Toxicological Effects of Selective Herbicides on Plant Growth Promoting Activities of Phosphate Solubilizing Klebsiella sp. Strain PS19

Characterization of Growth-Promoting Activities of Consortia of Chlorpyrifos Mineralizing Endophytic Bacteria Naturally Harboring in Rice Plants-A Potential Bio-Stimulant to Develop a Safe and Sustainable Agriculture

Eighteen pesticide-degrading endophytic bacteria were isolated from the roots, stems, and leaves of healthy rice plants and identified through 16S rRNA gene sequencing. Furthermore, biochemical properties, including enzyme production, dye degradation, anti-bacterial activities, plant-growth-promoting traits, including N-fixation, P-solubilization, auxin production, and ACC-deaminase activities of these naturally occurring endophytic bacteria along with their four consortia, were characterized. Enterobacter cloacae HSTU-ABk39 and Enterobacter sp. HSTU-ABk36 displayed inhibition zones of 41.5 ± 1.5 mm, and 29 ± 09 mm against multidrug-resistant human pathogenic bacteria Staphylococcus aureus and Staphylococcus epidermidis, respectively. FT-IR analysis revealed that all eighteen isolates were able to degrade chlorpyrifos pesticide. Our study confirms that pesticide-degrading endophytic bacteria from rice plants play a key role in enhancing plant growth. Notably, rice plants grown in pots containing reduced urea (30%) mixed with either endophytic bacterial consortium-1, consortium-2, consortium-3, or consortia-4 demonstrated an increase of 17.3%, 38.6%, 18.2%, and 39.1% yields, respectively, compared to the control plants grown in pots containing 100% fertilizer. GC-MS/MS analysis confirmed that consortia treatment caused the degradation of chlorpyrifos into different non-toxic metabolites, including 2-Hydroxy-3,5,6 trichloropyridine, Diethyl methane phosphonate, Phorate sulfoxide, and Carbonochloridic. Thus, these isolates could be deployed as bio-stimulants to improve crop production by creating a sustainable biological system.

Kinetic and mechanism investigation on the gamma irradiation induced degradation of quizalofop-p-ethyl

An efficient gamma radiolytic decomposition of one of the extensively used herbicides in the world quizalofo-p-ethyl (QPE) was explored under different experimental conditions. Aqueous solutions of QPE were irradiated by gamma rays emitted by a Cobalt 60 source. QPE aqueous solutions were irradiated at doses of 0.5-3 kGy with 26.31 Gy min^-1 dose rate. Obtained results indicated that removal efficiency of 98.5% and 73% of QPE were obtained, respectively, in absence and in presence of dissolved oxygen. Change of absorption spectra, pH effect and Total Organic Carbon (TOC) were carried out and studied. It was found that all absorption bands decreased with increasing irradiation dose and disappear totally after 3 kGy applied dose. Three pH conditions (pH = 10, pH = 6.2 and pH = 3) were applied in radiolytic degradation of QPE showing that the best removal efficiency has been found for neutral pH. Interestingly, the % TOC removal reaches 98% at 3 kGy indicated practically total mineralization. Furthermore, spectrophotometric analyses argued in favour of a pseudo-first-order kinetic of QPE degradation. The resulting apparent rate constant value is approximately k_app = (0.012 ± 0.001) min^-1. Finally, several by-products such as 6-chloroquinoxalin -2-ol, 2-(4-hydroxy-phenyoxy) propionate, 1,4-hydroquinone, quinone, 4-chlorobenzene-1,2diol and 1,2,4-benzenetriol were identified by gas chromatography-mass spectrometry (GC/MS) evidencing that radiation process starting with the fragmentation of the molecule involving the hydroxyl radical, which is generated by the radiolysis of water. Based on the identification intermediates, a degradation mechanistic schema of QPE has been proposed.

Effects of Phosphonate Herbicides on the Secretions of Plant-Beneficial Compounds by Two Plant Growth-Promoting Soil Bacteria: A Metabolomics Investigation

Plant growth-promoting rhizobacteria (PGPR) that colonize plant roots produce a variety of plant-beneficial compounds, including plant-growth regulators, metal-scavenging compounds, and antibiotics against plant pathogens. Adverse effects of phosphonate herbicides, the most extensively used herbicides, on the growth and metabolism of PGPR species have been widely reported. However, the potential consequence of these effects on the biosynthesis and secretion of PGPR-derived beneficial compounds still remains to be investigated. Here, using high-resolution mass spectrometry and a metabolomics approach, we investigated both the intracellular metabolome and the extracellular secretions of biomass-normalized metabolite levels in two PGPR species (Pseudomonas protegens Pf-5, a Gram-negative bacterium; Priestia megaterium QM B1551, a Gram-positive bacterium) exposed to three common phosphonate herbicides (glyphosate, glufosinate, and fosamine; 0.1-1 mM) in either iron (Fe)-replete or Fe-deficient nutrient media. We quantified secreted auxin-type plant hormone compounds (phenylacetic acid and indole-3-acetic acid), iron-scavenging compounds or siderophores (pyoverdine and schizokinen), and antibiotics (2,4-diacetylphloroglucinol and pyoluteorin) produced by these PGPR species. The Fe-replete cells exposed to the phosphonate herbicides yielded up to a 25-fold increase in the production of both auxin and antibiotic compounds, indicating that herbicide exposure under Fe-replete conditions triggered metabolite secretions. However, the herbicide-exposed Fe-deficient cells exhibited a near 2-fold depletion in the secretion of these auxin and antibiotic compounds as well as a 77% decrease in siderophore production. Intracellular metabolomics analysis of the Fe-deficient cells further revealed metabolic perturbations in biosynthetic pathways consistent with the impaired production of the plant-beneficial compounds. Our findings implied that compromised cellular metabolism during nutrient deficiency may exacerbate the adverse effects of phosphonate herbicides on PGPR species.

A comprehensive synthesis unveils the mysteries of phosphate-solubilizing microbes

Phosphate-solubilizing microbes (PSMs) drive the biogeochemical cycling of phosphorus (P) and hold promise for sustainable agriculture. However, their global distribution, overall diversity and application potential remain unknown. Here, we present the first synthesis of their biogeography, diversity and utility, employing data from 399 papers published between 1981 and 2017, the results of a nationwide field survey in China consisting of 367 soil samples, and a genetic analysis of 12986 genome-sequenced prokaryotic strains. We show that at continental to global scales, the population density of PSMs in environmental samples is correlated with total P rather than pH. Remarkably, positive relationships exist between the population density of soil PSMs and available P, nitrate-nitrogen and dissolved organic carbon in soil, reflecting functional couplings between PSMs and microbes driving biogeochemical cycles of nitrogen and carbon. More than 2704 strains affiliated with at least nine archaeal, 88 fungal and 336 bacterial species were reported as PSMs. Only 2.59% of these strains have been tested for their efficiencies in improving crop growth or yield under field conditions, providing evidence that PSMs are more likely to exert positive effects on wheat growing in alkaline P-deficient soils. Our systematic genetic analysis reveals five promising PSM genera deserving much more attention.

Influence of imazethapyr and quizalofop-p-ethyl application on microbial biomass and enzymatic activity in peanut grown soil

A field experiment was conducted to examine the degradation and impact of two post-emergence herbicides (imazethapyr and quizalofop-p-ethyl) on soil ecosystems at a half recommended rate (HRE), recommended rate (RE), and double recommended rate (DRE) during kharif peanut cultivation. Herbicides were innocuous to soil microbial activity at HRE, however, showed some significant influences at RE and DRE, and exerted temporary toxic effects on microbial biomass carbon and fluorescein diacetate hydrolyzing activity. Dehydrogenase activity also declined for a shorter period except imazethapyr application at DRE. Acid phosphatase activity was inhibited whereas alkaline phosphatase activity fluctuated between promotion and inhibition, but promotion was predominant suggesting a direct role of alkaline soil environment. Soil NH₄⁺ and NO₃^- nitrogen were increased by the herbicides at initial (after 7 days) and last phases (after 30 days), respectively. After an early period of inhibition, urease activity returned to the control level after 30 days. Dissipation of imazethapyr residues fitted best to bi-exponential order rate kinetics at DRE and RE, whereas it followed first-order rate kinetics at HRE. The residues of quizalofop-p-ethyl were found only up to 1 day after application suggesting its rapid conversion to active acid metabolites. Both the herbicides had transient harmful effects on most of the soil microbiological parameters.

Derepression of Mineral Phosphate Solubilization Phenotype by Insertional Inactivation of iclR in Klebsiella pneumoniae

The mode of succinate mediated repression of mineral phosphate solubilization and the role of repressor in suppressing phosphate solubilization phenotype of two free-living nitrogen fixing Klebsiella pneumoniae strains was studied. Organic acid mediated mineral phosphate solubilization phenotype of oxalic acid producing Klebsiella pneumoniae SM6 and SM11 were transcriptionally repressed by IclR in presence of succinate as carbon source. Oxalic acid production and expression of genes of the glyoxylate shunt (aceBAK) was found only in glucose but not in succinate- and glucose+succinate-grown cells. IclR, repressor of aceBAK operon, was inactivated using an allelic exchange system resulting in derepressed mineral phosphate solubilization phenotype through constitutive expression of the glyoxylate shunt. Insertional inactivation of iclR resulted in increased activity of the glyoxylate shunt enzymes even in succinate-grown cells. An augmented phosphate solubilization up to 54 and 59% soluble phosphate release was attained in glucose+succinate-grown SM6Δ and SM11Δ strains respectively, compared to glucose-grown cells, whereas phosphate solubilization was absent or negligible in wildtype cells grown in glucose+succinate. Both wildtype and iclR deletion strains showed similar indole-3-acetic acid production. Wheat seeds inoculated with wildtype SM6 and SM11 improved both root and shoot length by 1.2 fold. However, iclR deletion SM6Δ and SM11Δ strains increased root and shoot length by 1.5 and 1.4 folds, respectively, compared to uninoculated controls. The repressor inactivated phosphate solubilizers better served the purpose of constitutive phosphate solubilization in pot experiments, where presence of other carbon sources (e.g., succinate) might repress mineral phosphate solubilization phenotype of wildtype strains.

Non-symbiotic N2-fixation and phosphate-solubility in Gangetic alluvial soil as influenced by pre-emergence herbicide residues

An experiment has been conducted under laboratory conditions to investigate the effect of two pre-emergence herbicides viz., thiobencarb (at 1.5 and 4.5 kg a.i. ha(-1)) and pretilachlor (at 0.5 and 1.5 kg a.i. ha(-1)), on the changes of growth and activities of aerobic non-symbiotic N2-fixing bacteria and phosphate-solubilizing microorganisms in relation to availability of mineral nitrogen and soluble phosphorus in the Gangetic alluvial soil (Typic Haplustept) of West Bengal, India. Application of herbicides, in general, significantly increased growth and activities of microorganisms, resulting in greater release of available nitrogen and soluble phosphorus in soil; and the stimulation was more pronounced when the herbicides were applied at their lower concentrations (recommended field application rates), more so with thiobencarb, as compared to pretilachlor. As compared to untreated control, application of thiobencarb at lower concentration increased the proliferation of aerobic non-symbiotic N2-fixing bacteria, phosphate-solubilizing microorganisms and non-symbiotic N2-fixing capacity of soil to the extent of 54.0, 44.6 and 31.7%, respectively; and accumulated the highest amount of available nitrogen (37.8%) and phosphorus (54.5%) in soil, while pretilachlor at field application rate highly induced (37.2%) phosphate-solubilizing capacity of soil. At higher concentration, pretilachlor was superior to thiobencarb in augmenting the growth and activities of phosphate-solubilizers. The results of the present study also indicated that gradual increase in concentration of the herbicides over their recommended field application rates was not much conducive for growth and activities of microorganisms, and subsequent release of nutrients in soil.

Phosphate-solubilizing bacteria-assisted phytoremediation of metalliferous soils: a review

Heavy metal pollution of soils is of great concern. The presence of the toxic metal species above critical concentration not only harmfully affects human health but also the environment. Among existing strategies to remediate metal contaminates in soils, phytoremediation approach using metal accumulating plants is much convincing in terms of metal removal efficiency, but it has many limitations because of slow plant growth and decreased biomass owing to metal-induced stress. In addition, constrain of metal bioavailability in soils is the prime factor to restrict its applicability. Phytoremediation of metals in association with phosphate-solubilizing bacteria (PSB) considerably overcomes the practical drawbacks imposed by metal stress on plants. This review is an effort to describe mechanism of PSB in supporting and intensifying phytoremediation of heavy metals in soils and to address the developmental status of the current trend in application of PSB in this context.

Toxic effects of four sulphonylureas herbicides on soil microbial biomass

The effect of four triazinyl-sulfonylurea herbicides (cinosulfuron, prosulfuron, thifensulfuron methyl, triasulfuron) on soil microbial biomass, soil respiration, metabolic activity, metabolic quotient, and some enzymatic activities (acid and alkaline phosphatase, β-glucosidase, arylsulphatase, and fluorescein diacetate hydrolysis) were monitored under controlled conditions over 30 days. The herbicides were applied at the normal field dose (FD) and at ten-fold (10 FD) the field dose, in order to mimic a long term toxic effect. The measured soil microbial parameters showed that the FD had slight effects on soil microflora, while at 10 FD the tested herbicides exerted a stronger detrimental effect on soil microbial biomass and its biochemical activities.