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.

Modulation in Plant Growth and Drought Tolerance of Wheat Crop upon Inoculation of Drought-tolerant-Bacillus Species Isolated from Hot Arid Soil of India

Drought is one of the most detrimental environmental stressors to plants with the potential to decrease crop yields and affect agricultural sustainability. Native bacteria with beneficial traits enhance plant growth and help avoid and reverse the effects of drought in plants to a greater extent. In the present study, we aimed to ( i ) isolate drought-tolerant Bacillus isolates from the rhizosphere soil of wheat crop grown at different locations in Jaisalmer district, Rajasthan state and (ii) further evaluate their ability to enhance plant growth and induce drought tolerance in wheat ( Var. HD-2967) grown under drought stress conditions. Of more than 100 isolates, two putative Bacillus isolates capable of tolerating 30 % polyethylene glycol-6000 (PEG-6000) [equivalent to -9.80 MPa (Megapascal)] were identified as Bacillus altitudinis DT-89 and Bacillus paramycoides DT-113. These isolates exhibited different plant growth promoting (PGP) attributes such as phosphate solubilization, and production of siderophore, exopolysaccharide, ammonia, indole acetic acid and cytokinin at low osmotic stress of 10% PEG-6000 but shown variable response at higher osmotic stress particularly at 30% PEG-6000. However, they did not show any antifungal activity and one isolate was negative for phosphate solubilization. Of two strains, B. altitudinis DT-89 function more prominently with respect to plant growth promotion and drought tolerance to plant in the early stage but protective traits of B. paramycoides DT-113 was more prominent after 75 days as evident by increased EPS (164%), root dry weight (144.44%), chlorophyll content (90.26%), SOD (389%) and proline (99.3%). The results support both the strains as a potential candidate to alleviate drought stress and enhance plant growth in the drought regions.

Exploration, isolation and characterization of indigenous rhizobacteria from patchouli rhizosphere as PGPR candidates in producing IAA and solubilizing phosphate

Microorganisms that are active and aggressive colonizing the rhizosphere are known as rhizobacteria. They are able to act as biofertilizers, bioprotectants, biostimulants and bioremediation. This study aims to identify and characterize groups of rhizobacteria present in the patchouli rhizosphere that can produce IAA compounds and have the ability to solubilize phosphate in the soil. Soil samples were taken from the patchouli rhizosphere at Purwosari Village, Nagan Raya, Aceh Province, Indonesia. This study used quantitative and qualitative descriptive analysis through serial dilutions to obtain rhizobacterial strains. Parameters observed were macroscopic and microscopic characteristics, gram test, IAA production and phosphate solubilization. The study obtained 37 isolates of rhizobacteria from Purwosari (PS), comprising 25 isolates of gram positive and 12 isolates of gram negative. The rhizobacteria PS 5/1 produced the lowest IAA at 21.66 ppm, whereas isolate 5/6 C produced the highest IAA at 83.38 ppm. Twenty-five isolates of rhizobacteria could solubilize phosphate while the remaining 12 isolates did not have this ability. The rhizobacteria PS 7/1 resulted in the highest PSI at 2.55 and isolates PS 8/7 produced the lowest PSI at 1.33. The rhizobacteria isolates that can produce IAA and phosphate solubilizing have the potential to be used as PGPR candidates.

Plant Growth-promoting and Bio-control Activity of Micrococcus luteus Strain AKAD 3-5 Isolated from the Soybean (Glycine max (L.) Merr.) Rhizosphere

Background: Applications of bioinoculants for improving crop productivity may be an eco-friendly alternative to chemical fertilizers. Rhizosphere or soil-inhabiting beneficial microbes can enhance plant growth and productivity through direct and indirect mechanisms, i.e., phosphate solubilization, nutrient acquisition, phytohormone production, etc. Objective: This study is based on the hypothesis that diseases resistant plants can act as a source of potential microbes that can have good plant growth-promoting traits and bio-control potential. Methods: In this study, we have isolated the rhizobacterial strains (AKAD 2-1, AKAD 2-10, AKAD 3-5, AKAD 3-9) from the rhizosphere of a disease-resistant variety of soybean (JS-20-34) (Glycine max (L.) Merr.). These bacterial strains were further screened for various plant growth-promoting traits (phosphate solubilization, indole acetic acid (IAA), ammonia, biofilm, HCN, Exopolysaccharide (EPS), and enzyme production activity (catalase, cellulase, and chitinase)). Results: Among four, only bacterial strain AKAD 3-5 has shown plant-growth-promoting and biocontrol (98%) activity against Fusarium oxysporum. Morphological, biochemical, and molecular characterization (16S rRNA) revealed that this rhizobacterial isolate AKAD 3-5 closely resembles Micrococcus luteus (Gene bank accession: MH304279). Conclusion: Here, we conclude that this strain can be utilized to promote soybean growth under varied soil stress conditions.

Plant Growth Promoting Microorganism Selection and Activity Test for Reforestation of Topsoil Restoration Site

Objectives : This study was conducted to investigate the applicability of plant growth promoting microorganisms during restoration through re-vegetation of damaged topsoil.Methods : As the vegetation to be applied to the restoration site, Weigela subsessilis, Spiraea prunifolia, Pine densiflora, Pennisetum alopecuroides were selected. An attempt was made to isolate plant growth promoting microorganisms from the root zone of plants of the same species inhabiting domestic park sites and hiking trails. Plant growth promoting activities such as phosphate solubilization ability, siderophore production ability, IAA production ability, and ACC deaminase production ability were examined, and the species to be finally applied was selected and then identified. Among the strains whose plant growth promoting activity was confirmed, Arthrobacter sp. 1B2 and Paraburkholderia terrae 1P2 were applied to the genitalia and pine, respectively, and a pot experiment was conducted to confirm the activity.Results and Discussion : Forty-five strains were isolated from Weigela subsessilis, Spiraea prunifolia, Pine densiflora, Pennisetum alopecuroides and the IAA-producing ability and ACC deaminase-producing ability were confirmed for 16 strains whose phosphate solubilizing ability and siderophore-producing ability were confirmed. After selecting and identifying strains with excellent plant growth promoting ability, strains such as Cupriavidus sp, Arthrobacter sp., Pseudomonas fluorescens, Pseudomonas sp., Paraburkholderia terrae were obtained. Among them, Arthrobacter sp. 1B2 and Paraburkholderia terrae 1P2 strains were applied to genitalia and pine, respectively, and it was confirmed that plant growth was promoted.Conclusions : Bioassay experiments and field applications using plant growth promoting microorganisms have been mainly studied for herbaceous species (Grandaceae, corn, oats, etc.). However, in this study, the applied plants are shrubs class, which do not significantly grow in length, targeting damaged areas with high subsoil content, which are poor in environment and insufficient in organic and inorganic matter. Therefore, it is meaningful in that the activity of plant growth promoting microorganisms focused on absorption of inorganic substances, such as phosphate solubilization activity and siderophore ability, was investigated and the activity was confirmed by performing a bioassay.

Evaluation of Inorganic Phosphate Solubilizing Efficiency and Multiple Plant Growth Promoting Properties of Endophytic Bacteria Isolated From Root Nodules Erythrina Brucei

Background: The majority of phosphorous in the soil is fixed and unavailable to plant nutrition, hence in scarcity. Phosphate solubilizing bacteria, the ecological engineers, are considered as the best, sustainable and eco-friendly options. The objectives of this study were to screen and evaluate inorganic phosphate solubilizing efficiency and assess multiple plant growth promoting traits of E. brucei root nodule bacterial endophytes.Results: A total of 304 nodule bacterial endophytes were screened for phosphate solubilization potential on solid PA medium among which 119 (39%) were potential tricalcium phosphate solubilizers. None of these isolates were able to form clearly visible halos on aluminum phosphate (AlPO4), Al-P or iron phosphate (FePO4), Fe-P supplemented PA medium. Out of 119 inorganic phosphate solubilizing endophytes, 40.3% were IAA producers. Based on phosphate solubilization index, the potential bacterial endophytes were identified to Gluconobacter cerinus, Acinetobacter soli, Achromobacter xylosoxidans and Bacillus thuringiensis using the 16S rRNA gene sequences analysis. All the selected isolates were potential solubilizers of the three inorganic phosphates (Al-P, Fe-P and tricalcium phosphate, TCP) included in liquid NBRIP medium. The highest values of solubilized TCP were recorded by isolates AU4 and RG6 (A. soli), 108.96 mg L-1 and 107.48 mg L-1, respectively at sampling day3 and 120.36 mg L-1 and 112.82 mg L-1, respectively at day 6. The highest values of solubilized Al-P and Fe-P were recorded by isolate RG6, 102.14 mg L-1 and 96.07 mg L-1, respectively at sampling days 3 and 6, respectively. The highest IAA, 313.61µg mL-1 was recorded by isolate DM17 (B. thuringiensis). These selected potential isolates were also HCN, NH3, and hydrolytic enzymes producers. The isolates were also varied in tolerance to eco-physiological stressors and exhibited versatility to carbon and nitrogen substrate utilization. Conclusions: The genera and species Gluconobacter cerinus, Acinetobacter soli, Achromobacter xylosoxidans and Bacillus thuringiensis are the first reports from E .brucei root nodules and Gluconobacter is also the first report to the science as phosphate solubilizer. Isolates AU4 and RG (A. soli) could be potential bio-inoculant candidates for the growth enhancement of the host plant for better agro-forestry practices in acidic and alkaline soils in Ethiopia.

Inorganic and organic phosphate solubilization potential of Stenotrophomonas maltophilia

Most soils contain Phosphorus (P) in insoluble compounds as organic and inorganic forms which is unavailable to plants. Furthermore, phosphate solubilizing bacteria (PSB) play an important role in converting insoluble P to a bioavailable form through solubilizing and mineralizing inorganic or organic P. Therefore, this study aims to determine the ability of isolate a phosphate solubilizing bacterium (PSB), Stenotrophomonas maltophilia to solubilize inorganic and organic P. The bacterium was isolated from peat soil of West Kalimantan using Pikovskaya medium added with Ca3 (PO4)2 as the P source. The activity of organic P (acid phosphatase and phytase) was measured using p-nitrophenyl phosphate disodium (pNPP 0.115 M) and sodium phytate as the substrate of bacterial broth culture. Also, the optimization for inorganic and organic P solubilization by the bacterium was conducted. The maximum values of inorganic and organic phosphate solubilization was recorded to be 52.26 μg/mL and 44.51 U/mL (acid phosphatase), 0.13 U/mL (phytase) respectively. Optimum conditions found were temperature at 30 °C, pH 6.0 and in the presence of sucrose and beef extract, which serve as carbon and nitrogen sources, respectively. Therefore, it can probably be used as a biological fertilizer for plants.

Antagonistic potential of some phosphate solubilizing fungi against some phyto-pathogenic fungi

Background Little phosphorus availability in soil is one of the major forces for crop production. Phosphate solubilizing fungi enhance available phosphorus from soils and contribute to achieve the plants phosphorus requirement. Phosphate solubilizing fungi are important component of soil microbiota and play an essential role in the biogeochemical cycling of phosphorus in natural ecosystems. Results The study tested the potential of the isolated phosphate solubilizing fungi to inhibit the growth of Alternaria alternata, Fusarium solani, Geotrichum candidum, Rhizoctonia solani and Sclerotium rolfsii. A total of 137 fungal isolates were isolated and identified from 30 soil samples. Nine isolates solubilized inorganic phosphate (tricalcium phosphate). Eight of these isolates were isolated from the rhizosphere of Alhagi graecorum Boiss., Allium cepa L., Ehrharta calycina Sm., Ludwigia stolonifera (Guill. & Perr.) Raven, Mentha longifolia L., Phragmites communis Trin.,Triticum aestivum L. and Zea mays L., respectively and one isolate was isolated from island's free soil. Purified single spore cultures of these nine isolates were identified to species level by multi loci DNA barcoding using internal transcribed spacer1and 2 (ITS1 and ITS 2) of the rRNA gene cluster, Calmodulin (CaM) and β-tubulin)BenA) gene markers. Conclusions The isolated phosphate solubilizing fungi belonged to two genera, Aspergillus and Penicillium. The phosphate solubilization index ranged from 1 to 1.5. Aspergillius japonicus 2 had the highest solubilization index (1.5) and also had the highest inhibition percentage (70%) against Alternaria alternata. This isolate is promising for development of biocontrol agent of Fusarium solani, Geotricum candidium and Alternaria alternata. The highest phosphate solubilization by Aspergillius japonicus 2 was obtained by using sucrose as carbon source and also ammonium chloride and tryptophan as nitrogen source. The highest solubilization percentage was 78.2% at optimum glucose concentration (5%) while the pH value that gave the highest solubilization percentage (90%) was 3. After eight days of incubation of Aspergillius japonicus 2, the solubilization percentage reached its maximum value (80.2%).