Effect of insecticide-tolerant and plant growth-promoting Mesorhizobium on the performance of chickpea grown in insecticide stressed alluvial soils

Biodegradation of monocrotophos by Brucella intermedia Msd2 isolated from cotton plant

Widespread and inadequate use of Monocrotophos has led to several environmental issues. Biodegradation is an ecofriendly method used for detoxification of toxic monocrotophos. In the present study, Msd2 bacterial strain was isolated from the cotton plant growing in contaminated sites of Sahiwal, Pakistan. Msd2 is capable of utilizing the monocrotophos (MCP) organophosphate pesticide as its sole carbon source for growth. Msd2 was identified as Brucella intermedia on the basis of morphology, biochemical characterization and 16S rRNA sequencing. B. intermedia showed tolerance of MCP up to 100 ppm. The presence of opd candidate gene for pesticide degradation, gives credence to B. intermedia as an effective bacterium to degrade MCP. Screening of the B. intermedia strain Msd2 for plant growth promoting activities revealed its ability to produce ammonia, exopolysaccharides, catalase, amylase and ACC-deaminase, and phosphorus, zinc and potassium solubilization. The optimization of the growth parameters (temperatures, shaking rpm, and pH level) of the MCP-degrading isolate was carried out in minimal salt broth supplemented with MCP. The optimal pH, temperature, and rpm for Msd2 growth were observed as pH 6, 35 °C, and 120 rpm, respectively. Based on optimization results, batch degradation experiment was performed. Biodegradation of MCP by B. intermedia was monitored using HPLC and recorded 78% degradation of MCP at 100 ppm concentration within 7 days of incubation. Degradation of MCP by Msd2 followed the first order reaction kinetics. Plant growth promoting and multi-stress tolerance ability of Msd2 was confirmed by molecular analysis. It is concluded that Brucella intermedia strain Msd2 could be beneficial as potential biological agent for an effective bioremediation for polluted environments.

Isolation of indole-3-acetic acid-producing Azospirillum brasilense from Vietnamese wet rice: co-immobilization of isolate and microalgae as a sustainable biorefinery

Production of indole-3-acetic acid (IAA) is well documented in various studies for the bacteria that inhabit the rhizosphere of plants, but with roots of wet rice, the outstandings have been not yet elucidated. This study began with the isolation of bacteria type strain Azospirillum sp. and developed the investigation to a screening of their ability in IAA production. This screening conducted a selection of only bacteria that was capable of the production of IAA with its content of over 25µg. mL^-1 for sequencing. Of 10 isolates only one resulted from the type strain Azospirillum brasilense (A. brasilense) with a similarity of 100%. Various factors that influence A. brasilense in biosynthesizing IAA such as temperature, pH, nitrogen presence and concentration of tryptophan in the culture medium were examined. The results indicated that the culture conditions were suitable for IAA biosynthesis at pH 6.5, 30°C, culture media with nitrogen, and 0.1% trytophan. The next survey on the role of the immobilization of this bacteria with microalgae in alginate was highlighted to its support in microalgal growth. With the co-immobilization of bacteria and microalgae, the density of Chlorella vulgaris was significantly increased during 15-day culture, inducing 2.2 times of cell content in culture batch microalgae immobilized A. brasilense higher than that free-bacteria.

Ecological performance of multifunctional pesticide tolerant strains of Mesorhizobium sp. in chickpea with recommended pendimethalin, ready-mix of pendimethalin and imazethpyr, carbendazim and chlorpyrifos application

The present study was designed to screen the Mesorhizobium strains (50) for tolerance with four recommended pesticides in chickpea. In-vitro, robust pesticide tolerant strains were developed in pesticides amended media over several generations. Further, verification of the multifunctional traits of pesticide tolerant mesorhizobia under pesticide stress was conducted in-vitro. Among different pesticides, significantly high tolerance in Mesorhizobium strains was observed with recommended doses of pendimethalin (37%) and ready-mix (36%) followed by chlorpyrifos (31%) and carbendazim (30%), on an overall basis. Based on multifunctional traits, Mesorhizobium strains viz. MR2, MR17 and recommended MR33 were the most promising. Ecological performance of the potential Mesorhizobium strains alone and in dual-inoculation with recommended PGP rhizobacterium strain RB-1 (Pseudomonas argenttinensis JX239745.1) was subsequently analyzed in field following standard pesticide application in PBG-7 and GPF-2 chickpea varieties for two consecutive rabi seasons (2015 and 2016). Dual-inoculant treatments; recommended RB-1 + MR33 (4.1%) and RB-1 + MR2 (3.8%) significantly increased the grain yield over Mesorhizobium alone treatments viz MR33 and MR2, respectively. Grain yield in PBG7 variety was significantly affected (7.3%) by the microbial inoculant treatments over GPF2 variety. Therefore, the potential pesticide tolerant strains MR2 and MR33 can be further explored as compatible dual-inoculants with recommended RB-1 for chickpea under environmentally stressed conditions (pesticide application) at multiple locations. Our approach using robust multifunctional pesticide tolerant Mesorhizobium for bio-augmentation of chickpea might be helpful in the formulation of effective bio-inoculants consortia in establishing successful chickpea-Mesorhizobium symbiosis.

Immobilization of Bradyrhizobium and Azospirillum in alginate matrix for long time of storage maintains cell viability and interaction with peanut

Immobilizarion of PGPR for agricultural applications aims to provide temporary physical protection from stressful environmental conditions and the gradual release of cells for successful root colonization, release the cells gradually. In this work, we immobilized Bradyrhizobium sp. SEMIA6144 or Azospirillum brasilense Az39 cells in 2% alginate beads prepared by ionic gelation process, and then stored up to 12 months at 4 °C. Alginate matrix showed interaction with the immobilized bacteria (FTIR), allowed a constant release of cells, and improved their viability and capability to interact with Arachis hypogaea. Cell number into beads reached 10⁷ CFU.bead^-1; however, viability decreased from 4 months of storage for Az39, while it was maintained up to 12 months for SEMIA6144, showing a low metabolic activity measured by the MTT assay. Adhesion of SEMIA6144 and Az39 from new beads to peanut root was 11.5% and 16%, respectively, higher than non-immobilized bacteria. Peanut inoculation with 12 months storage SEMIA6144 beads significantly increased root length and biomass at 30 days of growth, and under restrictive water condition (RWC), nodulation and total plant N content increased compared with liquid inoculation. Our results demonstrate that immobilization of SEMIA6144 and Az39 in alginate matrix is a potential alternative to enhance peanut growth even under RWC. KEY POINTS: • Alginate encapsulation enhances viability of SEMIA6144 or Az39 under storage at 4 °C for 1 year. • Alginate beads 2% ensure the gradual release of the microorganisms. • Cells from beads stored for long periods present chemotaxis and adhesion to peanut root. • Peanut inoculation with 1-year-old SEMIA6144 beads improves nodulation and growth in RWC.

Induction of Systemic Resistance in Chickpea (Cicer arietinum L.) Against Fusarium oxysporum f. sp. ciceris by Antagonistic Rhizobacteria in Assistance with Native Mesorhizobium

In the present study five potent rhizobacterial antagonists of Fusarium oxysporum f. sp. ciceris alone and in combination with Mesorhizobium (M) were evaluated for their potential to elicit the defence response reactions to reduce the total loss of plants and enhance the growth of two chickpea cultivars i.e. resistant GPF-2 and susceptible JG-41. Observations revealed that maximum phenolic, peroxidase (PO) and polyphenol oxidase (PPO) activity was induced after 30th day of germination. Maximum phenol concentration of 745.8 and 724.1 μg/gfw root tissues was recorded by Ps45 when co-inoculated with Mesorhizobium in both the varieties i.e. GPF-2 and JG-41 respectively. Isolates Ps45, Ps47 and Ps44 were found most promising to induce PO and PPO activity, in combination with Mesorhizobium and recorded superior over the fungicide with respect to negative control. Similar results were recorded for the phenylalanine ammonia lyase (PAL), maximally induced on 20th day after germination, where dual inoculation of Ps44+M and Ps45+M induced 57.0 and 54.2 nmol of cinnamic acid min^-1 gfw^-1 in GPF-2. However in case of JG-41, Ps45 and Ba1a exhibited highest PAL activity of 54.2 and 41.4 nmol of cinnamic acid min^-1 gfw^-1. Malonic aldehyde concentration in stem tissues at 30th day revealed that lipid peroxidation was effectively reduced in rhizobacterial treated plants compared to fungicide and negative control, signifying the role of antagonistic plant growth promoting rhizobacteria in reducing the stress and enhancing the plant's defence response to reduce the disease incidence and thus improving the plant growth and yield. Moreover the dual inoculations were observed superior over the fungicide treatment as well as single inoculations in terms of growth (root/shoot length and weight), signifying the synergistic effect of screened antagonists and native Mesorhizobium in suppressing the pathogen and thereby enhancing the plant growth.

Assessment of edibility and effect of arbuscular mycorrhizal fungi on Solanum melongena L. grown under heavy metal(loid) contaminated soil

Arbuscular mycorrhizal fungi (AMF) aids in plant establishment at heavy metal(loid) (HM) contaminated soils, strengthening plant defense system along with promoting growth. A pot experiment was carried out to evaluate the effect of AMF on eggplants grown under HM stress. Further, the potential health risks of HM exposure to the humans via dietary intake of eggplant were also estimated. Results showed that AMF application improved growth, biomass and antioxidative defense response of plants against HM stress. Significant difference (p ≤ 0.001) in parameters under study was found on increasing metal dose and on application of AMF. Among metal(loid)s maximum uptake was recorded for Pb (29.64mgkg^-1 in roots; 23.08mgkg^-1 in shoot) followed by As (3.84mgkg^-1 in roots; 8.20mgkg^-1 in shoot) and, Cd (0.96mgkg^-1 in roots; 2.12mgkg^-1 in shoot). Based on the accumulation of HM in edible part, Hazard Quotient (HQ) was calculated. HQ was found to be > 1 for Pb, which highlights the risks associated with consumption of Eggplants grown on Pb contaminated soil. However this potential, which was further enhanced by application of AMF, can be harnessed for on-site remediation of Pb contaminated soils. The content of Cd and As in the edible part was found to be within safe limits (HQ < 1) when compared to chronic reference dose stated by USEPA.

Lead heavy metal toxicity induced changes on growth and antioxidative enzymes level in water hyacinths [Eichhornia crassipes (Mart.)]

BACKGROUND

Lead (Pb) heavy metal pollution in water bodies is one of the serious problems across the world. This study was designed to find out the effect of Pb toxicity on physiological and biochemical changes in Eichhornia crassipes (water hyacinth) seedlings.

RESULTS

The plant growth was significantly inhibited (50%) at 1000 mg/L Pb concentration. Accumulation of Pb was higher in root than in shoot tissues. The maximum level of Pb accumulation was noticed in roots (5.45%) followed by petiole (2.72%) and leaf tissues (0.66%). Increasing the Pb concentration gradually decreased the chlorophyll content. Intracellular distribution of Pb was also studied using SEM-EDX, where the Pb deposition was observed in both root and leaf tissues. MDA content increased in both the leaf and root tissues up to the 400 mg/L Pb treatment and slightly decreased at higher concentrations. The activity of antioxidative enzymes, such as APX and POX, positively correlated with Pb treatment, while in the case of SOD and CAT enzymes increased up to 800 mg/L treatment and then slightly decreased at higher concentration in both leaf and root tissues.

CONCLUSIONS

These results suggest that water hyacinth plants have efficient mechanism to tolerate Pb toxicity, as evidenced by an increased level of antioxidative enzymes. Results clearly indicate that water hyacinth is a feasible plant for hyperaccumulation of heavy metals from polluted wetlands.

Microcystin-tolerant Rhizobium protects plants and improves nitrogen assimilation in Vicia faba irrigated with microcystin-containing waters

Irrigation of crops with microcystins (MCs)-containing waters-due to cyanobacterial blooms-affects plant productivity and could be a way for these potent toxins entering the food chain. This study was performed to establish whether MC-tolerant rhizobia could benefit growth, nodulation, and nitrogen metabolism of faba bean plants irrigated with MC-containing waters. For that, three different rhizobial strains-with different sensitivity toward MCs-were used: RhOF96 (most MC-sensitive strain), RhOF125 (most MC-tolerant strain), or Vicz1.1 (reference strain). As a control, plants grown without rhizobia and fertilized by NH4NO3 were included in the study. MC exposure decreased roots (30-37 %) and shoots (up to 15 %) dry weights in un-inoculated plants, whereas inoculation with rhizobia protects plants toward the toxic effects of MCs. Nodulation and nitrogen content were significantly impaired by MCs, with the exception of plants inoculated with the most tolerant strain RhOF125. In order to deep into the effect of inoculation on nitrogen metabolism, the nitrogen assimilatory enzymes (glutamine synthetase (GS) and glutamate synthase (GOGAT)) were investigated: Fertilized plants showed decreased levels (15-30 %) of these enzymes, both in shoots and roots. By contrast, inoculated plants retained the levels of these enzymes in shoots and roots, as well as the levels of NADH-GOGAT activity in nodules. We conclude that the microcystin-tolerant Rhizobium protects faba bean plants and improves nitrogen assimilation when grown in the presence of MCs.

Plant growth promoting rhizobia: challenges and opportunities

Modern agriculture faces challenges, such as loss of soil fertility, fluctuating climatic factors and increasing pathogen and pest attacks. Sustainability and environmental safety of agricultural production relies on eco-friendly approaches like biofertilizers, biopesticides and crop residue return. The multiplicity of beneficial effects of microbial inoculants, particularly plant growth promoters (PGP), emphasizes the need for further strengthening the research and their use in modern agriculture. PGP inhabit the rhizosphere for nutrients from plant root exudates. By reaction, they help in (1) increased plant growth through soil nutrient enrichment by nitrogen fixation, phosphate solubilization, siderophore production and phytohormones production (2) increased plant protection by influencing cellulase, protease, lipase and β-1,3 glucanase productions and enhance plant defense by triggering induced systemic resistance through lipopolysaccharides, flagella, homoserine lactones, acetoin and butanediol against pests and pathogens. In addition, the PGP microbes contain useful variation for tolerating abiotic stresses like extremes of temperature, pH, salinity and drought; heavy metal and pesticide pollution. Seeking such tolerant PGP microbes is expected to offer enhanced plant growth and yield even under a combination of stresses. This review summarizes the PGP related research and its benefits, and highlights the benefits of PGP rhizobia belonging to the family Rhizobiaceae, Phyllobacteriaceae and Bradyrhizobiaceae.

Effect of fungicides on plant growth promoting activities of phosphate solubilizing Pseudomonasputida isolated from mustard (Brassica compestris) rhizosphere

This study was navigated to examine the effects of fungicide-stress on the activities of plant-growth-promoting rhizobacterium Pseudomonasputida with inherent phosphate solubilizing activity. The fungicide-tolerant and phosphate solubilizing P.putida strain PS9 was isolated from the mustard rhizosphere and tentatively identified following standard morphological, physiological and biochemical tests. To further consolidate the identity of the strain PS9, the 16S rDNA sequence analysis was performed. Following the BLAST program, the strain PS9 was identified as P.putida. In the presence of the varying concentrations (0-3200 μg mL(-1); at a two fold dilution interval) of four fungicides of different chemical families (tebuconazole, hexaconazole, metalaxyl and kitazin) amended in minimal salt agar medium, the P.putida strain PS9 showed a variable tolerance levels (1400-3200 μg mL(-1)) against the tested fungicides. The strain PS9 produced plant-growth-promoting (PGP) substances in significant amount in the absence of fungicides. In general, fungicides applied at the recommended, two and three times of the recommended rates, decreased the PGP attributes of P.putida the strain PS9 and affected the PGP activities in concentration-dependent manner. Fungicides at the recommended dose had minor reducing effect while the doses higher than the recommended dose significantly reduced the PGP activities (phosphate solubilization, salicylic acid, 2,3-dihydroxy benzoic acid, and indole-3-acetic acid production except exo-polysaccharides, hydrogen cyanate and ammonia production). Of the four fungicides, tebuconazole generally, showed maximum toxicity to the PGP activities of the strain PS9. This study inferred that fungicides must be examined in vitro for their possible adverse effects on soil micro flora before their application in agricultural fields. Moreover, the results also suggested the prerequisite of application of fungicide-tolerant PGPR strains as bioinoculants so that their PGP activities may not be suppressed under fungicide stress.

Insecticide-tolerant and plant-growth-promoting Rhizobium improves the growth of lentil (Lens esculentus) in insecticide-stressed soils

BACKGROUND

Application of insecticides in modern agriculture in order to enhance legume production has led to their accumulation in soils to levels that adversely affect soil microflora such as rhizobia and exert a negative impact on the physiological activities associated with them. This study was therefore designed to identify rhizobial strains expressing higher tolerance to insecticides fipronil and pyriproxyfen and synthesising plant growth regulators even amid insecticide stress.

RESULTS

The fipronil- and pyriproxyfen-tolerant Rhizobium sp. strain MRL3 produced plant-growth-promoting substances in substantial amounts, both in the presence and in the absence of the insecticides. In general, both insecticides at recommended and higher rates reduced plant dry biomass, symbiotic properties, nutrient uptake and seed yield of lentil plants. Interestingly, when applied with any concentration of the two insecticides, Rhizobium sp. strain MRL3 significantly increased the measured parameters compared with plants grown in soils treated solely with the same concentration of each insecticide but without inoculant.

CONCLUSION

This study suggests that Rhizobium strain MRL3 may be exploited as a bioinoculant to augment the efficiency of lentil exposed to insecticide-stressed soils.