https://cyberleninka.org/article/n/69550.pdf

Rhizosphere engineering: Enhancing sustainable plant ecosystem productivity

About 2,500 papers dated 2014-2016 were recovered by searching the PubMed database for Streptomyces, which are the richest known source of antibiotics. This review integrates around 100 of these papers in sections dealing with evolution, ecology, pathogenicity, growth and development, stress responses and secondary metabolism, gene expression, and technical advances. Genomic approaches have greatly accelerated progress. For example, it has been definitively shown that interspecies recombination of conserved genes has occurred during evolution, in addition to exchanges of some of the tens of thousands of non-conserved accessory genes. The closeness of the association of Streptomyces with plants, fungi, and insects has become clear and is reflected in the importance of regulators of cellulose and chitin utilisation in overall Streptomyces biology. Interestingly, endogenous cellulose-like glycans are also proving important in hyphal growth and in the clumping that affects industrial fermentations. Nucleotide secondary messengers, including cyclic di-GMP, have been shown to provide key input into developmental processes such as germination and reproductive growth, while late morphological changes during sporulation involve control by phosphorylation. The discovery that nitric oxide is produced endogenously puts a new face on speculative models in which regulatory Wbl proteins (peculiar to actinobacteria) respond to nitric oxide produced in stressful physiological transitions. Some dramatic insights have come from a new model system for Streptomyces developmental biology, Streptomyces venezuelae, including molecular evidence of very close interplay in each of two pairs of regulatory proteins. An extra dimension has been added to the many complexities of the regulation of secondary metabolism by findings of regulatory crosstalk within and between pathways, and even between species, mediated by end products. Among many outcomes from the application of chromosome immunoprecipitation sequencing (ChIP-seq) analysis and other methods based on "next-generation sequencing" has been the finding that 21% of Streptomyces mRNA species lack leader sequences and conventional ribosome binding sites. Further technical advances now emerging should lead to continued acceleration of knowledge, and more effective exploitation, of these astonishing and critically important organisms.

Recent advances in understanding streptomyces

Actinobacteria are wide spread in nature and represent the largest taxonomic group within the domain Bacteria. They are abundant in soil and have been extensively explored for their therapeutic applications. This versatile group of bacteria has adapted to diverse ecological habitats, which has drawn considerable attention of the scientific community in recent times as it has opened up new possibilities for novel metabolites that may help in solving some of the most challenging problems of the day, for example, novel drugs for drug-resistant human pathogens, affordable means to maintain ecological balance in various habitats, and alternative practices for sustainable agriculture. Traditionally, free dwelling soil actinobacteria have been the subject of intensive research. Of late, symbiotic actinobacteria residing as endophytes within the plant tissues have generated immense interest as potential source of novel compounds, which may find applications in medicine, agriculture, and environment. In the light of these possibilities, this review focuses on the diversity of endophytic actinobacteria isolated from the plants of extreme habitats and specific ecological niches. Furthermore, an attempt has been made to assign chemical class to the compounds obtained from endophytic actinobacteria. Potential therapeutic applications of these compounds and the utility of endophytic actinobacteria in agriculture and environment are discussed.

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Diversity and Applications of Endophytic Actinobacteria of Plants in Special and Other Ecological Niches.

Plant growth promoting rhizobacteria (PGPR) are found to control the plant diseases by adopting various mechanisms. Induced systemic resistance (ISR) is an important defensive strategy manifested by plants against numerous pathogens especially infecting at aerial parts. Rhizobacteria elicit ISR by inducing different pathways in plants through production of various metabolites. In the present study, potential of Bacillus spp. KFP-5, KFP-7, KFP-17 was assessed to induce antioxidant enzymes against Pyricularia oryzae infection in rice. The antagonistic Bacillus spp. significantly induced antioxidant defense enzymes i-e superoxide dismutase (1.7-1.9-fold), peroxidase (3.5-4.1-fold), polyphenol oxidase (3.0-3.8-fold), phenylalanine ammonia-lyase (3.9-4.4-fold), in rice leaves and roots under hydroponic and soil conditions respectively. Furthermore, the antagonistic Bacillus spp significantly colonized the rice plants (2.0E+00-9.1E+08) and secreted multiple biocontrol determinants like protease (1.1-5.5 U/mg of soil or U/mL of hydroponic solution), glucanase, (1.0-1.3 U/mg of soil or U/mL of hydroponic solution), siderophores (6.5-42.8 μg/mL or mg) in the rhizosphere of different rice varieties. The results showed that treatment with Bacillus spp. enhanced the antioxidant defense activities in infected rice, thus alleviating P. oryzae induced oxidative damage and suppressing blast disease incidence.

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Bacillus spp., a bio-control agent enhances the activity of antioxidant defense enzymes in rice against Pyricularia oryzae

Endophytic fungi are abundant and have been reported from all tissues such as roots, stems, leaves, flowers, and fruits. In recent years, research into the beneficial use of endophytic fungi has increased worldwide. In this chapter, we critically review the production of a wide range of secondary metabolites, bioactive compounds from fungal endophytes that are a potential alternative source of secondary plant metabolites and natural producers of high-demand drugs. One of the major areas in endophytic research that holds both economic and environmental potential is bioremediation. During their life span, microbes adapt fast to environmental pollutants and remediate their surrounding microenvironment. In the last two decades, bioremediation has arisen as a suitable alternative for remediating large polluted sites. Endophytic fungi producing ligninolytic enzymes have possible biotechnological applications in lignocellulosic biorefineries. This chapter highlights the recent progress that has been made in screening endophytic fungi for the production and commercialization of certain biologically active compounds of fungal endophytic origin.

Endophytic Fungi: Biodiversity, Ecological Significance, and Potential Industrial Applications

Aims
To evaluate the potential of chitinolytic endophytic Actinomycetes isolated from medicinal plants in order to diminish the collar rot infestation induced by Sclerotium rolfsii in chickpea.

Methods and Results
Sixty-eight chitinolytic endophytic Actinomycetes were recovered from various medicinal plants and evaluated for their chitinase activity. Among these isolates, 12 were screened for their plant growth promoting abilities and antagonistic potential against Sc. rolfsii. Further, these isolates were validated in vivo for their ability to protect chickpea against Sc. rolfsii infestation under greenhouse conditions. The isolates significantly (P < 0·05) increased the biomass (1·2–2·0 fold) and reduced plant mortality (42–75%) of chickpea. On the basis of 16S rDNA profiling, the selected antagonistic strains were identified as Streptomyces diastaticus, Streptomyces fradiae, Streptomyces olivochromogenes, Streptomyces collinus, Streptomyces ossamyceticus and Streptomyces griseus.

Conclusion
This study is the first report of the isolation of endophytic Actinomycetes from various medicinal plants having antagonistic and plant growth promoting abilities. The isolated species showed potential for controlling collar rot disease on chickpea and could be useful in integrated control against diverse soil borne plant pathogens.

Significance and Impact of the Study
Our investigation suggests that endophytic Actinomycetes associated with medicinal plants can be used as bioinoculants for developing safe, efficacious and environment-friendly biocontrol strategies in the near future.

Evaluation of antagonistic and plant growth promoting activities of chitinolytic endophytic actinomycetes associated with medicinal plants against Sclerotium rolfsii in chickpea

Endophytic Streptomyces spp. underscore induction of defense regulatory genes and confers resistance against Sclerotium rolfsii in chickpea

The ability of Streptomyces species to act as biocontrol agents for plant pathogens via induced systemic resistance has been demonstrated and considerable efforts have been made in elucidating the underlying mechanisms of Streptomyces–host plant–phytopathogen interactions. Here, we have assessed the ability of Streptomyces coelicolor, Streptomyces griseus, Streptomyces albus, Streptomyces antibioticus and Streptomyces champavatii to provide disease protection against Rhizoctonia solani in Solanum lycopersicon and have also examined associated changes in hydrogen peroxide (H2O2) production, lipid peroxidation (LPO) and antioxidant enzymes. The production of H2O2 at the second day after pathogen inoculation (dapi) was observed to be 1.1-fold higher in Streptomyces-treated plants, when compared to untreated inoculated control plants. A similar increase in catalase and ascorbate peroxidase activity was observed at fourth dapi whereas increased activities of guaiacol reductase and glutathione peroxidase were observed at fifth dapi. Likewise, LPO reached a maximum at sixth dapi in untreated inoculated plants while in Streptomyces-treated plants it was observed to be 1.3–1.5-fold less when compared to untreated inoculated control plants. This study offers novel insights into the mechanisms of priming by Streptomyces and highlights their capacity to activate plant defence responses generated by biotic stress through induction of antioxidant enzymes along with improved reactive oxygen species management.

Streptomyces spp. alleviate Rhizoctonia solani-mediated oxidative stress in Solanum lycopersicon

Actinomycetes are a major group of beneficial microbes, which can be explored as spanking alternative to chemical fungicides for providing defense against phytopathogens. Rhizoctonia solani is a major havoc causing severe loss to many crops. Biological measures for fungal disease management are desired over the available chemical/synthetic fungicides owing to their safety towards non-target organisms. In the present study, 34 actinomycetes were isolated from vermicompost. Out of them, twelve revealed antifungal activity related to Indole acetic acid (IAA) production, siderophores and plant growth promotion. Under greenhouse and field conditions, these potent strains remarkably enhanced yield attributes and disease diminution as compared to untreated control. A significant disease reduction of 47–63 % against R. solani was observed in tomato plants pretreated with actinomycetes. Furthermore, induction in defense related enzymes such as peroxidase, polyphenol oxidase, phenylalanine ammonia lyase, accumulation of phenolics and flavonoids were also observed in actinomycetes treated plants. Morphological and molecular characterization analysis identified these potent isolates as Streptomyces sp. NBM3, Streptomyces sp. NBM2, Streptomyces sp. NBM1, Streptomyces sp. NBM12 and Streptomyces sp. NBM8. The present findings suggest that these microbes can be utilized for significant enhancement of plant growth and augmentation of defense related enzymes in order to cope up with R. solani induced stress, thereby contributing to crop health.

Satyendra P. Singh

Papers

Evaluation of antagonistic and plant growth promoting activities of chitinolytic endophytic actinomycetes associated with medicinal plants against Sclerotium rolfsii in chickpea

Endophytic Streptomyces spp. underscore induction of defense regulatory genes and confers resistance against Sclerotium rolfsii in chickpea

Streptomyces spp. alleviate Rhizoctonia solani-mediated oxidative stress in Solanum lycopersicon

Antagonistic Actinomycetes Mediated Resistance in Solanum lycopersicon Mill. Against Rhizoctonia solani Kühn