<i>Pythium</i> -Induced Changes in Bacterial and Fungal Community Structure in Hydroponically Grown Lettuce

The composition and structure of plant-associated microbial communities are shaped by abiotic and biotic factors, with host-related traits and plant-microbe interactions playing key roles. While studies on soil-based cultivation systems have shown that pathogen infection can disrupt plant microbial communities, little is known about how pathogen interactions affect hydroponically grown crops, particularly leafy greens. Despite the controlled conditions of hydroponic cultivation, these systems remain vulnerable to waterborne pathogens like Pythium and Globisporangium spp. These oomycetes can rapidly spread through recirculated nutrient solutions and cause severe root diseases. This study aimed to examine how Pythium aphanidermatum inoculation affects the bacterial and fungal communities in the roots of hydroponically grown lettuce in deep-water culture systems under research greenhouse conditions. Our findings revealed that root inoculation with P. aphanidermatum significantly impacted lettuce biomass and microbial communities. Bacterial and fungal alpha diversity and evenness were significantly higher in inoculated roots compared with non-inoculated controls. Beta diversity and network analyses indicated different modes of microbial response to pathogen invasion. Bacterial communities responded with compositional shifts regardless of disease severity. In contrast, fungal communities showed dramatic reorganization of network structure. Differential abundance and network analyses identified Pseudomonas and Olpidium as taxa consistently associated with Pythium-inoculated roots. A deeper understanding of hydroponics-associated microbial communities, their interactions, and functional roles will allow us to leverage these relationships for improved food security and environmental sustainability. Moreover, understanding the microbial composition dynamics during pathogen infection can have significant implications for disease management and the effective use of biological control agents.