspectives are considered in further exploring the PSB mechanism in utilizing a synergistic multi-omics approach, exploring PSB-related regulatory genes in different phosphorus levels and investigating the application of PSB as a microbial fungicide. This paper aims to provide theoretical support for improving the utilization of soil insoluble phosphorus and providing optimal management of elemental phosphorus in the future.

Phosphate-solubilizing bacteria: Their agroecological function and optimistic application for enhancing agro-productivity

Phosphorus (P) is a limiting nutrient in the soil-plant nutrient cycling. Although the exogenous application of chemical P fertilizers can satisfy crop P requirements during critical growth phases. While excessive P fertilizers use results in low phosphorus acquisition efficiency (PAE), it has serious environmental consequences and hastens the depletion of P mineral reserves. Phosphate-solubilizing bacteria (PSB) have the potential to make insoluble phosphate available to plants through solubilization and mineralization, increasing crop yields while maintaining environmental sustainability. Existing reviews mainly focus on the beneficial effects of PSB on crop performance and related mechanisms, while few of them elucidate the action mechanisms of PSB in soil-microbe-plant interactions for crop cultivation with high yield efficiency. Hence, this study provides a comprehensive review of the physicochemical and molecular mechanisms (e.g., root exudates, extracellular polysaccharides, organic acids, phosphatases, and phosphate-specific transport systems) of PSB to facilitate the P cycle in the soil-plant systems. Further, the potential of commercial applications of PSB (e.g., genetic engineering, seed priming and coating) are also discussed in order to highlight their contribution to sustainable agriculture. Finally, existing challenges and future prospects in agricultural applications are proposed. In conclusion, we firmly believe that PSB represent a highly significant biotechnological tool for enhancing agricultural productivity and offers a wide range of extensive potential applications.

Growth improvement of wheat (Triticum aestivum) and zinc biofortification using potent zinc-solubilizing bacteria

Zinc (Zn) is an indispensable element for proper plant growth. A sizeable proportion of the inorganic Zn that is added to soil undergoes a transformation into an insoluble form. Zinc-solubilizing bacteria (ZSB) have the potential to transform the insoluble Zn into plant-accessible forms and are thus promising alternatives for Zn supplementation. The current research was aimed at investigating the Zn solubilization potential of indigenous bacterial strains and to evaluate their impact on wheat growth and Zn biofortification. A number of experiments were conducted at the National Agriculture Research Center (NARC), Islamabad, during 2020-21. A total of 69 strains were assessed for their Zn-solubilizing ability against two insoluble Zn sources (ZnO and ZnCO₃) using plate assay techniques. During the qualitative assay, the solubilization index and solubilization efficiency were measured. The qualitatively selected Zn-solubilizing bacterial strains were further tested quantitatively using broth culture for Zn and phosphorus (P) solubility. Tricalcium phosphate was used as insoluble source of P. The results showed that broth culture pH was negatively correlated with Zn solubilization, i.e., ZnO (r^{2 =} 0.88) and ZnCO₃ (r^{2 =} 0.96). Ten novel promising strains, i.e., Pantoea sp. NCCP-525, Klebsiella sp. NCCP-607, Brevibacterium sp. NCCP-622, Klebsiella sp. NCCP-623, Acinetobacter sp. NCCP-644, Alcaligenes sp. NCCP-650, Citrobacter sp. NCCP-668, Exiguobacterium sp. NCCP-673, Raoultella sp. NCCP-675, and Acinetobacter sp. NCCP-680, were selected from the ecology of Pakistan for further experimentation on wheat crop based on plant growth-promoting rhizobacteria (PGPR) traits, i.e., solubilization of Zn and P in addition to being positive for nifH and acdS genes. Before evaluating the bacterial strains for plant growth potential, a control experiment was also conducted to determine the highest critical Zn level from ZnO to wheat growth using different Zn levels (0.1, 0.05, 0.01, 0.005, and 0.001% Zn) against two wheat varieties (Wadaan-17 and Zincol-16) in sand culture under glasshouse conditions. Zinc-free Hoagland nutrients solution was used to irrigate the wheat plants. As a result, 50 mg kg^-1 of Zn from ZnO was identified as the highest critical level for wheat growth. Using the critical level (50 mg kg^-1 of Zn), the selected ZSB strains were inoculated alone and in consortium to the seed of wheat, with and without the use of ZnO, in sterilized sand culture. The ZSB inoculation in consortium without ZnO resulted in improved shoot length (14%), shoot fresh weight (34%), and shoot dry weight (37%); with ZnO root length (116%), it saw root fresh weight (435%), root dry weight (435%), and Zn content in the shoot (1177%) as compared to the control. Wadaan-17 performed better on growth attributes, while Zincol-16 had 5% more shoot Zn concentration. The present study concluded that the selected bacterial strains show the potential to act as ZSB and are highly efficient bio-inoculants to combat Zn deficiency, and the inoculation of these strains in consortium performed better in terms of growth and Zn solubility for wheat as compared to individual inoculation. The study further concluded that 50 mg kg^-1 Zn from ZnO had no negative impact on wheat growth; however, higher concentrations hampered wheat growth.

Characterization and genome analysis of Acinetobacter oleivorans S4 as an efficient hydrocarbon-degrading and plant-growth-promoting rhizobacterium

Plant-growth-promoting rhizobacteria (PGPR) have received increasing attention for assisting phytoremediation. However, the effect of PGPR on total petroleum hydrocarbon (TPH) degradation and plant growth promotion and its underlying mechanism is not well understood. In this study, phenotypic analysis and whole genome sequencing were conducted to comprehensively characterize a newly isolated rhizobacterium strain S4, which was identified as Acinetobacter oleivorans, from a TPH-contaminated soil. The strain degraded 62.5% of initially spiked diesel (1%) in minimal media within six days and utilized n-alkanes with a wide range of chain length (i.e., C₁₂ to C₄₀). In addition, the strain showed phenotypic traits beneficial to plant growth, including siderophore production, indole-3-acetic acid synthesis and phosphate solubilization. Potential metabolic pathways and genes encoding proteins responsible for the phenotypic traits were identified. In a real TPH-contaminated soil, inoculation of Acinetobacter oleivorans S4 significantly enhanced the growth of tall fescue relative to the soil without inoculation. In contrast, inoculation of Bacillus sp. Z7, a hydrocarbon-degrading strain, showed a negligible effect on the growth of tall fescue. The removal efficiency of TPH with inoculation of Acinetobacter oleivorans S4 was significantly higher than those without inoculation or inoculation of Bacillus sp. Z7. These results suggested that traits of PGPR beneficial to plant growth are critical to assist phytoremediation. Furthermore, heavy metal resistance genes and benzoate and phenol degradation genes were found in the genome of Acinetobacter oleivorans S4, suggesting its application potential in broad scenarios.

Mineral-Solubilizing Bacteria-Mediated Enzymatic Regulation and Nutrient Acquisition Benefit Cotton’s (Gossypium hirsutum L.) Vegetative and Reproductive Growth

Many farmers’ incomes in developing countries depend on the cultivation of major crops grown in arid and semi-arid regions. The agricultural productivity of arid and semi-arid areas primarily relies on chemical fertilizers. The effectiveness of chemical fertilizers needs to improve by integration with other sources of nutrients. Plant growth-promoting bacteria can solubilize nutrients, increase plant nutrient uptake, and supplement chemical fertilizers. A pot experiment evaluated the promising plant growth-promoting bacterial strain’s effectiveness in promoting cotton growth, antioxidant enzymes, yield, and nutrient uptake. Two phosphate solubilizing bacterial strains (Bacillus subtilis IA6 and Paenibacillus polymyxa IA7) and two zinc solubilizing bacterial strains (Bacillus sp. IA7 and Bacillus aryabhattai IA20) were coated on cotton seeds in a single as well as co-inoculation treatments. These treatments were compared with uninoculated controls in the presence and absence of recommended chemical fertilizer doses. The results showed the co-inoculation combination of Paenibacillus polymyxa IA7 and Bacillus aryabhattai IA20 significantly increased the number of bolls, seed cotton yield, lint yield, and antioxidants activities, including superoxide dismutase, guaiacol peroxidase, catalase, and peroxidase. Co-inoculation combination of Bacillus subtilis IA6 and Bacillus sp. IA16 promoted growth attributes, including shoot length, root length, shoot fresh weight, and root fresh weight. This co-inoculation combination also increased soil nutrient content. At the same time, Paenibacillus polymyxa IA7 + Bacillus aryabhattai IA20 increased nutrient uptake by plant shoots and roots compared.

Whole genome analysis of Enterobacter cloacae Rs-2 and screening of genes related to plant-growth promotion

The genus Enterobacter is widely recognized for its biotechnology potential in improving soil environment and crop growth promotion. To further explore these biotechnological potentials, we sequenced and analyzed the whole genome of Enterobacter cloacae Rs-2. The analysis showed that the total length of the Rs-2 genome was 6,965,070,514 bp, and GC content was 55.80%; the annotation results of GO and COG databases showed that the genome contains a variety of growth-promoting genes, such as iscU, glnA, glnB (nitrogen fixation); iucABCD (siderophore synthesis) and fepA, fcuA, fhuA, and pfeA, etc. (siderophore transport); ipdC (secreted IAA) and gcd, pqqBCDEF (dissolved phosphorus), etc. No pathogenic factors such as virulence genes were found. The application of Rs-2 as a soil inoculant in pot experiments showed great potential for growth promotion. This study proved the plant growth-promoting ability of Rs-2 at the molecular level through genetic screening and analysis, which provided guidance for the further improvement of the strain and laid a foundation for its application in agricultural production.

Integrated analysis of potential microbial consortia, soil nutritional status, and agro-climatic datasets to modulate P nutrient uptake and yield effectiveness of wheat under climate change resilience

Climate change has a devastating effect on wheat production; therefore, crop production might decline by 2030. Phosphorus (P) nutrient deficiency is another main limiting factor of reduced yield. Hence, there is a dire need to judiciously consider wheat yield, so that human requirements and nutrition balance can be sustained efficiently. Despite the great significance of biostimulants in sustainable agriculture, there is still a lack of integrated technology encompassing the successful competitiveness of inoculated phosphate-solubilizing bacteria (PSB) in agricultural systems in the context of climatic conditions/meteorological factors and soil nutritional status. Therefore, the present study reveals the modulation of an integrated P nutrient management approach to develop potential PSB consortia for recommended wheat varieties by considering the respective soil health and agro-climatic conditions. The designed consortia were found to maintain adequate viability for up to 9 months, verified through field emission scanning electron microscopy and viable count. Furthermore, a significant increase in grain yield (5%-8%) and seed P (4%) content was observed in consortia-inoculated wheat plants with 20% reduced Diammonium phosphate (DAP) application under net house conditions. Fluorescence in situ hybridization analysis of roots and amplification of the gcd gene of Ochrobactrum sp. SSR indicated the survival and rhizosphere competency of the inoculated PSB. Categorical principal component analysis (CAT-PCA) showed a positive correlation of inoculated field-grown wheat varieties in native soils to grain yield, soil P content, and precipitation for sites belonging to irrigated plains and seed P content, soil organic matter, and number of tillers for sites belonging to Northern dry mountains. However, the impact of inoculation at sites belonging to the Indus delta was found significantly correlated to soil potassium (K) content, electrical conductivity (EC), and temperature. Additionally, a significant increase in grain yield (15%) and seed P (14%) content was observed in inoculated wheat plants. Thus, the present study demonstrates for the first time the need to integrate soil biological health and agro-climatic conditions for consistent performance of augmented PSB and enhanced P nutrient uptake to curtail soil pollution caused by the extensive use of agrochemicals. This study provides innovative insights and identifies key questions for future research on PSB to promote its successful implementation in agriculture.

The Metabolic Potential of Endophytic Actinobacteria Associated with Medicinal Plant Thymus roseus as a Plant-Growth Stimulator

Bio-fertilizer practice considers not only economical but also environmentally friendly, sustainable agriculture. Endophytes can play important beneficiary roles in plant development, directly, indirectly, or synergistically. In this study, the majority of our endophytic actinobacteria were able to possess direct plant growth-promoting (PGP) traits, including auxin (88%), ammonia (96%), siderophore production (94%), and phosphate solubilization (24%), along with cell-wall degrading enzymes such as protease (75%), cellulase (81%), lipase (81%), and chitinase (18%). About 45% of tested strains have an inhibitory effect on the phytopathogen Fusarium oxysporum, followed by 26% for Verticillium dahlia. Overall, our results showed that strains XIEG63 and XIEG55 were the potent strains with various PGP traits that caused a higher significant increase (p ≤ 0.05) in length and biomass in the aerial part and roots of tomato and cotton, compared to the uninoculated plants. Our data showed that the greatest inhibition percentages of two phytopathogens were achieved due to treatment with strains XIEG05, XIEG07, XIEG45, and XIEG51. The GC-MS analysis showed that most of the compounds were mainly alkanes, fatty acid esters, phenols, alkenes, and aromatic chemicals and have been reported to have antifungal activity. Our investigation emphasizes that endophytic actinobacteria associated with medicinal plants might help reduce the use of chemical fertilization and potentially lead to increased agricultural productivity and sustainability.

K-solubilizing bacteria (Bacillus) promote theanine synthesis in tea roots (Camellia sinensis) by activating CsTSI activity

Theanine is an important quality parameter referring to tea quality. Applying nitrogen fertilizers is one strategy to improve the level of theanine; however, the effect of plant growth-promoting rhizobacteria on theanine synthesis in tea roots has been less studied. In this study, the bacteria isolated from Qimen County with the maximum potassium (K) solubilization were identified as Bacillus by biochemical and molecular analyses. We show that tartaric and pyruvic acids produced by Bacillus were important components related to K solubilization in vitro. Pot experiments and enzymatic assays in vitro showed that inoculation with Bacillus-secreted organic acids increased the level of available potassium in the soil. The increased K level activated recombinant CsTSI activity (theanine biosynthesis enzyme) and increased ethylamine content (the synthesis precursor of theanine), resulting in promoted theanine synthesis in tea roots. Therefore, our study indicates that Bacillus can be a potential bioinoculant for biofortification of tea.

Insight into soilless revegetation of oligotrophic and heavy metal contaminated gold tailing pond by metagenomic analysis

Soilless revegetation is an efficient way for gold tailing remediation, and micro-ecological environments in plant rhizosphere are important for vegetation establishment and pollution removal. In the present study, a field experiment of soilless revegetation has been carried out in a gold tailings pond, and the key genera and functional genes in the plant rhizosphere of gold mine tailings were revealed by metagenomics analysis. Soilless revegetation significantly decreased rhizosphere tailing pH from 8.54 to 7.43-7.87, reduced heavy metal (HM) concentration by 29.81-44.02% and enhanced the nutrient content by 50.30-169.50% averagely. Such improvements were strongly and closely correlated to microbial community and functional gene composition variation. The relative abundance of ecologically beneficial genus such as Actinobacteria (increased 9.7-18.8%) and functional genes involved in carbon, nitrogen and phosphorus cycling such as pyruvate metabolism (relatively increased 8.7-15.0%), assimilatory (increased to 1.44-2.08 times), phosphate ester mineralization (increased to 1.12-1.29 times) and phosphate transportation (increased to 1.28-1.85 times) were significantly increased. Moreover, the relative abundance of most As and Zn resistance genes were reduced, which may relate to the decrease of As and Zn concentration in the rhizosphere tailings. These results revealed the correlation among HM concentrations, microbial composition and functional genes, and provided clear strategies for improving gold mine tailing ecological restoration efficiency.

The impact of the rice production system (irrigated vs lowland) on root-associated microbiome from farmer's fields in western Burkina Faso

Due to their potential applications for food safety, there is a growing interest in rice root-associated microbial communities, but some systems remain understudied. Here, we compare the assemblage of root-associated microbiota in rice sampled in 19 small farmer's fields from irrigated and rainfed lowlands in Burkina Faso, using an amplicon metabarcoding approach of the 16S rRNA gene (prokaryotes, three plant sample per field) and ITS (fungi, one sample per field). In addition to the expected structure by root compartments (root vs. rhizosphere) and geographical zones, we showed that the rice production system is a major driver of microbiome structure. In irrigated systems, we found a higher diversity of prokaryotic communities from the rhizosphere and more complex co-occurrence networks, compared to rainfed lowlands, while fungal communities exhibited an opposite pattern (higher richness in rainfed lowlands). Core taxa were different between the two systems, and indicator species were identified: mostly within Bacillaceae in rainfed lowlands, and within Burkholderiaceae and Moraxellaceae in irrigated areas. Finally, a higher abundance in rainfed lowlands was found for mycorrhizal fungi (both compartments) and rhizobia (rhizosphere only). Our results highlight deep microbiome differences induced by contrasted rice production systems that should consequently be considered for microbial engineering applications.

Draft Genome Sequence of Pseudomonas sp. Strain RGM 3321, a Phyllosphere Endophyte from Fragaria chiloensis subsp. chiloensis f. patagonica

Pseudomonas sp. strain RGM 3321 is a phyllosphere endophyte from Fragaria chiloensis subsp. chiloensis f. patagonica that harbors genes associated with plant growth promotion pathways, as well as genes typically found in plant pathogens.

PCycDB: a comprehensive and accurate database for fast analysis of phosphorus cycling genes

BACKGROUND

Phosphorus (P) is one of the most essential macronutrients on the planet, and microorganisms (including bacteria and archaea) play a key role in P cycling in all living things and ecosystems. However, our comprehensive understanding of key P cycling genes (PCGs) and microorganisms (PCMs) as well as their ecological functions remains elusive even with the rapid advancement of metagenome sequencing technologies. One of major challenges is a lack of a comprehensive and accurately annotated P cycling functional gene database.

RESULTS

In this study, we constructed a well-curated P cycling database (PCycDB) covering 139 gene families and 10 P metabolic processes, including several previously ignored PCGs such as pafA encoding phosphate-insensitive phosphatase, ptxABCD (phosphite-related genes), and novel aepXVWPS genes for 2-aminoethylphosphonate transporters. We achieved an annotation accuracy, positive predictive value (PPV), sensitivity, specificity, and negative predictive value (NPV) of 99.8%, 96.1%, 99.9%, 99.8%, and 99.9%, respectively, for simulated gene datasets. Compared to other orthology databases, PCycDB is more accurate, more comprehensive, and faster to profile the PCGs. We used PCycDB to analyze P cycling microbial communities from representative natural and engineered environments and showed that PCycDB could apply to different environments.

CONCLUSIONS

We demonstrate that PCycDB is a powerful tool for advancing our understanding of microbially driven P cycling in the environment with high coverage, high accuracy, and rapid analysis of metagenome sequencing data. The PCycDB is available at https://github.com/ZengJiaxiong/Phosphorus-cycling-database . Video Abstract.

Recent Advances in Agronomic and Physio-Molecular Approaches for Improving Nitrogen Use Efficiency in Crop Plants

The efficiency with which plants use nutrients to create biomass and/or grain is determined by the interaction of environmental and plant intrinsic factors. The major macronutrients, especially nitrogen (N), limit plant growth and development (1.5-2% of dry biomass) and have a direct impact on global food supply, fertilizer demand, and concern with environmental health. In the present time, the global consumption of N fertilizer is nearly 120 MT (million tons), and the N efficiency ranges from 25 to 50% of applied N. The dynamic range of ideal internal N concentrations is extremely large, necessitating stringent management to ensure that its requirements are met across various categories of developmental and environmental situations. Furthermore, approximately 60 percent of arable land is mineral deficient and/or mineral toxic around the world. The use of chemical fertilizers adds to the cost of production for the farmers and also increases environmental pollution. Therefore, the present study focused on the advancement in fertilizer approaches, comprising the use of biochar, zeolite, and customized nano and bio-fertilizers which had shown to be effective in improving nitrogen use efficiency (NUE) with lower soil degradation. Consequently, adopting precision farming, crop modeling, and the use of remote sensing technologies such as chlorophyll meters, leaf color charts, etc. assist in reducing the application of N fertilizer. This study also discussed the role of crucial plant attributes such as root structure architecture in improving the uptake and transport of N efficiency. The crosstalk of N with other soil nutrients plays a crucial role in nutrient homeostasis, which is also discussed thoroughly in this analysis. At the end, this review highlights the more efficient and accurate molecular strategies and techniques such as N transporters, transgenes, and omics, which are opening up intriguing possibilities for the detailed investigation of the molecular components that contribute to nitrogen utilization efficiency, thus expanding our knowledge of plant nutrition for future global food security.

Contribution of K solubilising bacteria (Burkholderia sp.) promotes tea plant growth (Camellia sinesis) and leaf polyphenols content by improving soil available K level

K+ availability is important for growth and quality of tea (Camellia sine sis L.). K solubilising bacteria convert insoluble K to available K. This study was conducted to screen K solubilising bacteria isolated from tea rhizosphere soil in Qimen county, Anhui province, China. The maximum K solubilisation colony (the ratio of diameter halo/colony was 2.54) was identified as Burkholderia sp. (storage number: M2021105) by biochemistry and molecular analysis. Pot experiments (Laterite) showed that the inoculation of Burkholderia sp. significantly improved tea plant height (Zhongcha108, 1 year old) and total polyphenols content by 21.14% and 21.58% compared with the control, respectively. Higher polyphenol level promoted the formation of theaflavin in the fermentation experiments. Further experiments showed that tartaric acid and pryuvic acid produced by Burkholderia sp. are important components associated with K solubilisation in vitro . Burkholderia sp. significantly increased soil available K by 15.12%; however, there was no significant difference in available N and P, and Cu, Mg, Zn and Ca compared with the control. K content in inoculated tea roots and leaves was significantly higher (50% and 10%, respectively) than the control. Compared with the control, exogenous supply of 60mgkg-1 K significantly increased levels of polyphenol (53.97%), theaflavin (16.31%), theaflavin-3-gallate (20%), theaflavin 3'-gallic acid ester (32.24%) and theaflavin 3,3'-gallic acid ester (40.95%). Due to its ability to enable higher available soil K, ur study indicated that Burkholderia sp. have potential to increase total polyphenols content be a bio-inoculant for biofortification of tea.

Psychrophilic Bacterial Phosphate-Biofertilizers: A Novel Extremophile for Sustainable Crop Production under Cold Environment

Abiotic stresses, including low-temperature environments, adversely affect the structure, composition, and physiological activities of soil microbiomes. Also, low temperatures disturb physiological and metabolic processes, leading to major crop losses worldwide. Extreme cold temperature habitats are, however, an interesting source of psychrophilic and psychrotolerant phosphate solubilizing bacteria (PSB) that can ameliorate the low-temperature conditions while maintaining their physiological activities. The production of antifreeze proteins and expression of stress-induced genes at low temperatures favors the survival of such organisms during cold stress. The ability to facilitate plant growth by supplying a major plant nutrient, phosphorus, in P-deficient soil is one of the novel functional properties of cold-tolerant PSB. By contrast, plants growing under stress conditions require cold-tolerant rhizosphere bacteria to enhance their performance. To this end, the use of psychrophilic PSB formulations has been found effective in yield optimization under temperature-stressed conditions. Most of the research has been done on microbial P biofertilizers impacting plant growth under normal cultivation practices but little attention has been paid to the plant growth-promoting activities of cold-tolerant PSB on crops growing in low-temperature environments. This scientific gap formed the basis of the present manuscript and explains the rationale for the introduction of cold-tolerant PSB in competitive agronomic practices, including the mechanism of solubilization/mineralization, release of biosensor active biomolecules, molecular engineering of PSB for increasing both P solubilizing/mineralizing efficiency, and host range. The impact of extreme cold on the physiological activities of plants and how plants overcome such stresses is discussed briefly. It is time to enlarge the prospects of psychrophilic/psychrotolerant phosphate biofertilizers and take advantage of their precious, fundamental, and economical but enormous plant growth augmenting potential to ameliorate stress and facilitate crop production to satisfy the food demands of frighteningly growing human populations. The production and application of cold-tolerant P-biofertilizers will recuperate sustainable agriculture in cold adaptive agrosystems.

Differential Root Exudation and Architecture for Improved Growth of Wheat Mediated by Phosphate Solubilizing Bacteria

Phosphorous (P) deficiency is a major challenge faced by global agriculture. Phosphate-solubilizing bacteria (PSB) provide a sustainable approach to supply available phosphates to plants with improved crop productivity through synergistic interaction with plant roots. The present study demonstrates an insight into this synergistic P-solubilizing mechanism of PSB isolated from rhizosphere soils of major wheat-growing agro-ecological zones of Pakistan. Seven isolates were the efficient P solubilizers based on in vitro P-solubilizing activity (233-365 μg ml-1) with a concomitant decrease in pH (up to 3.5) by the production of organic acids, predominantly acetic acid (∼182 μg ml-1) and gluconic acid (∼117 μg ml-1). Amplification and phylogenetic analysis of gcd, pqqE, and phy genes of Enterobacter sp. ZW32, Ochrobactrum sp. SSR, and Pantoea sp. S1 showed the potential of these PSB to release orthophosphate from recalcitrant forms of phosphorus. Principal component analysis indicates the inoculation response of PSB consortia on the differential composition of root exudation (amino acids, sugars, and organic acids) with subsequently modified root architecture of three wheat varieties grown hydroponically. Rhizoscanning showed a significant increase in root parameters, i.e., root tips, diameter, and surface area of PSB-inoculated plants as compared to uninoculated controls. Efficiency of PSB consortia was validated by significant increase in plant P and oxidative stress management under P-deficient conditions. Reactive oxygen species (ROS)-induced oxidative damages mainly indicated by elevated levels of malondialdehyde (MDA) and H2O2 contents were significantly reduced in inoculated plants by the production of antioxidant enzymes, i.e., superoxide dismutase, catalase, and peroxidase. Furthermore, the inoculation response of these PSB on respective wheat varieties grown in native soils under greenhouse conditions was positively correlated with improved plant growth and soil P contents. Additionally, grain yield (8%) and seed P (14%) were significantly increased in inoculated wheat plants with 20% reduced application of diammonium phosphate (DAP) fertilizer under net house conditions. Thus, PSB capable of such synergistic strategies can confer P biofortification in wheat by modulating root morphophysiology and root exudation and can alleviate oxidative stress under P deficit conditions.

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

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

Characterization of Phosphate Solubilizing Bacterial Endophytes and Plant Growth Promotion In Vitro and in Greenhouse

Phosphate is one of the most important nutrients for plant growth and development, and only 0.1% of the phosphate in soils is available to plants. Currently, the use of excess phosphate fertilizer has caused surface and ground water pollution and water eutrophication, resulting in algal blooms in lakes and oceans. Therefore, it is imperative to explore alternative ways to solve these problems for sustainable agricultural production and improvement of soil fertility, while protecting the environment. Microorganisms from the rhizosphere and within plants are able to solubilize insoluble soil phosphate, making it available to plants. Five high phosphate solubilizing bacteria from our bacterial endophyte library were chosen for this study and identified as Pantoea vagans IALR611, Pseudomonas psychrotolerans IALR632, Bacillus subtilis IALR1033, Bacillus safensis IALR1035 and Pantoea agglomerans IALR1325. All five bacteria significantly promoted tall fescue growth in vitro. Greenhouse experiments showed that IALR1325 significantly promoted pepper and tomato growth, and IALR632 was the best in promoting tomato growth. In addition, all these bacteria had extracellular acid phosphatase and phytase activities. One of the mechanisms for phosphate solubilization by bacteria is pH reduction caused by gluconic acid production. Our results indicate that P. agglomerans IALR1325 is a promising bacterium for future applications.

Crc Regulates Succinate-Mediated Repression of Mineral Phosphate Solubilization in Acinetobacter sp. SK2 by Modulating Membrane Glucose Dehydrogenase

The plant growth-promoting Acinetobacter sp. SK2 isolated from Vigna radiata rhizosphere was characterized for mineral phosphate solubilization (MPS). To understand the contribution of the membrane glucose dehydrogenase (mGDH) and soluble glucose dehydrogenase (sGDH) in glucose oxidation and MPS, insertional inactivation of the corresponding genes was carried out. The disruption of mGDH encoding gene gdhA resulted in complete loss of mGDH activity, which confirmed its role in periplasmic glucose oxidation and gluconate-mediated MPS phenotype. The inactivation of sGDH encoding gene gdhB resulted in loss of sGDH activity, which did not alter the MPS or mGDH activity. Thus, it was also concluded that the sGDH was dispensable in gluconate-mediated MPS. Supplementation of succinate in glucose-containing medium suppressed the activity of mGDH (and sGDH) and therefore repressed the MPS phenotype. The catabolite repression control protein (Crc) of Pseudomonas was implicated in Acinetobacter sp. for a similar function in the presence of preferred and non-preferred carbon sources. To understand the regulatory linkage between Crc and genes for glucose oxidation, crc mutants were generated. The inactivation of crc resulted in increased activity of the mGDH in glucose + succinate-grown cells, indicating derepression. An increase in phosphate solubilization up to 44% in glucose + succinate-grown crc ^- compared with glucose-grown cells was recorded, which was significantly repressed in the wild-type strain under similar conditions. It is therefore proposed that in Acinetobacter sp. SK2, Crc is involved in the succinate-provoked repression of the MPS phenotype. The gene expression data indicated that Hfq may also have a regulating role in preferential utilization of carbon source by perhaps modulating Crc-Hfq functionality. V. radiata plants inoculated with the wild type improved both root and shoot length by 1.3 to 1.4-fold. However, crc ^- increased the root and shoot length by 1.6-fold, compared with the uninoculated controls. In mimicking the soil condition (in the presence of multiple carbon sources, e.g., succinate along with glucose), the crc ^- strain of Acinetobacter sp. SK2 performed better in supporting the growth of V. radiata in pot experiments.

Rhizosphere Engineering With Plant Growth-Promoting Microorganisms for Agriculture and Ecological Sustainability

The rhizosphere is undoubtedly the most complex microhabitat, comprised of an integrated network of plant roots, soil, and a diverse consortium of bacteria, fungi, eukaryotes, and archaea. The rhizosphere conditions have a direct impact on crop growth and yield. Nutrient-rich rhizosphere environments stimulate plant growth and yield and vice versa. Extensive cultivation exhaust most of the soils which need to be nurtured before or during the next crop. Chemical fertilizers are the major source of crop nutrients but their uncontrolled and widespread usage has posed a serious threat to the sustainability of agriculture and stability of an ecosystem. These chemicals are accumulated in the soil, drained in water, and emitted to the air where they persist for decades causing a serious threat to the overall ecosystem. Plant growth-promoting rhizobacteria (PGPR) present in the rhizosphere convert many plant-unavailable essential nutrients e.g., nitrogen, phosphorous, zinc, etc. into available forms. PGPR produces certain plant growth hormones (such as auxin, cytokinin, and gibberellin), cell lytic enzymes (chitinase, protease, hydrolases, etc.), secondary metabolites, and antibiotics, and stress alleviating compounds (e.g., 1-Aminocyclopropane-1- carboxylate deaminase), chelating agents (siderophores), and some signaling compounds (e.g., N-Acyl homoserine lactones) to interact with the beneficial or pathogenic counterparts in the rhizosphere. These multifarious activities of PGPR improve the soil structure, health, fertility, and functioning which directly or indirectly support plant growth under normal and stressed environments. Rhizosphere engineering with these PGPR has a wide-ranging application not only for crop fertilization but developing eco-friendly sustainable agriculture. Due to severe climate change effects on plants and rhizosphere biology, there is growing interest in stress-resilient PGPM and their subsequent application to induce stress (drought, salinity, and heat) tolerance mechanism in plants. This review describes the three components of rhizosphere engineering with an explicit focus on the broader perspective of PGPM that could facilitate rhizosphere engineering in selected hosts to serve as an efficient component for sustainable agriculture.

The wheat growth-promoting traits of Ochrobactrum and Pantoea species, responsible for solubilization of different P sources, are ensured by genes encoding enzymes of multiple P-releasing pathways

Production and release of organic acids and phosphatase enzymes by microbes are important for inorganic and organic phosphorus cycling in soil. The presence of microorganisms with corresponding traits in the plant rhizosphere lead to improved plant P uptake and ultimately growth promotion. We studied the potential of two rhizosphere-competent strains, Pantoea sp. MR1 and Ochrobactrum sp. SSR, for solubilization of different organic and inorganic P sources in vitro. In a pot experiment we further revealed the impact of the two strains on wheat seedling performance in soil amended with either phytate, rock phosphate or K₂HPO₄ as solely P source. To directly link P-solubilizing activity to the strain-specific genetic potential, we designed novel primers for glucose dehydrogenase (gcd), phosphatase (pho) and phytase (phy) genes, which are related to the organic and inorganic P solubilization potential. Quantitative tracing of these functional genes in the inoculated soils of the conducted pot experiment further allowed to compare strain abundances in the soil in dependency on the present P source. We observed strain- and P source-dependent patterns of the P solubilization in vitro as well as in the pot experiment, whereby P release, particularly from phytate, was linked to the strain abundance. We further revealed that the activity of microbial phosphatases is determined by the interplay between functional gene abundance, available soil P, and substrate availability. Moreover, positive impacts of microbial seed inoculation on wheat root architecture and aboveground growth parameters were observed. Our results suggest that screening for rhizosphere-competent strains with gcd, pho and phy genes may help to identify new microbial taxa that are able to solubilize and mineralize inorganic as well as organic bound P. Subsequently, the targeted use of corresponding strains may improve P availability in agricultural soils and consequently reduce fertilizer application.

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.

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

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

Integrated use of phosphate-solubilizing Bacillus subtilis strain IA6 and zinc-solubilizing Bacillus sp. strain IA16: a promising approach for improving cotton growth

Mineral nutrition of crop plants is one of the major challenges faced by modern agriculture, particularly in arid and semi-arid regions. In alkaline calcareous soils, the availability of phosphorus and zinc is critically less due to their fixation and precipitation as complexes. Farmers use fertilizers to fulfill crop requirements, but their efficacy is less, which increases production costs. Plant growth-promoting rhizobacteria (PGPR) can improve the availability of crop nutrients through solubilizing the insoluble compounds of phosphorus and zinc in soil. In the present study, a total of 40 rhizobacterial isolates were isolated from cotton rhizosphere and screened for improving cotton growth through the solubilization of phosphorus and zinc. Out of these 40 isolates, seven isolates (IA2, IA3, IA6, IA7, IA8, IA13, and IA14) efficiently solubilized insoluble rock phosphate while seven isolates (IA10, IA16, IA20, IA23, IA24, IA28, and IA30) were more efficient in solubilizing insoluble zinc oxide. In liquid media, strain IA7 (2.75 μg/mL) solubilized the highest amount of phosphate while the highest concentration of soluble zinc was observed in the broth inoculated with strain IA20 (3.94 μg/mL). Seven phosphate-solubilizing and seven zinc-solubilizing strains were evaluated using jar trial to improve the growth of cotton seedlings, and the results were quite promising. All the inoculated treatments showed improvement in growth parameters in comparison with control. Best results were shown by the combined application of IA6 and IA16, followed by the combination of strains IA7 and IA20. Based on the jar trial, the selected isolates were further characterized by plant growth-promoting characters such as siderophores production, HCN production, ammonia production, and exopolysaccharides production. These strains were identified through 16S rRNA sequencing as Bacillus subtilis IA6 (accession # MN005922), Paenibacillus polymyxa IA7 (accession # MN005923), Bacillus sp. IA16 (accession # MN005924), and Bacillus aryabhattai IA20 (accession # MN005925). It is hence concluded that the integrated use of phosphate-solubilizing and zinc-solubilizing strains as potential inoculants can be a promising approach for improving cotton growth under semi-arid conditions.

Microbial Diversity of Upland Rice Roots and Their Influence on Rice Growth and Drought Tolerance

Among abiotic stresses, drought is one of the most important factors limiting plant growth. To increase their drought tolerance and survival, most plants interact directly with a variety of microbes. Upland rice (Oryza sativa L.) is a rice ecotype that differs from irrigated ecotype rice; it is adapted to both drought-stress and aerobic conditions. However, its root microbial resources have not been explored. We isolated bacteria and fungi from roots of upland rice in Xishuangbanna, China. Four hundred sixty-two endophytic and rhizospheric isolates (337 bacteria and 125 fungi) were distributed. They were distributed among 43 genera on the basis of 16S rRNA and internal transcribed spacer (ITS) gene sequence analysis. Notably, these root microbes differed from irrigated rice root microbes in irrigated environments; for example, members of the Firmicutes phylum were enriched (by 28.54%) in the roots of the upland plants. The plant growth-promoting (PGP) potential of 217 isolates was investigated in vitro. The PGP ability of 17 endophytic and 10 rhizospheric isolates from upland rice roots was evaluated under well-irrigated and drought-stress conditions, and 9 fungal strains increased rice seedling shoot length, shoot and root fresh weight (FW), antioxidant capability, and proline (Pro) and soluble sugar contents. Our work suggests that fungi from upland rice roots can increase plant growth under irrigated and drought-stress conditions and can serve as effective microbial resources for sustainable agricultural production in arid regions.

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

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

Lead was mobilized in acid silty clay loam paddy soil with potassium dihydrogen phosphate (KDP) amendment

The immobilization effectiveness between Pb and phosphorus in soil varies with soil types. To clarify the effect of phosphate on the availability of Pb in agricultural soil, a culture experiment with three types of paddy soil was performed with potassium dihydrogen phosphate (KDP) added. EDTA, DGT and in-situ solution extraction methods were used to represent different available Pb content. Results showed that the concentration of EDTA-Pb in HN soil was slightly elevated after exogenous KDP added. The supplement of 300 mg/kg KDP significantly increased the content of soluble Pb in both acid silty clay loam soil and neutral silty loam soil (increased by 104.65% and 65.12%, respectively). However, there was no significant influence of KDP on the concentration of DGT extracted Pb. XANES results showed that Pb(OH)₂, PbHPO₄, humic acid-Pb and GSH-Pb were the major speciation of Pb in soil colloids. The proportion of Pb(OH)₂ and humic acid-bounded Pb in soil colloids were elevated after exogenous KDP added. Our results indicated that there was a mobilization effect of KDP on Pb by increasing the amount of colloidal Pb in soil solution, especially in acid silty clay loam paddy soil. Such colloid-facilitated transport might promote the uptake of Pb in rice and pose a potential threat to human health.

Physical and chemical mutagens improved Sporotrichum thermophile, strain ST20 for enhanced Phytase activity

OBJECTIVE

Phosphorous is an essential micronutrient of plants and involved in critical biological functions. In nature, phosphorous is mostly present in immobilized inorganic mineral and in the fixed organic form including phytic acid and phosphoesteric compounds. However, the bioavailability of bound phosphorous could be enhanced by the use of phosphate solubilizing microorganisms such as bacteria and fungi. The phytases are widespread in an environment and have been isolated from different sources comprising bacteria and fungi.

METHODOLOGY

In current studies, we show the successful use of gamma rays and EMS (Ethyl Methane Sulphonate) mutagenesis for enhanced activity of phytases in a fungal strain Sporotrichum thermophile.

RESULTS

We report an improved strain ST2 that could produce a clear halo zone around the colony, up to 24 mm. The maximum enzymatic activity was found of 382 U/mL on pH 5.5. However, the phytase activity was improved to 387 U/ml at 45 °C. We also report that the mutants produced through EMS showed the greater potential for phytase production.

CONCLUSION

The current study highlights the potential of EMS mutagenesis for strain improvement over physical mutagens.

Isolation, characterization, and effect of phosphate-zinc-solubilizing bacterial strains on chickpea (Cicer arietinum L.) growth

OBJECTIVE

Phosphate (P) and zinc (Zn) are essential plant nutrients required for nodulation, nitrogen-fixation, plant growth and yield. Mostly applied P and Zn nutrients in the soil are converted into unavailable form. A small number of soil microbes have the ability to transform unsolvable forms of P and Zn to an available form. P-Zn-solubilizing rhizobacteria are potential alternates for P and Zn supplement. In the present study, the effect of two P-Zn-solubilizing bacterial strains (Bacillus sp. strain AZ17 and Pseudomonas sp. strain AZ5) was evaluated on the growth of chickpea plant.

METHODOLOGY

Both strains were purified from the rhizospheric soil of chickpea plant grown-up in sandy soil and rain-fed area (Thal desert). In vitro, both strains solubilize P and Zn as well both strain produce IAA and organic acids. In the field experiments, conducted in the rain-fed area, the positive influence of inoculation with both bacterial isolates AZ5 and AZ17 on chickpea growth was observed.

RESULTS

The application of inoculum (strains AZ5 and AZ17) resulted in up to 17.47% and 17.34% increase in grain yield of both types of chickpea grown in fertilized and non-fertilized soil, respectively over non-inoculated control. Strain AZ5 was the most effective inoculum, increasing up to 17.47%, 16.04%, 26.32%, 22.53%, 26.12% and 22.59% in grain yield, straw weight, nodules number, dry weight of nodules, Zn uptake and P uptake respectively, over control.

CONCLUSION

These results indicated that Pseudomonas sp. strain AZ5 and Bacillus sp. strain AZ17 can serve as effective microbial inocula for chickpea, particularly in the rain-fed area.