Isolation, Identification, Biocontrol Activity, and Plant Growth Promoting Capability of a Superior <i>Streptomyces tricolor</i> Strain HM10

Abstract

<i>Streptomyces</i> is a genus with known biocontrol activity, producing a broad range of biologically active substances. Our goal was to isolate local <i>Streptomyces</i> species, evaluate their capacity to biocontrol the selected phytopathogens, and promote the plant growth via siderophore and indole acetic acid (IAA) production and phosphate solubilization. Eleven isolates were obtained from local soil samples in Saudi Arabia via the standard serial dilution method and identified morphologically by scanning electron microscope (SEM) and 16S rRNA amplicon sequencing. The biocontrol of phytopathogens was screened against known soil-borne fungi and bacteria. Plant growth promotion capacity was evaluated based on siderophore and IAA production and phosphate solubilization capacity. From eleven isolates obtained, one showed 99.77% homology with the type strain <i>Streptomyces tricolor</i> AS 4.1867, and was designated <i>S. tricolor</i> strain HM10. It showed aerial hyphae in SEM, growth inhibition of ten known phytopathogens in <i>in vitro</i> experiments, and the production of plant growth promoting compounds such as siderophores, IAA, and phosphate solubilization capacity. <i>S. tricolor</i> strain HM10 exhibited high antagonism against the fungi tested (i.e., <i>Colletotrichum gloeosporides</i> with an inhibition zone exceeding 18 mm), whereas the lowest antagonistic effect was against <i>Alternaria solani</i> (an inhibition zone equal to 8 mm). Furthermore, the most efficient siderophore production was recorded to strain HM8, followed by strain HM10 with 64 and 22.56 h/c (halo zone area/colony area), respectively. Concerning IAA production, <i>Streptomyces</i> strain HM10 was the most effective producer with a value of 273.02 μg/ml. An autochthonous strain <i>S. tricolor</i> HM10 should be an important biological agent to control phytopathogens and promote plant growth.