Rhizosphere impacts bacterial community structure in the tea (Camellia sinensis (L.) O. Kuntze.) estates of Darjeeling, India.

India contributes 28% of the world's tea production, and the Darjeeling tea of India is a world-famous tea variety known for its unique quality, flavour and aroma. This study analyzed the spatial distribution of bacterial communities in the tea rhizosphere of six different tea estates at different altitudes. The organic carbon, total nitrogen and available phosphate were higher in the rhizosphere soils than the bulk soils, irrespective of the sites. Alpha and beta diversities were significantly (p < 0.05) higher in the bulk soil than in the rhizosphere. Among the identified phyla, the predominant ones were Proteobacteria, Actinobacteria and Acidobacteria. At the genus level, only four out of 23 predominant genera (>1% relative abundance) could be classified, viz., Candidatus Solibacter (5.36 ± 0.36%), Rhodoplanes (4.87 ± 0.3%), Candidatus Koribacter (2.3 ± 0.67%), Prevotella (1.49 ± 0.26%). The rhizosphere effect was prominent from the significant depletion of more ASVs (n = 39) compared to enrichment (n = 11). The functional genes also exhibit a similar trend with the enrichment of N2 fixation genes, disease suppression and Acetoin synthesis. Our study reports that the rhizobiome of tea is highly selective by reducing the alpha and beta diversity while enriching the significant functional genes.

Evaluation of plant growth promotion properties and induction of antioxidative defense mechanism by tea rhizobacteria of Darjeeling, India.

A total of 120 rhizobacteria were isolated from seven different tea estates of Darjeeling, West Bengal, India. Based on a functional screening of in vitro plant growth-promoting (PGP) activities, thirty potential rhizobacterial isolates were selected for in-planta evaluation of PGP activities in rice and maize crops. All the thirty rhizobacterial isolates were identified using partial 16S rRNA gene sequencing. Out of thirty rhizobacteria, sixteen (53.3%) isolates belong to genus Bacillus, five (16.6%) represent genus Staphylococcus, three (10%) represent genus Ochrobactrum, and one (3.3%) isolate each belongs to genera Pseudomonas, Lysinibacillus, Micrococcus, Leifsonia, Exiguobacterium, and Arthrobacter. Treatment of rice and maize seedlings with these thirty rhizobacterial isolates resulted in growth promotion. Besides, rhizobacterial treatment in rice triggered enzymatic [ascorbate peroxidase (APX), catalase (CAT), chitinase, and phenylalanine ammonia-lyase (PAL)], and non-enzymatic [proline and polyphenolics] antioxidative defense reactions indicating their possible role in the reduction of reactive oxygen species (ROS) burden and thereby priming of plants towards stress mitigation. To understand such a possibility, we tested the effect of rhizobacterial consortia on biotic stress tolerance of rice against necrotrophic fungi, Rhizoctonia solani AG1-IA. Our results indicated that the pretreatment with rhizobacterial consortia increased resistance of the rice plants towards the common foliar pathogen like R. solani AG1-IA. This study supports the idea of the application of plant growth-promoting rhizobacterial consortia in sustainable crop practice through the management of biotic stress under field conditions.

Molecular and culture-based surveys of metabolically active hydrocarbon-degrading archaeal communities in Sundarban mangrove sediments.

Archaea remain important players in global biogeochemical cycles worldwide, including in the highly productive mangrove estuarine ecosystems. In the present study, we have explored the diversity, distribution, and function of the metabolically active fraction of the resident archaeal community of the Sundarban mangrove ecosystem, using both culture-independent and culture-dependent approaches. To evaluate the diversity and distribution pattern of the active archaeal communities, RNA based analysis of the 16S rRNA gene was performed on an Illumina platform. The active Crenarchaeal community was observed to remain constant while active Euryarchaeal community underwent considerable change across the sampling sites depending on varying anthropogenic factors. Haloarchaea were the predominant group in hydrocarbon polluted sediments, leading us to successfully isolate eleven p-hydroxybenzoic acid degrading haloarchaeal species. The isolates could also survive in benzoic acid, naphthalene, and o-phthalate. Quantitative estimation of p-hydroxybenzoic acid degradation was studied on select isolates, and their ability to reduce COD of polluted saline waters of Sundarban was also evaluated. To our knowledge, this is the first ever study combining culture-independent (Next Generation sequencing and metatranscriptome) and culture-dependent analyses for an assessment of archaeal function in the sediment of Sundarban.

Effective rhizoinoculation and biofilm formation by arsenic immobilizing halophilic plant growth promoting bacteria (PGPB) isolated from mangrove rhizosphere: A step towards arsenic rhizoremediation.

Arsenic (As) uptake by plants is largely influenced by the presence of microbial consortia and their interactions with As. In the coastal region of Bengal deltaic plain of Eastern India, the As-contaminated groundwater is frequently used for irrigation purposes resulting in an elevated level of soil As in agricultural lands. The health hazards associated with As necessitates development of cost-effective remediation strategies to reclaim contaminated agricultural lands. Among the available technologies developed in recent times, bioremediation using bacteria has been found to be the most propitious. In this study, two As-resistant halophilic bacterial strains Kocuria flava AB402 and Bacillus vietnamensis AB403 were isolated, identified and characterized from mangrove rhizosphere of Sundarban. The isolates, AB402 and AB403, could tolerate 35mM and 20mM of arsenite, respectively. The effect of As on the exopolysaccharide (EPS) synthesis, biofilm formation, and root association was evaluated for both the bacterial strains. Arsenic adsorption on the cell surfaces and intracellular accumulation in both the bacterial strains were promising under culture conditions. Moreover, both the strains when used as inoculum, not only promoted the growth of rice seedlings but also decreased As uptake and accumulation in plants.

Genome-Guided Insights into the Plant Growth Promotion Capabilities of the Physiologically Versatile Bacillus aryabhattai Strain AB211.

Bacillus aryabhattai AB211 is a plant growth promoting, Gram-positive firmicute, isolated from the rhizosphere of tea (Camellia sinensis), one of the oldest perennial crops and a major non-alcoholic beverage widely consumed all over the world. The whole genome of B. aryabhattai AB211 was sequenced, annotated and evaluated with special focus on genomic elements related to plant microbe interaction. It's genome sequence reveals the presence of a 5,403,026 bp chromosome. A total of 5226 putative protein-coding sequences, 16 rRNA, 120 tRNA, 8 ncRNAs, 58 non-protein coding genes, and 11 prophage regions were identified. Genome sequence comparisons between strain AB211 and other related environmental strains of B. aryabhattai, identified about 3558 genes conserved among all B. aryabhattai genomes analyzed. Most of the common genes involved in plant growth promotion activities were found to be present within core genes of all the genomes used for comparison, illustrating possible common plant growth promoting traits shared among all the strains of B. aryabhattai. Besides the core genes, some genes were exclusively identified in the genome of strain AB211. Functional annotation of the genes predicted in the strain AB211 revealed the presence of genes responsible for mineral phosphate solubilization, siderophores, acetoin, butanediol, exopolysaccharides, flagella biosynthesis, surface attachment/biofilm formation, and indole acetic acid production, most of which were experimentally verified in the present study. Genome analysis and experimental evidence suggested that AB211 has robust central carbohydrate metabolism implying that this bacterium can efficiently utilize the root exudates and other organic materials as an energy source. Genes for the production of peroxidases, catalases, and superoxide dismutases, that confer resistance to oxidative stresses in plants were identified in AB211 genome. Besides these, genes for heat shock tolerance, cold shock tolerance, glycine-betaine production, and antibiotic/heavy metal resistance that enable bacteria to survive biotic/abiotic stress were also identified. Based on the genome sequence information and experimental evidence as presented in this study, strain AB211 appears to be metabolically diverse and exhibits tremendous potential as a plant growth promoting bacterium.