Continuous recirculation of hydroponic-nutrient solutions shifts bacterial communities and induces plant-defense gene...

Biotic and abiotic factors influence bacterial communities in the rhizosphere. This project aimed to characterize bacterial communities in the lettuce rhizosphere, assess the relationships between bacteria and root rot, and evaluate the impact of reused nutrient solutions on bacterial communities. Lettuce (Lactuca sativa cv. Rex) was grown in deep-water culture hydroponics in reused autoclaved or non-autoclaved nutrient solutions for five cycles, with or without Pythium myriotylum. Bacterial composition was determined via high-throughput sequencing of the 16S rRNA gene. Quantitative real-time PCR was used to detect P. myriotylum. Spearman's correlation coefficient (ρ) was performed to assess relationships between variables. The expression of PR1, PDF1.2, and LOX1 was quantified to determine if there were differences in plant defenses. The predominant phyla and genera across all cycles in the nutrient solution and roots were Proteobacteria (67%) and Cyanobacteria (47%) and Acinetobacter (13%) and Ideonella (22%), respectively. Bacterial communities in the nutrient solution (R²= 0.35) and roots (R²= 0.24) were significantly dissimilar between cycles. Bacterial communities were significantly dissimilar by the presence of P. myriotylum in nutrient solution (R²= 0.02) and roots (R²= 0.10). Bacterial correlations with health variables mostly differed by the presence/absence of P. myriotylum. Bacterial communities or individual bacterial isolates in recirculating hydroponic solutions that activate plant defenses resulting in the suppression or prevention of Pythium root rot in lettuce should be investigated further to be utilized as viable strains or synthesized compounds for the development of commercial products.

Pythium myriotylum is the causal agent of root rot and wilt disease, which can cause significant damage to lettuce in hydroponic systems. Root rot can be challenging to treat with traditional methods once it develops, often resulting in the destruction of the entire crop. Reused nutrient solutions have been reported to harbor microorganisms that may affect disease suppression. Examining how bacterial communities in recycled nutrient solutions change and trigger plant-defense genes may contribute to the reduction of Pythium root rot and provide chemical-free and cost-effective alternatives for soilless cultivation systems. Future studies focusing on specific microorganisms and their bioactive compounds will be essential for advancing biological control methods in hydroponic crop systems.