Nematode predation mitigates microbial competition to sustain the plant growth-promoting effect of Streptomyces

Streptomyces is a well-known plant growth-promoting rhizobacterium, but its perfor-mance in soil is influenced by interactions with native competitors and predators, which remain poorly understood. This study used factorial pot experiments to investi-gate how microbial competition and nematode predation influence the growth-promoting effect of Streptomyces on Arabidopsis thaliana (A. thaliana), through co-inoculation with indigenous soil bacteria and nematodes. Compared to in-oculation with Streptomyces alone, co-inoculation with either indigenous bacteria or nematodes significantly reduced the aboveground dry weight of A. thaliana. High-throughput sequencing showed that microbial competition reshaped bacterial community composition and enriched pathways related to nutrient acquisition and an-tibiotic biosynthesis, potentially limiting the effectiveness of Streptomyces. Nematode identification and feeding assays revealed that over 90% of the nematode community were bacterivores capable of consuming Streptomyces spores, contributing to reduction in plant growth promotion effect. However, when Streptomyces, indigenous bacteria, and nematodes were co-inoculated, the growth-promoting effect of Streptomyces was restored. This allowed Streptomyces to regain a competitive advantage, as reflected by the enrichment of the ansamycins biosynthesis pathway. These findings elucidate the intricate tripartite interactions among Streptomyces, indigenous soil bacteria, and bac-terivorous nematodes, thereby offering valuable insights into optimizing the applica-tion of Streptomyces as plant growth-promoting agents within natural soil ecosystems.