High-yield production of indole-3-acetic acid by Enterobacter sp. DMKU-RP206, a rice phyllosphere bacterium that possesses plant growth-promoting traits

Bioprospecting for Isoetes cangae Endophytes with Potential to Promote Plant Growth

Isoetes cangae is a native plant found only in a permanent pond in Serra dos Carajás in the Amazon region. Plant-associated microbial communities are recognized to be responsible for biological processes essential for the health, growth, and even adaptation of plants to environmental stresses. In this sense, the aims of this work were to isolate, identify, and evaluate the properties of endophytic bacteria isolated from I. cangae. The bioprospecting of potentially growth-promoting endophytes required the following steps to be taken: isolation of endophytic colonies, molecular identification by 16S rDNA sequence analysis, and evaluation of the bacterial potential for nitrogen fixation, production of indole acetic acid and siderophores, as well as phosphate solubilization and mineralization. Bacillus sp., Rhizobium sp., Priestia sp., Acinetobacter sp., Rossellomorea sp., Herbaspirillum sp., Heyndrickxia sp., and Metabacillus sp., among other bacterial species, were identified. The isolates showed to be highly promising, evidencing the physiological importance for the plant and having the potential to promote plant growth.

Identification and Optimisation of Indole-3-Acetic Acid Production of Endophytic Bacteria and Their Effects on Plant Growth

Indole-3-acetic acid (IAA) is one of the most physiologically active auxins produced by rhizobacteria and is potentially applied for agriculture. Two endophytic bacteria, VR2 and MG9, isolated from the root of Chrysopogon zizanioides (L.) collected at Cha-Am, and the leaf of Bruguiera cylindrica (L.) Blume collected from a mangrove forest at Ban Laem, Phetchaburi Province, Thailand, were taxonomic characterised based on their phenotypic characteristics and 16S rRNA gene analysis. Strain VR2 was closely related to Enterobacter hormaechei CIP 103441^T (99.6% similarity), while strain MG9 was closely related to Bacillus aryabhattai B8W22^T (99.9% similarity). Consequently, they were identified as Enterobacter hormaechei and Bacillus aryabhattai, respectively. The IAA production of VR2 and MG9 strains are determined and applied to rice seeds for their root and shoot germination. Strains VR2 and MG9 greatly produced a yield of IAA, 246.00 and 195.55 μg/mL in 1,000 μg/mL of L-tryptophan at pH 6 for 48 h. They showed no significant differences in IAA to root and shoot development. However, the bacterial IAA exhibited potential nearby synthetic IAA, which had a significant effect compared to the control. IAA produced from these two strains might preferably trim down the use of synthetic IAA and could contribute to sustainable agriculture.

Regulation of rhizospheric microbial network to enhance plant growth and resist pollutants: Unignorable weak electric field

The union of Plant Growth-Promoting Bacteria (PGPB) and rhizosphere confers a series of functions beneficial to plant. However, the lack of an opearable in situ method limits the further understanding on the mechanism. In this study, a weak electric field was designed to regulate rhizospheric microflora in a constructed root-splitting reactor. Compared with the control, the aboveground and underground biomass of rice seedling increased by 17 % and 18 % (p < 0.05) respectively under the exist of weak electric field of 0.14 V/cm. The joint action of rhizosphere and PGPB displayed the detoxification ability in the condition of soluble petroleum hydrocarbons, where the height, stem diameter, biomass and root vigor of the plant was increased by 58 %, 32 %, 43 % and 48 % respectively than the control. The selective reproduction of endophytes and ectophytes (denitrifying, auxin-producing, hydrocarbon-degrading and electroactive bacteria) was observed under applied weak electric field, which enhanced the nitrogen utilization, cellular metabolic activity and resistance to toxic organics of plant. This was further confirmed by the up-regulated OTUs related to the hydrocarbon degradation function, tryptophan metabolism and metabolism of nicotinate and nicotinamide. Moreover, the weak electric field also enhanced the transfer ability of partial endophytes grown in the root to improve plant stress resistance. The results in this work inspired an exercisable method for in situ enrichment of PGPB in the rhizosphere to cope with food crisis and provided a new way to deal with sudden environmental events.

Application of data integration for rice bacterial strain selection by combining their osmotic stress response and plant growth-promoting traits

Agricultural application of plant-beneficial bacteria to improve crop yield and alleviate the stress caused by environmental conditions, pests, and pathogens is gaining popularity. However, before using these bacterial strains in plant experiments, their environmental stress responses and plant health improvement potential should be examined. In this study, we explored the applicability of three unsupervised machine learning-based data integration methods, including principal component analysis (PCA) of concatenated data, multiple co-inertia analysis (MCIA), and multiple kernel learning (MKL), to select osmotic stress-tolerant plant growth-promoting (PGP) bacterial strains isolated from the rice phyllosphere. The studied datasets consisted of direct and indirect PGP activity measurements and osmotic stress responses of eight bacterial strains previously isolated from the phyllosphere of drought-tolerant rice cultivar. The production of phytohormones, such as indole-acetic acid (IAA), gibberellic acid (GA), abscisic acid (ABA), and cytokinin, were used as direct PGP traits, whereas the production of hydrogen cyanide and siderophore and antagonistic activity against the foliar pathogens Pyricularia oryzae and Helminthosporium oryzae were evaluated as measures of indirect PGP activity. The strains were subjected to a range of osmotic stress levels by adding PEG 6000 (0, 11, 21, and 32.6%) to their growth medium. The results of the osmotic stress response experiments showed that all bacterial strains accumulated endogenous proline and glycine betaine (GB) and exhibited an increase in growth, when osmotic stress levels were increased to a specific degree, while the production of IAA and GA considerably decreased. The three applied data integration methods did not provide a similar grouping of the strains. Especially deviant was the ordination of microbial strains based on the PCA of concatenated data. However, all three data integration methods indicated that the strains Bacillus altitudinis PB46 and B. megaterium PB50 shared high similarity in PGP traits and osmotic stress response. Overall, our results indicate that data integration methods complement the single-table data analysis approach and improve the selection process for PGP microbial strains.

Genotype and organ effect on the occupancy of phyllosphere prokaryotes in different rice landraces

Bacteria community provides essential ecological services to rice plants. The bacterial diversity of rice varies across host plant genotype and organs. This study employed 16S rDNA amplicon sequencing to characterise the bacterial community associated with three rice landraces using leaf blade and stem samples. The prokaryotic community found in these rice landraces comprised of two kingdoms, 12 phyla, 25 classes, 40 orders, 80 families, and 118 genera. Proteobacteria (53.9%) was the most abundant phylum. The most abundant genus was an undefined genus under Cyanobacteria (33.0%). Homogeneity of prokaryotic community was observed across the three rice landraces, which may suggest a high similarity in biological and genetical properties of the rice landraces. The difference in prokaryotic composition between leaf blade and stem was depicted based on principal coordinate analysis. This study observed that the prokaryotic inhabitants in rice plants is predominantly determined by rice plant organs.

Optimization, Characterization and Quantification of Indole Acetic Acid Produced by a Potential Plant Growth Promoting Rhizobacterium Bacillus safensis YKS2 from Yercaud Hills, Eastern Ghats

Plant growth-promoting rhizobacteria (PGPR) have proved to be an effective solution for enhancing growth of various plant species. Five different bacterial isolates extracted from rhizosphere soil were extensively studied for the production of indole acetic acid (IAA) and among those Bacillus safensis YKS2 strain was found to produce substantial quantities of IAA. B. safensis YKS2 strain was characterized and submitted to National Centre for Biotechnology Information (NCBI) (Gen Bank No. MH539636). Optimization of IAA production with varying pH and temperature revealed that IAA production was maximum at pH 7 and at a temperature of 37°C. The production of IAA was confirmed and quantified by Fourier-transform infrared spectroscopy (FTIR), Thin-layer chromatography (TLC), Gas chromatography-mass spectrometry (GC-MS). The PGPR inoculum showed significant (p<0.05) shoot increase (60.00 – 89.00%) and root increase (30.00 – 90.00%) relative to the controls in Vigna radiata. This study showed that IAA producing ability of B. safensis YKS2 can be used in the large-scale production of IAA for plant growth promotion.

Sinomicrobium kalidii sp. nov., an indole-3-acetic acid-producing endophyte from a shoot of halophyte Kalidium cuspidatum

To better understand the effects of endophytic bacteria on halophytes, a bacteria that produced indole-3-acetic acid and 1-aminocyclopropane-1-carboxylic acid deaminase, designated HD2P242^T, was isolated from a shoot of Kalidium cuspidatum collected in Tumd Right Banner, Inner Mongolia, PR China. The cells of strain HD2P242^T were Gram-stain-negative, strictly aerobic, motile by gliding, non-spore-forming and rod-shaped. Strain HD2P242^T grew at pH 6.0-9.0 (optimum, pH 7.0) and 10-45 °C (optimum 37 °C), in the presence of 0-8 % (w/v) NaCl (optimum, 4 %). The strain was positive for oxidase and catalase. The phylogenetic trees based on the 16S rRNA gene sequences and the whole genome sequences both showed that strain HD2P242^T clustered with Sinomicrobium pectinilyticum 5DNS001^T and S. oceani SCSIO 03483^T, and had 95.6, 94.3 and <94.3 % 16S rRNA gene similarities to S. pectinilyticum 5DNS001^T, S. oceani SCSIO 03483^T and all the other current type strains. Strain HD2P242^T contained menaquinone 6 as its sole respiratory quinone. Its major polar lipids were phosphatidylethanolamine, two unidentified aminolipids, two unidentified phospholipids and an unidentified lipid. The major fatty acids were iso-C_17 : 0, iso-C_16 : 0 3-OH, anteiso-C_17 : 0 and summed feature 6 (C_19 : 1  ω9c and/or C_19 : 1  ω11c). The genome consisted of a 5 364 211 bp circular chromosome, with a G+C content of 45.1 mol%, predicting 4391 coding sequence genes, 47 tRNA genes and two rRNA operons. The average nucleotide identity based on blast and the digital DNA-DNA hybridization values of strain HD2P242^T with S. oceani SCSIO 03483^T and S. pectinilyticum 5DNS001^T were 73.8 and 77.0%, and 22.3 and 22.2%, respectively. The comparative genome analysis showed that the pan-genomes of strain HD2P242^T and three Sinomicrobium type strains possessed 4236 clusters, whereas the core genome possessed 2162 clusters, which accounted for 52.3 % of all the clusters. The genomic analysis revealed that all four Sinomicrobium members could utilize d-glucose by the glycolysis-gluconeogenesis pathway or the pentose phosphate pathway. The tricarboxylic acid cycle was utilized as a metabolic centre. The phylogenetic, physiological and phenotypic characteristics allowed the discrimination of strain HD2P242^T from its phylogenetic relatives. Therefore, Sinomicrobium kalidii sp. nov. is proposed, and the type strain is HD2P242^T (=CGMCC 1.19025^T=KCTC 92136^T).

Marinilactibacillus kalidii sp. nov., an Indole Acetic Acid-Producing Endophyte Isolated from a Shoot of Halophyte Kalidium cuspidatum

A Gram-stain-positive, facultatively anaerobic, non-sporulating, motile with single polar flagellum, rod-shaped, indole-3-acetic acid (IAA)-producing bacterium, named M4U5P12^T, was isolated from a shoot of Kalidium cuspidatum, Inner Mongolia, China. Strain M4U5P12^T grew at pH 6.0-11.0 (optimum 7.5), 4-40 °C (optimum 25 °C), and in the presence of 0-15% (w/v) NaCl (optimum 4%). Positive for catalase, urease, methyl red (M.R.) reaction, and hydrolysis of starch; and negative for oxidase, Voges-Proskauer (V-P) test, and hydrolysis of cellulose. The phylogenetic trees based on the 16S rRNA gene sequences and the whole genome sequences both revealed that it clustered with Marinilactibacillus piezotolerans JCM 12337^T (99.3%) and Marinilactibacillus psychrotolerans M13-2^T (99.1%). The dDDH and ANIb values of strain M4U5P12^T to M. piezotolerans DSM 16108^T and M. psychrotolerans M13-2^T were 19.3 and 18.9%, and 74.3 and 74.0%, respectively. The polar lipids were diphosphatidylglycerol, phosphatidylglycerol, phosphatidylethanolamine, an unidentified phospholipid, and two unidentified lipids. The major fatty acids were C_16:0, C_18:1 ω9c, C_16:1 ω9c, and C_15:1 ω5c. The genomic DNA G + C content was 37.3%. On the basis of physiological, phenotypic, and phylogenetic characteristics, strain M4U5P12^T should be classified as a novel species. Therefore, Marinilactibacillus kalidii sp. nov. is proposed, and the type strain is M4U5P12^T (= CGMCC 1.17696^T = KCTC 43247^T).

Salt-tolerant bacteria enhance the growth of mung bean (Vigna radiata L.) and uptake of nutrients, and mobilize sodium ions under salt stress condition

Salinity is one of the significant abiotic stresses that exert harmful effects on plant growth and crop production. It has been reported that the harmfulness of salinity can be mitigated by the use of salt-tolerant plant growth-promoting (PGP) bacteria. In this study, four bacteria were selected from a total of 30 cultures, based on salt-tolerant and PGP properties. The isolates were found to produce indole acetic acid (8.49-19.42 μg/ml), siderophore (36.04-61.77%), and solubilize potassium and inorganic phosphate. Identification based on 16S rRNA gene sequencing revealed that the isolates belonged to Cronobacter (two isolates) and Enterobacter (two isolates). Inoculation of PGP bacteria under 2 and 10% salinity stress showed enhanced plant growth parameters in Vigna radiata compared to both salinity and non-salinity control plants. The rate of germination (113.32-206.64%), root length (128.79-525.31%), shoot length (34.09-50.32%), fresh weight, and dry weight were 3-fold higher in bacteria-treated seeds than control plants. The estimation of chlorophyll (1-5-fold), carotenoids (1-4-fold), and proline content (3.65-14.45%) was also higher compared to control plants. Further, the bacterized seeds showed enhanced nitrogen and phosphorous uptake and mobilized sodium ions from roots to leaves. Overall the strains SS4 and SS5 performed well in both 2 and 10% salt-amended soils. These strains could be formulated as a bioinoculant to mitigate the salinity stress in salinized soils.

Gracilibacillus suaedae sp. nov., an indole acetic acid-producing endophyte isolated from a root of Suaeda salsa

A Gram-stain-positive, facultatively anaerobic, spore-forming, motile with unipolar biflagella, rod-shaped, indole acetic acid-producing bacterium, named LD4P30^T, was isolated from a root of Suaeda salsa collected in Inner Mongolia, northern China. Strain LD4P30^T grew at pH 6.0-11.0 (optimum, pH 7.0), 10-40 °C (35 °C) and in the presence of 1-15% (w/v) NaCl (5%). The strain was positive for oxidase and negative for catalase. The major cellular fatty acids of strain LD4P30^T were iso-C_15:0, C_15:1 ω5c and anteiso-C_15:0; the major polar lipids were diphosphatidylglycerol and phosphatidylglycerol; and menaquinone-7 was the only respiratory quinone. The genomic DNA G+C content was 36.7 mol%. A phylogenetic tree based on 16S rRNA gene sequences showed that strain LD4P30^T clustered with Gracilibacillus thailandensis TP2-8^T, Gracilibacillus saliphilus YIM 91119^T and Gracilibacillus lacisalsi BH312^T, and showed 99.0, 98.9, 98.0 and <97.7% 16S rRNA gene similarity to G. thailandensis TP2-8^T, G. saliphilus YIM 91119^T, G. lacisalsi BH312^T and all other current type strains, respectively. The digital DNA-DNA hybridization and average nucleotide identity based on blast values between strain LD4P30^T and G. saliphilus YIM 91119^T, G. thailandensis TP2-8^T and G. lacisalsi BH312^T were 44.9, 44.7 and 44.4%, and 91.1, 91.0 and 90.8%, respectively. Based on its phenotypic, physiological and phylogenetic characteristics, strain LD4P30^T represents a novel species, for which the name Gracilibacillus suaedae is proposed. The type strain is LD4P30^T (=CGMCC 1.17697^T=KCTC 82375^T).

A highly efficient auxin-producing bacterial strain and its effect on plant growth

Background

Various bacteria promote plant root growth in the rhizosphere, as a measure of securing and enlarging their ecological niche. These interactions are mediated by plant growth regulators (PGRs) such as auxin, and indole-3-acetic acid (IAA) is one of the physiologically active auxin. In this study, we isolated an unusual bacterial strain from food process waste with high efficiency and demonstrated its effects on plant rooting and early-stage growth.

Results

The efficiency of this bacterial strain in producing IAA was 16.6 mg/L/h in Luria-Bertani broth containing 0.05% l-tryptophan (Trp) at room temperature (24 ± 2 °C). Its IAA production was highly dependent on the presence of precursor, Trp. This bacterium was identified as Ignatzschineria sp. by 16S rDNA sequencing. Its bacterial culture supernatant (BCS) enhanced plant root initiation, root growth, and plant growth in the early stages. The root mass formed BCS-treated in apple mint cuttings was twofold of that formed in the control. The root number and length were 46% and 18% higher, respectively, in BCS-treated chrysanthemum cuttings than in the control.

Conclusions

These results show that the BCS of Ignatzschineria sp. CG20001 isolate obtained in this study can be used for agricultural applications. In addition, the novelty of this strain makes it a valuable genetic resource for biotechnological applications.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-021-00252-w.

Understanding rice growth-promoting potential of Enterobacter spp. isolated from long-term organic farming soil in India through a supervised learning approach

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Diazotrophic bacteria were screened and characterized from long-term organic rice fields of Sikkim

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Screened the diazotrophic bacteria for nitrogen fixing ability and plant growth promoting traits

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Enterobacter was commonly found in paddy soils of Sikkim

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Enterobacter species were significantly enhanced the above and below ground responses in rice plants

This study addresses the plant beneficial enterobacteria present in rice rhizosphere and their efficiency for enhancing nitrogen uptake in rice plant. Using culturable approaches, the population of total diazotrophs present in rhizosphere samples collected from different organic rice fields of Sikkim were studied and recorded in the range between 4.62 to 4.97 log₁₀ CFU/g soil. All the isolated commonly occurred diazotrophic bacterial isolates were screened based on their ability to fix nitrogen in milligram per gram of sugar consumed under in-vitro condition with the reference check. In addition to nitrogen fixation, plant growth promoting traits such as production of indole-3-acetic acid and gibberellic acid were estimated using spectrophotometric approaches and compared against Bacillus subtilis as reference multi-potent plant growth promoting strain. In-vivo evaluation of these diazotrophic species in rice found improvement in both above and below ground responses in rice plant evaluated by estimating changes in chlorophyll concentration, plant biomass, root architecture, nitrogen uptake, microbial biomass and associated biochemical activity of soil. Further, the selected isolates were identified through DNA targeted analysis of 16S rRNA gene present in diazotrophs and which identified that the isolates belonged to the Enterobacter genus. Statistical models were prepared for deciphering the dynamics of plant growth improvement due to selective enrichment of rhizosphere bacteria and found significant (p<0.05) correlation between soil and plant parameters. This study concludes that Enterobacter spp. present in organic paddy soils of Sikkim having good nitrogen fixing abilities and whose selective enrichment in rhizosphere improved nitrogen uptake and plant growth promotion in rice plant.

Humic Substances in Combination With Plant Growth-Promoting Bacteria as an Alternative for Sustainable Agriculture

Plant growth-promoting bacteria (PGPB) and humic substances (HSs) are promising options for reducing the use of pesticides and mineral fertilizers. Although many studies have shown the effects of PGPB and HSs separately, little information is available on plant responses to the combined application of these biostimulants despite the great potential for the simultaneous action of these biological inputs. Thus, the objective of this review is to present an overview of scientific studies that addressed the application of PGPB and HSs to different crops. First, we discuss the effect of these biostimulants on biological nitrogen fixation, the various effects of the inoculation of beneficial bacteria combined with the application of HSs on promoting the growth of nonleguminous plants and how this combination can increase bacterial colonization of plant hosts. We also address the effect of PGPB and HSs on plant responses to abiotic stresses, in addition to discussing the role of HSs in protecting plants against pathogens. There is a lack of studies that address the role of PGPB + HSs in biocontrol. Understanding the factors involved in the promotion of plant growth through the application of PGPB and HSs can assist in the development of efficient biostimulants for agricultural management. This approach has the potential to accelerate the transition from conventional cultivation to sustainable agrosystems.

Epiphytic Bacteria from Sweet Pepper Antagonistic In Vitro to Ralstonia solanacearum BD 261, a Causative Agent of Bacterial Wilt

Biological control of plant pathogens, particularly using microbial antagonists, is posited as the most effective, environmentally-safe, and sustainable strategy to manage plant diseases. However, the roles of antagonists in controlling bacterial wilt, a disease caused by the most devastating and widely distributed pathogen of sweet peppers (i.e., R. solanacearum), are poorly understood. Here, amplicon sequencing and several microbial function assays were used to depict the identities and the potential antagonistic functions of bacteria isolated from 80 red and green sweet pepper fruit samples, grown under hydroponic and open soil conditions, with some plants, fungicide-treated while others were untreated. Amplicon sequencing revealed the following bacterial strains: Bacillus cereus strain HRT7.7, Enterobacter hormaechei strain SRU4.4, Paenibacillus polymyxa strain SRT9.1, and Serratia marcescens strain SGT5.3, as potential antagonists of R. solanacearum. Optimization studies with different carbon and nitrogen sources revealed that maximum inhibition of the pathogen was produced at 3% (w/v) starch and 2,5% (w/v) tryptone at pH 7 and 30 °C. The mode of action exhibited by the antagonistic isolates includes the production of lytic enzymes (i.e., cellulase and protease enzymes) and siderophores, as well as solubilization of phosphate. Overall, the results demonstrated that the maximum antimicrobial activity of bacterial antagonists could only be achieved under specific environmental conditions (e.g., available carbon and nitrogen sources, pH, and temperature levels), and that bacterial antagonists can also indirectly promote crop growth and development through nutrient cycling and siderophore production.

Metagenomic analyses, isolation and characterization of endophytic bacteria associated with Eucalyptus urophylla BRS07-01 in vitro plants

Eucalyptus is the main species for the forestry industry in Brazil. Biotechnology and, more recently, gene editing offer significant opportunities for rapid improvements in Eucalyptus breeding programs. However, the recalcitrance of Eucalyptus species to in vitro culture is also a major limitation for commercial deployment of biotechnology techniques in Eucalyptus improvement. We evaluated various clones of Eucalyptus urophylla for their in vitro regeneration potential identified a clone, BRS07-01, with considerably higher regeneration rate (85%) in organogenesis, and significantly higher than most works described in literature. Endophytic bacteria are widely reported to improve in vitro plant growth and development. Hence, we believe that inclusion of endophytic plant growth promoting bacteria enhanced was responsible for the improved plantlets growth and development of this clone under in vitro culture. Metagenomic analysis was performed to isolate and characterize the prominent endophytic bacteria on BRS07-01 leaf tissue in vitro micro-cultures, and evaluate their impact on plant growth promotion. The analysis revealed the presence of the phyla Firmicutes (35%), Proteobacteria (30%) and much smaller quantities of Actinobacteria, Bacteroidetes, Gemmatimonadetes, Crenarchaeota, Euryarchaeota and Acidobacteria. Of the thirty endophytic bacterial strains isolated, eleven produced indole-3-acetic acid. Two of the isolates were identified as Enterobacter sp. and Paenibacillus polymyxa, which are nitrogen-fixing and capable of phosphate and produce ammonium. These isolates also showed similar positive effects on the germination of common beans (Phaseolus spp.). The isolates will now be tested as a growth promoter in Eucalyptus in vitro cultures. Graphical abstract for the methodology using cultivation independent and dependent methodologies to investigate the endophytic bacteria community from in vitro Eucalyptus urophylla BRS07-01.

Production of Indole Auxins by Enterobacter sp. Strain P-36 under Submerged Conditions

Bioactive compounds produced by plant growth-promoting bacteria through a fermentation process can be valuable for developing innovative second-generation plant biostimulants. The purpose of this study is to investigate the biotechnological potential of Enterobacter on the production of auxin—a hormone with multiple roles in plant growth and development. The experiments were carried in Erlenmeyer flasks and a 2-L fermenter under batch operating mode. The auxin production by Enterobacter sp. strain P-36 can be doubled by replacing casein with vegetable peptone in the culture medium. Cultivation of strain P36 in the benchtop fermenter indicates that by increasing the inoculum size 2-fold, it is possible to reduce the fermentation time from 72 (shake flask cultivation) to 24 h (bioreactor cultivation) and increase the auxin volumetric productivity from 6.4 to 17.2 mg [IAAequ]/L/h. Finally, an efficient storage procedure to preserve the bacterial auxin was developed. It is noteworthy that by sterilizing the clarified fermentation broth by filtration and storing the filtrated samples at +4 °C, the level of auxin remains unchanged for at least three months.

Large scale production of indole-3-acetic acid and evaluation of the inhibitory effect of indole-3-acetic acid on weed growth

Indole-3-acetic acid (IAA) is the most common plant hormone of the auxin class and regulates various plant growth processes. The present study investigated IAA production by the basidiomycetous yeast Rhodosporidiobolus fluvialis DMKU-CP293 using the one-factor-at-a-time (OFAT) method and response surface methodology (RSM). IAA production was optimized in shake-flask culture using a cost-effective medium containing 4.5% crude glycerol, 2% CSL and 0.55% feed-grade l-tryptophan. The optimized medium resulted in a 3.3-fold improvement in IAA production and a 3.6-fold reduction in cost compared with those obtained with a non-optimized medium. Production was then scaled up to a 15-L bioreactor and to a pilot-scale (100-L) bioreactor based on the constant impeller tip speed (V_tip) strategy. By doing so, IAA was successfully produced at a concentration of 3569.32 mg/L at the pilot scale. To the best of our knowledge, this is the first report of pilot-scale IAA production by microorganisms. In addition, we evaluated the effect of crude IAA on weed growth. The results showed that weed (Cyperus rotundus L.) growth could be inhibited by 50 mg/L of crude IAA. IAA therefore has the potential to be developed as a herbicidal bioproduct to replace the chemical herbicides that have been banned in various countries, including Thailand.

Streptomyces sp. strain TOR3209: a rhizosphere bacterium promoting growth of tomato by affecting the rhizosphere microbial community

Aiming at revealing the possible mechanism of its growth promoting effect on tomato, the correlations among Streptomyces sp. TOR3209 inoculation, rhizobacteriome, and tomato growth/production traits were investigated in this study. By analyses of Illumina sequencing and plate coating, differences in rhizosphere microbial communities were found in different growth stages and distinct inoculation treatments. The plant biomass/fruit yields and relative abundances of families Flavobacteriaceae, Sphingobacteriaceae, Polyangiaceae and Enterobacteriaceae in treatments T (tomato inoculated with TOR3209) and TF (tomato inoculated with TOR3209 + organic fertilizer) were higher than that in the controls (CK and CK+ organic fertilizer), respectively. The analysis of Metastats and LEfSe revealed that the genera Flavobacterium and Sorangium in seedling stage, Klebsiella in flowering stage, Collimonas in early fruit setting stage, and genera Micrococcaceae, Pontibacte and Adhaeribacter in late fruit setting stage were the most representative rhizobacteria that positively responded to TOR3209 inoculation. By cultivation method, five bacterial strains positively correlated to TOR3209 inoculation were isolated from rhizosphere and root endosphere, which were identified as tomato growth promoters affiliated to Enterobacter sp., Arthrobacter sp., Bacillus subtilis, Rhizobium sp. and Bacillus velezensis. In pot experiment, TOR3209 and B. velezensis WSW007 showed joint promotion to tomato production, while the abundance of inoculated TOR3209 was dramatically decreased in rhizosphere along the growth of tomato. Conclusively, TOR3209 might promote the tomato production via changing of microbial community in rhizosphere. These findings provide a better understanding of the interactions among PGPR in plant promotion.

Seed biopriming with P- and K-solubilizing Enterobacter hormaechei sp. improves the early vegetative growth and the P and K uptake of okra (Abelmoschus esculentus) seedling

Limited information is available that seed biopriming with plant growth-promoting Enterobacter spp. play a prominent role to enhance vegetative growth of plants. Contrary to Enterobacter cloacae, Enterobacter hormaechei is a less-studied counterpart despite its vast potential in plant growth-promotion mainly through the inorganic phosphorus (P) and potassium (K) solubilization abilities. To this end, 18 locally isolated bacterial pure cultures were screened and three strains showed high P- and K-solubilizing capabilities. Light microscopy, biochemical tests and 16S rRNA gene sequencing revealed that strains 15a1 and 40a were closely related to Enterobacter hormaechei while strain 38 was closely related to Enterobacter cloacae (Accession number: MN294583; MN294585; MN294584). All Enterobacter spp. shared common plant growth-promoting traits, namely nitrogen (N₂) fixation, indole-3-acetic acid production and siderophore production. The strains 38 and 40a were able to produce gibberellic acid, while only strain 38 was able to secrete exopolysaccharide on agar. Under in vitro germination assay of okra (Abelmoschus esculentus) seeds, Enterobacter spp. significantly improved overall germination parameters and vigor index (19.6%) of seedlings. The efficacy of root colonization of Enterobacter spp. on the pre-treated seedling root tips was confirmed using Scanning Electron Microscopy (SEM). The pot experiment of bioprimed seeds of okra seedling showed significant improvement of the plant growth (> 28%) which corresponded to the increase of P and K uptakes (> 89%) as compared to the uninoculated control plants. The leaf surface area and the SPAD chlorophyll index of bioprimed plants were increased by up to 29% and 9% respectively. This report revealed that the under-explored species of P- and K-solubilizing Enterobacter hormaechei sp. with multiple plant beneficial traits presents a great potential sustainable approach for enhancement of soil fertility and P and K uptakes of plants.

Seed biopriming with P- and K-solubilizing Enterobacter hormaechei sp. improves the early vegetative growth and the P and K uptake of okra (Abelmoschus esculentus) seedling

Limited information is available that seed biopriming with plant growth-promoting Enterobacter spp. play a prominent role to enhance vegetative growth of plants. Contrary to Enterobacter cloacae, Enterobacter hormaechei is a less-studied counterpart despite its vast potential in plant growth-promotion mainly through the inorganic phosphorus (P) and potassium (K) solubilization abilities. To this end, 18 locally isolated bacterial pure cultures were screened and three strains showed high P- and K-solubilizing capabilities. Light microscopy, biochemical tests and 16S rRNA gene sequencing revealed that strains 15a1 and 40a were closely related to Enterobacter hormaechei while strain 38 was closely related to Enterobacter cloacae (Accession number: MN294583; MN294585; MN294584). All Enterobacter spp. shared common plant growth-promoting traits, namely nitrogen (N2) fixation, indole-3-acetic acid production and siderophore production. The strains 38 and 40a were able to produce gibberellic acid, while only strain 38 was able to secrete exopolysaccharide on agar. Under in vitro germination assay of okra (Abelmoschus esculentus) seeds, Enterobacter spp. significantly improved overall germination parameters and vigor index (19.6%) of seedlings. The efficacy of root colonization of Enterobacter spp. on the pre-treated seedling root tips was confirmed using Scanning Electron Microscopy (SEM). The pot experiment of bioprimed seeds of okra seedling showed significant improvement of the plant growth (> 28%) which corresponded to the increase of P and K uptakes (> 89%) as compared to the uninoculated control plants. The leaf surface area and the SPAD chlorophyll index of bioprimed plants were increased by up to 29% and 9% respectively. This report revealed that the under-explored species of P- and K-solubilizing Enterobacter hormaechei sp. with multiple plant beneficial traits presents a great potential sustainable approach for enhancement of soil fertility and P and K uptakes of plants.

Effects of auxin derivatives on phenotypic plasticity and stress tolerance in five species of the green alga Desmodesmus (Chlorophyceae, Chlorophyta)

Green microalgae of the genus Desmodesmus are characterized by a high degree of phenotypic plasticity (i.e. colony morphology), allowing them to be truly cosmopolitan and withstand environmental fluctuations. This flexibility enables Desmodesmus to produce a phenotype–environment match across a range of environments broader compared to algae with more fixed phenotypes. Indoles and their derivatives are a well-known crucial class of heterocyclic compounds and are widespread in different species of plants, animals, and microorganisms. Indole-3-acetic acid (IAA) is the most common, naturally occurring plant hormone of the auxin class. IAA may behave as a signaling molecule in microorganisms, and the physiological cues of IAA may also trigger phenotypic plasticity responses in Desmodesmus. In this study, we demonstrated that the changes in colonial morphs (cells per coenobium) of five species of the green alga Desmodesmus were specific to IAA but not to the chemically more stable synthetic auxins, naphthalene-1-acetic acid and 2,4-dichlorophenoxyacetic acid. Moreover, inhibitors of auxin biosynthesis and polar auxin transport inhibited cell division. Notably, different algal species (even different intraspecific strains) exhibited phenotypic plasticity different to that correlated to IAA. Thus, the plasticity involving individual-level heterogeneity in morphological characteristics may be crucial for microalgae to adapt to changing or novel conditions, and IAA treatment potentially increases the tolerance of Desmodesmus algae to several stress conditions. In summary, our results provide circumstantial evidence for the hypothesized role of IAA as a diffusible signal in the communication between the microalga and microorganisms. This information is crucial for elucidation of the role of plant hormones in plankton ecology.

Deciphering operation of tryptophan-independent pathway in high indole-3-acetic acid (IAA) producing Micrococcus aloeverae DCB-20

Genus Micrococcus is considered a high IAA producer. However, interestingly, there is no report on the tryptophan- independent pathway operation in this genus. Consequently, the present study was undertaken to evaluate high IAA production by Micrococcus aloeverae DCB-20 and generate reasonable evidence for the occurrence of the tryptophan-independent pathway. Strain DCB-20 produced a high quantity of 880.51 µM or 154.3 µg/mL IAA in LB broth supplemented with L-tryptophan. The tryptophan-independent pathway operation was supported by IAA production in Tris-minimal broth (TM broth) medium supplemented with acid hydrolyzed casein hydrolysate (casein acid hydolysate), which lacks tryptophan. The HPLC analysis showed the absence of tryptophan either from exogenous or endogenous sources in TM broth in the presence of casein acid hydrolysate inoculated with M. aloeverae DCB-20. The absence of tryptophan was further confirmed by the appearance of non-pigmented colonies of Chromobacterium violaceum strain TRFM-24 on Tris-minimal agar (TM agar) containing acid-hydrolyzed casein. This is probably the first report on IAA biosynthesis by M. aloeverae DCB-20 employing tryptophan-independent pathway. This simple technique can also be adapted to detect operation of the tryptophan-independent pathway in other bacteria.

Differences in Distribution of Potassium-Solubilizing Bacteria in Forest and Plantation Soils in Myanmar

Potassium (K) has been recognized as an essential element in intensive agricultural production systems, and deficiency of K usually results in a decrease in crop yields. The utilization of potassium-solubilizing bacteria (KSB) to increase the soluble K content in soil has been regarded as a desirable pathway to increase plant yields. Following the inoculation of KSB in the soil, potassium can be released (in the form of K⁺) and consumed by plants. This study aims to investigate and compare the distribution characteristics of potassium-solubilizing bacteria between forest and plantation soils in Myanmar. In this study, 14 KSB strains were isolated from rhizosphere samples collected from forest soil, as well as fertilized rubber tree rhizosphere soil and fertilized bare soil from a plantation. Broadleaf forests with high levels of canopy cover mainly comprised the forest environment, and rubber trees were planted in the plantation environment. The Chao and abundance-based coverage estimator (ACE) indices showed that the microbial abundance of the plantation soil was higher than that of the forest soil. According to the Illumina MiSeq sequencing analysis results, the Shannon index of the forest soil was lower while the Simpson index was higher, which demonstrated that the microbial diversity of the forest soil was higher than that of the plantation soil. Potassium-solubilizing test results showed that the strains E, I, M, and N were the most effective KSB under liquid cultivation conditions. Additionally, KSB only accounted for less than 5.47% of the total bacteria detected in either of the sample types, and the distribution of dominant KSB varied with the soil samples. As another result, the abundance of Pseudomonas spp. in S1 was higher than in S2 and S3, indicating a negative impact on the growth of Pseudomonas in the fertilized rubber tree rhizosphere soil. The significance of our research is that it proves that the increasing use of KSB for restoring soil is a good way to reduce the use of chemical fertilizers, which could further provide a relatively stable environment for plant growth.

Optimization of indole acetic acid production by isolated bacteria from Stevia rebaudiana rhizosphere and its effects on plant growth

The ability to synthesize Indole-3-acetic acid (IAA) is widely associated with the plant growth promoting rhizobacteria (PGPR). The present work deals with isolation and characterization of such bacteria from the rhizosphere of medicinal plant Stevia rebaudiana and optimization of IAA production from its isolates. The optimization of IAA production was carried out at different pH and temperature with varied carbon and nitrogen sources of culture media. Out of different isolates obtained, three of them were screened as efficient PGPRs on the basis of different plant growth promoting attributes. Isolates CA1001 and CA2004 showed better production of IAA at pH 9 (91.7 µg ml⁻¹) and at temperature 37 °C (81.7 µg ml⁻¹). Dextrose (1%) was found to be the best carbon source for isolate CA1001 with 104 µg ml⁻¹ IAA production. Isolate CA 2004 showed best production of IAA 36 µg ml⁻¹ and 34 µg ml⁻¹ at 1.5% and 1% Beef extract as nitrogen source respectively. Isolate CA 1001 showed 32 µg ml⁻¹ IAA production at 0.5% nicotinic acid concentration. From the current study, CA1001 and CA2004 emerged as noble alternatives for IAA production further which also resulted in root and shoot biomass generation in crop plants, hence can be further used as bio-inoculants for plant growth promotion.