Isolation and characterization of plant growth-promoting rhizobacteria from wheat rhizosphere and their effect on plant growth promotion

Biopriming with multifarious sulphur-oxidizing bacteria improve in vitro Vigna radiata L. (mung bean) and Brassica juncea L. (mustard) seed germination

Biopriming seeds with beneficial bacteria has potential to enhance seed germination. Therefore, in this investigation, five sulphur-oxidizing bacterial cultures, viz., Pantoea dispersa SOB2, Bacillus velezensis SN06, Bacillus cereus SN20, Bacillus tropicus SN16, and Bacillus megaterium SN11, were evaluated for different plant growth-promoting traits and their impact on Vigna radiata L. (mung bean) and Brassica juncea L. (mustard) seed germination. Among these, three bacterial cultures Pantoea dispersa SOB2, Bacillus velezensis SN06, and Bacillus megaterium SN11 evinced potential for mineral solubilization on solid medium where Pantoea dispersa SOB2 had the maximum solubilization indices-3.06, 5.14, and 2.48 for phosphate, zinc, and potassium respectively. The culture also displayed higher indole acetic acid (113.12 µg/mL), gibberellic acid (162.66 µg/mL), ammonia (5.23 µg/mL), and siderophore (69.53%) production than other bacterial cultures whereas Bacillus cereus SN20 showed maximum exopolysaccharide production (9.26 g/L). Bacterial culture Pantoea dispersa SOB2 significantly ameliorated the germination rate (3.73 no./day) and relative seed germination (208%) of Brassica juncea L., while Bacillus velezensis SN06 and Bacillus cereus SN20 followed with germination rate and relative seed germination of 2.86 no./day and 207%, respectively. Pantoea dispersa SOB2 displayed lowest mean germination time 2.91 days followed by Bacillus megaterium SN11 with 3.19 days. Biopriming with sulphur-oxidizing bacterial cultures, germination parameters of Vigna radiata L. were also markedly improved. Pantoea dispersa SOB2 demonstrated the highest germination rate (6.72 no./day), relative seed germination (115.56%), and minimum mean generation time (1.73 days). Bacillus velezensis SN06 inoculation had a beneficial effect on the seedling growth of Vigna radiata L., whereas Pantoea dispersa SOB2 greatly aided the seedling growth of Brassica juncea L. Results corroborated a prominent positive correlation between seed germination and plant growth-promoting traits. This is the first study on Pantoea dispersa as sulphur oxidizer, displaying plant growth promoting traits and seed germination potential. The potent sulphur-oxidizing bacterial cultures possessing plant growth promoting activities enhanced seed germination under in vitro conditions that could be further explored in field as biofertilizers to enhance the growth and yield of Brassica juncea L. and Vigna radiata L. crop.

Isolation of bacteria with plant growth-promoting properties from microalgae-bacterial flocs produced in high-rate oxidation ponds

Exploring plant growth-promoting (PGP) bacterial activity of microbial components aggregated by wastewater treatment can reduce dependence on fossil fuel-derived fertilisers. This study describes the isolation and identification of bacteria from microalgae-bacteria flocs (MaB-flocs) generated in high-rate algal oxidation ponds (HRAOP) of an integrated algal pond system (IAPS) remediating municipal wastewater. Amplified 16S rRNA gene sequence analysis determined the molecular identity of the individual strains. Genetic relatedness to known PGP rhizobacteria in the NCBI GenBank database was by metagenomics. Isolated strains were screened for the production of indoles (measured as indole-3-acetic acid; IAA) and an ability to mineralise NH 4 + , PO 4 3 - , and K + . Of the twelve bacterial strains isolated from HRAOP MaB-flocs, four produced indoles, nine mineralised NH 4 + , seven solubilised P, and one K. Potential of isolated strains for PGP activity according to one-way ANOVA on ranks was: ECCN 7b > ECCN 4b > ECCN 6b > ECCN 3b = ECCN 10b > ECCN 1b = ECCN 5b > ECCN 8b > ECCN 2b > ECCN 12b > ECCN 9b = ECCN 11b. Further study revealed that cell-free filtrate from indole-producing cultures of Aeromonas strain ECCN 4b, Enterobacter strain ECCN 7b, and Arthrobacter strain ECCN 6b promoted mung bean adventitious root formation suggestive of the presence of auxin-like biological activity.

A meta-narrative review of research traditions on hidden workers in aging population for transdisciplinary implementation research

Hidden workers are defined as the three vulnerable subgroups of workers: the underemployed, the unemployed, and the discouraged workers. Hidden workers indeed the group with multiple identities; a transitioning retiree, jobseeker, caring for some, who may also have long term health conditions and ethnic minority all at the same time. Designing an intervention for this group necessitates the transdisciplinary knowledge. Transdisciplinary knowledge is crucial because it can inform how the intersectoral challenges might be addressed in interventions, and how the intersectoral implementation design and evaluation on hidden workers might be designed. This paper maps the intellectual landscape of the hidden workers in aging population literature to identify key disciplinary research clusters; and to find out how those research clusters are investigating hidden workers. With the meta-narrative review methodology on studies retrieved from the Web of Science Core Collection, five research clusters were identified: (1) public health approaches to hidden workers, (2) welfare state and aging workforce, (3) older jobseekers, (4) life course perspective, (5) retirement transitions. Each research cluster focuses on different aspects of hidden workers, with varying research questions and rationales. These include conceptualising the determinants of the hidden workers in aging populations and the complex interrelation with public health. Furthermore, we suggest an analytical framework to allow for better understanding between the research traditions based on (1) the chosen socioecological level of analysis, (2) whether the research question is on the determinant for hidden workers or on the outcome of being hidden and (3) the chronosystem (early/middle/later life) timeframe of research question that is addressed. Through this study, we can identify the main issues faced by hidden workers among the older adults and the measures to address these issues as well as opening up a possibility for cross-sectoral policy responses.

Isolation and Characterization of Plant-Growth-Promoting, Drought-Tolerant Rhizobacteria for Improved Maize Productivity

Drought is one of the main abiotic factors affecting global agricultural productivity. However, the application of bioinocula containing plant-growth-promoting rhizobacteria (PGPR) has been seen as a potential environmentally friendly technology for increasing plants' resistance to water stress. In this study, rhizobacteria strains were isolated from maize (Zea mays L.) and subjected to drought tolerance tests at varying concentrations using polyethylene glycol (PEG)-8000 and screened for plant-growth-promoting activities. From this study, 11 bacterial isolates were characterized and identified molecularly, which include Bacillus licheniformis A5-1, Aeromonas caviae A1-2, A. veronii C7_8, B. cereus B8-3, P. endophytica A10-11, B. halotolerans A9-10, B. licheniformis B9-5, B. simplex B15-6, Priestia flexa B12-4, Priestia flexa C6-7, and Priestia aryabhattai C1-9. All isolates were positive for indole-3-acetic acid (IAA), siderophore, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, ammonia production, nitrogen fixation, and phosphate solubilization, but negative for hydrogen cyanide production. Aeromonas strains A1-2 and C7_8, showing the highest drought tolerance of 0.71 and 0.77, respectively, were selected for bioinoculation, singularly and combined. An increase in the above- and below-ground biomass of the maize plants at 100, 50, and 25% water-holding capacity (WHC) was recorded. Bacterial inoculants, which showed an increase in the aerial biomass of plants subjected to moderate water deficiency by up to 89%, suggested that they can be suitable candidates to enhance drought tolerance and nutrient acquisition and mitigate the impacts of water stress on plants.

Draft genome sequence of Bacillus velezensis strains AOA1 and AKS2, the potential plant growth-promoting rhizobacteria

This report describes the draft genome sequence of Bacillus velezensis strains AOA1 and AKS2 isolated from maize rhizosphere soil in South Africa. Bacillus velezensis plays important biological roles as plant growth promoting rhizobacterium (PGPR). Bacillus velezensis strains also exhibit numerous biotechnological application potentials in agriculture and diverse industrial settings.

Biofertilizer and biocontrol properties of Stenotrophomonas maltophilia BCM emphasize its potential application for sustainable agriculture

Introduction

Microbial biofertilizers or biocontrol agents are potential sustainable approaches to overcome the limitations of conventional agricultural practice. However, the limited catalog of microbial candidates for diversified crops creates hurdles in successfully implementing sustainable agriculture for increasing global/local populations. The present study aimed to explore the wheat rhizosphere microbiota for microbial strains with a biofertilizer and biocontrol potential.

Methods

Using a microbial culturing-based approach, 12 unique microbial isolates were identified and screened for biofertilizer/biocontrol potential using genomics and physiological experimentations.

Results and discussion

Molecular, physiological, and phylogenetic characterization identified Stenotrophomonas maltophilia BCM as a potential microbial candidate for sustainable agriculture. Stenotrophomonas maltophilia BCM was identified as a coccus-shaped gram-negative microbe having optimal growth at 37°C in a partially alkaline environment (pH 8.0) with a proliferation time of ~67 minutes. The stress response physiology of Stenotrophomonas maltophilia BCM indicates its successful survival in dynamic environmental conditions. It significantly increased (P <0.05) the wheat seed germination percentage in the presence of phytopathogens and saline conditions. Genomic characterization decoded the presence of genes involved in plant growth promotion, nutrient assimilation, and antimicrobial activity. Experimental evidence also correlates with genomic insights to explain the potential of Stenotrophomonas maltophilia BCM as a potential biofertilizer and biocontrol agent. With these properties, Stenotrophomonas maltophilia BCM could sustainably promote wheat production to ensure food security for the increasing population, especially in native wheat-consuming areas.

The PGPR Bacillus aryabhattai promotes soybean growth via nutrient and chlorophyll maintenance and the production of butanoic acid

Plant growth-promoting rhizobacteria (PGPR) colonize plant roots, establish a mutualistic relationship with the plants and help them grow better. This study reports novel findings on the plant growth-promoting effects of the PGPR Bacillus aryabhattai. Soil was collected from a soybean field, PGPR were isolated, identified, and characterized for their ability to promote plant growth and development. The bacterium was isolated from the soybean rhizosphere and identified as B. aryabhattai strain SRB02 via 16s rRNA sequencing. As shown by SEM, the bacterium successfully colonized rice and soybean roots within 2 days and significantly promoted the growth of the GA-deficient rice cultivar Waito-C within 10 days, as well as the growth of soybean plants with at least six times longer shoots, roots, higher chlorophyll content, fresh, and dry weight after 10 days of inoculation. ICP analysis showed up to a 100% increase in the quantity of 18 different amino acids in the SRB02-treated soybean plants. Furthermore, the 2-DE gel assay indicated the presence of several differentially expressed proteins in soybean leaves after 24 hrs of SRB02 application. MALDI-TOF-MS identified β-conglycinin and glycinin along with several other proteins that were traced back to their respective genes. Analysis of bacterial culture filtrates via GCMS recorded significantly higher quantities of butanoic acid which was approximately 42% of all the metabolites found in the filtrates. The application of 100 ppm butanoic acid had significantly positive effects on plant growth via chlorophyll maintenance. These results establish the suitability of B. aryabhattai as a promising PGPR for field application in various crops.

Enhancement of mangrove growth performance using fish emulsion and halotolerant plant growth-promoting actinobacteria for sustainable management in the UAE

The combination of fish emulsion (FE) and the actinobacterial isolate, Streptomyces griseorubens UAE1 (Sg) capable of producing plant growth regulators (PGRs) and 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, was evaluated on mangrove (Avicennia marina) in the United Arab Emirates. Under greenhouse and field conditions, sediments amended with the biostimulant FE effectively enhanced mangrove growth compared to those inoculated with Sg only. Plant growth promotion by Sg was more pronounced in the presence of FE (+FE/+Sg) than in individual applications. Our data showed that Sg appeared to use FE as a source of nutrients and precursors for plant growth promotion. Thus, in planta PGR levels following the combined +FE/+Sg were significantly induced. This is the first report in the field of marine agriculture that uses FE as a nutrient base for soil microorganisms to promote mangrove growth. This study will support mangrove restoration along the Arabian Gulf coastline as a nature-based solution to changing climate and economic activities.

Polysaccharides Produced by Plant Growth-Promoting Rhizobacteria Strain Burkholderia sp. BK01 Enhance Salt Stress Tolerance to Arabidopsis thaliana

Salt stress is one of the most serious abiotic stresses leading to reduced agricultural productivity. Polysaccharides from seaweed have been used as biostimulants to promote crop growth and improve plant resistance to abiotic stress. In this study, PGPR strain Burkholderia sp. BK01 was isolated from the rhizosphere of wheat, and it was characterized for phosphorus (Pi) dissolution, indole-3-acetic acid (IAA) production, ammonia (NH3) and exopolysaccharides (EPS). In particular, strain BK01 can efficiently produce extracellular polysaccharide with a yield of 12.86 g/L, using sorbitol as carbon source. BK01 EPS was identified as an heteropolysaccharide with Mw 3.559 × 106 Da, composed of (D)-galactose (75.3%), (D)-glucose (5.5%), (L)-rhamnose (5.5%), (D)-galactouronic acid (4.9%) and (D)-glucuronic acid (8.8%). The present work aims to highlight the effect of the BK01 EPS on growth and biochemical changes in Arabidopsis thaliana under salt stress (100 mM). The purified BK01 EPS at a concentration of 100 mg/L efficiently promoted the growth of plants in pot assays, improved the chlorophyll content, enhanced the activities of SOD, POD and CAT, and decreased the content of MDA. This results suggested that the polysaccharides produced by PGPR strain Burkholderia sp. BK01 can be used as biostimulants to promote plant growth and improve plant resistance to salt stress.

Isolation and evaluation of Qatari soil rhizobacteria for antagonistic potential against phytopathogens and growth promotion in tomato plants

Plant growth promoting rhizobacteria are a diverse group of microorganisms that enhance the growth of plants under various conditions. In this study, 55 isolates of endogenous rhizobacteria were collected from the rhizosphere of Avicennia marina, Suaeda vermiculata, Salsola soda, Anabasis setifera, Salicornia europaea, Arthrocnemum macrostachyum, Limonium axillare, Tetraena qatarensis, Aeluropus lagopoides, and Prosopis juliflora. The isolates were evaluated in-vitro for their antagonist potential against Fusarium oxysporum and Botrytis cinerea using the dual culture technique, where the maximum growth inhibition reached 49% and 57%, respectively. In-vivo evaluation was accomplished to determine the growth-promoting potential of the rhizobacteria under greenhouse conditions where the strain ANABR3 (Bacillus subtilis) showed the strongest growth-promoting effects. Further in-vivo testing regarding the effectiveness of rhizobacteria in the presence of the phytopathogen was also completed using the Hoagland medium. LEMR3 and SALIR5 (both identified as two strains of B. subtilis) supported the tomato seedlings to overcome the disease and significantly (p ≤ 0.05) increased above and belowground biomass compared to the control. Additionally, several characterizing tests were carried out on the selected strains, these strains were found to possess numerous features that promote plant growth directly and indirectly such as the production of IAA, HCN, hydrolytic enzymes, ACC deaminase, NH3, and some rhizobacteria were capable of phosphate solubilization. In conclusion, this study showed that local rhizobacterial isolates collected from arid lands possess valuable traits, making them promising bio-control agents and bio-fertilizers for agricultural purposes.

Potential of psychrotolerant rhizobacteria for the growth promotion of wheat (Triticum aestivum L.)

Wheat is the second most important staple crop grown and consumed worldwide. Temperature fluctuations especially the cold stress during the winter season reduces wheat growth and grain yield. Psychrotolerant plant growth-promoting rhizobacteria (PGPR) may improve plant stress-tolerance in addition to serve as biofertilizer. The present study aimed to isolate and identify PGPR, with the potential to tolerate cold stress for subsequent use in supporting wheat growth under cold stress. Ten psychrotolerant bacteria were isolated from the wheat rhizosphere at 4 °C and tested for their ability to grow at wide range of temperature ranging from -8 °C to 36 °C and multiple plant beneficial traits. All bacteria were able to grow at 4 °C to 32 °C temperature range and solubilized phosphorus except WR23 at 4 °C, whereas all the bacteria solubilized phosphorus at 28 °C. Seven bacteria produced indole-3-acetic acid at 4 °C, whereas all produced indole-3-acetic acid at 28 °C. Seven bacteria showed the ability to fix nitrogen at 4 °C, while all the bacteria fixed nitrogen at 28 °C. Only one bacterium showed the potential to produce cellulase at 4 °C, whereas four bacteria showed the potential to produce cellulase at 28 °C. Seven bacteria produced pectinase at 4 °C, while one bacterium produced pectinase at 28 °C. Only one bacterium solubilized the zinc at 4 °C, whereas six bacteria solubilized the zinc at 28 °C using ZnO as the primary zinc source. Five bacteria solubilized the zinc at 4 °C, while seven bacteria solubilized the zinc at 28 °C using ZnCO3 as the primary zinc source. All the bacteria produced biofilm at 4 °C and 28 °C. In general, we noticed behavior of higher production of plant growth-promoting substances at 28 °C, except pectinase assay. Overall, in vitro testing confirms that microbes perform their inherent properties efficiently at optimum temperatures rather than the low temperatures due to high metabolic rate. Five potential rhizobacteria were selected based on the in vitro testing and evaluated for plant growth-promoting potential on wheat under controlled conditions. WR22 and WR24 significantly improved wheat growth, specifically increasing plant dry weight by 42% and 58%, respectively. 16S rRNA sequence analysis of WR22 showed 99.78% similarity with Cupriavidus campinensis and WR24 showed 99.9% similarity with Enterobacter ludwigii. This is the first report highlighting the association of C. campinensis and E. ludwigii with wheat rhizosphere. These bacteria can serve as potential candidates for biofertilizer to mitigate the chilling effect and improve wheat production after field-testing.

Metabolic Profiling of Endophytic Bacteria in Relation to Their Potential Application as Components of Multi-Task Biopreparations

Agricultural crops are exposed to various abiotic and biotic stresses that can constrain crop productivity. Focusing on a limited subset of key groups of organisms has the potential to facilitate the monitoring of the functions of human-managed ecosystems. Endophytic bacteria can enhance plant stress resistance and can help plants to cope with the negative impacts of stress factors through the induction of different mechanisms, influencing plant biochemistry and physiology. In this study, we characterise endophytic bacteria isolated from different plants based on their metabolic activity and ability to synthesise 1-aminocyclopropane-1-carboxylic acid deaminase (ACCD), the activity of hydrolytic exoenzymes, the total phenolic compounds (TPC) and iron-complexing compounds (ICC). Test GEN III MicroPlate indicated that the evaluated endophytes are highly metabolically active, and the best used substrates were amino acids, which may be important in selecting potential carrier components for bacteria in biopreparations. The ACCD activity of strain ES2 (Stenotrophomonas maltophilia) was the highest, whereas that of strain ZR5 (Delftia acidovorans) was the lowest. Overall, the obtained results indicated that ∼91.3% of the isolates were capable of producing at least one of the four hydrolytic enzymes. In addition, most of the tested strains produced ICC and TPC, which play a significant role in reducing stress in plants. The results of this study suggest that the tested endophytic bacterial strains can potentially be used to mitigate climate change-associated stresses in plants and to inhibit plant pathogens.

Biochemical, Catabolic, and PGP Activity of Microbial Communities and Bacterial Strains from the Root Zone of Baccharis linearis in a Mediterranean Mine Tailing

The management of mine tailings (MT) is commonly workload heavy, intrusive, and expensive. Phytostabilization offers a promising approach for MT management; however, it poses challenges due to the unfavorable physicochemical properties of these wastes. Nevertheless, native microorganisms capable of supporting plant growth and development could enhance the efficacy of phytostabilization. This study assesses the biological activity of microbial communities from the root zone of Baccharis linearis, which is naturally present in MT, in order to evaluate their biotechnological potential for phytostabilization. The root zone and bulk samples were collected from B. linearis plants located within a MT in the Mediterranean zone of Chile. Enzyme activities related to the cycling of C, N, and P were assessed. The community-level physiological profile was evaluated using the MicroRespTM system. Bacterial plant growth-promoting (PGP) traits and colony forming units (CFU) were evaluated through qualitative and microbiological methods, respectively. CFU, enzyme activities, and CLPP were higher in the root zone compared with the bulk samples. Five bacterial strains from the root zone exhibited PGP traits such as P solubilization and N acquisition, among others. The presence of microbial communities in the root zone of B. linearis with PGP traits suggests their potential to enhance the ecological management of MT through phytostabilization programs.

Bacillus velezensis BVE7 as a promising agent for biocontrol of soybean root rot caused by Fusarium oxysporum

Introduction

Soybean root rot (SRR), caused by Fusarium oxysporum, is a severe soil-borne disease in soybean production worldwide, which adversely impacts the yield and quality of soybean. The most effective method for managing crop soil-borne diseases and decreasing reliance on chemical fungicides, such as Bacillus spp., is via microbial biocontrol agents.

Methods and Results

In this study, a soil-isolated strain BVE7 was identified as B. velezensis, exhibiting broad-spectrum activity against various pathogens causing soybean root rot. BVE7 sterile filtrate, at a concentration of 10%, demonstrated significant antifungal activity by inhibiting the conidial germination, production, and mycelial growth of F. oxysporum by 61.11%, 73.44%, and 85.42%, respectively, causing hyphal malformations. The antifungal compound produced by BVE7 demonstrated adaptability to a standard environment. The pot experiment showed that BVE7 suspension could effectively control soybean root rot, with the highest control efficiency of 75.13%. Furthermore, it considerably enhanced the activity of catalase, phenylalanine ammonia lyase, superoxide dismutase, and peroxidase in soybean roots, while also preventing an increase in malondialdehyde activity. By improving the host resistance towards pathogens, the damage caused by fungi and the severity of soybean root rot have been reduced.

Discussion

This study presents the innovative utilization of B. velezensis, isolated from soybean roots in cold conditions, for effectively controlling soybean root rot caused by F. oxysporum. The findings highlight the remarkable regional and adaptive characteristics of this strain, making it an excellent candidate for combating soybean root rot in diverse environments. In conclusion, B. velezensis BVE7 demonstrated potential in effectively reducing SRR incidence and can be considered as a viable option for SRR management.

Characterizing indigenous plant growth promoting bacteria and their synergistic effects with organic and chemical fertilizers on wheat (Triticum aestivum)

The excessive use of chemical fertilizers is deteriorating both the environment and soil, making it a big challenge faced by sustainable agriculture. To assist the efforts for the solution of this burning issue, nine different potential native strains of plant growth-promoting bacteria (PGPB) namely, SA-1(Bacillus subtilis), SA-5 (Stenotrophomonas humi),SA-7(Azospirillum brasilense), BH-1(Azospirillum oryzae), BH-7(Azotobacter armeniacus), BH-8(Rhizobium pusense), BA-3(Azospirillum zeae), BA-6(Rhizobium pusense), and BA-7(Pseudomonas fragi) were isolated that were characterized morphologically, biochemically and molecularly on the basis of 16S rRNA sequencing. Furthermore, the capability of indigenous PGPB in wheat (Triticum aestivum, Chakwal-50) under control, DAP+FYM, SA-1,5,7, BH-1,7,8, BA-3,6,7, DAP+ FYM + SA-1,5,7, DAP+FYM+ BH-1,7,8 and DAP+FYM+ BA-3,6,7 treatments was assessed in a randomized complete block design (RCBD). The results of the study showed that there was a significant increase in plant growth, nutrients, quality parameters, crop yield, and soil nutrients at three depths under SA-1,5,7, BH-1,7,8, and BA-3,6,7 in combination with DAP+FYM. Out of all these treatments, DAP+ FYM + BA-3,6,7 was found to be the most efficient for wheat growth having the highest 1000-grain weight of 55.1 g. The highest values for plant height, no. of grains/spike, spike length, shoot length, root length, shoot dry weight, root dry weight, 1000 grain weight, biological yield, and economic yield were found to be 90.7 cm, 87.7 cm, 7.20 cm, 53.5 cm, 33.5 cm, 4.87 g, 1.32 g, 55.1 g, 8209 kg/h, and 4572 kg/h, respectively, in the DAP+FYM+BA treatment. The DAP+FYM+BA treatment had the highest values of TN (1.68 µg/mL), P (0.38%), and K (1.33%). Likewise, the value of mean protein (10.5%), carbohydrate (75%), lipid (2.5%), and available P (4.68 ppm) was also highest in the DAP+FYM+BA combination. C:P was found to be significantly highest (20.7) in BA alone but was significantly lowest (11.9) in DAP+FYM+BA. Hence, the integration of strains BA-3, BA-5, and BA-7 in fertilizers can be regarded as the most suitable choice for agricultural growth in the sub-mountainous lower region of AJK. This could serve as the best choice for sustainable wheat growth and improved soil fertility with lesser impacts on the environment.

Growth stimulation of two legumes (Vicia faba and Pisum sativum) using phosphate-solubilizing bacteria inoculation

The application of chemical fertilizers for plant growth and protection is one of the reasons for the environment and ecosystem destruction, thus, sustainable agriculture is gaining popularity in research and among farming communities. Although most soils are high in total phosphorus (P), a large portion is unavailable to plants and regarded as a growth-limiting factor. P-solubilizing bacteria (PSB) exploitation is a newly developed bio-solution for enhancing rhizosphere P availability; however, the effect of these bacteria on soil quality and the different phases of plant growth remains unknown. This study aims to evaluate the impact of five strains of PSB, isolated from legume rhizosphere, on the growth of two plants (Vicia faba and Pisum sativum) and certain soil properties. The efficient strains of PSB used are characterized by the P-solubilization, the ACC deaminase activity, the fixation of N, and the IAA, HCN, and siderophores production. The activity of these bacteria is tested in vitro and in vivo under controlled conditions on the growth of the two plants supplemented with the rock P (RP). According to our findings, all PSBs strains outperformed the control in terms of enhancing the growth of the tested legumes with a percentage ranging from 77.78 to 88.88%, respectively. The results showed that all treatments significantly improved plant parameters like nitrogen- (N) and P-content in the plants (67.50, 23.11%), respectively. Also, an increase in the fresh and dry weights of above- (41.17, 38.57%) and below-ground biomasses (56.6, 42.28%), respectively. Compared to the control, this leads to an increase of 72% in root length, 40.91% in plant dry weight, and 40.07% in fresh weight. Rhizospheric soil in PSBs treatments displayed high levels of N, P, and organic matter. All treatments were found to have significantly higher levels of alkaline phosphatase, basal soil respiration, and β-glucosidase activity than the control. It is concluded that multi-traits PSB can be an alternative for utilizing chemical fertilizers to enhance soil quality and plant growth. Despite the potency of PSBs, its use as a source for the development of sustainable agriculture implies focusing on crop species and adaptation, stress tolerance and climate resilience.

Assessment of Tunisian Trichoderma Isolates on Wheat Seed Germination, Seedling Growth and Fusarium Seedling Blight Suppression

Beneficial microorganisms, including members of the Trichoderma genus, are known for their ability to promote plant growth and disease resistance, as well as being alternatives to synthetic inputs in agriculture. In this study, 111 Trichoderma strains were isolated from the rhizospheric soil of Florence Aurore, an ancient wheat variety that was cultivated in an organic farming system in Tunisia. A preliminary ITS analysis allowed us to cluster these 111 isolates into three main groups, T. harzianum (74 isolates), T. lixii (16 isolates) and T. sp. (21 isolates), represented by six different species. Their multi-locus analysis (tef1, translation elongation factor 1; rpb2, RNA polymerase B) identified three T. afroharzianum, one T. lixii, one T. atrobrunneum and one T. lentinulae species. These six new strains were selected to determine their suitability as plant growth promoters (PGP) and biocontrol agents (BCA) against Fusarium seedling blight disease (FSB) in wheat caused by Fusarium culmorum. All of the strains exhibited PGP abilities correlated to ammonia and indole-like compound production. In terms of biocontrol activity, all of the strains inhibited the development of F. culmorum in vitro, which is linked to the production of lytic enzymes, as well as diffusible and volatile organic compounds. An in planta assay was carried out on the seeds of a Tunisian modern wheat variety (Khiar) by coating them with Trichoderma. A significant increase in biomass was observed, which is associated with increased chlorophyll and nitrogen. An FSB bioprotective effect was confirmed for all strains (with Th01 being the most effective) by suppressing morbid symptoms in germinated seeds and seedlings, as well as by limiting F. culmorum aggressiveness on overall plant growth. Plant transcriptome analysis revealed that the isolates triggered several SA- and JA-dependent defense-encoding genes involved in F. culmorum resistance in the roots and leaves of three-week-old seedlings. This finding makes these strains very promising in promoting growth and controlling FSB disease in modern wheat varieties.

Exploration of plant growth promoting traits and regulatory mechanisms of Bacillus anthracis PM21 in enhancing salt stress tolerance in maize

Bacillus species have been reported to reduce the negative effects of salt stress on plants; the involvement of Bacillus anthracis PM21 and the internal mechanisms involved in this process are unclear. The effects of PM21 inoculation on maize plants under salt stress were investigated in this study. The study aimed to assess the ability of Bacillus anthracis PM21 to endure high levels of salinity stress while preserving the concentration of plant growth regulators. The biomass, photosynthetic pigments, relative water content (RWC), antioxidants, osmoprotectants, inorganic ion contents, regulation of plant hormones and expression of antioxidants enzyme encoded genes were investigated under normal and salinity stress conditions. Bacillus anthracis PM21 produced a significant amount of 1-aminocyclopropane-1-carboxylate deaminase enzyme (ACC deaminase) and exopolysaccharides (EPS) under salt stress and normal conditions. PM21 also produced plant growth stimulants including indole acetic acid, gibberellic acid (GA3), kinetin, and siderophore under salinity stress and normal conditions. Under salt stress, PM21 inoculation markedly increased plant growth indices, stimulate antioxidant enzyme mechanisms, osmoprotectants, and chlorophyll content. The use of qRT-PCR to analyze the transcription of targeted genes indicated greater expression of antioxidant-encoded genes and inferred their possible function in salinity stress tolerance. Our findings shed light on the functions of PM21 and its regulatory mechanisms in plant salt stress tolerance, as well as the importance of PM21 in this process. This study will provide a thorough analysis of the theoretical framework for adopting PM21 in agricultural production as an eco-friendly method to enhance crop growth and yield under salinity stress.

Isolation and characterization of halotolerant plant growth promoting rhizobacteria from mangrove region of Sundarbans, India for enhanced crop productivity

Halotolerant plant growth promoting rhizobacteria (PGPR) are beneficial microorganisms utilized to mitigate the biotic and abiotic stresses in plants. The areas of Sundarban mangroves of West Bengal, India have been reported to be rich in halotolerant microflora, yet major area remains unexplored. The present study, therefore, aims to map down the region-specific native microbial community potent of salt tolerance, plant growth promoting (PGP) activity and antagonistic activity against fungal pathogens. Bacterial samples were isolated from the saline soil of the Sundarban mangroves. A total of 156 bacterial samples were isolated and 20 were screened for their salt tolerance potential. These isolates were characterised using morphological, biochemical, and molecular approaches. Based on 16s rRNA sequencing, they were classified into 4 different genera, including Arthrobacter sp. (01 isolate), Pseudomonas plecoglossicida (01 isolate), Kocuria rosea (01 isolate), and Bacillus (17 isolates). The halotolerant isolates which possessed plant growth promoting traits including phosphate, and zinc solubilization, indole acetic acid production, siderophore, and ammonia generation were selected. Further, the effect of two halotolerant isolates GN-5 and JR-12 which showed most prominent PGP activities was evaluated in pea plant under high salinity conditions. The isolates improved survival by promoting germination (36 to 43%) and root-shoot growth and weight of pea plant in comparison to non-inoculated control plants. In a subsequent dual culture confrontation experiment, both these halo-tolerant isolates showed antagonistic activities against the aggressive root rot disease-causing Macrophomina phaseolina (Tassi) Goid NAIMCC-F-02902. The identified isolates could be used as potential bioagents for saline soils, with potential antagonistic effect on root rot disease. However, further studies at the physiological and molecular level would help to delineate a detail mechanistic understanding of broad-spectrum defence against salinity and potential biotic pathogen.

'A plant's major strength in rhizosphere': the plant growth promoting rhizobacteria

Human activities, industrialization and civilization have deteriorated the environment which eventually has led to alarming effects on plants and animals by heightened amounts of chemical pollutants and heavy metals in the environment, which create abiotic stress. Environmental conditions like drought, salinity, diminished macro-and micro-nutrients also contribute in abiotic stress, resulting in decrement of survival and growth of plants. Presence of pathogenic and competitive microorganisms, as well as pests lead to biotic stress and a plant alone can not defend itself. Thankfully, nature has rendered plant's rhizosphere with plant growth promoting rhizobacteria which maintain an allelopathic relationship with host plant to defend the plant and let it flourish in abiotic as well as biotic stress situations. This review discusses the mechanisms behind increase in plant growth via various direct and indirect traits expressed by associated microorganisms in the rhizosphere, along with their current scenario and promising future for sustainable agriculture. It also gives details of ten such bacterial species, viz. Acetobacter, Agrobacterium, Alcaligenes, Arthrobacter, Azospirillum, Azotobacter, Bacillus, Burkholderia, Enterobacter and Frankia, whose association with the host plants is famed for enhancing plant's growth and survival.

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.

Wheat genome architecture influences interactions with phytobeneficial microbial functional groups in the rhizosphere

Wheat has undergone a complex evolutionary history, which led to allopolyploidization and the hexaploid bread wheat Triticum aestivum. However, the significance of wheat genomic architecture for beneficial plant-microbe interactions is poorly understood, especially from a functional standpoint. In this study, we tested the hypothesis that wheat genomic architecture was an overriding factor determining root recruitment of microorganisms with particular plant-beneficial traits. We chose five wheat species representing genomic profiles AA (Triticum urartu), BB {SS} (Aegilops speltoides), DD (Aegilops tauschii), AABB (Triticum dicoccon) and AABBDD (Triticum aestivum) and assessed by quantitative polymerase chain reaction their ability to interact with free-nitrogen fixers, 1-aminocyclopropane-1-carboxylate deaminase producers, 2,4-diacetylphloroglucinol producers and auxin producers via the phenylpyruvate decarboxylase pathway, in combination with Illumina MiSeq metabarcoding analysis of N fixers (and of the total bacterial community). We found that the abundance of the microbial functional groups could fluctuate according to wheat genomic profile, as did the total bacterial abundance. N fixer diversity and total bacterial diversity were also influenced significantly by wheat genomic profile. Often, rather similar results were obtained for genomes DD (Ae. tauschii) and AABBDD (T. aestivum), pointing for the first time that the D genome could be particularly important for wheat-bacteria interactions.

Bio-efficacy of Geobacillus thermodenitrificans PS41 against larvicidal, fungicidal, and plant growth-promoting activities

The microbial interactions with plant hosts were known to establish plant growth and beneficial productivity. Some bacteria, fungi, actinomycetes, yeast, and algae have proven as potential effective microbes in agricultural field. In this study, the insecticidal effect of Geobacillus thermodenitrificans PS41 secondary metabolites was tested against third instar larvae of Spodoptera litura, with mortality rate 60.26 ± 1.5% which might influence the agropest management. The test bacterial metabolites were subjected to GC-MS analysis. Totally, 17 different compounds were identified from the ethyl acetate extract metabolites of PS41 strain. The highest peak was obtained with behenic alcohol compound followed by 1-octadecene and penta erythrityl tetrachloride. The Geobacillus thermodenitrificans PS41 secondary metabolites showed potential antifungal activity against plant pathogenic fungi. The highest inhibit was attained against Cladosporium sp., (25 mm) followed by Rhizoctonia solani and Alternaria brassicola (23 mm). However, no toxic effect was exerted upon earthworm (Perionyx excavatus) when treated with PS41 bacterial metabolites. The potential PS41 strain was also found supporting the plant growth. The potential bacterial strain PS41 did not show antagonistic activity against soil bacteria such as Rhizobium sp., Azotobacter sp., Azospirullum brasilense, Pseudomonas fluorescens and Bacillus megaterium. The potential test organism, Geobacillus thermodenitrificans PS41, possessing biopesticide and biofertilizer properties can be a suitable ecofriendly organic applicant in agricultural field for enhancing crop production.

Comparison of the Rhizobacteria Serratia sp. H6 and Enterobacter sp. L7 on Arabidopsis thaliana Growth Promotion

The genera Serratia and Enterobacter belong to the Enterobacteriaceae family and several members have been described as plant growth-promoting rhizobacteria (PGPR). However, how these bacteria influence growth and development is unclear. We performed in vitro interaction assays between either Serratia sp. H6 or Enterobacter sp. L7 with Arabidopsis thaliana seedlings to analyze their effects on plant growth. In experiments of co-cultivation distant from the root tip, Enterobacter sp. decreased root length, markedly increased lateral root number, and slightly increased plant biomass by 33%, 230%, and 69%, respectively, and relative to the control. The volatile organic compounds (VOCs) emitted from Serratia sp. H6 but not those from Enterobacter sp. L7 promoted Arabidopsis growth. A blend of volatile compounds from the two bacteria had effects on plant growth that were similar to those observed for volatile compounds from H6 only. At several densities, the direct contact of roots with Serratia sp. H6 had phytostimulant properties but Enterobacter sp. L7 had clear deleterious effects. Together, these results suggest that direct contact and VOCs of Serratia sp. H6 were the main mechanisms to promote plant growth of A. thaliana, while diffusible compounds of Enterobacter sp. L7 were predominant in their PGPR activity.

From guest to host: parasite Cistanche deserticola shapes and dominates bacterial and fungal community structure and network complexity

BACKGROUND

Rhizosphere and plant microbiota are assumed to play an essential role in deciding the well-being of hosts, but effects of parasites on their host microbiota have been rarely studied. Also, the characteristics of the rhizosphere and root microbiota of parasites and hosts under parasitism is relatively unknown. In this study, we used Cistanche deserticola and Haloxylon ammodendron from cultivated populations as our model parasites and host plants, respectively. We collected samples from BULK soil (BULK), rhizosphere soil of H. ammodendron not parasitized (NCD) and parasitized (RHA) to study how the parasite influenced the rhizosphere microbiota of the host. We also collected samples from the rhizosphere soil and roots of C. deserticola (RCD and ECD) and Haloxylon ammodendron (RHA and EHA) to explore the difference between the microbiota of the parasite and its host under parasitism.

RESULTS

The parasite reduced the compositional and co-occurrence network complexities of bacterial and fungal microbiota of RHA. Additionally, the parasite increased the proportion of stochastic processes mainly belonging to dispersal limitation in the bacterial microbiota of RHA. Based on the PCoA ordinations and permutational multivariate analysis of variance, the dissimilarity between microbiota of C. deserticola and H. ammodendron were rarely evident (bacteria, R² = 0.29971; fungi, R² = 0.15631). Interestingly, four hub nodes of H. ammodendron in endosphere fungal microbiota were identified, while one hub node of C. deserticola in endosphere fungal microbiota was identified. It indicated that H. ammodendron played a predominant role in the co-occurrence network of endosphere fungal microbiota. Source model of plant microbiome suggested the potential source percentage from the parasite to the host (bacteria: 52.1%; fungi: 16.7%) was lower than host-to-parasite (bacteria: 76.5%; fungi: 34.3%), illustrating that microbial communication was bidirectional, mainly from the host to the parasite.

CONCLUSIONS

Collectively, our results suggested that the parasite C. deserticola shaped the diversity, composition, co-occurrence network, and community assembly mechanisms of the rhizosphere microbiota of H. ammodendron. Additionally, the microbiota of C. deserticola and H. ammodendron were highly similar and shared. Our findings on parasite and host microbiota provided a novel line of evidence supporting the influence of parasites on the microbiota of their hosts.

Brevibacillus DesertYSK and Rhizobium MAP7 stimulate the growth and pigmentation of Lactuca sativa L

BACKGROUND

Applying microbial biostimulants during crop cultivation allows for higher sustainability levels. It reduces the need for fertilizers and environmental contaminants while enhancing plant quality. This study assessed 13 endophytic bacteria, 4 newly isolated, and 9 donated, for plant growth-promoting capabilities. Quantitative assessments of indole acetic acid (IAA), gibberellic acid (GA₃), siderophores, ammonia, exopolysaccharides, volatile HCN, and phosphate solubilization, along with Bray-Curtis cluster analyses were performed.

RESULTS

Upon the results  we selected RhizobiumMAP7, Brevibacillus DesertYSK, Pseudomonas MAP8, BacillusMAP3, Brevibacillus MAP, and Bacillus DeltaYSK to evaluate their effects on Lactuca sativa growth and pigmentation in a 30-day greenhouse pot experiment. Both Brevibacillus DesertYSK and Rhizobium MAP7surpassed other strains in growth promotional effects. They doubled shoot length (12 and 12.3 cm, respectively, when compared with 7 cm for control after 30 days), and fresh weight (0.079 and 0.084 g, respectively, when compared with 0.045 g for control after 30 days), and increased root length by at least 3-fold when compared with control (4.5 and 3.5 cm, respectively, when compared with 1.2 cm for control after 30 days). Chlorophyll content also exhibited at least a 2-fold significant increase in response to bacterization compared with control.

CONCLUSIONS

This strain superiority was consistent with the in vitro assays data that showed strains capability as IAA and GA₃producers. Also, strains were highly capable ammonia and siderophore producers and phosphate solubilizers, providing considerable hormone and nutrient levels for L. sativa plantsleading to improved growth parameters and appearance. These data support the notion that nodule-based bacteria are potential plant growth-promoting bacteria (PGPB) that may be used on a wider scale rather than just for legumes.

Dominance of Bacillus species in the wheat (Triticum aestivum L.) rhizosphere and their plant growth promoting potential under salt stress conditions

Wheat (Triticum aestivum L.) is a major source of calorific intake in its various forms and is considered one of the most important staple foods. Improved wheat productivity can contribute substantially to addressing food security in the coming decades. Soil salinity is the most serious limiting factor in crop production and fertilizer use efficiency. In this study, 11 bacteria were isolated from wheat rhizosphere and examined for salt tolerance ability. WGT1, WGT2, WGT3, WGT6, WGT8, and WGT11 were able to tolerate NaCl salinity up to 4%. Bacterial isolates were characterized in vitro for plant growth-promoting properties including indole-3-acetic acid (IAA) production, phosphate solubilization, nitrogen fixation, zinc solubilization, biofilm formation, and cellulase-pectinase production. Six isolates, WGT1, WGT3, WGT4, WGT6, WGT8, and WGT9 showed IAA production ability ranging from 0.7-6 µg m/L. WGT8 displayed the highest IAA production. Five isolates, WGT1, WGT2, WGT5, WGT10, and WGT11, demonstrated phosphate solubilization ranging from 1.4-12.3 µg m/L. WGT2 showed the highest phosphate solubilization. Nitrogen fixation was shown by only two isolates, WGT1 and WGT8. Zinc solubilization was shown by WGT1 and WGT11 on minimal media. All isolates showed biofilm formation ability, where WGT4 exhibited maximum potential. Cellulase production ability was noticed in WGT1, WGT2, WGT4, and WGT5, while pectinase production was observed in WGT2 and WGT3. Phylogenetic identification of potential bacteria isolates confirmed their close relationship with various species of the genus Bacillus. WGT1, WGT2, and WGT3 showed the highest similarity with B. cereus, WGT6 with B. tianshenii, WGT8 with B. subtilis, and WGT11 with B. thuringiensis. Biofertilizer characteristics of salt-tolerant potential rhizospheric bacteria were evaluated by inoculating wheat plants under controlled conditions and field experiments. B. cereus WGT1 and B. thuringiensis WGT11 displayed the maximum potential to increase plant growth parameters and enhance grain yield by 37% and 31%, respectively. Potential bacteria of this study can tolerate salt stress, have the ability to produce plant growth promoting substances under salt stress and contribute significantly to enhance wheat grain yield. These bacterial isolates have the potential to be used as biofertilizers for improved wheat production under salinity conditions and contribute to the sustainable agriculture.

Biopriming of seed with plant growth-promoting bacteria for improved germination and seedling growth

Several seed priming methods can be used to improve seed germination, seedling vigor, and to overcome abiotic stress. In addition to these benefits, only the biopriming method provides the additional benefit of biotic stress management, earning it special attention. Seed biopriming is useful in almost all crops around the world and is an environmentally friendly alternative to chemical fungicides. Biopriming usually refers to use of beneficial microorganisms, in particular plant growth-promoting bacteria (PGPB) able to survive under various harsh environmental conditions. In this study, various bacterial strains were isolated from samples of different origins, i.e., rhizospheric soil, desert sand, and sea mud. Preliminary screening of 156 bacterial isolates was conducted on the basis of their potassium (K), phosphorus (P) solubilization ability, and production of plant growth hormone, i.e., indole acetic acid (IAA). The most efficient bacteria were identified by 16S rRNA gene nucleotide sequences and further examined for their ACC deaminase activity, ammonia production, and biocontrol activity (defined via chitinolytic activity, HCN, and siderophores production). Finally, carrot seed germination assay was conducted with 10 shortlisted most potent isolates. 68.6, 58.3, and 66.7% of tested bacterial isolates were capable of P, K, and Zn solubilization, respectively. Klebsiella aerogenes AF3II1 showed the highest P and K solubilization, while isolate AF4II5, AF7II3, and PC3 showed the highest IAA synthesis ability. Serratia plymuthica EDC15 and Pseudomonas putida AF1I1 showed the strongest chitinolytic and siderophore production activity, respectively. Seven isolates demonstrated strong HCN production ability. Five isolates improved carrot seed germination. Only selected isolates with plant growth-promoting properties can improve carrot germination. The results of this study demonstrate that mainly auxins are involved in seed germination. Furthermore, the data suggest that phosphate solubilization ability may play an additional role in seed germination.

Native bacteria for field biocontrol of black root rot in Egyptian strawberry

Background

Increasing cultivation of strawberry in Egypt has boosted efforts to increase its yield. Biocontrol agents (BCAs) may avoid side effects and health risks caused by chemical fungicides used to control black root rot disease in strawberry. Some BCAs control the disease and augment strawberry yield, but additional research is needed to fit BCAs into emerging control strategies. The impact of six bacterial isolates of Paenibacillus polymyxa and Bacillus brevis on this disease and on berry yield is reported and compared to a common chemical fungicide.

Results

The bacterial isolates reduced the growth of the black root rot causal agents Fusarium solani, Rhizoctonia solani, and Macrophomina phaseolina, in dual culture bioassays. The greatest fungal inhibition was caused by P. polymyxa isolates 1 and 2 and B. brevis isolate 2. They suppressed the growth of F. solani, R. solani, and M. phaseolina by more than 74, 76, and 79%, respectively. Disease severity and incidence were significantly reduced on naturally infected strawberry plants in the field by the six isolates. The best results were obtained by the superior bioassay isolates which suppressed the disease incidence by 73, 77, and 71%, and its severity by 72, 78, and 70%, respectively. Disease suppression by bacteria was comparable to that by fungicide Actamyl. Bacteria surpassed Actamyl with regard to strawberry yield and enhancement of peroxidase and chitinase activities in the leaves.

Conclusions

These isolates are potential benign alternatives to fungicides used against black root rot in strawberry in Egypt. More studies are needed to examine their economic use on a wider scale.

New insights into engineered plant-microbe interactions for pesticide removal

Over the past decades, rapid industrialization along with the overutilization of organic pollutants/pesticides has altered the environmental circumstances. Moreover, various anthropogenic, xenobiotics and natural activities also affected plants, soil, and human health, in both direct and indirect ways. To counter this, several conventional methods are currently practiced, but are uneconomical, noxious, and is yet inefficient for large-scale application. Plant-microbe interactions are mediated naturally in an ecosystem and are practiced in several areas. Plant growth promoting rhizobacteria (PGPR) possess certain attributes affecting plant and soil consequently performing decontamination activity via a direct and indirect mechanism. PGPR also harbors indispensable genes stimulating the mineralization of several organic and inorganic compounds. This makes microbes potential candidates for contributing to sustainably remediating the harmful pesticide contaminants. There is a limited piece of information about the plant-microbe interaction pertaining predict and understand the overall interaction concerning a sustainable environment. Therefore, this review focuses on the plant-microbe interaction in the rhizosphere and inside the plant's tissues, along with the utilization augmenting the crop productivity, reduction in plant stress along with decontamination of pesticides/organic pollutants in soil for sustainable environmental management.

Phosphorus-Solubilizing Capacity of Mortierella Species Isolated from Rhizosphere Soil of a Poplar Plantation

Phosphorus is one of the main nutrients necessary for plant growth and development. Phosphorus-dissolving microorganisms may convert insoluble phosphorus in soil into available phosphorus that plants can easily absorb and utilize. In this study, four phosphorus-solubilizing fungi (L3, L4, L5, and L12) were isolated from the rhizosphere soil of a poplar plantation in Dongtai, Jiangsu Province, China. Phylogenetic analysis based on the internal transcribed spacer (ITS) and large subunit (LSU) of the ribosomal DNA sequences showed that the ITS and 28S sequences of isolates were the most similar to those of Mortierella. Morphological observation showed that most colonies grew in concentric circles and produced spores under different culture conditions. These results and further microscopic observations showed that these isolated fungi belonged to the genus Mortierella. Pikovskaya (PKO) medium, in which tricalcium phosphate was the sole phosphorus source, was used to screen strain L4 with the best phosphorus-solubilizing effect for further study. When the carbon source was glucose, the nitrogen source was ammonium chloride, the pH was 5, and the available phosphorus content was the highest. By exploring the possible mechanism of phosphorus release by phosphorus-solubilizing fungi, it was found that strain L4 produces several organic acids, such as oxalic acid, lactic acid, acetic acid, succinic acid, tartaric acid, malic acid, and citric acid. At 24 h, the alkaline phosphatase and acid phosphatase activities reached 154.72 mol/(L·h) and 120.99 mol/(L·h), respectively.

Draft Genome Sequence of Aeromonas caviae Strain A1-2, a Potential Plant Growth-Promoting Rhizospheric Bacterium

The genomic analysis of the plant growth-promoting rhizospheric Aeromonas caviae strain A1-2, which was isolated from a maize plant in Northwest Province, South Africa, is presented in this paper. Aeromonas caviae strain A1-2 demonstrates its potential to promote plant growth and enhance the tolerance of maize plants to drought stress.

Draft Genome Sequence of Enterobacter mori AYS9, a Potential Plant Growth-Promoting Rhizobacterium

Here, we report the draft genome sequence of Enterobacter mori AYS9, a rhizobacterium isolated from the rhizosphere of sorghum plants in South Africa. The genome sequence comprised 4,852,175 bp and exhibited a GC content of 55.5% and 4,567 genes, with 4,453 coding sequences, 3 rRNAs, 64 tRNAs, and 1 CRISPR.

Identification, characterization, and genome sequencing of Brevibacterium sediminis MG-1 isolate with growth-promoting properties

In recent years, plant growth-promoting rhizobacteria (PGPR) have received increased attention due to their prospective use as biofertilizers for the enhancement of crop growth and yields. However, there is a growing need to identify new PGPR isolates with additional beneficial properties. In this paper, we describe the identification of a new strain of a non-sporulating Gram-positive bacterium isolated from the rhizosphere of potato plants, classified as Brevibacterium sediminis MG-1 based on whole-genome sequencing. The bacteria are aerobic; they grow in a pH range of 6.0-10.0 (optimum 6.0), and a temperature range of 20-37 °C (optimum 30 °C). At 96 h of cultivation, strain MG-1 synthesizes 28.65 µg/ml of indole-3-acetic acid (IAA) when 500 µg/ml of l-tryptophan is added. It is a producer of catechol-type siderophores and ACC deaminase (213 ± 12.34 ng/ml) and shows halotolerance. Treatment of pea, rye, and wheat seeds with a suspension of MG-1 strain cells resulted in the stimulation of stem and root biomass accumulation by 12-26% and 6-25% (P < 0.05), respectively. Treatment of seeds with bacteria in the presence of high salt concentration reduced the negative effects of salt stress on plant growth by 18-50%. The hypothetical gene lin, encoding the bacteriocin Linocin-M18, RIPP-like proteins, and polyketide synthase type III (T3PKS) loci, gene clusters responsible for iron acquisition and metabolism of siderophores, as well as gene clusters responsible for auxin biosynthesis, were identified in the B. sediminis MG-1 genome. Thus, the rhizosphere-associated strain B. sediminis MG-1 has growth-stimulating properties and can be useful for the treatment of plants grown on soils with high salinity.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03392-z.

Molecular Traits Underlying the Growth Promotion and Metabolite Accumulation in Rheum palmatum Inoculated with Endophytic Trichoderma citrinoviride HT-1

Trichoderma spp. are an important plant-growth-promoting fungi. Trichoderma citrinoviride HT-1 was isolated from Rheum palmatum root, which has beneficial effects on growth and metabolite accumulation. However, the improvement mechanisms for growth and metabolite accumulation of T. citrinoviride HT-1 are unclear. In this study, RNA sequencing (RNA-seq) and high-performance liquid chromatography (HPLC) were used to measure the effect of different concentrations of conidial suspension of the HT-1 strain on the growth promotion and metabolite accumulation of R. palmatum seedlings. The results showed that the highest biomass and metabolites of R. palmatum seedlings were obtained through treatment with the HT-1 strain at a final spore concentration of 107 spores/mL. RNA sequencing indicated that 1662 genes were upregulated and 2155 genes were downregulated after inoculation with 107 spores/mL of the HT-1 strain. This strain induced significant upregulation of related genes in the phenylpropanoid biosynthesis pathway, plant hormone signal transduction pathway, biosynthesis of secondary metabolites pathway, and plant–pathogen interaction pathway in R. palmatum. The gene expression trends were revealed through quantitative real-time polymerase chain reaction (qRT-PCR) and were consistent with those determined by RNA-seq. Our results will help us to understand the growth-promoting mechanisms of the HT-1 strain on R. palmatum and provide a theoretical basis for the application of T. citrinoviride HT-1 as a biological fertilizer.

Effects of dazomet combined with Rhodopsesudomonas palustris PSB-06 on root-knot nematode, Meloidogyne incognita infecting ginger and soil microorganisms diversity

Root-knot nematode, Meloidogyne incognita is one of the most important nematodes affecting ginger crop. Rhodopseudomonas palustris PSB-06, as effective microbial fertilizer in increasing plant growth and suppressing soil-borne disease of many crops has been reported. The combination of R. palustris PSB-06 and dazomet treatments had been proved to inhibit root-knot nematode on ginger and increase ginger yield in our preliminary study. The field experiments were conducted to elucidate the reasons behind this finding, and followed by next-generation sequencing to determine the microbial population structures in ginger root rhizosphere. The results showed that combination of R. palustris PSB-06 and dazomet treatment had a synergetic effect by achieving of 80.00% reduction in root-knot nematode numbers less than soil without treatment, and also could increase 37.37% of ginger yield through increasing the contents of chlorophyll and total protein in ginger leaves. Microbiota composition and alpha diversity varied with treatments and growth stages, soil bacterial diversity rapidly increased after planting ginger. In addition, the combined treatment could increase diversity and community composition of probiotic bacteria, and decrease those of soil-borne pathogenic fungi comparing to the soil treated with dazomet alone. Meanwhile, it could also effectively increase soil organic matter, available phosphorus and available potassium. Analysis of correlation between soil microorganisms and physicochemical properties indicated that the soil pH value and available phosphorus content were important factors that could affect soil microorganisms structure at the harvest stage. The bacterial family was more closely correlated with the soil physicochemical properties than the fungal family. Therefore, the combination of R. palustris PSB-06 and dazomet was considered as an effective method to control root-knot nematode disease and improve ginger soil conditions.

Bacterial bioinoculants adapted for sustainable plant health and soil fertility enhancement in Namibia

The increase in dryland agriculture elicits the need to develop sustainable practices that improve crop yield and protect soil fertility. The use of biofertilisers adapted to nutrient deficient soils and arid climates would help achieve this. In this review, the use of plant growth-promoting bacteria is explored as a possible solution to the current state of dryland agriculture and climate change threats to agriculture. Plant microbe interactions form the basis of this review as evidence has shown that these interactions often exist to improve the health of plants. This is achieved by the production of important biochemicals and enzymes like indole acetic acid and amino cyclopropane-1-carboxylate deaminase while also actively protecting plants from pathogens including fungal pathogens. Research, therefore, has shown that these plant-growth promoting bacteria may be exploited and developed into biofertilisers. These biofertilisers are both economically and environmentally sustainable while improving soil quality and crop yield. The literature presented in this review is in context of the Namibian climate and soil profiles.

Plant Growth-Promoting Activity of Bacteria Isolated from Asian Rice (Oryza sativa L.) Depends on Rice Genotype

Asian rice is one of the most important crops because it is a staple food for almost half of the world's population. To have production of rice keep pace with a growing world population, it is anticipated that the use of fertilizers will also need to increase, which may cause environmental damage through runoff impacts. An alternative strategy to increase crop yield is the use of plant growth-promoting bacteria. Thousands of microbial species can exist in association with plant roots and shoots, and some are critical to the plant's survival. We isolated 140 bacteria from two distantly related rice accessions and investigated whether their impact on the growth of four different rice accessions. The bacterial isolates were screened for their ability to solubilize phosphate, a known plant growth-promoting characteristic, and 25 isolates were selected for further analysis. These 25 phosphate-solubilizing isolates were also able to produce other potentially growth-promoting factors. Five of the most promising bacterial isolates were chosen for whole-genome sequencing. Four of these bacteria, isolates related to Pseudomonas mosselii, a Microvirga sp., Paenibacillus rigui, and Paenibacillus graminis, improved root and shoot growth in a rice genotype-dependent manner. This indicates that while bacteria have several known plant growth-promoting functions, their effects on growth parameters are rice genotype dependent and suggest a close relationship between plants and their microbial partners. IMPORTANCE In this study, endophytic bacterial isolates from roots and shoots of two distantly related rice accessions were characterized phenotypically and genotypically. From the isolated bacterial species, five of the most promising plant growth-promoting bacteria were selected to test their abilities to enhance growth of the four rice accessions. Interestingly, plant growth enhancement was both bacterial isolate specific and plant genotype specific. However, the positive interactions between plant and bacteria could not easily be predicted because rice growth-promoting bacteria isolated from their host plants did not necessarily stimulate growth of their own host.

Inoculation With Azospirillum spp. Acts as the Liming Source for Improving Growth and Nitrogen Use Efficiency of Potato

Nitrogen (N) is one of the limiting factors for plant growth, and it is mainly supplied exogenously by fertilizer application. It is well documented that diazotrophic rhizobacteria improve plant growth by fixing atmospheric N in the soil. The present study investigates the nitrogen-fixing potential of two Azospirillum spp. strains using the 15N isotope-dilution method. The two diazotrophic strains (TN03 and TN09) native to the rhizosphere of potato belong to the genus Azospirillum (16S rRNA gene accession numbers LN833443 and LN833448, respectively). Both strains were able to grow on an N-free medium with N-fixation potential (138-143 nmol mg-1 protein h-1) and contained the nifH gene. Strain TN03 showed highest indole acetic acid (IAA) production (30.43 μg/mL), while TN09 showed highest phosphate solubilization activity (249.38 μg/mL) while both diazotrophs showed the production of organic acids. A 15N dilution experiment was conducted with different fertilizer inputs to evaluate the N-fixing potential of both diazotrophs in pots. The results showed that plant growth parameters and N contents increased significantly by the inoculations. Moreover, reduced 15N enrichment was found compared to uninoculated controls that received similar N fertilizer levels. This validates the occurrence of N-fixation through isotopic dilution. Strain TN09 showed higher N-fixing potential than TN03 and the uninoculated controls. Inoculation with either strain also showed a remarkable increase in plant growth under field conditions. Thus, there were remarkable increases in N use efficiency, N uptake and N utilization levels. Confocal laser scanning and transmission electron microscopy showed that TN03 is an ectophyte, i.e., present outside root cells or within the grooves of root hairs, while TN09 is an endophyte, i.e., present within root cells, forming a strong association withroot it. This study confirms that diazotrophic Azospirillum spp. added to potato systems can improve plant growth and N use efficiency, opening avenues for improvement of potato crop growth with reduced input of N fertilizer.

Exploring plant growth-promoting, biocatalytic, and antimicrobial potential of salt tolerant rhizospheric Georgenia soli strain TSm39 for sustainable agriculture

To explore the in vivo and in vitro plant growth promoting activities, biocatalytic potential, and antimicrobial activity of salt tolerance rhizoactinobacteria, rhizospheric soil of a halotolerant plant Saueda maritima L. was collected from Rann of Tiker, near Little Rann of Kutch, Gujarat (India). The morphology analysis of the isolated strain TSm39 revealed that the strain belonged to the phylum actinobacteria, as it was stained Gram-positive, displayed filamentous growth, showed spore formation and red pigment production on starch casein agar (SCA). It was identified as Georgenia soli based on 16S rRNA gene sequencing. The Georgenia soli strain TSm39 secreted extracellular amylase, pectinase, and protease. It showed in vitro plant growth-promoting (PGP) activities such as indole acetic acid (IAA) production, siderophore production, ammonia production, and phosphate solubilization. In vivo plant growth-promoting traits of strain TSm39 revealed 30% seed germination on water agar and vigor index 374.4. Additionally, a significant increase (p ≤ 0.05) was found in growth parameters such as root length (16.1 ± 0.22), shoot length (15.2 ± 0.17), the fresh weight (g), and dry weight (g) of the roots (0.43 ± 0.42 and 0.32 ± 0.12), shoots (0.62 ± 0.41 and 0.13 ± 0.03), and leaves (0.42 ± 0.161 and 0.14 ± 0.42) in treated seeds of Vigna radiata L. plant with the strain TSm39 compared to control. The antibiotic susceptibility profile revealed resistance of the strain TSm39 to erythromycin, ampicillin, tetracycline, and oxacillin, while it displayed maximum sensitivity to vancomycin (40 ± 0.72), chloramphenicol (40 ± 0.61), clarithromycin (40 ± 1.30), azithromycin (39 ± 0.42), and least sensitivity to teicoplanin (15 ± 0.15). Moreover, the antimicrobial activity of the strain TSm39 was observed against Gram's positive and Gram's negative microorganisms such as Shigella, Proteus vulgaris, and Bacillus subtilis. These findings indicated that the Georgenia soli strain TSm39 has multiple plant-growth-promoting properties and biocatalytic potential that signifies its agricultural applications in the enhancement of crop yield and quality and would protect the plant against plant pathogens.

Plant Development of Early-Maturing Spring Wheat (Triticum aestivum L.) under Inoculation with Bacillus sp. V2026

The effect of a plant growth-promoting bacterium (PGPB) Bacillus sp. V2026, a producer of indolyl-3-acetic acid (IAA) and gibberellic acid (GA), on the ontogenesis and productivity of four genotypes of early-maturing spring wheat was studied under controlled conditions. The inoculation of wheat plants with Bacillus sp. V2026 increased the levels of endogenous IAA and GA in wheat of all genotypes and the level of trans-Zeatin in Sonora 64 and Leningradskaya rannyaya cvs but decreased it in AFI177 and AFI91 ultra-early lines. Interactions between the factors “genotype” and “inoculation” were significant for IAA, GA, and trans-Zeatin concentrations in wheat shoots and roots. The inoculation increased the levels of chlorophylls and carotenoids and reduced lipid peroxidation in leaves of all genotypes. The inoculation resulted in a significant increase in grain yield (by 33–62%), a reduction in the time for passing the stages of ontogenesis (by 2–3 days), and an increase in the content of macro- and microelements and protein in the grain. Early-maturing wheat genotypes showed a different response to inoculation with the bacterium Bacillus sp. V2026. Cv. Leningradskaya rannyaya was most responsive to inoculation with Bacillus sp. V2026.

Comparison of bacterial communities and their functional profiling using 16S rRNA gene sequencing between the inherent serpentine-associated sites, hyper-accumulator, downgradient agricultural farmlands, and distal non-serpentine soils

This study aims to determine and compare the bacterial community and functional profiles associated with serpentine sites, innate hyper-accumulating weed, downgradient agricultural farmlands and non-serpentine sites using 16S rRNA gene sequencing. Elemental analysis revealed that the serpentine rock and weathered soil have higher magnesium, nickel, chromium, magnesium/calcium and lower calcium/magnesium ratios and agricultural farmlands have recorded elevated chromium. Proteobacteria were found predominant, except the non-serpentine site which was rich in Cyanobacteria. PCA analysis at the genus level indicates the uniqueness of different experimental groups, except the hyperaccumulators which exhibited relatively less dissimilarity. The shift analysis showed the serpentine sites were characterized by the abundance of bacteria having heavy metal effluxion. The hyper-accumulating weeds were higher in plant growth-promoting bacteria expressing tolerance against heavy metals toxicity such as nickel, chromium, cobalt and arsenic. Besides, the agricultural lands were abundant in wetland-associated methanogens and metal (manganese, iron and zinc) transporting function related bacteria. The results suggest that the inherent edaphic factors including heavy metal content, the interacting behavior of hyperaccumulator's rhizosphere microbiota with soil and anthropogenic activities such as agricultural practices could be a major determinant of the variation in the bacterial community selection and abundance in the respective study sites.

Validation and Evaluation of Plant Growth Promoting Potential of Rhizobacteria Towards Paddy Plants

This study aimed to characterize, validate, and evaluate the plant growth potential of bacterial isolates (E-2, T-2, and T-1) to determine their suitability for application as biofertilizers and/or plant-biostimulants. The plant growth-promoting potential of bacteria (E-2, T-2, and T-1) has been validated in a hydroponic study on paddy plants by inoculating bacterial isolates and monitoring the phenotypic and plant growth responses. The applicability of bacteria was tested based on their tolerance to salinity, susceptibility to antibiotics, and identification based on 16S rDNA sequencing. The isolates E-2, T-2, and T-1 improved plant growth variably and significantly (P < 0.05 at 95% confidence interval) when inoculated into the plant growth matrix, ensuring nutrient availability to the plants grown under a nutrient (nitrate or phosphate) deprived growth matrix. Isolates E-2, T-2, and T-1 grew at salt (NaCl) concentrations of 7%, 6%, and 6%, respectively, and were tolerant to saline conditions. Although these three isolates exhibited resistance to certain antibiotics, they were susceptible to a large number of readily available antibiotics. Isolates E-2, T-2, and T-1 were identified as Klebsiella sp. strain BAB-6433, Citrobacter freundii strain R2A5, and Citrobacter sp. DY1981 respectively, and all of these may be assigned to Risk-Group-2 and hence are safe in view of their susceptibility to readily available antibiotics. Hence, these isolates are promising for extensive evaluation as bioinoculants to ecologically improve soil quality, fertility, crop growth, and yield.

Minerals solubilizing and mobilizing microbiomes: A sustainable approaches for managing minerals deficiency in agricultural soil

Agriculture faces challenges to fulfill the rising food demand due to shortage of arable land and various environmental stressors. Traditional farming technologies help in fulfilling food demand but they are harmful to humans and environmental sustainability. The food production along with agro-environmental sustainability could be achieved by encouraging farmers to use agro-environmental sustainable products such as biofertilizers and biopesticides consisting of live microbes or plant extract instead of chemical-based inputs. The ecofriendly formulations play a significant role in plant growth promotion, crop yield, and repairing degraded soil texture and fertility sustainably. Mineral solubilizing microbes that provide vital nutrients like phosphorus, potassium, zinc, and selenium are essential for plant growth and development and could be developed as biofertilizers. These microbes could be plant-associated (rhizospheric, endophytic, and phyllospheric) or inhabits the bulk soil, and diverse extreme habitats. Mineral solubilizing microbes from soil, extreme environments, surface and internal parts of the plant belong to diverse phyla such as Ascomycota, Actinobacteria, Basidiomycota, Bacteroidetes, Chlorobi, Cyanobacteria, Chlorophyta, Euryarchaeota, Firmicutes, Gemmatimonadetes, Mucoromycota, Proteobacteria, and Tenericutes. Mineral solubilizing microbes (MSMs) directly or indirectly stimulate plant growth and development either by releasing plant growth regulators; solubilizing phosphorus, potassium, zinc, selenium, and silicon; biological nitrogen fixation; and production of siderophores, ammonia, hydrogen cyanide, hydrolytic enzymes, and bioactive compound/secondary metabolites. Biofertilizer developed using mineral solubilizing microbes is an eco-friendly solution to the sustainable food production system in many countries worldwide. The present review deals with the biodiversity of mineral solubilizing microbes, and potential roles in crop improvement and soil well-being for agricultural sustainability.

Molecular Identification and Characterization of Plant Growth Promoting Rhizobacteria and their Effect on Seed Germination and Vigour Index of Barley (Hordeum vulgare L.)

Plant growth promoting rhizobacteria (PGPR) are a group of useful bacteria that colonize the plant roots and significantly enhances the plant growth promotion. Keeping this in mind, an investigation was performed for the screening of potent PGPR strains for enhancing seed germination and vigour index of Hordeum vulgare (commonly called barley). Rhizobacterial strains were isolated and screened for various plant growth promoting traits, their effect on seed germination and vigour index of barley plant through pot trial, and resistant ability under various temperature and pH range. Based upon 16S rRNA sequencing data, Pseudomonas punonensis LMT03 (R1), Pseudomonas plecoglossicida R4, Pseudomonas aeruginosa DSM50071 (R2) and Alcaligenes faecalis (DBHU5) isolates were selected and showed positive result for IAA production, Phosphate solubilization, ammonia production, catalase activity, siderophore production and MR-VP test. Barley plants treated with P. punonensis and P. plecoglossicida both showed 94.44% of highest seed germination %, while P. aeruginosa and A. faecalis showed 83.11% and 77.33% respectively in comparison to the control plant shows which 49.99% seed germination only. These respective isolates also showed 2.57, 2.37, 2.0 and 1.69 fold of increase in vigour index as compare to the control plants. The above increase in fold in vigour index and seed germination is much higher as compared to earlier reports. Collectively, the data of current study underpin that addition of these PGPRs to barley rhizosphere appears a promising strategy to enhance root and shoot biomass of this important agriculture crop. To the best of our knowledge this is the first report demonstrating the effect of P. punonensis and P. plecoglossicda on barley crop.

The Role of Soil Microbial Diversity in the Conservation of Native Seed Bacterial Microbiomes

Research into understanding the structure, composition and vertical transmission of crop seed microbiomes has intensified, although there is much less research into the seed microbiomes of crop wild relatives. Our previous study showed that the standard seed storage procedures (e.g., seed drying and storage temperature) can influence the seed microbiome of domesticated Glycine max. In this study, we characterized the seed microbiota of Glycine clandestina, a perennial wild relative of soybean (G. max (L.) Merr.) to expand our understanding about the effect of other storage procedures such as the periodic regeneration of seed stocks to bulk up seed numbers and secure viability on the seed microbiome of said seed. The G. clandestina microbiota was analysed from Generation 1 (G1) and Generation 2 (G2) seed and from mature plant organs grown in two different soil treatments T (treatment [native soil + potting mix]) and C (control [potting mix only]). Our dataset showed that soil microbiota had a strong influence on next generation seed microbiota, with an increased contribution of root microbiota by 90% and seed transmissibility by 36.3% in G2 (T) seed. Interestingly, the G2 seed microbiota primarily consisted of an initially low abundance of taxa present in G1 seed. Overall, our results indicate that seed regeneration can affect the seed microbiome composition and using native soil from the location of the source plant can enhance the conservation of the native seed microbiota.

Genetic Diversity and Characterization of Plant Growth-Promoting Effects of Bacteria Isolated from Rhizospheric Soils

Plant growth-promoting rhizobacteria (PGPR) have the potential to make a significant contribution to the development of sustainable agricultural systems. Generally, PGPRs function in three different ways, summarized as the synthesis of certain compounds for plants, facilitating the uptake of certain nutrients from the soil and protecting plants from diseases. This study aims to isolate plant growth-promoting bacteria from different plant rhizospheres from Ankara province, to reveal their genetic diversity, and to determine their plant growth-promoting properties. The identification of the 69 isolates was made according to the 16S rDNA sequence results and ARDRA analyses were also performed using AluI, HeaIII, and MspI enzymes. Nitrogen fixation, phosphate dissolving, IAA (indole-3-acetic acid) and siderophore production capacities of the 69 bacterial strains including 12 different genera (30 Pseudomonas, 13 Arthrobacter, 7 Bacillus, 4 Phyllobacter, 4 Variovorax, 3 Olivibacter, 3 Enterobacter, 2 Paenarthrobacter, 1 Stenotrophomonas, 1 Flavobacterium, 1 Caulobacter, 1 Paenibacillus) were evaluated in in vitro conditions. Nitrogen fixation capacities of 55 isolates varied between 2.29 and 46.11 µg mL^-1 according to micro-kjeldahl method. Among the strains studied, nifH gene was detected only in Paenibacillus polymyxa H8/2 strain. The highest Phosphorus dissolving and IAA production capacity (in tryptophan-added medium) of isolates were  186.52 µg mL^-1, and 50.05 μg mL-1  respectively, and 31 of 69 isolates were able to produce siderophore. Regarding antifungal activities, results showed that 31 bacterial isolates had antagonistic activities against at least one of the tested pathogens. Nitrogen fixation and phosphate solubilizing potential of the promising bacterial strains were determined through two-independent pot experiments with wheat and it has been found that they have positive effects on the yield parameters of wheat.

Functional Genetic Diversity and Plant Growth Promoting Potential of Polyphosphate Accumulating Bacteria in Soil

Polyphosphate (polyP) accumulation is an important trait of microorganisms. Implication of polyP accumulating bacteria (PAB) in enhanced biological phosphate removal, heavy metal sequestration, and dissolution of dental enamel is well studied. Phosphorous (P) accumulated within microbial biomass also regulates labile P in soil; however, abundance and diversity of the PAB in soil is still unexplored. Present study investigated the genetic and functional diversity of PAB in rhizosphere soil. Here, we report the abundance of Pseudomonas spp. as high PAB in soil, suggesting their contribution to global P cycling. Additional subset analysis of functional genes i.e., polyphosphate kinase (ppk) and exopolyphosphatase (ppx) in all PAB, indicates their significance in bacterial growth and metabolism. Distribution of functional genes in phylogenetic tree represent a more biologically realistic discrimination for the two genes. Distribution of ppx gene disclosed its phylogenetic conservation at species level, however, clustering of ppk gene of similar species in different clades illustrated its environmental condition mediated modifications. Selected PAB showed tolerance to abiotic stress and strong correlation with plant growth promotary (PGP) traits viz. phosphate solubilization, auxin and siderophore production. Interaction of PAB with A. thaliana enhanced the growth and phosphate status of the plant under salinity stress, suggestive of their importance in P cycling and stress alleviation.

IMPORTANCE Study discovered the abundance of Pseudomonas genera as a high phosphate accumulator in soil. The presence of functional genes (polyphosphate kinase [ppk] and exopolyphosphatase [ppx]) in all PAB depicts their importance in polyphosphate metabolism in bacteria. Genetic and functional diversity reveals conservation of the ppx gene at species level. Furthermore, we found a positive correlation between PAB and plant growth promotary traits, stress tolerance, and salinity stress alleviation in A. thaliana.

Multifarious Indigenous Diazotrophic Rhizobacteria of Rice (Oryza sativa L.) Rhizosphere and Their Effect on Plant Growth Promotion

A diverse group of rhizobacteria persists in the rhizospheric soil, on the surface of roots, or in association with rice plants. These bacteria colonize plant root systems, enhance plant growth and crop yield. Indigenous rhizobacteria are known to promote soil health, grain production quality and serve as sustainable bioinoculant. The present study was aimed to isolate, identify and characterize indigenous plant growth promoting (PGP) diazotrophic bacteria associated with the rhizosphere of rice fields from different areas of Jammu and Kashmir, India. A total of 15 bacteria were isolated and evaluated for various PGP traits, antagonistic activity against phytopathogens, production of hydrolytic enzymes and biofilm formation under in-vitro conditions. The majority of the isolated bacteria were Gram-negative. Out of 15 bacterial isolates, nine isolates produced IAA (12.24 ± 2.86 to 250.3 ± 1.15 μg/ml), 6 isolates exhibited phosphate solubilization activity (36.69 ± 1.63 to 312.4 ± 1.15 μg/ml), 7 isolates exhibited rock phosphate solubilization while 5 isolates solubilized zinc (10–18 mm), 7 isolates showed siderophore production, 8 isolates exhibited HCN production, 6 isolates exhibited aminocyclopropane-1-carboxylate (ACC) deaminase activity, 13 isolates exhibited cellulase activity, nine isolates exhibited amylase and lipase activity and six isolates exhibited chitinase activity. In addition, 5 isolates showed amplification with the nifH gene and showed a significant amount of nitrogenase activity in a range of 0.127–4.39 μmol C₂H₄/mg protein/h. Five isolates viz., IHK-1, IHK-3, IHK-13, IHK-15 and IHK-25 exhibited most PGP attributes and successfully limited the mycelial growth of Rhizoctonia solani and Fusarium oxysporum in-vitro. All the five bacterial isolates were identified based on morphological, biochemical and 16S rDNA gene sequencing study, as Stenotrophomonas maltophilia, Enterobacter sp., Bacillus sp., Ochrobactrum haematophilum and Pseudomonas aeruginosa. Rice plants developed from seeds inoculated with these PGP strains individually had considerably higher germination percentage, seed vigor index and total dry biomass when compared to control. These findings strongly imply that the PGP diazotrophic bacteria identified in this work could be employed as plant growth stimulators in rice.