Plant Growth-promoting Rhizobacterial Strains Research Articles

The rhizobacterial Priestia megaterium strain SH-19 mitigates the hazardous effects of heat stress via an endogenous secondary metabolite elucidation network and molecular regulation signalling

Global warming is a leading environmental stress that reduces plant productivity worldwide. Several beneficial microorganisms reduce stress; however, the mechanism by which plant–microbe interactions occur and reduce stress remains to be fully elucidated. The aim of the present study was to elucidate the mutualistic interaction between the plant growth-promoting rhizobacterial strain SH-19 and soybeans of the Pungsannamul variety. The results showed that SH-19 possessed several plant growth-promoting traits, such as the production of indole-3-acetic acid, siderophore, and exopolysaccharide, and had the capacity for phosphate solubilisation. The heat tolerance assay showed that SH-19 could withstand temperatures up to 45 °C. The strain SH-19 was identified as P. megaterium using the 16S ribosomal DNA gene sequence technique. Inoculation of soybeans with SH-19 improved seedling characteristics under high-temperature stress. This may be due to an increase in the endogenous salicylic acid level and a decrease in the abscisic acid level compared with the negative control group. The strain of SH-19 increased the activity of the endogenous antioxidant defense system, resulting in the upregulation of GSH (44.8%), SOD (23.1%), APX (11%), and CAT (52.6%). Furthermore, this study involved the transcription factors GmHSP, GmbZIP1, and GmNCED3. The findings showed upregulation of the two transcription factors GmbZIP1 (17%), GmNCED3 (15%) involved in ABA biosynthesis and induced stomatal regulation, similarly, a downregulation of the expression pattern of GmHSP by 25% was observed. Overall, the results of this study indicate that the strain SH-19 promotes plant growth, reduces high-temperature stress, and improves physiological parameters by regulating endogenous phytohormones, the antioxidant defense system, and genetic expression. The isolated strain (SH-19) could be commercialized as a biofertilizer.

Characterization and Field Evaluation of Potential Fluorescent Pseudomonas Isolates on Growth and Yield Attributing Characters in Chickpea

The present investigation was conducted at the research and instructional farm, College of Agriculture, Indira Gandhi Krishi Vishwavidyalaya, Raipur, Chhattisgarh, India during rabi (October–December 2019) to evaluate the influence of fluorescent Pseudomonas isolates on chickpeas through characterization and field evaluation. Eighty-eight rhizosphere soil samples were derived from different locations in Bastar and Bilaspur districts of Chhattisgarh, India. Potential isolates were required for fundamental research and practical application of plant growth-promoting rhizobacterial strains on different crops. Among eighty-eight isolates, ten fluorescent Pseudomonas (visible under UV light at 360 nm) emitting strong fluorescence were selected for the course of the investigation and characterized for the production of siderophores, phosphate solubilization, indole acetic acid, 1-aminocyclopropane-1-carboxylic acid deaminase, and analysis of antagonistic potential. Of the 10 Pseudomonas isolates, 9704 was identified as the highest phosphate solubilizer, BSP-23 was identified as the highest indole acetic acid producer, and BSP-19 was the highest siderophore producer. Fluorescent Pseudomonas isolates BS-1, BS-4, 9809, BSP-14, and BSP-19 were identified to possess 1-aminocyclopropane-1-carboxylic acid deaminase-producing ability following qualitative analysis. Our present investigation indicates that beneficial effects of fluorescent Pseudomonas could be achieved through simple and cost-effective seed treatment. Three fluorescent Pseudomonas strains, 9704, BS-14, and 9829, consistently increased seed germination and improved plant growth and yield (BS-37.9% and 9704-35.7%) attributes in chickpeas. Besides, confrontation assays performed against R. bataticola expressed varying levels of inhibition. The phylogenetic affinities of 16S rRNA gene sequencing resolved the species identities of selected isolates.

Phenotypic and Genomic Characterization of Pseudomonas wuhanensis sp. nov., a Novel Species with Promising Features as a Potential Plant Growth-Promoting and Biocontrol Agent.

Plant growth-promoting rhizobacterial strain FP607T was isolated from the rhizosphere of beets in Wuhan, China. Strain FP607T exhibited significant antagonism toward several phytopathogenic bacteria, indicating that FP607T may produce antimicrobial metabolites and has a stronger biocontrol efficacy against plant pathogens. Growth-promoting tests showed that FP607T produced indole-3-acetic acid (IAA), NH3, and ferritin. The genome sequence of strain FP607T was 6,590,972 bp long with 59.0% G + C content. The optimum temperature range was 25-30 °C, and the optimum pH was 7. The cells of strain FP607T were Gram-negative, short, and rod-shaped, with polar flagella. The colonies on the King's B (KB) agar plates were light yellow, smooth, and circular, with regular edges. A phylogenetic analysis of the 16S rRNA sequence and a multilocus sequence analysis (MLSA) showed that strain FP607T was most closely related to the type of strain Pseudomonas farris SWRI79T. Based on a polyphasic taxonomic approach, strain FP607T was identified as a novel species within the genus Pseudomonas, for which the name Pseudomonas wuhanensis sp. nov. was proposed. The type of strain used was FP607T (JCM 35688, CGMCC 27743, and ACCC 62446).

Single and co-inoculation of biofilm-forming rhizobacteria with macroalgae extract increase barley productivity in organic matter-limited soil

Context Limitation in soil organic matter interferes with plant growth and productivity. Chemical fertilisers can provide limiting nutrients to plants, especially in barren soil, but they have detrimental effects on living organisms; therefore, plant growth-promoting (PGP) rhizobacteria may be an alternative to synthetic fertilisers to improve plant tolerance to nutrient stress. Aims This study evaluated the effects of single and co-inoculation of two PGP rhizobacterial strains with macroalgae (Cladophora glomerata) extract on barley growth. We also tested the ability of the respective strains to produce biofilm and exopolysaccharide (EPS) at different pH values under in vitro conditions. Methods Plants were cultivated in an organic matter-limited alkaline soil under field conditions. Key results All treatments increased barley growth parameters to some extent. Growth parameters increased most in Bacillus + macroalgae extract (B + MA) and Bacillus + Azotobacter + macroalgae extract (B + A + MA) treatments. These two treatments significantly enhanced shoot length by 22.9–25.7%, dry weight of shoot + spike by 118.2–154.5%, total weight of grains by 76.5–78.1%, and number of grains per spike by 94.8–112.1% in comparison with the control. The respective two treatments also increased biofilm formation and EPS production in response to alkaline pH. Conclusions Barley productivity may be increased by using PGP strains with greater potential to produce biofilm and EPS, and inoculating them with macroalgae extract. The B + MA treatment could be applied singly or in combination with Azotobacter. Implications Single or co-inoculation of such PGP bacteria with macroalgae extract may be commercialised to enhance barley productivity in organic matter-limited soils.

Assessment of the Protective Potential of Inoculums and Metabolites of Rhizobacteria on Soybean (Glycine max) Seedlings against Bacterial and Fungal Pathogens.

Plant growth-promoting bacteria (PGPR), while generally considered to aid plant growth with the provision of nutrients, can also be used as biocontrol agents for plant pathogens. The study assessed the protective potential of inoculums and metabolites of plant growthpromoting rhizobacterial strains against bacterial and fungal pathogens on soybean seedlings. Inoculums and metabolites of 15 rhizobacterial strains were used for the study. Five pathogens (Alternaria sp., Aspergillus niger, Corynespora sp., Fusarium oxysporum and Xanthomonas campestris) were employed for the study. Four experimental setups: treated-only seeds, infected-only seeds, infected then inoculum or metabolite treated seeds, and infected then distilled water treated seeds. In the setup infected with Alternaria sp., final germination values of seeds in the presence of the respective inoculums showed no significant variation between the treated only and the infected then treated setup. In the case of seeds infected with Aspergillus niger, higher germination and vigor index values were observed in the treated-only seeds when compared with the infected then-treated seeds. For seeds infected with Corynespora sp., significantly lower germination and vigor index values were observed in the infected then-treated seeds than the treated-only seeds in the presence of the respective inoculums. With regards to setup infected with Fusarium oxysporum, significantly higher final germination and vigor index values were recorded for the treated only seeds when compared with the infected then treated setups. For the Xanthomonas campestris infected seeds, the majority of the infected then metabolite-treated seeds showed significantly lower final germination values when compared with the treated-only seeds. The study findings were able to establish the efficacy of some bacteria agents against economically important species of plant pathogens.

Phytostabilization of metal(loid)s by ten emergent macrophytes following a 90-day exposure to industrially contaminated groundwater

Better understanding of macrophyte tolerance under long exposure times in real environmental matrices is crucial for phytoremediation and phytoattenuation strategies for aquatic systems. The metal(loid) attenuation ability of 10 emergent macrophyte species (Carex riparia, Cyperus longus, Cyperus rotundus, Iris pseudacorus, Juncus effusus, Lythrum salicaria, Menta aquatica, Phragmites australis, Scirpus holoschoenus, and Typha angustifolia) was investigated using real groundwater from an industrial site, over a 90-day exposure period. A “phytobial” treatment was included, with 3 plant growth-promoting rhizobacterial strains. Plants exposed to the polluted water generally showed similar or reduced aerial biomass compared to the controls, except for C. riparia. This species, along with M. aquatica, exhibited improved biomass after bioaugmentation. Phytoremediation mechanisms accounted for more than 60% of As, Cd, Cu, Ni, and Pb removal, whilst abiotic mechanisms contributed to ∼80% removal of Fe and Zn. Concentrations of metal(loid)s in the roots were generally between 10–100 times higher than in the aerial parts. The macrophytes in this work can be considered “underground attenuators”, more appropriate for rhizostabilization strategies, especially L. salicaria, M. aquatica, S. holoschoenus, and T. angustifolia. For I. pseudacorus, C. longus, and C. riparia; harvesting the aerial parts could be a complementary phytoextraction approach to further remove Pb and Zn. Of all the plants, S. holoschoenus showed the best balance between biomass production and uptake of multiple metal(loid)s. Results also suggest that multiple phytostrategies may be possible for the same plant depending on the final remedial aim. Phytobial approaches need to be further assessed for each macrophyte species.

Suppression of tomato bacterial speck disease (Pseudomonas syringae pv. tomato (Okabe) Young, Dye, & Wilkie) via induced systemic resistance by Pseudomonas and Bacillus strains

Bacterial speck Pseudomonas syringae pv. tomato (Pst) (Okabe) Young, Dye, & Wilkie is a widespread disease in tomato plants. Four plant growth-promoting rhizobacterial (PGPR) strains 5(3), 68(2), 36(1), and 47(3) played a significant role (50% and higher) in reducing spot disease severity. Selected strains were identified as Pseudomonas koreensis 5(3), Bacillus mycoides 68(2), Bacillus mojavensis 36(1), and Bacillus simplex 47(3) using the MALDI Biotyper classification system. In planta assay using tomato seedlings were inoculated with the bacterial strains alone or in dual combination. Pseudomonas koreensis 5(3) (51.9%–74.29%) and Bacillus mycoides 68(2) (36.70%–65.56%) both provided a significant reduction in foliar severity caused by bacterial speck disease agent Pseudomonas syringae pv. tomato (Okabe) Young, Dye, & Wilkie. Bacillus simplex 47(3) and Bacillus mojavensis 36(1) were successful only in combined treatments. Defense enzymes Proline, Peroxidase, and Catalase were induced by PGPR strains in comparison with those of control plants. Hydrogen peroxide (H2O2) and callose deposition were evident at reaction sites induced by PGPR strains. The accumulation of callose, H2O2, and high levels of defense enzymes via the treatment of PGPRs might play a significant role in a practical, safe, and effective way to control Pseudomonas syringae pv. tomato.

Antagonistic Activity of Pseudomonas fluorescens Strain X1 Against Different Fusaria and it’s In Vivo Analysis Against Fusarium udum Infected Pigeon Pea

A plant growth-promoting rhizobacterial strain, Pseudomonas fluorescens X1 isolated from the garden soil was employed for antagonistic activity against different species of fusaria. Strain X1 inhibited four different fusaria (Fusarium moniliforme, Fusarium oxysporum, Fusarium semitectum and Fusarium udum) in dual culture plate assay, and in broth culture using cell-free culture filtrate. Scanning electron microscopic (SEM) analysis revealed deformation and shrinkage in mycelia of fusaria after treatment with strain X1. Confocal micrographs showed degeneration of nuclei inside the cells of fusaria for the same effect. Strain X1 exhibited maximum antifungal activity, when it was grown in nutrient broth yeast (NBY) medium amended with 1mM NH4MoO4 and 1% glucose. The antifungal extracts eluted from thin-layer chromatography (TLC) followed by high performance liquid chromatography (HPLC) showed two fractions active against different fusaria. Liquid chromatography-mass spectrometry (LCMS) analysis of the two fractions 1 and 2 corresponded to molecular ions at m/z 177.16 and m/z 177.09, respectively. Infra-red (IR) analysis showed five similar absorption bands in both the fractions analysed. In vivo analysis of strain X1 alone and along with fungicide inhibited the growth of F. udum and improved the biomass and growth of pigeon pea. These results indicated that strain X1 could be possibly used as a biocontrol agent to inhibit the growth of soil-borne diseases of different fusaria including F. udum that causes wilting in pigeon pea.

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.

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.

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.

Improved salinity and dust stress tolerance in the desert halophyte Haloxylon aphyllum by halotolerant plant growth-promoting rhizobacteria.

Because of global warming, desertification is increasing. One of the best strategies for combating desertification is reforestation of forests and biological operations of vegetation. However, events like soil salinity and dust storms, as the most important manifestations of desertification, prevent vegetation from settling in these areas. In this study, the effects of two halotolerant plant growth-promoting rhizobacterial strains, Bacillus pumilus HR and Zhihengliuella halotolerans SB, on physiological and nutritional status of the desert halophyte Haloxylon aphyllum under the stress of salinity (0, 300, and 600 mM NaCl) and dust (0 and 1.5 g m−2 month−1) were examined. Under dust application, the Z. halotolerans SB strain compared to the B. pumilus HR strain and the combination of these two bacterial strains improved the content of total chlorophyll (247 and 316%), carotenoid (94 and 107%), phosphorus (113 and 209%), magnesium (196 and 212%), and total dry biomass (13 and 28%) in H. aphyllum at salinity levels of 300 and 600 mM NaCl, respectively. Under conditions of combined application of dust and salinity, B. pumilus HR compared to Z. halotolerans SB and the combination of two strains at salinity levels of 300 and 600 mM NaCl, respectively, had better performance in increasing the content of iron (53 and 69%), calcium (38 and 161%), and seedling quality index (95 and 56%) in H. aphyllum. The results also showed that both bacterial strains and their combination were able to reduce the content of ascorbic acid, flavonoid, total phenol, proline, and malondialdehyde, and catalase activity, and ultimately improve the antioxidant capacity of H. aphyllum. This showed that the use of halotolerant rhizobacteria can stop the production of free radicals and thus prevent cell membrane damage and the formation of malondialdehyde under salinity and dust stress. The results of this study for the first time showed that halotolerant rhizobacteria can increase the seedling quality index of H. aphyllum under combined conditions of salinity and dust. The use of these bacteria can be useful in the optimal afforestation of H. aphyllum species in arid and semi-arid ecosystems.

Halotolerant rhizobacteria enhance the tolerance of the desert halophyte Nitraria schoberi to salinity and dust pollution by improving its physiological and nutritional status

Playas have a high potential for producing dust, sand, and salt storms, which can influence human health and agricultural land in the near future. Growing vegetation is an effective way to control wind erosion, but events like soil salinity and dust storms prevent vegetation from settling in these areas. In this study, the effects of two halotolerant plant growth-promoting rhizobacterial strains, Bacillus pumilus HR and Zhihengliuella halotolerans SB, on morpho-physiological and nutritional status of the desert halophyte Nitraria schoberi under the stress of salinity (0, 300, and 600 mM NaCl) and dust (0 and 1.5 g m−2 month−1) were examined. The results showed that salinity, dust and the combination of the two stresses had a negative effect on most morphological and physiological traits of the plant. Salinity stress reduced potassium, iron, and manganese of N. schoberi by 28, 30, and 21 %, and increased anthocyanins, proline, and peroxidase by 21, 92, and 16 %, respectively. Both bacterial strains and their combination improved plant growth against such stresses. The effect of B. pumilus HR strain was more than that of Z. halotolerans SB strain on the plant. HR strain and SB strain increased chlorophyll a by 62 and 25 %, total dry biomass by 66 and 31 % at 300 mM NaCl level, respectively, and significantly increased seedling quality index by 72 and 76 %, respectively, under dust stress. The results of principal component analysis (PCA) showed that the presence of bacterial strains decreased antioxidant compounds (negative correlation) and increased mineral elements, chlorophyll, total dry biomass, and seedling quality index. This information can help us in the optimal afforestation of this plant species and the fixation of salt dust in the playa under the conditions of climate change in arid and semi-arid ecosystems.

Volatile Organic Compounds of the Plant Growth-Promoting Rhizobacteria JZ-GX1 Enhanced the Tolerance of Robinia pseudoacacia to Salt Stress.

Salt stress is one of the major abiotic stresses that affects plant growth and development. The use of plant growth-promoting rhizobacteria to mitigcate salt stress damage in plants is an important way to promote crop growth under salt stress conditions. Rahnella aquatilis JZ-GX1 is a plant growth-promoting rhizobacterial strain, but it is not clear whether it can improve the salt tolerance of plants, and in particular, the role of volatile substances in plant salt tolerance is unknown. We investigated the effects of volatile organic compounds (VOCs) from JZ-GX1 on the growth performance, osmotic substances, ionic balance and antioxidant enzyme activities of acacia seedlings treated with 0 and 100mm NaCl and explored the VOCs associated with the JZ-GX1 strain. The results showed that compared to untreated seedlings, seedlings exposed to plant growth-promoting rhizobacterium JZ-GX1 via direct contact with plant roots under salt stress conditions exhibited increases in fresh weight, lateral root number and primary root length equal to approximately 155.1, 95.4, and 71.3%, respectively. Robinia pseudoacacia seedlings exposed to VOCs of the JZ-GX1 strain showed increases in biomass, soil and plant analyser development values and lateral root numbers equal to 132.1, 101.6, and 166.7%, respectively. Additionally, decreases in malondialdehyde, superoxide anion (O2−) and hydrogen peroxide (H2O2) contents and increases in proline contents and superoxide dismutase, peroxidase and glutathione reductase activities were observed in acacia leaves. Importantly, the sodium-potassium ratios in the roots, stems, and leaves of acacia exposed to VOCs of the JZ-GX1 strain were significantly lower than those in the control samples, and this change in ion homeostasis was consistent with the upregulated expression of the (Na+, K+)/H+ reverse cotransporter RpNHX1 in plant roots. Through GC-MS and creatine chromatography, we also found that 2,3-butanediol in the volatile gases of the JZ-GX1 strain was one of the important signaling substances for improving the salt tolerance of plants. The results showed that R. aquatilis JZ-GX1 can promote the growth and yield of R. pseudoacacia under normal and salt stress conditions. JZ-GX1 VOCs have good potential as protectants for improving the salt tolerance of plants, opening a window of opportunity for their application in salinized soils.

Synergistic effects of plant growth promoting rhizobacteria and silicon dioxide nano-particles for amelioration of drought stress in wheat

Drought tolerant plant growth-promoting rhizobacteria (PGPR) can confer drought tolerance in plants, when inoculated, and this effect can be more pronounced by their combined application with silicon oxide nanoparticles (SiO2 NPs). In this research, drought-tolerant and plant growth-promoting rhizobacterial strains were isolated from the rhizospheric soil of wheat plants growing in the arid region of Pakistan. Out of 30 isolated strains, three rhizobacterial strains were selected based on their drought tolerance, higher phytohormones (indole acetic acid (IAA), abscisic acid (ABA), and cytokinin (CK), and osmolyte (proline and sugar) production ability. These strains were identified as Bacillus sp. Azospirillum lipoferum and Azospirillum brasilense by 16S rRNA sequencing and accession numbers (MT482404, MT742664, and MT 742666, respectively) were obtained. Inoculation of these strains, alone and in combination, improved the germination attributes of wheat seeds under drought stress conditions. However, the combination of all three bacterial strains gave the best results. SiO2 NPs were prepared from silicon dioxide and characterized by scanning electron microscopy (SEM), Energy dispersive X-rays pattern (EDX), and UV–visible spectrum. The effect of SiO2 NPs was also tested on wheat seeds under drought stress and it was observed that SiO2 NPs (150 mg/L) create pronounced drought ameliorative potential in wheat seedlings. In the pot experiment, the combined application of SiO2 NPs and PGPR exhibited a synergistic role and improved the growth and yield of wheat. The interaction between SiO2 NPs and bacterial combination improved biomass (fresh and dry weight), and chlorophyll-a, b content by 138.78%, 65.70%, 128.57%, and 283.33% respectively as compared to untreated but drought exposed plants. They also improved relative water content (71.66%), gas exchange attributes, increased nutrients uptake, and osmolytes production of wheat. Up-regulation of antioxidant enzymes; superoxide dismutase (60.49%), peroxidase (55.99%), and catalase (81.69%) was also observed. This research work suggested that the application of SiO2 NPs and PGPR strains induced drought tolerance in wheat by modulating different physiological and metabolic processes in plants which ultimately improved the growth and yield of wheat under drought stress.

Potential use of Solanum lycopersicum and plant growth promoting rhizobacterial (PGPR) strains for the phytoremediation of endosulfan stressed soil

This study aimed to assess the role of Solanum lycopersicum and plant growth promoting rhizobacterial (PGPR) strains to remove endosulfan present in the soil. S. lycopersicum was grown in endosulfan amended soil (5, 10, 25, and 50 mg kg−1) inoculated with PGPR strains for 40, 80, and 120 days. The influence of PGPR inoculation on endosulfan accumulation in plant tissues, endosulfan degradation in soil, and plant growth parameters were evaluated. The oxidative stress tolerance was assessed by determining the malondialdehyde formation in S. lycopersicum planted in endosulfan stressed soil inoculated with PGPR strains. The results showed that uptake of endosulfan followed root > shoot pathway in association with a reduction in endosulfan accumulation in inoculated plants as related to un-inoculated plants. Moreover, inoculation of PGPR strains showed a beneficial influence on the degradation of endosulfan, Bacillus sp. PRB101 showed maximum degradation (89% at 5 mg kg−1 of soil) of endosulfan at 120 days after sowing. Furthermore, the content of malondialdehyde was lower in inoculated plants as related to un-inoculated plants. Inoculation of PGPR strains efficiently enhanced plant biomass. The findings showed the effectiveness of PGPR strains to increase the decontamination of endosulfan stressed soil and decline endosulfan concentration in the plant tissues.

Isolation and characterization of streptomycetes and pseudomonad strains with antagonistic activity against the plant parasitic nematode Pratylenchus penetrans and fungi associated with replant disease

Pratylenchus penetrans is one of the most important plant-parasitic nematodes causing losses in economically significant crops such as apple, cherry, corn, grapes, oat, onion, peach, potato, soybean, strawberry, and wheat. The objective of this study was to isolate and characterize plant growth promoting rhizobacterial (PGPR) strains from sweet cherry rhizospheres with biocontrol potential against P. penetrans, and to evaluate the antagonistic activity of the selected strains on a model plant (onion), in a growth chamber experiment. Five strains: three actinomycetes (Streptomyces fulvissimus A12, S. venezuelae A30, and S. annulatus A34) and two pseudomonads (Pseudomonas sp. P3 and P. donghuensis P17) with nematicidal activity were isolated. The selected PGPR strains decreased P. penetrans populations on onion roots and enhanced onion root length and dry weight. Among possible mechanisms of action, the strains displayed the capacity to produce nematicidal lytic enzymes such as chitinases and proteases, and form biofilms, which is an advantage in colonizing the plant rhizosphere. Some of them also showed antifungal activity against the phytopathogenic fungi Fusarium oxysporum C1-1, Ilyonectria macrodidyma C1-1, and Ilyonectria spp. C2-1 isolated from roots of cherry trees affected by replant disease. The multifunctionality of the strains, and the fact that they were isolated from the cherry rhizosphere, makes them good candidates to be used as biocontrol agents in sweet cherry orchards affected by P. penetrans.

Characterization of a Versatile Plant Growth-Promoting Rhizobacterium Pseudomonas mediterranea Strain S58.

Plant growth-promoting rhizobacterial strain S58 was isolated from the tobacco rhizosphere. It showed strong antagonism against a battery of plant pathogenic fungi and bacteria, and controlled wheat sharp eyespot and tobacco wildfire diseases efficiently. Further tests showed that strain S58 solubilized organic phosphate and produced siderophore, protease, ammonia, and indole-3-acetic acid. In Arabidopsis thaliana, it promoted plant growth and changed root system architecture by restricting the growth of primary roots and increasing lateral root numbers. We relied on morphological, biochemical, physiological characteristics, and molecular phylogenic analysis to identify strain S58 as Pseudomonas mediterranea. The complete genome of strain S58 has a single circular chromosome of 6,150,838 bp with a 61.06% G+C content. The bacterial genome contained 5,312 predicted genes with an average length of 992.90 bp. A genome analysis suggested that P. mediterranea S58 was a rich cyclic lipopeptide (CLP)-producing strain that possessed seven non-ribosomal peptide gene clusters for CLP synthesis. Leaf inoculation of the bacterial culture and supernatants triggered cell death-like immunity in tobacco. Quantitative real-time PCR assays showed that the strain S58 induced the expression of pattern-triggered immunity and cell death marker genes, but not jasmonic acid marker genes. The results suggested that P. mediterranea S58 is a novel, versatile plant growth-promoting agent with multiple beneficial traits for plants.

Amelioration of salinity stress and growth stimulation of mustard (Brassica juncea L.) by salt-tolerant Pseudomonas species

Indian mustard (Brassica juncea) is a winter oilseed crop grown across the Northern Indian planes. The growth and yield of the crop have been declining over the recent years due to increase in soil salinity. Applications of plant growth-promoting rhizobacterial strains have been found to enhance crop productivity under salinity conditions. Therefore, the objective of this study was to identify bacterial isolates from the mustard rhizosphere that may help in improving the mustard growth under salt stress conditions. Various plant growth-promoting activities, i.e., production of aminolevulinic acid, indole acetic acid, 1-aminocyclopropane-1-carboxylate deaminase, and solubilization of potassium and phosphorus, were analyzed in isolated salt-tolerant rhizobacterial strains, which may contribute towards salt tolerance and crop productivity in mustard. The most promising rhizobacterial isolates HMM57 and JMM15 were identified as Pseudomonas argentinensis and P. azotoformans, respectively by 16SrRNA sequencing. In primary screening, out of ninety-four rhizobacterial isolates tested, only ten isolates HMM6, HMM13, HMM34, HMM39, HMM44, HMM57, HMM65, HMM88, JMM15 and JMM42 showed salt tolerance up to 8% NaCl concentration (0.1–20.0 mm colony size). During secondary screening on agar plates, these rhizobacterial isolates were checked for their potential to enhance seed germination of mustard and growth of seedlings at the high level of salinity (20 dS m−1). Moreover, bacterial isolates HMM57 and JMM15, which exhibited different plant growth-promoting traits, were used as inoculants on the mustard plants grown under high salinity conditions (8 and 12 dS m−1) in a greenhouse. Rhizobacterial inoculation revealed 139 to 291% increase in root and shoot dry weight even after 80 days of sowing. The results indicated that application of two potential rhizobacterial isolates HMM57, and JMM15, which were able to tolerate high salt concentration (8% NaCl) and having IAA and ALA production along with ACC utilization activities, showed stimulation of the mustard growth even up to 12 dS m−1 in plates and also under controlled greenhouse conditions. These plant growth-promoting rhizobacterial isolates may be used for the enhancement of mustard crop productivity under salinity stress and could be used as biofertilizer.

Genome Sequence of Novoherbaspirillum sp. UKPF54, a Plant Growth-Promoting Rhizobacterial Strain with N2O-Mitigating Abilities, Isolated from Paddy Soil.

Novoherbaspirillum sp. strain UKPF54, a plant growth-promoting rhizobacterium with the ability to mitigate nitrous oxide emission from agriculture soils, has been successfully isolated from paddy soil in Kumamoto, Japan. Here, we report the whole-genome sequence of this strain.