Characterization of rhizosphere Streptomyces virginiae IRHB6 as a dual-agent for disease suppression and growth promotion in soybean

• The rhizobacterium IRHB6 is identified as Streptomyces virginiae . • It shows broad-spectrum antagonism against multiple phytopathogens. • The strain produces siderophores, IAA, hydrolytic enzymes, and solubilizes phosphorus. • Key genes for siderophore and polyketide biosynthesis are upregulated. • IRHB6 controls Fusarium root rot and promotes soybean growth in vivo . Streptomyces spp. are renowned for producing biologically active compounds and serving as biological control agents against plant diseases. In a previous study, Streptomyces strain IRHB6, isolated from the soybean rhizosphere, exhibited significant inhibitory activity against mycelial growth of Fusarium oxysporum, the causative agent of soybean root rot. To further characterize its biocontrol potential, we conducted comprehensive in vitro and in vivo assays to evaluate antagonistic effects on multiple phytopathogens, and assess plant growth-promoting capabilities. Our results showed that strain IRHB6 exhibited a colony morphology characterized by a wrinkled, rough surface texture and was identified as Streptomyces virginiae through multilocus sequence analysis (MLSA). The strain produced hydrolytic enzymes, siderophores, and indole-3-acetic acid (IAA), and exhibited phosphorus-solubilizing activity. A dual-cultural confrontation assay revealed that IRHB6 exhibited antagonistic traits against phytopathogens, particularly, Fusarium species responsible for soybean root rot and Pseudomonas syringae pv. actinidiae for kiwifruit canker. The induced expression of biosynthetic genes for siderophore pyoverdine ( pvdA ) as well as antibiotic polyketide ( T1PKS/NRPS-like and T2PKS ) further substantiates its capacity for producing bioactive compounds against phytopathogens. Furthermore, treatment with IRHB6 cell suspension significantly promoted plant growth. Critically, IRHB6 conferred substantial protective capabilities against Fusarium root rot. Based on these findings, S. virginiae IRHB6 represents a promising biocontrol agent for soil-borne disease as well as plant growth promotion.