Pseudomonas fluorescens ATCC 13525 containing an artificial oxalate operon and Vitreoscilla hemoglobin secretes oxalic acid and solubilizes rock phosphate in acidic alfisols

Molecular Characterization and Mineralizing Potential of Phosphorus Solubilizing Bacteria Colonizing Common Bean (Phaseolus vulgaris L.) Rhizosphere in Western Kenya

Phosphorus solubilizing bacteria (PSB) are a category of microbes that transform insoluble phosphates in soil into soluble forms that crops can utilize. Phosphorus in natural soils is abundant but poorly soluble. Hence, introducing PSB is a safer way of improving its solubility. The aim of this study was to genetically characterize and determine the mineralization capability of selected PSB colonizing rhizospheres of common beans in Western Kenya. Seven potential phosphorus solubilizing bacteria (PSB) were isolated from various subregions of Western Kenya. 16S ribosomal RNA gene sequencing and National Center for Biotechnology Information (NCBI), Basic Local Alignment Search Tool (BLAST) identified the isolates. The phosphate solubilization potential of the isolates was evaluated under agar and broth medium of National Botanical Research Institute's phosphate (NBRIP) supplemented with tricalcium calcium phosphate (TCP). Identified isolates were as follows: KK3 as Enterobacter mori, B5 (KB5) as Pseudomonas kribbensis, KV1 as Enterobacter asburiae, KB3 as Enterobacter mori, KK1 as Enterobacter cloacae, KBU as Enterobacter tabaci, and KB2 as Enterobacter bugandensis. The strains B5 and KV1 were the most effective phosphorus solubilizers with 4.16 and 3.64 indices, respectively. The microbes converted total soluble phosphate concentration in broth medium which was 1395 and 1471 P μg/mL, respectively. The least performing isolate was KBU with a 2.34 solubility index. Significant (p ≤ 0.05) differences in plant biomass for Rose coco and Mwitemania bean varieties were observed under inoculation with isolates B5 and KV1. PSB isolates found in common bean rhizospheres exhibited molecular variations and isolates B5 and KV1 are the potential in solving the insufficiency of phosphorus for sustainable crop production.

Enhanced production of poly-γ-glutamic acid via optimizing the expression cassette of Vitreoscilla hemoglobin in Bacillus licheniformis

Poly-γ-glutamic acid (γ-PGA) is a natural polymer with various applications, and its high-viscosity hinders oxygen transmission and improvement of synthesis level. Vitreoscilla hemoglobin (VHB) has been introduced into various hosts as oxygen carrier, however, its expression strength and contact efficiency with oxygen hindered efficient oxygen transfer and metabolite synthesis. Here, we want to optimize the expression cassette of VHB for γ-PGA production. Firstly, our results implied that γ-PGA yields were enhanced when introducing twin-arginine translocation (Tat) signal peptides (SP_YwbN, SP_PhoD and SP_TorA) into VHB expression cassette, and the best performance was attained by SP_YwbN from Bacillus subtilis, the γ-PGA yield of which was 18.53% higher than that of control strain, and intracellular ATP content and oxygen transfer coefficient (K_La) were increased by 29.71% and 73.12%, respectively, indicating that VHB mediated by SP_YwbN benefited oxygen transfer and ATP generation for γ-PGA synthesis. Furthermore, four promoters were screened, and P_vgb was proven as the more suitable promoter for VHB expression and γ-PGA synthesis, and γ-PGA yield of attaining strain WX/pPvgb-YwbN-Vgb was further increased to 40.59 g/L by 10.18%. Finally, WX/pPvgb-YwbN-Vgb was cultivated in 3 L fermentor for fed-batch fermentation, and 46.39 g/L γ-PGA was attained by glucose feeding, increased by 49.26% compared with the initial yield (31.01 g/L). Taken together, this study has attained an efficient VHB expression cassette for oxygen transfer and γ-PGA synthesis, which could also be applied in the production of other metabolites.

Recent progress in understanding the ecology and molecular genetics of soil mineral weathering bacteria

Mineral weathering bacteria play essential roles in nutrient cycling and plant nutrition. However, we are far from having a comprehensive view of the factors regulating their distribution and the molecular mechanisms involved. In this review, we highlight the extrinsic factors (i.e., nutrient availability, carbon source) and the intrinsic properties of minerals explaining the distribution and functioning of these functional communities. We also present and discuss the progress made in understanding the molecular mechanisms and genes that are used by bacteria during the mineral weathering process, or regulated during their interaction with minerals, that have been recently unraveled by omics approaches.

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.

Plant Growth Promotion at Low Temperature by Phosphate-Solubilizing Pseudomonas Spp. Isolated from High-Altitude Himalayan Soil

Scarcity of arable land, limited soil nutrient availability, and low-temperature conditions in the Himalayan regions need to be smartly managed using sustainable approaches for better crop yields. Microorganisms, able to efficiently solubilize phosphate at low temperatures, provide an opportunity to promote plant growth in an ecofriendly way. In this study, we have investigated the ability of psychrotolerant Pseudomonas spp., isolated from high altitudes of Indian Himalaya to solubilize P at low temperature. Quantitative estimation of phosphate solubilization and production of relevant enzymes at two different temperatures (15 and 25 °C) was performed for 4 out of 11 selected isolates, namely, GBPI_506 (Pseudomonas sp.), GBPI_508 (Pseudomonas palleroniana), GBPI_Hb61 (Pseudomonas proteolytica), and GBPI_CDB143 (Pseudomonas azotoformans). Among all, isolate GBPI_CDB143 showed highest efficiency to solubilize tri-calcium phosphate (110.50 ± 3.44 μg/mL) at 25 °C after 6 days while the culture supernatants of isolate GBPI_506 displayed the highest phytase activity (15.91 ± 0.35 U/mL) at 15 °C and alkaline phosphatase (3.09 ± 0.07 U/mL) at 25 °C in 6 and 9 days, respectively. Out of five different organic acids quantified, oxalic acid and malic acid were produced in maximum quantity by all four isolates. With the exception of GBPI_508, inoculation of bacteria promoted overall growth (rosette diameter, leaf area, and biomass) of Arabidopsis thaliana plants as compared to uninoculated control plants in growth chamber conditions. The plant growth promotion by each bacterial isolate was further validated by monitoring root colonization in the inoculated plants. These bacterial isolates with low-temperature phosphate solubilization potential along with phosphatases and phytase activity at low temperature could be harnessed for sustainable crop production in P-deficient agricultural soils under mountain ecosystems.

Stress preadaptation and overexpression of rpoS and hfq genes increase stress resistance of Pseudomonas fluorescens ATCC13525

Pseudomonas fluorescens ATCC13525 is an important growth-promoting rhizobacteria (PGPR) and plant disease biocontrol bacterium. However, due to poor stress resistance, it is prone to be inactivated by preparation, drying and storage. In this study, we investigated the effects of different stress preadaptation methods (2.0∼3.0 wt% NaCl, 0.01∼0.20 wt% H₂O₂, and 35∼44 °C) and two stress adaptation genes (rpoS, and hfq) on the stress resistance of P. fluorescens ATCC13525 (PF-WT). After stress preadaptation with low stress of 3.0 wt% NaCl, 0.05 wt% H₂O₂, and 41 °C for 30 min, the tolerance of PF-WT toward high lethal stress environments (20.0 wt% NaCl, 1.00 wt% H₂O₂, and 47 °C) were significantly improved. Moreover, knockout of rpoS and hfq genes resulted in slower culture growth than PF-WT under the sublethal stress culture conditions (5.0 wt% NaCl, 0.08 wt% H₂O₂, and 35 °C), whereas rpoS and hfq overexpressed strains (PF-pBBR2-rpoS and PF-pBBR2-hfq) obviously grew better than the control strain PF-pBBR2. Further, we prepared biocontrol agents (BACs) of different strains after different stress preadaptation treatments. Compared to PF-WT without stress preadaptation, preadaptation by 0.05 wt% H₂O₂ for 30 min resulted in 5.65 times higher survival rate, while treatment with 3.0 wt% NaCl for 30 min of PF-pBBR2-rpoS led to 5.60 times higher survival rate. This finding provides the simple and effective protection methods for P. fluorescens ATCC13525 BACs preparation by stress preadaptation and overexpression of stress adaptation genes.

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

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