Plant materials
In April 2019, 2-year-old Chinese fir seedlings were collected from the Yalin Center of the Chinese Academy of Forestry in good condition and free from pests and diseases. (The use of Chinese fir seedlings in the experiment complies with national regulations).
Medium
Pikovskava (PVK) solid medium: glucose 10 g, Ca3(PO4) 25 g, CaCO35g, (NH4)2SO40.5 g, NaCl 0.2 g, MgSO4 7H2O 0.1 g, KCl 0.1 g, MnSO4 0.002 g, FeSO4 7H2O 0.002 g, agar 18 g, Distilled water 1000 mL, pH 7.0.
PVK liquid medium: PVK solid medium without agar.
Luria-Bertan (LB) medium: tryptone 10 g, yeast extract powder 5 g, NaCl 10 g, agar 18 g, distilled water 1000 mL, pH7.0.
LB liquid medium:LB solid medium without agar.
Isolation and purification of endophytes from Chinese fir seedlings
The roots, stems, and leaves of the selected Chinese fir seedlings were washed away with running water to remove the surface soil, and then washed with running water for 24 h to 36 h, and the surface moisture was absorbed with sterile filter paper. Weigh 1 g of roots, stems and leaves in a petri dish, and then carry out surface disinfection in a sterile operating table with 75% alcohol (C2H5OH) for 30 s, 5% sodium hypochlorite (NaClO) 10 min, wash the sterile water 7 times, and use sterile filter paper to absorb the water. The material after surface sterilization is cut into 2 mm × 2 mm with sterile surgical scissors, placed in a sterilized mortar and grated with a small amount of sterile quartz sand and ground into a homogenate, then diluted with sterile water to 10–1, 10–2 and 10–3, pipette to draw 150 µL of sample grinding fluid, spread on the medium, set the sterile water of the last rinse of Chinese fir tissue as a blank control, incubate with other plates under the same conditions, and verify whether the surface disinfection. Cultivate in a 28 ℃ incubator according to the characteristics of the colony phenotype, and use the streak separation method to further purify and isolate the strains until the isolation of the colony morphology is uniform for each isolate.
Note: 75% C2H5OH: 75 mL absolute ethanol + 25 mL sterile deionized water; 5% NaClO: sodium hypochlorite solution with 10% available chlorine: sterile deionized water = 1:1.
Characterization of PGP traits
Determination of phosphorus solubilizing ability
The strain was inoculated into PVK liquid medium and cultured at 28 ℃ for 180 days/min on a reciprocating shaker for 7 days, and then the pH value of the medium was measured. The culture solution was centrifuged at 8000 rpm for 15 min to remove bacterial cells. Take the supernatant and use the molybdenum antimony scandium colorimetric method23 to determine the soluble phosphorus content in the culture broth.
Determination of nitrogenase activity
An aliquot of 200 µL fresh culture was inoculated to 20 mL of nutrient broth and incubated overnight at 30℃. Bacterial growth was collected by centrifugation and was washed twice using sterile water, and resuspended by liquid limited nitrogen culture medium (OD600 = 0.2). The 3 mL suspension was transferred to a 25 mL sterilized serum vial and 2.4 mL acetylene gas (99.9999%) was driven into the serum bottle, and then incubated at 30 °C for 12 h. The ethylene content and the protein of bacterial suspension were determined as You et al.24.
1-Aminocyclopropane-1-carboxylate (ACC) deaminase activity determination
ACC deaminase activity was determined by the method of Glick et al.25 using N-free medium (Nfb)26 for bacteria and minimal medium (MM)27 for actinomycetes containing 0.3 m mol L−1 ACC (Sigma, USA) as a sole nitrogen source. MM with 0.1% (w/v) NH4(SO4)2 was used as a positive control and cultivation without ACC was used as a negative control. After incubation at 28 ℃ for 7 days for non-actinomycete bacteria and 14 days for actinomycetes, colony growth on Nfb or MM with addition of ACC indicated ACC deaminase activity.
Indole-3-acetic acid (IAA) production
IAA production was measured by colorimetric assay27. Bacterial isolates were cultured for 3 days in TY broth (without L-tryptophan or supplemented with 500 μg/mL of l-tryptophan) in the dark at 28 °C. Cells were removed from the culture medium by centrifugation at 13,000×g for 10 min; then, 1 mL of the supernatant was mixed vigorously with 2 mL of Salkowski’s reagent (4.5 g of FeCl3 per L in 10.8 M H2SO4). Samples were incubated at room temperature for 30 min and the IAA production was estimated from the optical density at 600 nm (OD600) by comparison with a standard curve prepared from known concentrations of IAA.
Siderophore production
Siderophore production was examined by using chrome azurol S (CAS) agar28. Isolate was inoculated onto CAS agar, cultured at 28 °C for 2 days, and the positive strain was indicated by an orange halo around the bacterial colony. Determine the ratio (D/d) of orange aperture diameter (D) to colony diameter (d) to determine the iron-producing carrier capacity of the strain.
Physiological and biochemical tests of phosphate-solubilizing bacteria
The conventional physiological and biochemical identification of PSB is carried out according to the methods in the “Common Bacterial System Identification Manual”, which mainly includes Gram stain, glucose hydrolysis test, lactose hydrolysis test, methyl red test, Voges-Proskauer (VP) test, hydrogen sulfide production test, gelatin liquefaction Test, citrate utilization test, malonate utilization test, denitrification test.
16 SrRNA gene sequencing
Taking the screened multifunctional PSB as the object, the bacterial genomic DNA extraction kit of Beijing Bomed Biotechnology Co., Ltd. was used to extract the DNA of the strain, using the DNA as a template, and using the bacterial universal primer 27F (5′-AGAGTTTGATCCTGGCTCAG-3′) and 1492R (5′-GGTTACCTTGTTACGACTT-3′) PCR amplification, the amplification system is as follows: DNA template 1 uL, primer 27F 0.5 µL, 1492R 0.5 µL, 2 × TaqMix 12.5 µL, ddH2O10.5 µL. The PCR procedure is as follows: 93℃ for 3 min, 93℃ for 30 s, 56 ℃ for 30 s, 72℃ for 2 min, 32 cycles; 72 ℃ for 7 min. The amplified products were sequenced bidirectionally by BGI. After splicing the measured 16SrDNA sequences in ContigExpress, search in GenBank, EzTaxon, BIGSdb databases respectively, select the model strains with high homology. The phylogenetic tree was constructed by the neighbor-joining method using MEGA version 7.0 with the Kimura 2-parameter model29, the robustness of the tree was evaluated by performing bootstrap analyses based on 1000 replications30.
Evaluation of plant growth promotion by individual inoculation
Preparation of inoculum
The single colonies were picked out and incubated in LB broth at 180 rpm at 28 ℃ for 12 h. The above solution was inserted into 200 mL of LB broth at 1% inoculation, incubated at 180 rpm at 28℃ for 48 h and the cell pellet was resuspended in sterile distilled water and made up to a final concentration of 3 × 108 CFU/mL.
Experimental seedlings and soil
Chinese fir seedlings were provided by the Experimental Center of Subtropical Forestry, Chinese Academy of Forestry (117° 67′ E, 27° 82′ N), Jiangxi Province, China. The use of Chinese fir seedlings in the experiment complies with national regulations. Five months old seedlings with vigorous and apparently disease and pest free were used. The height and root collar diameters of seedlings were 8.7 cm and 1.36 mm, respectively. The seedling container was made of non-woven fabric, the specification was 4.5 cm × 8.0 cm (Diameter × Height). The soil was consisted of nursery medium and loess at a ratio of 9:1, was thoroughly mixed and homogenized with 3 kg slow-release fertilizer per cubic. The slow-release fertilizer is produced by American Abbes (180 g kg−1 total N, 80 g kg−1 available P, and 80 g kg−1 total K, the fertilizer effect period is 9 months). The soil exhibited the following properties: 6.34 g kg−1 total N, 0.80 g kg−1 total P, 2.50 g kg−1 total K, and a pH-value of 6.00.
Test design
The SSP2, JRP22 and HRP2, which were confirmed to have the characteristics of promoting plant growth, so pot experiments were conducted. To determine the effectiveness of phosphorus-solubilizing bacteria in plant growth of Chinese fir, a pot culture experiment was conducted between August and November 2019 in an open-sided greenhouse in Experimental Center of Subtropical Forestry, Chinese Academy of Forestry, Jiangxi Province, China. The experiment was carried out in three-factor orthogonal design with five replications for each treatment. The orthogonal experimental design of experiment is provided in Table 1, and strain, dilution ratio, inoculation method contained 3 levels. The pots with water was used as control (CK). For the irrigation of the rhizosphere (IR) treatments, 30 mL of diluted inoculum was added to the soil in the vicinity of the roots of Chinese fir. For the foliar spray (FS) treatments, 30 mL diluted bacterial cell suspension was inoculated in the leaves of Chinese fir by using a syringe. For the rhizosphere + foliar spray (IS) treatments, 15 mL of diluted inoculum was inoculated in the rhizosphere of seedlings, 15 mL was added to the leaves of seedlings by foliar spray. Each treatment contained sixteen seedlings for a total of nine treatments. A total of 3 inoculations were given in the middle of each month. The plant height and stem diameter were recorded before the first inoculation. The plants were harvested after 90 days (16 plantlets/replicate/treatment, i.e., a total of 80 plantlets per treatment) and the root biomass, stem biomass, leaf biomass, plant height and stem diameter were measured.
Determination of growth indicators
During the test, before each inoculation, the height of the seedlings was measured with a ruler and the ground diameter of the seedlings was measured with a vernier caliper. After the experiment, 10 plants of Chinese fir seedlings in average growth were randomly selected from each treatment, a total of 30 plants were washed with clean water to remove surface impurities, the filter paper was dried and the roots, stems, and leaves were put into paper bags respectively at 105 °C. After being degraded for 0.5 h, dried at 70 °C to a constant weight, weighed and recorded the biomass of each part.
Determination of leaf and soil nutrient content
Total N content of leaf and soil was measured by a 2300 Kjeltec Analyzer Unit (FOSS, Höganäs, Sweden). Total P and total K, total Mg and total Fe of leaf, soil TP, TK, AP and AK were extracted according to literature31 and were determined by ICP (Kleve, Germany).
Determination of soil enzyme activities
Activities of the soil urease, cellulase, sucrase, dehydrogenase and acid phosphatase were determined by spectrophotometry. Firstly, 0.05 g of soil was added to 450 mL of phosphate buffer solution (PBS, 0.1 mol L−1, pH 7.4). Then the solution was mixed by shaking, and centrifuged at 2000 rpm at 4 ℃ for 10 min and supernatant was collected with a new centrifugal tube. The supernatant and reagents were added according to the kit instructions (Shanghai Enzyme-linked Biotechnology Co., Ltd., Shanghai, China). Absorbance at 450 nm was measured on a SpectraMax Paradigm Multi-Mode detection platform (Molecular devices, San Jose, CA, USA).
Data processing and analysis
All statistical analyses were performed using SPSS24. Data are presented in terms of means (± SE; standard error). Statistical differences were tested by one-factor ANOVA to evaluate the differences in the nutrient content of soil and plant growth status. In MEGA7.0, the Neighbor-Joining method was used to construct the phylogenetic tree, and the Bootstrap value was 1000.
Source: Ecology - nature.com
