Production, Puriﬁcation, and Characterization of Bacillibactin Siderophore of Bacillus subtilis and Its Application for Improvement in Plant Growth and Oil Content in Sesame

: Siderophores are low molecular weight secondary metabolites produced by microorganisms under low iron stress as a speciﬁc iron chelator. In the present study, a rhizospheric bacterium was isolated from the rhizosphere of sesame plants from Salem district, Tamil Nadu, India and later identiﬁed as Bacillus subtilis LSBS2. It exhibited multiple plant-growth-promoting (PGP) traits such as hydrogen cyanide (HCN), ammonia, and indole acetic acid (IAA), and solubilized phosphate. The chrome azurol sulphonate (CAS) agar plate assay was used to screen the siderophore production of LSBS2 and quantitatively the isolate produced 296 mg/L of siderophores in succinic acid medium. Further characterization of the siderophore revealed that the isolate produced catecholate siderophore bacillibactin. A pot culture experiment was used to explore the effect of LSBS2 and its siderophore in promoting iron absorption and plant growth of Sesamum indicum L. Data from the present study revealed that the multifarious Bacillus sp. LSBS2 could be exploited as a potential bioinoculant for growth and yield improvement in S. indicum .


Introduction
Sesame (Sesamum indicum L.), an oilseed crop of the family Pedaliaceae, is one of the oldest cultivated crops of the world with a total production of 3.3 million tons. It is an economically important crop for its edible oil, fatty acids, antioxidants, and its nutritive additive values, medicinal, and industrial purposes [1]. On the other hand, the demand for edible oil is increasing gradually with the increase in world population; therefore, the cultivation of oilseed crops is expanding all over the world. The efforts are needed to increase the sustainable production of vegetable oils in an ecofriendly and effective manner. Sustainable measures are being explored to improve the quality and quantity of edible oil for human consumption. The utilization of multifarious plant-growthpromoting rhizobacteria (PGPR) as biofertilizers is increasingly being reported as one of the best practices of complementing conventional agrochemical inputs in agricultural systems [2][3][4]. These PGPRs promote plant growth by facilitating nutrient and nitrogen fixation [5,6], providing plant hormones such as including indole acetic acid (IAA) etc., phosphate solubilization [7,8] and iron nutrition through siderophore. Siderophores are iron scavenging ligands produced by a wide range of PGPR including Bacillus spp. to help the solubilization and transport of iron through the formation of soluble Fe 3+ [9,10].
Bacillus sp. is one of the most studied PGPRs. Members of this species produce a wide range of plant-growth-promoting (PGP) traits such as antibiotics, hydrolytic enzymes, phytohormones, antioxidant enzymes, and siderophores etc. [11]. Bacillus sp. produces the catecholate siderophore 2,3-dihydroxy benzoyl glycine. Bacillibactin is the archetypal triscatetholate siderophore. Owning to the incorporated threonine, bacillibactin is known for its highest affinity for iron (Fe 3+ ) of natural siderophores, and bacillibactin has been stated as the dominant extracellular ferric iron scavenger of Bacillus subtilis under iron limitations [12]. Inoculation of PGP cells suspension of Bacillus sp. promotes plant growth, plant nutrition and control plant pathogens. Addition of suspension of the purified siderophore also promotes plant growth and control phytopathogens; however, the effect is less as compared to the whole cell suspension [11]. Despite the plant growth and nutrition potential, the bioefficacy of Bacillus sp. has not been much studied in sesame. Therefore, the present study was aimed to isolate the siderophore and the other plantgrowth-promoting metabolite PGPR from sesame rhizosphere, to produce, purify and characterize the siderophore, and to evaluate the bioefficacy of the siderophore producing Bacillus sp. as well as the iron nutrition and plant growth promotion in sesame plants.

Soil Sample and Sesame Seeds
Soil samples were collected in five replicates, from the rhizosphere of sesame plant in Salem (11.8386 • N, 78.0723 • E), Tamil Nadu, India and subjected to further analysis [13]. Sesamum indicum (sesame) seeds were procured from the local market of Salem district of Tamil Nadu, India.

Soil Analysis
The pH of the soil samples was measured with a pH meter (Elico, Mumbai), and electrical conductivity (EC) was measured using an electrical conductivity bridge and expressed as dS/m [14]. The available nitrogen content in the soil was measured by the alkaline permanganate method [15,16]. Soil phosphorus was estimated according to Olsen's method [17]. The potassium and iron content of the soil were estimated according to Jackson's [18] and Emsens's method [19], respectively. The sulfur, zinc, and copper content of the soil samples were measured according to the method of Jackson [17].

Isolation of Bacillus sp.
The Bacillus sp. was isolated from the collected soil samples by serial dilution techniques. The isolate was subcultured and maintained on nutrient agar slants and incubation at 28 ± 2 • C for 24 h [20]. Each soil sample was processed in triplicate.

Production of Hydrogen Cyanide (HCN)
Hydrogen cyanide production of the isolate was tested as described by Castric [21]. The isolate was streaked on a nutrient agar medium amended with glycine (4.4 g/L). Plate was covered with a Whatman filter paper previously soaked in a mixture of 0.5% picric acid and 2% Na 2 CO 3 , and incubated at 28 ± 2 • C for 72 h. Following the incubation, the plate was observed for the development of dark brown color on the filter paper. For quantification of HCN production, LSBS2 was inoculated in glycine-nutrient broth and the OD values were measured at 625 nm.

Production of Ammonia
To test the production of ammonia, the isolate was grown in peptone broth (10 mL) at 28 ± 2 • C for 48 h. After incubation, 0.5 mL of Nessler's reagent was added to the bacterial suspension and observed for the development of brown to the yellow color [22].

Production of Indole Acetic Acid (IAA)
For this purpose, each NB broth containing L-tryptophan (100 mg/L) was separately inoculated with the isolate and incubated at 28 ± 2 • C for 48 h in the dark followed by centrifugation at 9730× g for 15 min, and cell-free supernatant was assayed for IAA content according to Salkovski's method [23].

Estimation of Phosphate Solubilization Activity
For this purpose, 5 µL (10 6 cells/mL) of each isolate was grown on Pikovskaia's agar medium at 28 ± 2 • C for 72 h. Following the incubation, the zone of P solubilization was measured [24].

Screening and Production of Siderophore
For siderophore production, 5 µL (10 6 cells/mL) of each isolate was spotted onto the center of each chrome azurol sulphonate (CAS) agar plates. Following the incubation at 30 • C for 48 h, plates were observed for the formation of orange or yellow halo zone around the colonies [25].
Quantitative estimation of siderophore production was performed by growing the isolate in a succinate medium at 28 ± 2 • C for 48 h [26]. This was followed by centrifugation at 10,000 rpm for 15 min. Cell-free supernatant was assayed for the amount of siderophore according to CAS assay [25]. Percent siderophore unit (PSU) was calculated according to the following formula [27].
Siderophore production (psu) Ar − As Ar × 100 where Ar = absorbance of reference (CAS solution and un-inoculated broth), and As = absorbance of the sample (CAS solution and cell-free supernatant of the sample).

Determiantion of Type of Siderophore
The catecholate type of siderophore was determined by the Arnow's test [28]. In this method, 1.0 mL of culture supernatant was added to 0.5 mL of HCl (5 M), 0.1 mL NaOH (10 N), and 1.0 mL of nitrite-molybdate reagent and observed for color change. The catecholate siderophore was confirmed by the formation of orange-red color.
Detection of hydroxamate type siderophore was carried out by tetrazolium test [29]. In this, 100 µL of culture supernatant was added to 1-2 drops of NaOH (2 N) mixed with a small quantity of tetrazolium salt and observed for the deep red color formation.

Biochemical Characterization
Physiological and biochemical characteristics of potent isolates were carried as per Bergey's Manual of Systematic Bacteriology [30] using pre-sterilized biochemical kits (Hi-Media, Mumbai, India). The ability of isolates to oxidize different carbon sources and the production of various enzymes was investigated.

16S rRNA Gene Sequence Analysis
The sequencing of 16S rRNA genes of the isolate was carried out as per the method described by Gangurde et al. [31]. Genomic DNA of the isolate was extracted by using Hi-PurATM Plant Genomic DNA Miniprep Purification Spin Kit (HI-Media, Mumbai, India) as per the method of Sambrook and Russel [32]. Amplification of 16S rRNA gene sequencing was performed using the primers fD1 (50-AGAGTTTG ATCCTGGCTCAG-30) and rP2 (30-ACGGCTACCTTGTTACGACTT-50) [33]. The amplified sequences were analyzed with gapped BLASTn (http://www.ncbi.nlm.nih.gov, 2 March 2020), and the evolutionary distances were computed using the neighbor-joining method [34].

Purification of Siderophore
For this purpose, B. subtilis LSBS2 (5 × 10 −5 cells/mL) was grown in 2 L of SM medium in a 5 L flask at 28 ± 2 • C for 24 h followed by centrifugation at 9370 for 15 min. After confirming the CAS test, the pH of the cell-free supernatant was adjusted to 6.0 with 12 N HCl. The acidified supernatant was concentrated (10 times, i.e., to obtain about 200 mL) on a rotary vacuum evaporator (Buchi, R-124, Flawil, Switzerland) at 100 rpm at 50 • C. The concentrated supernatant was then subjected to purification [35].

Solvent Purification
Concentrated supernatant was subjected to the extraction of siderophore by using various solvents like chloroform-phenol, ethyl acetate, water, etc. were used for extraction of siderophores [35].

Purification on Sep Pack C C18 Column
A 5 mL of concentrated supernatant was passed through Sep-Pak C18 cartridge (Waters, Milford, MA, USA); the cartridge was washed with 2 mL of distilled water followed by elution with acetonitrile (20% v/v) [36]. The fractions from elute were separately collected and subjected to the CAS test.

Purification on Amberlite-400 Resin Column
Fractional purification of the siderophore was performed using Amberlite IR120 (Na+) ion-exchange chromatography [35]. Amberlite XAD-400 resins were dissolved in distilled water, soaked overnight at 8 • C, loaded in a sintered glass column; pre-washed with distilled water; methanol, and then water. The concentrated siderophore-rich broth was loaded on Amberlite XAD-400 resin column. The loading was continued until the saturation of the columns, indicated by the browning of the column. The loaded supernatant was allowed to flow through the column at a flow rate of 5 mL/min. The column was then washed with distilled water to remove all unbound material and eluted with 50% (v/v) methanol [37]. Different fractions were separately collected and subjected to the CAS test. CAS-positive fractions were used for further analysis. Crude siderophore supernatant was spotted drop by drop on TLC plates (Merck, thickness 0.25 mm of Silica gel G) using mobile phase isopropanol: acetic acid: distilled water (12:3:5) following the procedure of Neilands, [38]. Fully developed TLC plates were sprayed with 0.1 M of FeCl 3 reagents to detect the siderophore and their types.

Fourier Transform Infra-Red (FTIR) Spectroscopic Analysis
Purified siderophore fraction was processed for FTIR in the range of 4000-400 cm −1 . The infrared spectrum wavelengths were determined based on their functional groups [39].

High-Performance Liquid Chromatography (HPLC) Analysis
The purified siderophore was subjected to HPLC analysis using the stationary phase pinnacle II C18 reverse-phase column (5 µM integrated pre-column, 250 × 4.6 mm) (Waters 2190 system, Milford, MA, USA) and methanol: water (8:2 v/v) as mobile phase [40]. Siderophore sample was injected at the flow rate of 1 mL/min at 25 • C at 400 nm. The preparatory separation of siderophore was done using the same mobile phase. The retention times (RT) of peaks with comparable heights were analyzed [41]. The NMR spectrum of the purified siderophores was recorded using NMR spectrometer (Brucker AM-500, 500 MHz, Basel, Switzerland) equipped with a 5 mm triple resonance HCN probe with Z-axis pulsed-field gradients [42]. Samples and reference compounds were dissolved in 90% H 2 O/10% D 2 O or 100% D 2 O and 4,4-dimethyl-4-silapentane-1sulfonic acid (DSS). Carbon and nitrogen chemical shifts were referenced using indirect chemical shift ratios to calculate the frequencies at 0 ppm for the respective nuclei. Spectra were recorded at 25 uC [42].

Plant Growth-Promotion Studies under Greenhouse Conditions
Seeds of Sesamum indicum were surface sterilized with 0.1% HgCl 2 for 5 min, washed three times with sterile distilled water, and air-dried at 28 • C and seeded in pots containing 500 g of a sterile mixture of garden soil, sand, and manure (2:1:1). Each pot was seeded with five sterilized seeds and treated as follows. For each treatment, five pots were used.
T0-Control uninocluated nutrient broth T1-with 200 mL of B. subtilis LSBS2 broth T2-with 200 mL of pure siderophore suspension (296 mg/L) Pots were incubated under greenhouse conditions (24/20 • C and 70% relative humidity, with the alternate cycle of day and night photoperiod of 18/6 h) for 60 days. Pots were arranged in randomized complete block design (RBD). The plants were irrigated with nitrogen-free sterile tap water every two days. Seedlings were harvested 60 days after sowing and plant growth parameters were measured. After harvest, the fresh weight, dry weight, and root and shoot lengths of the plants were measured. The shoots and roots were separated and dried in an oven at 68 ± 2 • C for 48 h and weighed. There were five pots for each treatment and three seedlings per pot.

Estimation of Photosynthetic Pigments
The sesame leaves were harvested from the treated seedlings 60 DAS and were used to estimate the total carotenoids and chlorophyll a, and chlorophyll b contents according to the procedure of Wellburn [43].

Estimation of Total Iron Content in Sesame Seedling
For the estimation of total iron content, sesame leaves, stem and seeds were collected after 60 DAS. About 0.5 g of each sample was separately digested with a mixture of 65% HNO 3 and 30% of H 2 O 2 (5:3, (v/v)). The iron content was measured according to the method of Bansal et al. [44].

Determination of Oil Content in Seeds
For the estimation of the oil content of sesame seeds, 100 g of dried ground seeds were used to extract the oil using chloroform followed by extraction of oil with the help of Sokule device [43][44][45]. Then the oil content was determined on a percentage basis based on the following formula: Ether extract (%) = (Weight of the flask + Extract) − Tare weight of the flask weight of the sample × 100

Estimation of Soil Nutrients
Soil samples were collected from the untreated and treated pots. Total nitrogen was calculated by the method of Labconco [46]. The amount of P from the soil was calculated according to Sims [47] and the amount of potassium was measured according to the method of Upadhyay and Sahu [48]. While calcium (Ca), copper (Cu), zinc (Zn), iron (Fe), magnesium (Mg), manganese (Mn), and sodium (Na) were analyzed was by the method of Sahrawat [49].

Statistical Analysis
All the experiments were performed in five replicates and the data is represented as mean ± standard error (SE). The data was analyzed by One Way Analysis of Variance (ANOVA) and Tukey's test. All the data were analyzed using SPSS software 20.0 version (SPSS Japan Inc., Tokyo, Japan). The significant differences in the means were analyzed based on Tukey's multiple comparison test (p < 0.05).

Soil Analysis
Soil sample was analyzed for various physicochemical properties, such as EC, pH, available macro and micronutrients such as phosphorus, nitrogen, and potassium, iron, zinc, calcium, magnesium, sulfur, and organic carbon contents (Table 1). Values are the average of triplicates and significant at p > 0.5.

Isolation of Bacillus sp.
A bacterium initially labeled as LSBS2 was isolated from the agriculture soil from Salem district, Tamil Nadu, India (Table 1).

Production of Hydrogen Cyanide (HCN)
The isolate LSBS2 produced HCN as evidenced by the change in the color of filter paper from yellow to moderate brown. In the presence of glycine and FeCl 3 , the deep brown color of filter paper was observed as an indication of HCN production by bacterial strain ( Figure 1A). In quantitative estimation, LSBS2 showed a strong OD value of 0.094.

Production of Ammonia
LSBS2 produced copious amounts of ammonia in peptone water. The addition of 0.5 mL of Nessler ' s reagent in LSBS2 inoculated peptone water resulted in the development of brown to yellow color ( Figure 1B). The isolate produced 8 µg/mL of ammonia.

Screening and Production of Siderophore
In CAS assay LSBS2 showed a positive reaction (orange color zone) for siderophore production. The strain LSBS2 produced a 10 mm-sized orange color zone in the CAS plate ( Figure 1E). CAS shuttle assay estimated the production of 296 µg/mL of siderophore. The addition of Salkowski's reagent in the cell-free supernatant produced pink color indicating the production of IAA by LSBS2 ( Figure 1C). Quantitative analysis revealed the production of 78.45 ± 1.9 µg/mL IAA.

Phosphate Solubilization Analysis
A clear zone of phosphate solubilization was observed around the colonies of LSBS2 on Pikovoskaya agar LSBS2. A maximum zone of P solubilization of 0.5 mm was observed. (Figure 1D). The isolate solubilized 540 µg/mL of P.

Screening and Production of Siderophore
In CAS assay LSBS2 showed a positive reaction (orange color zone) for siderophore production. The strain LSBS2 produced a 10 mm-sized orange color zone in the CAS plate. (Figure 1E). CAS shuttle assay estimated the production of 296 µg/mL of siderophore.

Qualitative Detection of Siderophore
In the Arnow 's assay, the addition of 0.5 mL of HCl (5 M) followed by the addition of 0.1 mL of NaOH (10 N), and 1.0 mL of nitrite-molybdate reagent resulted in the formation of a red-colored solution. This assay confirmed the production of catecholate siderophore by LSBS2.

Polyphasic Identification of LSBS2
Biochemical test followed by16S rRNA gene sequencing analysis using BLAST search of NCBI revealed 98.2% similarity of LSBS2 with the sequence of Bacillus subtilis available in the GenBank. Thus, the siderophore-producing isolate LSBS2 was identified as Bacillus subtilis LSBS2, and its 16SrRNA gene sequencing was deposited in NCBI GenBank under the accession number MH4832621 (Figure 2).

Qualitative Detection of Siderophore
In the Arnow 's assay, the addition of 0.5 mL of HCl (5 M) followed by the additio of 0.1 mL of NaOH (10 N), and 1.0 mL of nitrite-molybdate reagent resulted in the fo mation of a red-colored solution. This assay confirmed the production of catecholate s derophore by LSBS2.

Polyphasic Identification of LSBS2
Biochemical test followed by16S rRNA gene sequencing analysis using BLAST searc of NCBI revealed 98.2% similarity of LSBS2 with the sequence of Bacillus subtilisavailab in the GenBank. Thus, the siderophore-producing isolate LSBS2 was identified as Bacill subtilis LSBS2, and its 16SrRNA gene sequencing was deposited in NCBI GenBank und the accession number MH4832621 (Figure 2).

Solvent Purification
No extraction was obtained with any of the solvents used. None of the solvents e tracted the siderophore from the concentrated broth.

Purification on Sep-Pack C18 Column
All the fractions obtained from the Sep-Pak C18 column were CAS negative, whi the filtrate was strongly CAS positive, indicating the presence of siderophores. The CA positive filtrate and water-wash yielded 200 and 20 mg/L siderophore, respectively (Tab 2).  3.6. Purification of Siderophore 3.6.1. Solvent Purification No extraction was obtained with any of the solvents used. None of the solvents extracted the siderophore from the concentrated broth.

Purification on Sep-Pack C18 Column
All the fractions obtained from the Sep-Pak C18 column were CAS negative, while the filtrate was strongly CAS positive, indicating the presence of siderophores. The CAS-positive filtrate and water-wash yielded 200 and 20 mg/L siderophore, respectively ( Table 2).

Purification on Amberlite-400 Resin Column
Siderophore-rich concentrated supernatant when passed through Amberlite XAD-400 resins yielded five fractions. The first fraction showed a single peak with absorption Sustainability 2021, 13, 5394 9 of 18 maxima of 224 nm. This fraction contained a major amount of a single type of siderophore; the second CAS positive fraction (λmax 264) contained the minor siderophore (Table 3). The siderophore produced by crude Bacillus subtilis (LSBS2) extract upon being loaded on the TLC showed a red color spot when sprayed with 0.1 M FeCl 3 in 0.1 N HCl reagent. The red color spots developed reveal the presence of a catecholate type of siderophore in the tested extract of LSBS2 isolate.

Infra-Red (IR) Spectroscopic Analysis
The partial purification of siderophore resulted in a yield of 20 mg/L. Further, the FTIR spectrum of LSBS2 shows the adsorption bands at 3445, 2951, 1652, 1455, and 1143 cm 1, respectively, which indicates the presence of (-OH), aromatic (-CH), (-C=O), (-CH 2 ) and (C-O-C) linkage ( Figure 3).  These functional groups are present in the catecholate type of siderophores so that the purification of siderophore production in FTIR analysis conformed to catecholate type that are specific to compound 2,3dihydroxybenzoic acid are present in LSBS2 (Figure 3).

High-Performance Liquid Chromatography (HPLC) Analysis
The confirmation of purified siderophore was done by HPLC analysis using a mixture of methanol:water (80:20 v/v) as a solvent system. In HPLC analysis, the peaks appeared at retention time 1  These functional groups are present in the catecholate type of siderophores so that the purification of siderophore production in FTIR analysis conformed to catecholate type that are specific to compound 2,3dihydroxybenzoic acid are present in LSBS2 (Figure 3).

High-Performance Liquid Chromatography (HPLC) Analysis
The confirmation of purified siderophore was done by HPLC analysis using a mixture of methanol:water (80:20 v/v) as a solvent system. In HPLC analysis, the peaks appeared at retention time 1.621 min, 2.188 min, 2.473 min. The peaks appeared at 1-0.767 min; 2-2.033 min was used as a standard. From these results, the presence of purified 2-3 dihydroxy benzoic acid siderophore in the sample is revealed (Figure 4). By performing preparative HPLC, all three peaks were separated and checked again for the presence of no additional peak of any degraded product or impurity visible in the chromatogram supporting its purity. These functional groups are present in the catecholate type of siderophores so that the purification of siderophore production in FTIR analysis conformed to catecholate type that are specific to compound 2,3dihydroxybenzoic acid are present in LSBS2 (Figure 3).

High-Performance Liquid Chromatography (HPLC) Analysis
The confirmation of purified siderophore was done by HPLC analysis using a mixture of methanol:water (80:20 v/v) as a solvent system. In HPLC analysis, the peaks appeared at retention time 1.621 min, 2.188 min, 2.473 min. The peaks appeared at 1-0.767 min; 2-2.033 min was used as a standard. From these results, the presence of purified 2-3 dihydroxy benzoic acid siderophore in the sample is revealed (Figure 4). By performing preparative HPLC, all three peaks were separated and checked again for the presence of no additional peak of any degraded product or impurity visible in the chromatogram supporting its purity.   Overall, the accumulated peak values determine the chemical shifts as noticed from the NMR confirms the presence of catecholate type of siderophore in the tested compound (LSBS2). Furthermore, the chemical hydrolysis in the NMR is considered as the most authentic evidence for the bacillibactin structure, chelation of ferric iron which is a catecholate type of siderophore (Figure 7) produced by the isolated Bacillus subtilis LSBS2. Overall, the accumulated peak values determine the chemical shifts as noticed from the NMR confirms the presence of catecholate type of siderophore in the tested compound (LSBS2). Furthermore, the chemical hydrolysis in the NMR is considered as the most authentic evidence for the bacillibactin structure, chelation of ferric iron which is a catecholate type of siderophore (Figure 7) produced by the isolated Bacillus subtilis LSBS2.

Measurement of Plant Growth Parameters
Sesame plants treated with the B. subtilis LSBS2 broth significantly enhanced the lea length by 32%, shoot length by 39%, root length by 43%, fresh weight by 27%, and dry weight 24% as compared to the untreated control plants (Figure 8) ( Table 4). The applica tion of pure siderophore solution also improved the growth parameters in sesame. How ever, the plant-growth-promoting effect of a pure siderophore was comparatively less. I Figure 7. Elucidation of bacillibactin structure based on NMR spectral analysis.

Measurement of Plant Growth Parameters
Sesame plants treated with the B. subtilis LSBS2 broth significantly enhanced the leaf length by 32%, shoot length by 39%, root length by 43%, fresh weight by 27%, and dry weight 24% as compared to the untreated control plants (Figure 8) ( Table 4). The application of pure siderophore solution also improved the growth parameters in sesame. However, the plant-growth-promoting effect of a pure siderophore was comparatively less. It resulted in a 17% increase in leaf length, 24% increase in shoot length, 31% improvement in root length 19% increase in fresh weight 16% increase in dry weight 24% over the control plants.

Measurement of Plant Growth Parameters
Sesame plants treated with the B. subtilis LSBS2 broth significantly enhanced the leaf length by 32%, shoot length by 39%, root length by 43%, fresh weight by 27%, and dry weight 24% as compared to the untreated control plants (Figure 8) ( Table 4). The application of pure siderophore solution also improved the growth parameters in sesame. However, the plant-growth-promoting effect of a pure siderophore was comparatively less. It resulted in a 17% increase in leaf length, 24% increase in shoot length, 31% improvement in root length 19% increase in fresh weight 16% increase in dry weight 24% over the control plants.

Leaf Length (cm) Shoot Length (cm) Root Length (cm) Fresh Weight (g) Dry Weight (g) Number of Pods
Mean ± SD Mean ± SD Mean ± SD Mean ± SD Mean ± SD Mean ± SD 7.3 ± 0.9 29

Estimation of Total Iron, Seed Oil, and Photosynthetic Pigments
The iron content of B. subtilis-LSBS2-treated sesame plants was significantly higher. The iron content in the leaf, shoot, and seeds was 34%, 29%, and 47.0% higher as compared to the control (un-inoculated) plants. The application of pure siderophore solution also resulted in a substantial increase in iron. It resulted in a 1%, 11%, and 19.0% increase in iron content over the control. LSBS2-treated seeds showed 47% more oil content as compared to the control. Plants treated with pure siderophore solution showed a substantial

Estimation of Total Iron, Seed Oil, and Photosynthetic Pigments
The iron content of B. subtilis-LSBS2-treated sesame plants was significantly higher. The iron content in the leaf, shoot, and seeds was 34%, 29%, and 47.0% higher as compared to the control (un-inoculated) plants. The application of pure siderophore solution also resulted in a substantial increase in iron. It resulted in a 1%, 11%, and 19.0% increase in iron content over the control. LSBS2-treated seeds showed 47% more oil content as compared to the control. Plants treated with pure siderophore solution showed a substantial improvement of 11% in iron content as compared to control treatment. Application of LSBS2 improved the photosynthetic pigments over the control and pure siderophore solution application; it resulted in a 50% and 42% increase in chlorophyll and carotenoids content in treated as compared to the control (un-inoculated), and 36% and 29% increase in chlorophyll and carotenoids content over the pure siderophore-treated plants (Table 5).

Measurement of Soil Nutrients
Soil analysis was done after the experiment on 60DAS of treated seeds. The soil treated with LSBS2 broth showed a high amount of nitrogen content of 63.5 (mg/kg), followed by 19.5 mg/kg of phosphorus and potassium content with 154 mg/kg, whereas the control soil recorded a low amount of nitrogen phosphorous and potassium. In the case of iron, the treated plants recorded a 3.5-fold increase (5.22 ppm) in comparison with untreated  (Table 6).

Discussion
Plant-growth-promoting beneficial characters are produced by various groups of microorganisms [49,50]. These rhizosphere organisms are soil inhabitant, nonpathogenic capable of plant growth promotion [51]. Considering their ability in crop improvement, they are used as a substitute for chemical application in agriculture. Most of the rhizosphere bacteria secrete secondary metabolites called siderophores which act as chelating agents for ferric iron, produced under low iron stress [51][52][53][54][55].
In the present study, a bacterium (LSBS2) was isolated from the agricultural soil, based on biochemical and plant-growth-promoting traits and 16SrRNA gene the isolated bacterium was identified as Bacillus subtilis LSBS2. In the past, various reports are documented for the isolation of B. subtilis from rhizosphere soil from India by biochemical and plant growth-promoting (PGP) traits such as hydrogen cyanide (HCN), ammonia, and indole acetic acid (IAA), and solubilized phosphate and molecular characterization [56]. As a next step, the qualitative assessment of siderophores production was demonstrated from the isolated B. subtilis LSBS2 under nutrient agar medium with iron limiting stress. In earlier reports, B. subtilis can produce higher yields of siderophores under iron stress conditions [57]. In the current investigation, the siderophore was positively isolated using the CAS reagent which revealed a change in blue to orange color which resembled similar observations of siderophore production [58][59][60][61]. Mehri et al. [62] reported that different doses of zinc and magnesium also play a major role in the induction of siderophore production by the formation of yellow-green fluorescent pigment around them on nutrient agar medium.
To detect the type of siderophore, we have used the tetrazolium test for the estimation of siderophore type using spectrophotometric assay, the subjected extract showed the appearance of red color indicating the presence of catecholate type of siderophores. This result is well supported by previous findings of Santos [61] in which the authors described the biocontrollability of siderophore producing Burkholderiacepacia XXVI against Colletotrichumgloeosporioides.
It was noticed that a clear red spot was developed on the TLC plates which revealed the positive response of catecholate type siderophores in the test extract. In the same manner, the detection of siderophores was confirmed in the bacillus culture supernatant obtained from rhizosphere soil [63]. The partial purification of siderophores on the XAD Ambelite column yielded 20 mg/L. Sayyed and Chincholkar [35] purified siderophores of Alcaligenes faecalis on Amberlite XAD-400 resins. Further, the FTIR spectrum of LSBS2 showed the adsorption bands at 3445, 2951, 1652, 1455, and 1143 cm −1 , respectively, which indicates the presence of (-OH), aromatic (-CH), (-C=O), (-CH 2 ) and (C-O-C) linkage. These functional groups are present in the catecholate type of siderophore so that the purification of siderophore production in FTIR analysis confirmed the presence of a catecholate type of siderophore that is specific to 2,3dihydroxybenzoic acid [64]. In HPLC analysis, the peaks appeared at retention time 1.621 min, 2.188 min, 2.473 min. The peaks appeared at 1-0.767 min; 2-2.033 min was used as a standard. The results of the present study revealed the presence of purified 2-3 dihydroxy benzoic acid siderophore in the sample [65]. Microbial siderophores directly enhance the availability of iron present in the rhizosphere soil, thereby stimulating plant growth by supplying low iron [56,57,[66][67][68][69].
Inoculation of siderophore producing B. subtilis LSBS2 significantly enhanced plant biomass, pigment content, iron, and oil content in the bioinoculant-treated sesamum plants as compared to the control (uninoculated) plants. Several investigations on the application of different species of Pseudomonas provided evidence of siderophores directly involved in the stimulation of plant growth and also protecting the plants against various biotic stresses [58,[70][71][72][73]. Likewise, an increased carotenoids level was observed in siderophores producing strain (Ros2) treated to wheat plants in the presence of pesticide stress [70]. Kusale and co-workers [74] isolated a potential siderophores-producing organism from the rhizosphere; this bacterium treated to sesame plants showed a drastic increase in the seed germination, shoot, root length and also recorded enhanced levels of chlorophyll. Furthermore, in this study, the pH and electrical conductivity, NPK, and iron content in the soil are influenced by the presence of siderophores secreting microorganisms, this is in agreement with the results obtained by Kumar et al. [75]. The present findings also provide evidence of enhanced oil content, chlorophyll, carotenoids pigments, and micro and macronutrients in siderophore-producing bioinoculant-treated sesame plants. These enhanced beneficial characters are correlated with the previous studies [60,75,76]. In an independent study [60], a significant increase in the rice grain oil content after application of siderophore-producing rhizobacteria. Improvement in plant growth by Bacillus sp. is well documented and several mechanisms of plant growth promotion are known for this group of PGP. Bacillus sp. is known to stimulate plant growth and prevent pathogen infection [77] and also plays a significant role against biotic stresses through the production of phytohormones, volatile organic compounds, exopolysaccharides, siderophores, and P solubilization [78][79][80]. Application of B. subtilis has been reported to improve crop productivity. Bacillus species have been reported as the most promising PGPR, which is an ecologically sound and economically feasible alternative to pesticide practice agriculture [78]. Tahir al [80] reported the production of a wide variety of volatile organic compounds in B. subtilis SYST2. They found these VOCs responsible for triggering hormone activity and promoting plant growth. Qiao et al. [81] reported the plant-growth-promoting ability of Bacillus subtilis PTS-394. Trivedi [82] reported the promotion of plant growth in maize and wheat by using broth, coal and alginate beads formulation of Bacillus sp. Saxena et al. [83] reported Bacillus species as a natural soil resource for plant health and nutrition. Verma et al. [84] isolated various Bacillus species from the sesame rhizosphere and reported improved seed germination and plant growth promotion in sesame due to inoculation with these Bacilli.

Conclusions
The results from the present investigation provided evidence of a positive correlation between the production of various plant growth parameters, nutrient uptake, and improvement in the iron and oil content in sesamum plants inoculated with Bacillus sp. LSB2. This inoculation also improves soil nutrients. The results of the present study suggest the role of LSBS2 as multifarious PGPR for improvement in the growth parameters and nutrient content in sesame as well as soil nutrients. Such multifarious PGPR strains can be used as effective bioinoculants for sesame farming. However, field trials under different agroclimatic zones are needed to assure the bioefficacy of the isolate.