Engineered Nanomaterials in Soil: Their Impact on Soil Microbiome and Plant Health

A staggering number of nanomaterials-based products are being engineered and produced commercially. Many of these engineered nanomaterials (ENMs) are finally disposed into the soil through various routes in enormous quantities. Nanomaterials are also being specially tailored for their use in agriculture as nano-fertilizers, nano-pesticides, and nano-based biosensors, which is leading to their accumulation in the soil. The presence of ENMs considerably affects the soil microbiome, including the abundance and diversity of microbes. In addition, they also influence crucial microbial processes, such as nitrogen fixation, mineralization, and plant growth promoting activities. ENMs conduct in soil is typically dependent on various properties of ENMs and soil. Among nanoparticles, silver and zinc oxide have been extensively prepared and studied owing to their excellent industrial properties and well-known antimicrobial activities. Therefore, at this stage, it is imperative to understand how these ENMs influence the soil microbiome and related processes. These investigations will provide necessary information to regulate the applications of ENMs for sustainable agriculture and may help in increasing agrarian production. Therefore, this review discusses several such issues.

PLANT GROWTH PROMOTING ACTIVITIES OF RHIZOBACTERIA ISOLATED FROM RHIZOSPHERIC SOILS OF RURAL BANGALORE, INDIA

Soil is a dynamic ecosystem which provides support to plant life. Microorganisms inhabiting the rhizosphere region of soil play a key role in agriculture by promoting the exchange of plant nutrients and reducing the application of chemical fertilizers to a large extent. Engineering of rhizospheric region through exploitation of specific microorganisms leads to higher microbial diversity in the soil which in turn plays a significant role in maintaining the soil health. The present work envisages the isolation, screening and biochemical profiling of potent plant growth promoting rhizobacteria from various rhizospheric soils in and around Bangalore. Sixty isolates from rhizospheric region of fourteen different agricultural soils were screened for plant growth promoting traits such as phosphate solubilization, siderophore production, Ammonia, HCN & Phytohormone production. Twelve isolates that exhibited plant growth promotional traits were further subjected to screening for drought and salt tolerance. Among the twelve isolates, four potential isolates namely Serratia marcescens, Pseudomonas aeruginosa and Acinetobacter pittii were identified based on biochemical methods and 16SrRNA sequencing.

Diversity, biocontrol, and growth promotion potential of culturable endophytic fungi isolated from root, shoot, and leaf of wild Phoebe bournei (Hemsl.) Yang across the seasons

Endophytic fungi were isolated from Phoebe bournei and their diversity and antimicrobial and plant growth-promoting activities were investigated. Of the 389 isolated endophytic fungi, 88.90% belonged to phylum Ascomycota and 11.10% to phylum Basidiomycota. The isolates were grouped into four taxonomic classes, 11 orders, 30 genera, and 45 species based on internal transcribed spacer sequencing and morphologic analysis. The host showed a strong affinity for the genera Diaporthe and Phyllosticta. The diversity of the fungi was highest in autumn, followed by spring and summer, and was lowest in winter. The fungi exhibited notable tissue specificity in P. bournei, and the species richness and diversity were highest in the root across all seasons. Five isolates showed antimicrobial activity against eight plant pathogens, and reduced the incidence of leaf spot disease in P. bournei. Additionally, 9 biocontrol isolates showed plant growth-promoting activity, with five significantly promoting P. bournei seedling growth. This is the first report on the endophytic fungi of P. bournei and their potential applicability to plant disease control and growth promotion.

Diversity and plant growth-promoting potential of actinomycetes associated with the rhizosphere of Arnebia euchroma from Himachal Pradesh (India)

Aim: The present study aimed to explicate the diversity and plant growth promoting (PGP) potential of actinomycetes present in the rhizosphere of an endangered medicinal plant, Arnebia euchroma. Methodology: Rhizospheric soil samples of A. euchroma collected from trans-Himalayan region of Himachal Pradesh were processed for elucidating actinomycetes diversity and load by employing Standard Plate Count Technique. All recovered isolates were screened for their PGP potential using standard protocols. The potential strains were identified through 16SrRNA ribotyping and were deposited in the National Culture Collection Centre, NCMR, Pune, (India). Results: The rhizosphere of A. euchroma harboured a great deal of actinomycetes diversity (33 diverse morphotypes). Starch casein agar was best for isolating the actinomycetes. The same actinomycete isolate showed variations in their morphological features including pigments production on different isolation media. They exhibited multifarious plant growth-promoting activities like P- solubilization, phytase activity, N- fixation, siderophore production etc. The potential isolates were identified as Streptomyces silaceus CA7 (MK836019) and Streptomyces rectiviolaceus NA8 (MK836018). Interpretation: The actinomycetes evinced a considerable plant growth promoting potential that might be helpful in the adaptation and perpetuation of A. euchroma under prevailing harsh environmental conditions of Himachal Pradesh.

Screening and Potential Uses of Contaminated Spent Mushroom (Pleurotus spp.)

The commercial production of mushrooms generates a co-product, a virtually inexhaustible supply of spent mushroom substrate (SMS). It represents an ideal growth medium for plants and plant disease suppressive quality. Here we discussed about the contaminated microbial flora of SMS, potential antifungal and plant growth promoting activities, the results of these findings were also discussed in relation to the usage of SMS as a potential product for organic farming. SMS contained moisture content 72%, EC 1.75 mmho.cm−1 and had pH of 6.1. The cellulose and hemicellulose content of paddy straw substrate were 30.25%, 23.18% and 15.31% dry weight respectively. Growth in terms of root and shoot weight of the seedlings of green gram, black gram, tomato and chili were significantly higher when grown in 60% SMS amended soil. Spent mushroom compost from Pleurotus eous used in this study harbored bacterial population including, Bacillus sp., Clostridium sp., Pseudomonas sp. and E. coli. Bacterial isolate B1 was identified as Bacillus sp., isolate B2 was identified as Clostridium sp., isolate B3 as Pseudomonas sp. and B4 as Escherichia coli. These bacterial strains showed significant antagonistic activity against soil borne pathogenic fungi viz., Fusarium sp., Alternaria sp., Phytophthora sp. and Aspergillus sp.

Bioprospecting of microbial isolates from biodynamic preparations for PGPR and biocontrol properties

Aim: To study the role of microorganisms behind their bioenhancing, biocontrol properties, their enzymatic potential, and characterization of high performing microbial isolates on molecular basis. Methodology: Dominant culturable microbes including bacteria and fungi were isolated from biodynamic preparations and screened on the basis of plant-growth promoting (PGP) activities viz., ammonia production, phosphate solubilization, siderophore production, hydrogen cyanide production, seed germination efficacy and biocontrol properties. The dominant and effective microorganisms were screened for enzymes activities viz., pectinase, cellulase and amylase. The selected bacterial and fungal isolates, exhibiting higher enzyme activities, were subjected to molecular characterization. Results: Out of 68 bacterial and 25 fungal isolates from 8 biodynamic preparations (BD 500 - BD 507), 15 bacterial isolates exhibited high plant growth promoting activities while 10 bacterial isolates exhibited biocontrol activity against pathogens. Bacillus licheniformis isolated from BD 504, expressed high pectinase (2.595 U ml-1 min-1), cellulase (0.308 U ml-1 min-1) and amylase (0.418 U ml-1 min-1) activities. Fungal isolates with high enzymatic activities, were isolated from BD 500, 503 and 506, respectively. Interpretation: Microorganism isolates from biodynamic preparations possessed strong plant growth promoting, biocontrol and enzymatic properties, which might be responsible for the efficacy of organic preparations under field conditions.