Abstract. Aouar L, Boukelloul I, Benadjila A. 2020. Identification of antagonistic Streptomyces strains isolated from Algerian Saharan soils and their plant growth promoting properties. Biodiversitas 21: 5672-5683. To produce new bioactive substances of agricultural interest, extreme ecosystems can be a source of unexplored microorganisms. Accordingly, in this study, twenty-two actinobacteria strains were obtained from rhizospheric arid soils of palm groves collected from Biskra and El Oued in the Algerian Sahara. All isolates were examined for the in vitro antifungal potential towards phytopathogenic fungi: Aspergillus flavus, Verticillium dahlia, Rhizoctonia solani, Botrytis cinerea and Fusarium oxysporum as well as for their antibacterial property toward phytopathogenic bacteria: Streptomyces scabiei, Pectobacterium carotovorum and Agrobacterium tumefaciens. The three isolates (13%) that inhibited at least five pathogens were then selected, identified and assessed for their attributes to produce indole-3-acetic acid (IAA) and siderophores, to solubilize phosphate, and to antagonize Streptomyces scabiei in vivo. According to phylogenetic analysis performed with 16S rDNA sequence, chemotaxonomy and phenotypic characteristics, the strain SO1, which inhibited all tested pathogens, was assigned to Streptomyces flaveus. While, strains SO2 and SB1 were affiliated to Streptomyces enissocaesilis and Streptomyces albidoflavus, respectively. All strains produced IAA but only SO1 and SB1 were able to elaborate siderophores catecholate-type. Two strains SO1 and SO2 exhibited a capacity to solubilize phosphate and SO1 was able to suppress the pathogenic effect of Streptomyces scabiei on radish seedlings. The findings indicate that SO1 strain may reveal the potential for use as a biocontrol agent and plant growth promoter.

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Biocontrol is considered as an effective alternative to the application of agrochemicals, which are harmful to the environment, human, and animal health. In this study, twenty-six strains of actinomycetes were isolated from rhizospheric arid soil of the halophyte Atriplex halimus L. ῾Guettaf’ in Biskra province, Algeria. The six isolates that have inhibited at least three phytopathogenic fungi among the five tested (Fusarium oxysporum, Alternaria alternata, Fusarium solani, Aspergillus flavus and Botrytis cinerea) were selected, and have been tested in vitro against phytopathogenic bacteria (Pectobacterium carotovorum and Streptomyces scabies). They were also evaluated for their ability to hydrolyze phosphate, elaborate siderophores, produce indole-3-acetic acid (IAA), and to antagonize S. scabies in vivo (on radish seedlings). Based on the physicochemical analyses, soil samples were categorized as alkaline and extremely-saline. The antagonism results revealed varying antifungal potential among the selected isolates (Act11, Act16, Act17, Act18, Act23 and Act24), about 50% were able to inhibit the growth of F. solani and A. flavus, followed by 33.33% of those having antagonized F. oxysporum, while A. alternata was found to be the most sensitive. Only Act18 has antagonized S. scabies in vitro with an inhibition diameter zone of 19 ± 0.41 mm. However, in vivo trials showed that four isolates have counteracted S. scabies. Among them, Act18 and Act24 have significantly and positively affected the root surface (P = 0.0062) and prevented common scab. IAA was detected in all selected isolates with Act24 being the highest producer (77.45 μg mL−1). Additionally, degradation ability revealed that four isolates were able to hydrolyze phosphate while three exhibited the capacity of elaborating siderophores. The six isolates were assigned to Streptomyces genius according to their morphological, physiological and chemotaxonomical traits. Based on this study, Streptomyces sp. Act18 and Streptomyces sp. Act24 that tolerate 7.5% NaCl concentration, prevent common scab and exhibit some plant growth attributes, may be considered as promising biocontrol agents to be applied in arid and saline soils.