Efficacy of antifungal substances of three Streptomyces spp. against different plant pathogenic fungi

Mechanistic insights into the role of actinobacteria as potential biocontrol candidates against fungal phytopathogens

Worldwide mounting demand for better food production to nurture exasperating population emphasizes on reduced crop losses. The incidence of pathogens into the agricultural fields has tend to dwindle plethora of cereal, vegetable, and other fodder crops. This, in turn, has seriously impacted the economic losses on global scale. Apart from this, it is quite challenging to feed the posterity in the coming decades. To counteract this problem, various agrochemicals have been commercialized in the market that no doubt shows positive results but along with adversely affecting the ecosystem. Therefore, the excessive ill-fated use of agrochemicals to combat the plant pests and diseases highlights that alternatives to chemical pesticides are need of the hour. In recent days, management of plant diseases using plant-beneficial microbes is gaining interest as safer and potent alternatives to replace chemically based pesticides. Among these beneficial microbes, actinobacteria especially streptomycetes play considerable role in combating plant diseases along with promoting the plant growth and development along with their productivity and yield. The mechanisms exhibited by actinobacteria include antibiosis (antimicrobial compounds and hydrolytic enzymes), mycoparasitism, nutrient competition, and induction of resistance in plants. Thus, in cognizance with potential of actinobacteria as potent biocontrol agents, this review summarizes role of actinobacteria and the multifarious mechanisms exhibited by actinobacteria for commercial applications.

The Good, the Bad, and the Useable Microbes within the Common Alder (Alnus glutinosa) Microbiome-Potential Bio-Agents to Combat Alder Dieback

Common Alder (Alnus glutinosa (L.) Gaertn.) is a tree species native to Ireland and Europe with high economic and ecological importance. The presence of Alder has many benefits including the ability to adapt to multiple climate types, as well as aiding in ecosystem restoration due to its colonization capabilities within disturbed soils. However, Alder is susceptible to infection of the root rot pathogen Phytophthora alni, amongst other pathogens associated with this tree species. P. alni has become an issue within the forestry sector as it continues to spread across Europe, infecting Alder plantations, thus affecting their growth and survival and altering ecosystem dynamics. Beneficial microbiota and biocontrol agents play a crucial role in maintaining the health and resilience of plants. Studies have shown that beneficial microbes promote plant growth as well as aid in the protection against pathogens and abiotic stress. Understanding the interactions between A. glutinosa and its microbiota, both beneficial and pathogenic, is essential for developing integrated management strategies to mitigate the impact of P. alni and maintain the health of Alder trees. This review is focused on collating the relevant literature associated with Alder, current threats to the species, what is known about its microbial composition, and Common Alder-microbe interactions that have been observed worldwide to date. It also summarizes the beneficial fungi, bacteria, and biocontrol agents, underpinning genetic mechanisms and secondary metabolites identified within the forestry sector in relation to the Alder tree species. In addition, biocontrol mechanisms and microbiome-assisted breeding as well as gaps within research that require further attention are discussed.

Untargeted MS-Based Metabolomic Analysis of Termite Gut-Associated Streptomycetes with Antifungal Activity against Pyrrhoderma noxium

Pyrrhoderma noxium is a plant fungal pathogen that induces the disease of brown root rot in a large variety of tree species. It is currently infecting many of the amenity trees within Brisbane City of Queensland, Australia. Steering away from harmful chemical fungicides, biological control agents offer environmentally friendly alternatives. Streptomycetes are known for their production of novel bioactive secondary metabolites with biocontrol potential, particularly, streptomycete symbionts isolated from unique ecological niches. In this study, 37 termite gut-associated actinomycete isolates were identified using molecular methods and screened against P. noxium. A majority of the isolates belonged to the genus Streptomyces, and 15 isolates exhibited strong antifungal activity with up to 98.5% mycelial inhibition of the fungal pathogen. MS/MS molecular networking analysis of the isolates' fermentation extracts revealed several chemical classes with polyketides being among the most abundant. Most of the metabolites, however, did not have matches to the GNPS database, indicating potential novel antifungal compounds in the active extracts obtained from the isolates. Pathway enrichment and overrepresentation analyses revealed pathways relating to polyketide antibiotic production, among other antibiotic pathways, further confirming the biosynthetic potential of the termite gut-associated streptomycetes with biocontrol potential against P. noxium.

Formulation-based antagonistic endophyte Amycolatopsis sp. SND-1 triggers defense response in Vigna radiata (L.) R. Wilczek. (Mung bean) against Cercospora leaf spot disease

In the present work, Amycolatopsis sp. SND-1 (SND-1) was isolated from Cleome chellidonii Linn. (C. chellidonii) was performed as biocontrol and resistance elicitor in Vigna radiata (L.) R. Wilczek (mung bean) plants against Cercospora leaf spot causing pathogen Cercospora canescens (C. canescens). The SND-1 isolate showed 74% of inhibition against C. canescens in dual culture and GC-MS analysis revealed the presence of antifungal compounds. Molecular characterization through 16S rRNA showed that the isolated SND-1 belongs to Amycolatopsis sp. The in vitro plant growth trials exhibited production of indole acetic acid, gibberellic acid, cytokinin, ammonia, hydrogen cyanide, and siderophore and phosphate solubilization. In vivo study with talcum formulation of SND-1 revealed a significant increase in plant root length, shoots length, root and shoot fresh weight, and reduced the disease severity in treated mung bean plants. Triggering of resistance by SND-1 formulation was studied by histochemical depositions and biochemical defense enzymes that resulted in the acceleration in defense response in comparison with control plants. The bioactive endophytic Amycolatopsis sp. SND-1 enhanced the defense against C. canescens infection; hence, it can be used as a biological control agent in mung bean cultivars.

A Coating Based on Bioactive Compounds from Streptomyces spp. and Chitosan Oligomers to Control Botrytis cinerea Preserves the Quality and Improves the Shelf Life of Table Grapes

Botrytis cinerea is the most harmful postharvest disease of table grapes. Among the strategies that can be envisaged for its control, the use of coatings based on natural products is particularly promising. The study presented herein focuses on the assessment of the antagonistic capacity of two Streptomyces species and their culture filtrates against B. cinerea. Firstly, the secondary metabolites were characterized by gas chromatography-mass spectrometry, with N1-(4-hydroxybutyl)-N3-methylguanidine acetate and 2R,3S-9-[1,3,4-trihydroxy-2-butoxymethyl]guanine acetate as the main compounds produced by S. lavendofoliae DSM 40217; and cyclo(leucyloprolyl) and cyclo(phenylalanylprolyl) as the most abundant chemical species for S. rochei DSM 41729. Subsequently, the capacity of S. lavendofoliae DSM 40217 and S. rochei DSM 41729 to inhibit the growth of the pathogen was tested in dual culture plate assays, finding 85-90% inhibition. In agar dilution tests, their culture filtrates resulted in effective concentration values (EC₉₀) in the 246-3013 μg·mL^-1 range. Upon the formation of conjugate complexes with chitosan oligomers (COS) to improve solubility and bioavailability, a synergistic behavior was observed, resulting in lower EC₉₀ values, ranging from 201 to 953 μg·mL^-1. Ex situ tests carried out on 'Timpson' and 'Red Globe' table grapes using the conjugate complexes as coatings were found to maintain the turgor of the grapes and delay the appearance of the pathogen by 10-15 days at concentrations in the 750-1000 µg·mL^-1 range. Hence, the conjugate complexes of COS and the selected Streptomyces spp. culture filtrates may be put forward as promising protection treatments for the sustainable control of gray mold.