Evaluating the Antagonistic Potential of Actinomycete Strains Isolated From Sudan’s Soils Against Phytophthora infestans

Abstract

Soil microorganisms play crucial roles in soil fertility, e.g., through decomposing organic matter, cycling nutrients or through beneficial interactions with plants. Actinomycetes are a major component of soil inhabitants; they are prolific producers of specialized metabolites, among which many antibiotics. Here we report the isolation and characterization of 175 Actinomycetes from rhizosphere and bulk soil samples collected in 18 locations in Sudan. We evaluated the strains’ metabolic potential for plant protection by testing their ability to inhibit the mycelial growth of the oomycete Phytophthora infestans, which is one of the most devastating plant pathogens worldwide. Most strains significantly reduced the oomycete’s growth in direct confrontational in vitro assays. A significant proportion of the tested strains (15%) were able to inhibit P. infestans to more than 80%, 23% to 50%–80%, while the remaining 62% had inhibition percentages lesser than 50%. Different morphologies of P. infestans mycelial growth and sporangia formation were observed upon co-inoculation with some of the Actinomycetes isolates, such as the production of fewer, thinner hyphae without sporangia leading to a faint growth morphology, or on the contrary, of clusters of thick-walled hyphae leading to a bushy, or “frozen” morphology. These morphologies were caused by strains differing in activity levels but phylogenetically closely related with each other. To evaluate whether the isolated Actinomycetes could also inhibit the pathogen’s growth in planta, the most active strains were tested for their ability to restrict disease progress in leaf disc and full plant assays. Five of the active strains showed highly significant protection of potato leaves against the pathogen in leaf disc assays, as well as substantial reduction of disease progress in full plants assays. Using cell-free filtrates instead of the bacterial spores also led to full protection against disease on leaf discs, which highlights the strong crop protective potential of the secreted metabolites that could be applied as leaf spray. This study demonstrates the strong anti-oomycete activity of soil- and rhizosphere-borne Actinomycetes and highlights their significant potential for the development of sustainable solutions based on either cell suspensions or cell-free filtrates to safeguard potatoes from their most damaging pathogen.

Generation of a high quality library of bioactive filamentous actinomycetes from extreme biomes using a culture-based bioprospecting strategy

“…Actinomycetes are to the fore amongst microbial control agents given their ability to produce antifungal compounds and siderophores, secrete enzymes that degrade fungal cell walls and compete for nutrients ( Guo X. et al, 2015 ; Alblooshi et al, 2021 ). Taxonomically diverse actinomycetes, notably streptomycetes, inhibit the growth of phytopathogens, as exemplified by Abdelrahman et al (2022) who found that streptomycetes from rhizosphere and soil samples in the Sudan inhibited the growth of the devastating oomycete pathogen Phytophthora infestans . Further, an actinomycete closely related to Streptomyces spectabilis inhibited the growth of 11 plant pathogens ( Chen et al, 2018 ) whereas Nocardiopsis and Streptomyces were found to show in vitro and in planta activity against bacterial and fungal pathogens of carrots and tomatoes ( Djebaili et al, 2021 ).…”

Comparative genomic and metabolomic study of three Streptomyces sp. differing in biological activity

“…Three Streptomyces strains (B5, B91, and B135) isolated in Sudan and originating from different habitats were used in this project. Their isolation and characterization were described in Abdelrahman et al ( 2022 ). They were routinely grown at 28°C on V8 medium (Miller, 1955 ) containing (per liter of distilled water) 1 g of CaCO 3 (BioXtra ≥ 99%; Sigma‐Aldrich), 15 g of agar (Carl Roth), and 100 mL of V8 100% spicy hot vegetable juice (Campbell Soup Company).…”

“…In an earlier study, a collection of 175 Actinomycetes strains isolated from different Sudanese habitats were assessed for their inhibitory activity on P. infestans (Abdelrahman et al, 2022 ). Within this collection, we observed that three Streptomyces strains that showed close phylogenetic relationships based on 16S ribosomal RNA (rRNA) gene homology displayed very different levels of P. infestans mycelial growth inhibition.…”

Potential of actinobacteria as biocontrol agents to Xanthomonas oryzae pv. oryzae in vitro

“…The interaction of these bacteria with plants can promote plant growth while competing with and preventing the development of other microorganisms, primarily plant diseases. Through the plant [4] and Actinobacteria are some of the root bacteria genera that have been classified [6]. Actinobacteria are a prokaryotic microorganism group.…”

“…Actinobacteria are a prokaryotic microorganism group. These bacteria are classified as intermediate between bacteria and fungi because their morphology is similar to that of filamentous fungi [6]. Actinobacteria characterize gram-positive bacteria.…”