Rhizobacterial Strains Research Articles

Growth enhancement and extenuation of drought stress in maize inoculated with multifaceted ACC deaminase producing rhizobacteria

IntroductionMaize is a major staple cereal crop grown and consumed globally. However, due to climate change, extreme heat and drought stresses are greatly affecting its production especially in sub-Saharan Africa. The use of a bio-based approach to mitigate drought stress is therefore suggested using plant growth-promoting rhizobacteria (PGPR).MethodsThis study investigated the abilities of 1-aminocyclopropane-1-carboxylate (ACC) deaminase producing PGPR Pseudomonas sp. MRBP4, Pseudomonas sp. MRBP13 and Bacillus sp. MRBP10 isolated from maize rhizosphere soil, to ameliorate the effect of drought stress in maize genotypes MR44 and S0/8/W/I137TNW//CML550 under two water regimes; mild drought stress (50% FC) and well-watered conditions (100% FC). The rhizobacterial strains were identified by 16S rRNA sequencing and biochemical tests, and evaluated for plant growth-promoting and abiotic stress tolerance traits.Results and discussionThe synergistic effect of the bacterial strains had a highly significant (p < 0.001) effect on the total soluble sugar, soil moisture content and relative water content, which were enhanced under water-stress in the inoculated plants. Relative water content was significantly highest (p < 0.001) in maize plants co-inoculated with Pseudomonas sp. MRBP4 + Bacillus sp. MRBP10 (60.55%). Total chlorophyll content was significantly enhanced in maize seedlings sole inoculated with Pseudomonas sp. MRBP4, Pseudomonas sp. MRBP13, and co-inoculated with Pseudomonas sp. MRBP13 + Bacillus sp. MRBP10 by 15.91%, 14.99% and 15.75% respectively, over the un-inoculated control. Soil moisture content increased by 28.67% and 30.71% compared to the un-inoculated control when plants were inoculated with Pseudomonas sp. MRBP4 + Bacillus sp. MRBP10 and Pseudomonas sp. MRBP4 + Bacillus sp. MRBP10 respectively. The interactive effect of genotype × bacteria significantly enhanced biomass production. Leaf area was highest in maize plants co-inoculated with Pseudomonas sp. MRBP4 + Pseudomonas sp. MRBP13 (212.45 ± 0.87 cm2) under drought stress. Treatment of maize seeds with Pseudomonas sp. MRBP 4 + Pseudomonas sp. MRBP13 + Bacillus sp. MRBP10 significantly increased the root length (10.32 ± 0.48 cm) which enhanced survival of the maize seedlings. Bioinoculation of maize seeds with these strains could boost maize production cultivated in arid regions.

Exploring the Identity and Properties of Two Bacilli Strains and their Potential to Alleviate Drought and Heavy Metal Stress

Naturally available plant growth-promoting rhizobacteria (PGPR) have 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase enzymes, and are capable of processing the plant-borne ACC by converting it into α-ketobutyrate and ammonia. Thus, the PGPRs help in the depletion of ethylene levels, and enhance abiotic stress tolerance in plants. In the present study, two rhizobacterial strains, i.e., Bacillus cereus and B. haynesii, isolated from Vigna mungo and Phaseolus vulgaris, were used. These strains were taxonomically identified by 16S rDNA sequencing as B. cereus and B. haynesii, with NCBI accession numbers LC514122 and LC 514123, respectively. The phylogeny of these strains has also been worked out based on homology, with data available on NCBI GenBank. The strains were screened for their plant growth-promoting traits, and quantified in the same way. The enzymatic activity and molecular weight of the ACC deaminase obtained from both bacterial strains have also been determined. An in vitro drought tolerance study was done by using PEG 6000. These bacterial strains exhibited higher ACC deaminase activity (~5 to 6 µmol/mL), exopolysaccharide yield (15 to 18 mg/10 mL protein), and indole acetic acid (27–32 µg/mL). These characteristics indicate that the bacterial strains under present study may be helpful in enhancing the drought tolerance of the crops with enhanced yield. Bacillus cereus has been found to be a tolerant strain to As, Ba, and Ni, based on the plate assay method, and so it has the potential to be used as biofertilizer in fields affected by these metals.

New trends for the efficient bio-degradation of food contaminant zearalenone using a plant rhizobacterial strain: an in-vitro study

Zearalenone (ZEN) is one of the most damaging and well-known toxins present in foods and animal feeds throughout the world. Klebsiella pneumoniae strains isolated from maize roots through soil samples were able to completely remove ZEN from LB medium after a 72-h fermentation through the enzymatic degradation and absorption of the toxin. The removing properties, including ZEN concentrations, incubation pH, incubation temperatures, bacterial concentrations, culture supernatant, and cell wall effect on degradation were evaluated. The K. pneumoniae strain culture supernatant has a strong ability to remove ZEN, and the degradation rate was more than 70%. The cell wall also has a certain ability to remove ZEN up to 18.51%, while the bacterial content has almost 5.61% ability to degrade ZEN. The removal rate and degradation metabolites alpha/beta-zearalenol, alpha/beta-zearalanol, zearalanone, and ZEN-sulphate were evaluated and confirmed by High Performance Liquid Chromatography (HPLC) and Ultra-Performance Liquid Chromatography Tandem Mass Spectrometry analyses. The mechanism of ZEN and its derivatives degradation by this bacterial strain will be further studied so that novel degrading enzymes can be isolated which can be used as bio-detoxifier.

Characterization of plant growth promoting rhizobacterial strain Bacillus cereus with special reference to exopolysaccharide production

Exo-polysaccharides (EPS) possess unique water holding and cementing properties that help in soil aggregation. EPS also increase nutrient uptake in plants, increase water flow across the roots, reduce saline stress by up taking Na+ from soil and increases plant resistance to drought stress. All these factors lead to investigate for eco-friendly and economically reasonable sources of exo-polysaccharides. The exo-polysaccharides secreted by bacteria might play potential role in improvement of agricultural productivity. In the present study, seven bacterial isolates from rice rhizosphere were characterized for their plant growth promoting traits. The isolate NP6 showed most promising results with indole-3-acetic acid production, phosphate solubilization, hydrogen cyanide production, ammonia production, nitrogen fixation along with significant amount of exo-polysaccharides production. The strain was identified as Bacillus cereus by 16S rDNA sequencing method. The co-inoculation of NP6 with pathogenic fungal strain Alternaria alternata showed 86% inhibition of the mycelial growth. For NP6, fructose and peptone were proved to be most optimum carbon and nitrogen sources respectively for exo-polysaccharide production. At a C/N ratio of 20, highest exo-polysaccharides production was observed. In presence of salinity stress NP6 is also able to secrete considerable amount of EPS. All these properties of NP6 make it a candidate to be utilized in large scale exo-polysaccharides production as well as bio-inoculant for improvement of soil condition.

Single and combined effects of Pythium oligandrum Po37 and a consortium of three rhizobacterial strains on Sclerotinia stem rot severity and tomato growth promotion

Individual plant growth-promoting rhizobacterial (PGPR) strains (Bacillus thuringiensis B2, Bacillus subtilis B10 and Enterobacter cloacae B16) and their combination were used, with or without Pythium oligandrum, against Sclerotinia sclerotiorum. In vitro, all biological control agents (BCAs) reduced successfully hyphal growth of the targeted pathogenic fungus. They displayed antifungal activity by more than 50% compared to controls. In planta trials were conducted two years in the row and led to a significant decrease in stem rot severity two months after the antagonist’s application onto infected tomato plants. The reduction reached 75% using rhizobacterial mixtures and 72% using P. oligandrum alone, compared to controls. The plant growth-promoting potential of the three-strain consortium and P. oligandrum was also assessed. Increased height in disease-free plants was obtained with rhizobacterial mixtures (60%) compared to P. oligandrum (47%). The BCA’s mixture increased the height of treated plants inoculated with S. sclerotiorum (up to 80%) compared to inoculated and untreated plants. The fresh weight of the aerial parts and roots of disease-free plants was increased by 42 and 30% over control following their treatment with mixtures of rhizobacteria and P. oligandrum alone, respectively. On plants inoculated with S. sclerotiorum in both trials, the highest growth-enhancing effect was achieved using the combined treatment based on P. oligandrum and the three-strain rhizobacterial consortium (Po37 + B2 + B10 + B16). The rhizospheric microbial communities were assessed using Single Strand Conformational Polymorphism (SSCP). Differences in the genetic structure of the fungal and bacterial communities were observed following treatments applied in both trials.

ACC Deaminase Produced by PGPR Mitigates the Adverse Effect of Osmotic and Salinity Stresses in Pisum sativum through Modulating the Antioxidants Activities.

Salinity-induced ethylene production and reactive oxygen species (ROS) inhibit agricultural productivity. The plant synthesizes ethylene directly from aminocyclopropane-1-carboxylic acid (ACC). By using ACC as a nitrogen source, bacteria with ACC deaminase (ACCD) inhibit the overproduction of ethylene, thereby maintaining the ROS. The present study investigated the ACCD activity of previously identified rhizobacterial strains in Dworkin and Foster (DF) minimal salt media supplemented with 5 mM ACC (as N-source). Bacterial isolates GKP KS2_7 (Pseudomonas aeruginosa) and MBD 133 (Bacillus subtilis) could degrade ACC into α-ketobutyrate, exhibiting ACCD activity producing more than ~257 nmol of α-ketobutyrate mg protein−1 h−1, and were evaluated for other plant growth-promoting (PGP) traits including indole acetic acid production (>63 µg/mL), phosphate solubilization (>86 µg mL−1), siderophore (>20%) ammonia and exopolysaccharide production. Furthermore, Fourier Transform Infrared analysis also demonstrated α-ketobutyrate liberation from ACC deamination in DF minimal salt media, thereby confirming the ACCD activity. These isolates also showed enhanced tolerance to salinity stress of 3% w/v NaCl in vitro, in addition to facilitating multifarious PGP activities. Seed bacterization by these ACCD-producing bacterial isolates (GKP KS2_7 and MBD 133) revealed a significant decline in stress-stimulated ethylene levels and its associated growth inhibition during seedling germination. They also mitigated the negative effects of salt stress and increased the root-shoot length, fresh and dry weight of root and shoot, root-shoot biomass, total sugar, protein, reducing sugar, chlorophyll content, and antioxidants enzymes in Pisum sativum. As a result, these strains (GKP KS2_7 and MBD 133) might be applied as biofertilizers to counteract the negative effects of soil salinity.

Alleviation of Cr(VI) Toxicity and Improve Phytostabilization Potential of Vigna radiata Using a Novel Cr(VI) Reducing Multi-Stress-Tolerant Plant Growth Promoting Rhizobacterial Strain Bacillus flexus M2

Chromium (Cr) is a toxic heavy metal discharged into the environment through various anthropogenic sources, which affects soil properties and fertility. Hence, an effective soil restoration strategy is the need of the hour. In this study, a potent Cr(VI)-reducing strain M2 was isolated from the rhizosphere of Zea mays L. grown in leather industrial effluent contaminated sites and identified as Bacillus flexus through 16S rDNA sequencing. Strain M2 exhibited strong tolerance to multi-stresses such as temperature (up to 45 °C), pH (up to 9.0), Sodium chloride (NaCl) (up to 7%) and PEG 6000 (up to 50%) and showed strong Cr(VI) reduction with the presence of multi-stresses. The interaction of Cr(VI) with strain M2 was elucidated through various instrumentation analyses. Fourier Transform Infra-red (FTIR) Spectroscopy analysis confirmed that Cr(VI) exposures induce significant changes in the cell-surface functional groups. Raman spectrum and Transmission Electron Microscopy–Energy Dispersive X-ray spectroscopy (TEM-EDX) analysis confirmed the bio-reduction of Cr(VI) to Cr(III) and their intracellular localization as Cr(III). Further, strain M2 produced a significant quantity of Indole acetic acid (IAA), ammonia, and exopolysaccharide (EPS) and showed positive results for various plant-growth-promoting activities with the presence of Cr(VI). In greenhouse experiments, the strain M2 inoculation progressively increased the plant growth parameters and stabilized the antioxidant system of Vigna radiata under Cr stress. However, Cr(VI) exposure decreased the growth parameters and increased the level of proline content, Hydrogen peroxide (H2O2) accumulation, and antioxidant enzymes expression in V. radiata. Interestingly, strain M2 inoculation significantly reduced the accumulation of Cr in root and shoot of V. radiata when compared to the uninoculated Cr(VI) treatment. Hence, this study confirms that rhizobacterial inoculation markedly reduced the negative impact of Cr toxicity and improved V. radiata growth even in harsh environments by stabilizing the mobility of Cr in the rhizosphere.

Antifungal properties of rhizobacterial strains in relation to fungi of agricultural crops

On earth, pathogenic fungal infections are considered one of the most common crop problems, accounting for more than 80% of all plant diseases. The production of biologics and the fight against pathogens are highly relevant priorities. In this article, rhizosphere cultures isolated from wheat root tubers have been studied in detail for the development of pathogenic fungi. In the course of research, the inhibitory properties of rhizobacteria against common pathogenic fungi were studied. The main mechanism of antagonism is that bacteria directly affect phytopathogens or increase plant resistance to diseases by enhancing immunity. The antagonistic activity of rhizospheric microorganisms was studied and the level of their action was determined. It has been established that wheat rhizospheric bacteria Escherichia hermannii, Enterobacter cloacae and Rahnella aquatilis affect the development of pathogens that cause Alternaria and Fusarium wilt. In the experiments, 46 strains of rhizobacteria of the genus Enterobacter (strain E. cloacae CCIM1022), Rahnella (R. aquatilis CCIM1023), Pantoea, Pseudomonas, Bacillus, Escherichia were isolated and the effectiveness of their antifungal properties on the reproduction of pathogenic fungi was tested. Pathogenic fungi Alternaria, Fusarium, causing diseases of vegetable and melon crops, were isolated from host plants. It is noted that rhizobacteria have a negative impact and block the development of pure cultures of fungi isolated from tomatoes, zucchini, figs, melons, causing fungal diseases on cups. The 46 isolated and tested strains of rhizobacteria effectively inhibited the development of pathogenic fungi. Of these, Rahnella is the first bacterium studied for this purpose, and has been given great importance in experiments to block the development of Alternaria (A. alternata CCIM 1021). The causative agents of alternariosis are widely open. However, large-scale analysis of their presence and control of pathogens has always been difficult. In experiments to combat the pathogen in the laboratory, a new agent was used to spread nitrogen-fixing bacteria.

Salt-tolerant PGPR strain Priestia endophytica SK1 promotes fenugreek growth under salt stress by inducing nitrogen assimilation and secondary metabolites.

Soil salinity is a huge obstacle in crop production worldwide. Saline soil can reduce active chemical contents in medicinal plants of the Leguminosae family through crippled normal nodule function. Intensive efforts are underway to improve yield and medicinal value of leguminous herbs under salt stress condition by using benign microbes. Here, an attempt was made to explore the salt-tolerant bacteria associated with rhizosphere of fenugreek plant (Trigonella foenum-graecum L.) and to evaluate their impact on host plant growth and metabolite of pharmaceutical importance. A salt-tolerant plant growth promoting rhizobacterial (PGPR) strain Priestia endophytica SK1 isolated from fenugreek rhizospheric soil, which increased biomass and metabolite content in plants grown under saline stress. SK1 bacterial application induced nodule formation and enhanced nitrogen and phosphorus content under salt (100 mM NaCl) stress as compared to control plants. H2 O2 production and lipid peroxidation as a measure of stress were observed high in control plants, while a reduction in these parameters was observed in plants inoculated with SK1. In addition, a significant effect was found on the phenolic compounds and trigonelline content in fenugreek plant inoculated with SK1 bacterium. An increased trigonelline content of about 54% over uninoculated control was recorded under salt stress. The results of this study revealed that the application of salt-tolerant PGPR strain P. endophytica SK1 induced nitrogen fixation machinery that leads to alleviate salt stress and improved the biosynthesis of trigonelline content in fenugreek. This study extends our understanding on the significance of rhizosphere microbiome and their beneficial role in plant health under environmental stress to promote agro-eco-farming practices.

Molecular, biochemical, and comparative genome analysis of a rhizobacterial strain Klebsiella Sp. KBG6.2 imparting salt stress tolerance to Oryza sativa L.

One-third of global agricultural land is affected by salinity, thereby limiting the, productivity and sustainability of the cropping ecosystem. Rice is a globally important, economical crop but is largely affected by soil salinity , leading to loss of standing crop. Plant growth-promoting rhizobacteria (PGPR) can be a potential solution to this problem by improving soil fertility, tolerance to stress and ultimately enhancing crop yield. However, the usage of PGPR in alleviating stress is limited and requires further investigation. In the current work, a PGPR strain, namely, Klebsiella sp. KBG6.2 was isolated from the rice rhizosphere of salt-affected agricultural fields in Odisha, India., The strain was found to be imparting salt tolerance to rice seedlings grown up to 200 mM of sodium chloride. The rice seedlings when treated with sodium chloride and coinoculated with Klebsiella sp. KBG6.2 showed better germination percentage, biomass, shoot, root length, chlorophyll content, and reducing sugar accumulation. The PGPR treatment also led to a reduction in reactive oxygen species (ROS) production, as evidenced by reduced ROS scavenging enzyme (catalase, superoxide dismutase , and peroxidase) activities and lower accumulation of the corresponding mRNA as, verified by real time PCR analysis. This was further supported by the reduced, accumulation of malondialdehyde which is an indicator of stress-induced oxidative damage. The genome sequence analysis of the Klebsiella sp. KBG6.2 strain revealed, it to contain the required genes responsible for colonization and acclimation during stress conditions. The successful root colonization was further verified by scanning electron microscopy. Further, the obtained PGPR strain could provide saline stress tolerance to rice and serve as bioinoculant for sustainable agriculture in salt-affected areas. • Isolation and characterization of a potential PGPR strain named Klebsiella sp. KBG6.2. • The PGPR strain Klebsiella sp. KBG6.2 could be used to impart saline stress tolerance to rice. • A successful strain in maintaining cellular reactive oxygen species homeostasis.

Phytohormones producing rhizobacteria alleviate heavy metals stress in soybean through multilayered response

Aimof the current research was to use plant growth promoting rhizobacteria for sequestration and biotransformation of the toxic form of Cr and As into non-toxic form. Remediating these contaminants using microbes is a common technique and rhizo-microbiota not only relieves metal stress but also acts as biofertilizers. Role of plant growth-promoting rhizobacterial (PGPR) strains Acinetobacter beijerinckii (C5) and Raoultella planticola (C9) in counteracting chromium and arsenic stress in soybean seedlings was assessed. The isolated rhizobacteria were able to tolerate excessive quantities (up to 1200 ppm) of chromate and arsenate in liquid media. Beside their growth in heavy metal containing media, the strains were able to bio-transform chromate and arsenate to their least toxic form. They released significant quantities of stress related metabolites including phenols, flavonoids, proline, sugars and protein even in the presence of 1200 ppm of the heavy metals. They also released several plant hormones together with indole-3-acetic acid (IAA), salicylic acid (SA) and gibberellins. Another important feature of the isolates was their ability to solubilize phosphate and release siderophores and exposure to different levels of the selected heavy metals enhanced phosphate solubilization potential of both the isolates by up to 2-fold. Release of siderophore in A. beijerinckii C5 was enhanced by increasing heavy metals concentration in the media but in case of R. planticola C9 a decline was noted. When inoculated on soybean seedlings, the isolates modulated several metabolites of the hos plant enabling them to combat heavy metal toxicity at different levels. The PGPR strains boosted host’s antioxidants production which minimized the oxidative damage by scavenging excessive ROS produced under stress. Control plants showed upregulation of stress response metabolites compared to PGPR application, whereas, IAA and SA were significantly higher in PGPR associated seedlings. In conclusion, PGPR alters the physiological and metabolic responses of soybean enabling it to cope better with chromate and arsenic toxicity and grow well under the stress.

Potential of mineral-solubilizing bacteria for physiology and growth promotion of Chenopodium quinoa Willd.

Nutrient deficiency in wild plant species, including quinoa (Chenopodium quinoa Willd), can be overcome by applying mineral-solubilizing bacteria. Quinoa is a gluten-free, nutritious food crop with unique protein content. The present study aimed to characterize mineral-solubilizing rhizobacterial strains and to evaluate their plant growth-promoting potential in quinoa seedlings. More than sixty rhizobacterial strains were isolated from the quinoa rhizosphere and found eighteen strains to be strong phosphate solubilizers. Most of these bacterial strains showed zinc solubilization, and more than 80% of strains could solubilize manganese. The selected strains were identified as Bacillus altitudinis Cq-3, Pseudomonas flexibilis Cq-32, Bacillus pumilus Cq-35, Pseudomonas furukawaii Cq-40, Pontibacter lucknowensis Cq-48, and Ensifer sp. Cq-51 through 16S rRNA partial gene sequencing. Mainly, these strains showed the production of organic acids, including malic, gluconic, tartaric, ascorbic, lactic, and oxalic acids in insoluble phosphorus amended broth. All strains showed production of gluconic acids, while half of the strains could produce malic, ascorbic, lactic, and oxalic acids. These strains demonstrated the production of indole-3-acetic acid in the presence as well as in the absence of L-tryptophan. The bacterial strains also demonstrated their ability to promote growth and yield attributes, including shoot length, root length, leave numbers, root and shoot dry biomass, spike length, and spikes numbers of quinoa in pots and field trials. Increased physiological attributes, including relative humidity, quantum flux, diffusive resistance, and transpiration rate, were observed due to inoculation with mineral solubilizing bacterial strains under field conditions. P. lucknowensis Cq-48, followed by P. flexibilis Cq-32, and P. furukawaii Cq-40 showed promising results to promote growth, yield, and physiological attributes. The multi-traits characteristics and plant growth-promoting ability in the tested bacterial strains could provide an opportunity for formulating biofertilizers that could promote wild quinoa growth and physiology.

Postharvest Application of Bacillus amyloliquefaciens PMB04 Fermentation Broth Reduces Anthracnose Occurrence in Mango Fruit

Anthracnose is a destructive postharvest disease of mango fruit, especially after ripening. The application of antagonistic Bacillus spp. strains is one of many promising control strategies. B. amyloliquefaciens (PMB04) exhibits strong antagonistic activity against anthracnose in strawberry fruit, yet has not been explored for anthracnose control in mango fruit. Among seven tested rhizobacterial strains, PMB04 exhibited the strongest inhibition against Colletotrichum gloeosporioides (Cg) growth. PMB04 demonstrated better inhibitory activity against Cg mycelial growth than the fungicides pyraclostrobin and difenoconazole. A PMB04 fermentation broth (PMB04FB) was prepared and tested, and showed superior efficacy in the reduction of anthracnose disease severity in mangoes compared to a PMB04 suspension, difenoconazole, or pyraclostrobin. We further found PMB04FB increased the mortality ratio of Cg conidia. Our study demonstrates that treatment with PMB04FB is an effective control method for anthracnose in mango and represents a promising strategy for postharvest mango processing.

Potential of plant growth promoting bacterial consortium for improving the growth and yield of wheat under saline conditions.

Owing to inconsistent results of a single bacterial strain, co-inoculation of more than one strain under salinity stress could be a more effective strategy to induce salt tolerance. Co-inoculation of more than one bacterial strain could be more effective due to the presence of several growths promoting traits. This study was conducted to evaluate the effectiveness of multi-strains bacterial consortium to promote wheat growth under salinity stress. Several plant growth promoting rhizobacteria (PGPR) had been isolated and tested for their ability to grow in increasing concentrations of sodium chloride (NaCl). Those rhizobacterial strains having tolerance against salinity were screened to evaluate their ability to promote wheat growth in the presence of salinity by conducting jar trials under axenic conditions. The rhizobacteria with promising results were tested for their compatibility with each other before developing multi-strain inoculum of PGPR. The compatible PGPR strains were characterized, and multi-strain inoculum was then evaluated for promoting wheat growth under axenic conditions at different salinity levels, i.e., 2.1 (normal soil), 6, 12, and 18 dS m–1. The most promising combination was further evaluated by conducting a pot trial in the greenhouse. The results showed that compared to a single rhizobacterial strain, better growth-promoting effect was observed when rhizobacterial strains were co-inoculated. The multi-strain consortium of PGPR caused a significant positive impact on shoot length, root length, shoot fresh weight, and root fresh weight of wheat at the highest salinity level in the jar as well as in the pot trial. Results showed that the multi-strain consortium of PGPR caused significant positive effects on the biochemical traits of wheat by decreasing electrolyte leakage and increasing chlorophyll contents, relative water contents (RWC), and K/Na ratio. It can be concluded that a multi-strain consortium of PGPR (Ensifer adhaerens strain BK-30, Pseudomonas fluorescens strain SN5, and Bacillus megaterium strain SN15) could be more effective to combat the salinity stress owing to the presence of a variety of growth-promoting traits. However, further work is going on to evaluate the efficacy of multi-strain inoculum of PGPR under salt-affected field conditions.

Management of chromium(VI)-contaminated soils through synergistic application of vermicompost, chromate reducing rhizobacteria and Arbuscular mycorrhizal fungi (AMF) reduced plant toxicity and improved yield attributes in Ocimum basilicum L.

An integrated approach involving vermicompost, chromate reducing bacteria and AMF was tested to manage the toxic impacts of Cr(VI) on Ocimum basilicum as a model plant. Pot experiments were conducted on O. basilicum plants in an artificially Cr(VI)-contaminated soil in two phases of experiment as bioinoculants experiment and vermicompost experiment. In the first phase of the bioinoculants experiment the series of gradient concentrations of Cr(VI) (0, 25, 50 and 100 mg kg–1 in soil) were evaluated with previously isolated four efficient Cr(VI)-reducing rhizo-bacterial strains (Bacillus Cereus strain SUCR 44, BC; Microbacterium sp. strain SUCR 140, MB; Bacillus thuringiensis strain SUCR186, BT; and Bacillus subtilis strain SUCR188; BS) along with Arbuscular Mycorrhizal Fungus—Glomus fasciculatum (GF) in alone and in co-inoculation form. In the second experiment (vermicompost) the best performing strain (MB) was tested alone or in combination with GF along with different doses of vermicompost. It was observed that vermicompost by itself could be useful in decreasing the bioavailable Cr(VI), uptake of Cr besides improving the nutritional status of plants. The vermicompost also played an important and indirect role and improved herb yield by supporting the multiplication of MB (Microbacterium sp.), an efficient chromate reducing rhizobacteria, that further decreased the bioavailable and toxic form of Cr and improved population and colonization of GF too. The translocation of Cr(VI) was averted through improved colonization of GF, also prevented higher accumulation of Cr in aerial parts (leafy herb) of O. basilicum.Supplementary InformationThe online version contains supplementary material available at 10.1007/s00203-022-03230-z.

Metabolomic insight into the synergistic mechanism of action of a bacterial consortium in plant growth promotion

It has been established that a consortium consisting of compatible plant growth promoting rhizobacterial strains outperforms their individual impacts on plant attributes. While the phenomenon of synergism is extensively reported, the mechanism that underpins it is yet to be elucidated. In the present study the impact of three plant growth promoting bacteria, Azotobacter chroococcum (A), Priestia megaterium (formerly Bacillus megaterium) (B), and Pseudomonas sp. SK3 (P) was studied as a consortium on the growth attributes of pigeonpea. In addition, microbe-microbe interactions were investigated through metabolomic profiling to understand the mechanism of synergism. Plant growth experiments revealed that bacterial consortium A+ B+ P showed a significant increase in plant attributes such as shoot length, root length, fresh weight, and dry weight as compared to monocultures and two-membered consortia. Metabolomic profiling through high resolution liquid chromatograph mass spectrometer revealed the presence of a few bioactive compounds in the consortium that might play a potential role in the enhancement of biometric parameters of the plant. Several compounds, such as antipyrine, 6,6-dimethoxy-2,5,5-trimethyl-2-hexene, N-methyltryptamine, 2,2-dimethyl-3,4-bis(4-methoxyphenyl)-2H-1-benzopyran-7-ol acetate, N6-hydroxy-l-lysine, and l-furosin, were detected in the metabolome of the consortium, which was unique among all the treatments. The study also detected a few metabolites involved in sphingolipid biosynthesis (ketosphinganine and sphinganine) known for cell signaling in the consortium. This unravels the possible mechanism of synergism between bacterial strains in a consortium. The metabolomic profile would be helpful to strategically develop unique and more effective consortia that are tailored to the soil type.

Halotolerant rhizobacteria isolated from a mangrove forest alleviate saline stress in Musa acuminata cv. Berangan

Saline soils resulting from anthropogenic activity and climate change present a challenge to future food security. Towards addressing this, we isolated and characterized halotolerant bacteria from a Malaysian mangrove forest, and explored their effect on morpho-physiological and biochemical parameters of banana plantlets under salt stress. A total of 88 rhizobacterial and 16 endophytic bacterial isolates collected from the roots and rhizosphere of Rhizophora apiculata, Avicennia alba and Sonneratia alba, were found to tolerate up to 400 mM of sea salt. Based on best performance in multiple plant growth traits, three rhizobacterial strains RB1, RB3 and RB4 and three endophytic bacterial strains EB1, EB2 and EB3 were used for further analysis. The rhizobacterial strains were identified as Bacillus sp. and endophytic bacteria as Pseudomonas sp. based on 16 S rRNA gene sequence. SEM observation confirmed colonization of each strain on banana plantlet roots. When colonized plantlets were subjected to 90 mM salt and compared to uninoculated (control) and mock inoculated plants, improved plant growth was observed with each of the strains, especially with bacterial strains EB3 and RB3. Biochemical analysis of plantlets revealed that root colonization with EB3 and RB3 enhanced levels of plant chlorophyll (> 5-fold), carotenoid (> 2.85-fold) and proline (2.6-fold and 2.3-fold), while plantlets also showed reduced MDA content (0.45-fold and 0.51-fold), significantly reduced generation of ROS (0.23-fold and 0.47-fold) and lower levels of electrolyte leakage (0.77 and 0.51-fold). Antioxidant enzymes also showed enhanced activity with EB3 and RB3. Our results indicate that these halotolerant Bacillus and Pseudomonas strains from the mangrove have multifunctional plant growth promoting activity and can reduce salt stress in bananas. This data provides a reference for exploring halotolerant microbes from hypersaline environments to overcome salt stress in plants.

Halotolerant plant growth-promoting rhizobacteria-mediated alleviation of salinity and dust stress and improvement of forage yield in the desert halophyte Seidlitzia rosmarinus

In desert areas, a set of environmental stresses such as salinity and dust simultaneously affect plant biomass and nutritional value. In this study, the effects of two halotolerant plant growth-promoting rhizobacterial strains, Bacillus pumilus HR, isolated from the halophyte Halostachys belangeriana, and Zhihengliuella halotolerans SB, isolated from the halophyte Seidlitzia rosmarinus, on morpho-physiological, biochemical and ionomic properties of S. rosmarinus under salinity stress (0, 300, and 600 mM NaCl) and mineral dust (0 and 1.5 g m−2 month−1) were examined. Salinity stress reduced the morpho-physiological characteristics of S. rosmarinus plant more than dust stress. The dust application, similar to salinity stress, increased the content of Na, malondialdehyde (MDA), acidity, and glycine betaine in S. rosmarinus leaves. Z. halotolerans SB strain was more effective than B. pumilus HR strain in improving the characteristics of S. rosmarinus under salinity and dust stress. Under the conditions of combined application of salinity (at 600 mM NaCl level) and dust stress, this strain (SB) increased chlorophyll a content by 40%, protein content by 48%, Fe content by 45%, Mg content by 63%, Mn content by 21%, total dry biomass by 35%, seedling quality index by 104%, catalase activity by 47%, anthocyanin content by 85, and Na content by 53% in S. rosmarinus plant leaves compared to control. In addition, the results of PCA analysis showed that at salinity level of 300 and 600 mM NaCl, there were an increase in antioxidant indices and a sharp increase in Na and MDA content in S. rosmarinus leaves, respectively. It was also suggested that the bacterial strain (Z. halotolerans SB) can increase the quantity and quality of forage (seedling quality index) better than another bacterial strain (B. pumilus HR) under salinity and dust conditions. In general, the study reveals that the application of halotolerant rhizobacteria can be a cost effective and ecological sustainable method to improve the yield and quality of forage in the desert halophyte S. rosmarinus under salinity and dust stress conditions.

Hydroxamate type of Siderophore production by rhizobacterial strains isolated from root nodules of Vigna trilobata

21 rhizobacterial strains were isolated from root nodules of Vigna trilobata (L-Verdc) plants raised in soils collected from geographically different areas in Andhra Pradesh, India. The identification of rhizobacteria was done by biochemical and by 16 S rDNA sequencing analysis along with accession numbers (Rhizobium sp. MRR 106, KC428655 and Rhizobium sp. MRR 123, KC503884). Detection of siderophore production by microorganisms in solid medium is the universal Chrome Azurol’ S (CAS). Agar plate assay method was used for this study. Out of 21 strains, only 7 showed the siderophore production. Of the 4 siderophore positive, two strains Rhizobium sp. MRR 106 and Rhizobium sp. MRR 123 are hydroxamate type and rest of them are catechol type. Siderophore production was observed at 24 h of incubation and it reached maximum at 144 h of incubation. Qualitative and quantitative estimation of hydroxamate type of siderophore production shows much difference along with iron concentrations up to 20 to 80 μM in Rhizobium sp. MRR 106 and Rhizobium sp. MRR 123. Various carbon and nitrogen sources greatly influenced the siderophore production. Among them, glucose (1%) and glycine (0.1%) were found to be the best for siderophore production.

Development of plant systemic resistance by beneficial rhizobacteria: Recognition, initiation, elicitation and regulation.

A plant growing in nature is not an individual, but it holds an intricate community of plants and microbes with relatively stable partnerships. The microbial community has recently been demonstrated to be closely linked with plants since their earliest evolution, to help early land plants adapt to environmental threats. Mounting evidence has indicated that plants can release diverse kinds of signal molecules to attract beneficial bacteria for mediating the activities of their genetics and biochemistry. Several rhizobacterial strains can promote plant growth and enhance the ability of plants to withstand pathogenic attacks causing various diseases and loss in crop productivity. Beneficial rhizobacteria are generally called as plant growth-promoting rhizobacteria (PGPR) that induce systemic resistance (ISR) against pathogen infection. These ISR-eliciting microbes can mediate the morphological, physiological and molecular responses of plants. In the last decade, the mechanisms of microbial signals, plant receptors, and hormone signaling pathways involved in the process of PGPR-induced ISR in plants have been well investigated. In this review, plant recognition, microbial elicitors, and the related pathways during plant-microbe interactions are discussed, with highlights on the roles of root hair-specific syntaxins and small RNAs in the regulation of the PGPR-induced ISR in plants.