| Summary: | A total of 461 indigenous Streptomycetes strains recovered from various Greek rhizosphere habitats were tested for their bioactivity. All isolates were examined for their ability to suppress the growth of 12 specific target microorganisms. Twenty-six were found to exert antimicrobial activity and were screened for potential nematicidal action. <i>S. monomycini</i> ATHUBA 220, <i>S. colombiensis</i> ATHUBA 438, <i>S. colombiensis</i> ATHUBA 431, and <i>S. youssoufensis</i> ATHUBA 546 were proved to have a nematicidal effect and thus were further sequenced. Batch culture supernatants and solvent extracts were assessed for paralysis on <i>Meloidogyne javanica</i> and <i>Meloidogyne incognita</i> second-stage juveniles (J2). The solvent extracts of <i>S. monomycini</i> ATHUBA 220 and <i>S. colombiensis</i> ATHUBA 438 had the highest paralysis rates, so these Streptomycetes strains were further on tested for nematodes’ biological cycle arrest on two <i>Arabidopsis thaliana</i> plants; the wild type (Col-0) and the katanin mutant <i>fra2</i>, which is susceptible to <i>M. incognita</i>. Interestingly, <i>S. monomycini</i> ATHUBA 220 and <i>S. colombiensis</i> ATHUBA 438 were able to negatively affect the <i>M. incognita</i> biological cycle in Col-0 and <i>fra2</i> respectively, and increased growth in Col-0 upon <i>M. incognita</i> infection. However, they were ineffective against <i>M. javanica</i>. <i>Fra2</i> plants were also proved susceptible to <i>M. javanica</i> infestation, with a reduced growth upon treatments with the <i>Streptomyces</i> strains. The nematicidal action and the plant-growth modulating abilities of the selected Streptomycetes strains are discussed.
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