Molecular Diversity Analysis of Plant Growth Promoting Rhizobium Isolated from Groundnut and Evaluation of Their Field Efficacy

The Effect of Climate Variables, Soil Characteristics, and Peanut Cultivars on the Rhizobial Bacteria Community

Peanuts are widely cultivated across the world; however, peanut's rhizobial community and the determinant factors of their composition are still to be elucidated. This study investigates the biogeography and determinant soil environmental factors for peanut rhizobia. A total of 1001 rhizobial isolates were obtained from the peanut root nodules, mainly belonging to two cultivars (X9 and M6) cultivated in 20 sampling sites across China. According to recA sequence analysis, all the isolates were classified as 84 haplotypes, and a representative strain for each haplotype was randomly selected to perform subsequent analyses. Based on multilocus sequence analysis (MLSA) of housekeeping genes dnaK, glnII, gyrB, recA, and rpoB, all the representative strains were classified as 42 genospecies in the genus Bradyrhizobium, including 12 effectively published and 30 undefined genospecies. Strains belonging to six genospecies were predominant (>5%), including B. ottawaense, B. liaoningense, B. yuanmingense, Bradyrhizobium sp. XXIX, B. guangdongense, and B. nanningense. However, only a single isolate was obtained for 15 genospecies. The diversity indices of peanut rhizobia distributed in South China are obviously higher than those in North China, but no obvious peanut cultivar selection for rhizobial genospecies was found. Correlation analyses indicated that the community composition of peanut rhizobia was mainly affected by MAP, MAT, soil AP, and pH. Nodulation tests indicated that the 79 representative strains belonging to 37 genospecies with both nodC and nifH could perform nitrogen-fixing symbiosis with peanuts. This study revealed the great diversity and varied composition of communities of peanut rhizobia in different geographic regions across China.

Efficacy of molecularly diversified phosphorus-solubilizing rhizobacterial isolates in phytostimulation, antimicrobial attributes and phosphorus-transporter genes mediated plant growth performance in maize (Zea mays L.)

This study evaluated a dual management approach to enhance plant-growth by improving soil fertility, reducing pathogenic stress using PGPR that affect phosphorus-transporter (pht) genes. Among 213 maize rhizobacterial isolates, 40 demonstrated the ability to solubilize tri-calcium phosphate, potassium, zinc, and silicon, showing various PGP traits. Nine of these isolates exhibited significant antagonistic activity against the plant pathogens Colletotrichum chlorophyti and Xanthomonas axonopodis. These pathogens cause root infection, reduces plant-immunity and growth. In pot experiments, these nine strains significantly improved root length, shoot length, chlorophyll content, fresh weight, proline, APX, CAT, GR, NPK, and Zn content in maize plants after 60 days under pathogenic stress. Notably, PSB-25 increased root length by up to 66% under C. chlorophyti stress and 64% under X. axonopodis stress. PSB21 enhanced proline content by 49%, APX by 70%, and GR by 41%, while PSB-16 raised CAT activity by 55% under X. axonopodis stress. Molecular diversity analysis of the 40 PS-RB strains using ERIC, BOX, REP, and ARDRA showed two major clusters with Jaccard coefficients from 0.72 to 1.00. 16S rRNA gene sequencing identified PSB10, PSB16, and PSB25 as Serratia sp., Enterobacter cloacae, and Enterobacter sp., respectively. The effects of PSB10, PSB16, and PSB25 on growth parameters under pathogen stress were also studied. Field trials indicated that treatment T6 (100% RDF + PSB16) was most effective in promoting plant growth. Additionally, significant differences in the expression of six Pht1 transporter genes were noted between PS-RB treated and untreated maize seedlings, and these genes improving phosphorus acquisition.

Evaluation of commercial importance of endophytes isolated from Argemone mexicana and Papaver rhoeas

The paper industry is a composite one constituting different types of mills, processes, and products. The paper industries consume large amounts of resources, like wood and water. These industries also create huge amounts of waste that have to be treated. In our study, 23 endophytic bacteria were isolated from Argemone mexicana, and 16 endophytic bacteria were isolated from Papaver rhoeas. Seventeen and 15 bacterial endophytes from A. mexicana and P. rhoeas, respectively, showed cellulose-degrading activity. The biochemical and molecular characterization were done for endophytic bacteria with cellulolytic activity. The consortium of cellulose-degrading endophytic bacteria from A. mexicana showed endoglucanase activity (0.462 IU/ml) and FPCase enzyme activity (0.269 IU/ml) and from P. rhoeas gave endoglucanase activity (0.439 IU/ml) and FPCase enzyme activity (0.253 IU/ml). Degraded carboxy methylcellulose and filter paper were further treated by Saccharomyces cerevisiae and bioethanol was produced. Cellulose-degrading endophytic bacteria were also tested for auxin, siderophore production, and phosphate solubilization activities. Individual cellulose-degrading endophytic bacteria with plant growth-promoting activities were used as biofertilizers, tested for plant growth-promoting activities using Basmati Pusa 1121 rice, and plant growth parameters were recorded. The degraded paper enhances the growth of rice plants. Selected bacterial endophytes and their consortia from A. mexicana and P. rhoeas were powerful cellulose degraders, which can be further employed for ethanol production and as significant biofertilizers in agriculture.

Maize and peanut intercropping improves the nitrogen accumulation and yield per plant of maize by promoting the secretion of flavonoids and abundance of Bradyrhizobium in rhizosphere

Belowground interactions mediated by root exudates are critical for the productivity and efficiency of intercropping systems. Herein, we investigated the process of microbial community assembly in maize, peanuts, and shared rhizosphere soil as well as their regulatory mechanisms on root exudates under different planting patterns by combining metabolomic and metagenomic analyses. The results showed that the yield of intercropped maize increased significantly by 21.05% (2020) and 52.81% (2021), while the yield of intercropped peanut significantly decreased by 39.51% (2020) and 32.58% (2021). The nitrogen accumulation was significantly higher in the roots of the intercropped maize than in those of sole maize at 120 days after sowing, it increased by 129.16% (2020) and 151.93% (2021), respectively. The stems and leaves of intercropped peanut significantly decreased by 5.13 and 22.23% (2020) and 14.45 and 24.54% (2021), respectively. The root interaction had a significant effect on the content of ammonium nitrogen (NH₄ ⁺-N) as well as the activities of urease (UE), nitrate reductase (NR), protease (Pro), and dehydrogenase (DHO) in the rhizosphere soil. A combined network analysis showed that the content of NH₄ ⁺-N as well as the enzyme activities of UE, NR and Pro increased in the rhizosphere soil, resulting in cyanidin 3-sambubioside 5-glucoside and cyanidin 3-O-(6-Op-coumaroyl) glucoside-5-O-glucoside; shisonin were significantly up-regulated in the shared soil of intercropped maize and peanut, reshaped the bacterial community composition, and increased the relative abundance of Bradyrhizobium. These results indicate that interspecific root interactions improved the soil microenvironment, regulated the absorption and utilization of nitrogen nutrients, and provided a theoretical basis for high yield and sustainable development in the intercropping of maize and peanut.

Elite Bradyrhizobium strains boost biological nitrogen fixation and peanut yield in tropical drylands

Peanut (Arachis hypogaea L.) is an important crop for the family-based systems in the tropics, mainly in Brazil. In the Brazilian drylands, peanuts are cropped in low technological systems, and cheap and efficient technologies are needed to improve crop yield and sustainability. Despite this importance, few data are available on selecting efficient peanut rhizobia in experiments under different edaphoclimatic conditions. This work evaluated the agronomic efficiency and the biological nitrogen fixation (BNF) by two elite Bradyrhizobium strains under four different fields in the Brazilian semiarid region. We compared a new efficient strain Bradyrhizobium sp. ESA 123 with the reference strain B. elkanii SEMIA 6144, currently used in peanut rhizobial inoculants in Brazil. Besides the inoculated treatments, two uninoculated controls were assessed (with and without 80 kg ha^-1 of N-urea). The BNF was estimated by the δ¹⁵N approach in three out of four field assays. BNF contribution was improved by inoculation of both Bradyrhizobium strains, ranging from 42 to 51% in Petrolina and 43 to 60% in Nossa Senhora da Glória. Peanuts' yields benefited from the inoculation of both strains and N fertilization in all four assays. Nevertheless, the results showed the efficiency of both strains under different edaphoclimatic conditions, indicating the native strain ESA 123 as a potential bacterium for recommendation as inoculants for peanuts in Brazil, mainly in drylands.

The introduced strain Mesorhizobium ciceri USDA3378 is more competitive than an indigenous strain in nodulation of chickpea in newly introduced areas of China

The present study aimed to compare the competitive advantage of two chickpea nodulating rhizobia strains (an indigenous strain Mesorhizobium muleiense CCBAU 83963^T and an introduced strain Mesorhizobium ciceri USDA 3378) in different soils originated from new chickpea cultivation areas of China. The results showed that USDA 3378 had a significant competitive advantage in nodulation, with nodulation occupation rates ranging from 84.6% to 100% in all the sampled soils. According to the efficiency of symbiosis under single inoculation, chickpea plants inoculated with USDA 3378 showed better symbiotic performance based on the plant dry weight, leaf chlorophyll content and nodule numbers. The chickpea plants inoculated with USDA 3378 formed nodules about 2 days earlier than those inoculated with CCBAU 83963^T . The higher growth in media and the stronger adsorption on chickpea roots of USDA 3378 when mixed with CCBAU 83963^T may explain why USDA3378 shows a competitive advantage. The results from this study will contribute towards the development of effective chickpea rhizobial inoculants for soil conditioning and more environmentally friendly production of chickpeas in China.

A new pioneer colorimetric micro-plate method for the estimation of ammonia production by plant growth promoting rhizobacteria (PGPR)

The ability of Plant Growth Promoting Rhizobacteria (PGPR) to produce ammonia (NH3) is one of the key mechanisms for the growth and development of plants. Ammonia spectrophotometric quantification assay using Nessler’s reagent remains laborious and seems to be not suitable for large-scale PGP screening assay. Here, we performed a new accurate, easy, and cost-effective micro-plate protocol for large-scale ammonia quantification in bacterial supernatants as an alternative to the spectrophotometric method. In order to validate the accuracy of our innovative microplate assay, 9 bacterial strains were explored for their ability to produce ammonia using both the conventionally described assay and the newly performed micro-plate one. The ammonia 96-well microplate assay was successfully performed by scaling down the spectrophotometric methods to reduce the volume of bacterial supernatant as well as Nessler’s reagent. The reduction was estimated to 90%of the total used volume in comparison to the conventional test. The micro-test is 10-fold cheaper and 26 times faster than the conventional method. All bacterial isolates were positive for ammonia production. Bacillus inaquorsum and Bacillus mojavensis produced the highest ammonia concentration of about 371 and 370μM respectively. Furthermore, the application of the ammonia micro-plate assay reduces drastically the reagent waste and toxicity hazard of K2HgI4 (Nessler’s reagent) in the environment, thus, we can classify it as eco-friendly respecting the Green Chemistry concept according to Environmental Protection Agency (EPA). The statistical data obtained from both assays are significantly correlated (r = 0.985, R squared = 0.9329, and p <  0.001) proving the accuracy of the micro-plate assay. The proposed NH3 micro-assay is a reliable, rapid, eco-friendly, and cost-effective method to screen plant growth-promoting potential of more than 25 bacterial strains in one micro-plate. It could be an alternative for the conventional NH3 assay as a routine research tool.

Phenetic and Molecular Diversity of Nitrogen Fixating Plant Growth Promoting Azotobacter Isolated from Semiarid Regions of India

In the present study, 24 Azotobacter strains were isolated from soils of different areas of southern Rajasthan and characterized at biochemical, functional, and molecular levels. The isolated Azotobacter strains were gram negative and cyst forming when viewed under the microscope. These strains were also screened for their plant growth promoting activities and the ability of these isolates to survive under abiotic stress conditions viz. salt, pH, temperature, and drought stress. All the isolates showed IAA, siderophore, HCN, and ammonia production, whereas seven Azotobacter strains showed phosphate solubilization. Amplified Ribosomal DNA Restriction Analysis (ARDRA) revealed significant diversity among Azotobacter strains and the dendrogram obtained differentiated twenty-four of the strains into two major clusters at a similarity coefficient of 0.64. Qualitative and quantitative N2 fixation abilities of these strains were also detrained, and the amounts of acetylene reduced by Azotobacter strains were in the range of 1.31 to 846.56 nmol C2H4 mg protein−1 h−1. The strains showing high nitrogen fixation ability with multiple PGP activities were selected for further pot studies, and these Azotobacter strains significantly increased the various plant growth parameters of maize plantlets. Furthermore, the best Azotobacter isolates were subjected to 16S rRNA sequencing and confirmed their identities as Azotobacter sp. The indigenous Azotobacter strains with multiple PGP activities could be further used for commercial production.

Polyphasic Characterization of Plant Growth Promoting Cellulose Degrading Bacteria Isolated from Organic Manures

In the present study, twenty seven cellulose-degrading bacteria (CDB) were isolated from various organic manures and their cellulolytic activities were determined. The bacterial isolate CDB-26 showed the highest cellulolytic index, released 0.507 ± 0.025 mg/ml glucose and produced 0.196 ± 0.014 IU/ml cellulase enzyme under in vitro conditions. Biochemically, all the 27 isolates showed difference in the 6 biochemical tests performed. Further, all the 27 CDB isolates were subjected to various plant growth-promoting activities, and all CDB strains were positive for IAA production, GA₃ production and siderophore production, whereas 19 strains were positive for ACC deaminase activity, 21 strains showed NH₃ production and 19 strains were positive for HCN production. Out of 27 CDB isolates, 18 isolates were able to solubilize phosphate, 21 isolates were able to solubilize potash and 10 CDB isolates were found positive for silica solubilization. The molecular diversity among different CDB isolates was studied through ARDRA and demonstrated very high genetic diversity among these bacteria. The in vitro cellulose-degradation potential of these CDB isolates using vegetable waste as substrate were also assessed, and the 3 CDB isolates viz. Serratia surfactantfaciens (CDB-26), Stenotrophomonas rhizophila (CDB-16) and Pseudomonas fragi (CDB-5) showed the highest cellulose-degrading potential under in vitro conditions. Hence, the cellulolytic microbes isolated in the present study could be used for effective bioconversion of plant biomasses into enriched compost.