From sea to farm: Repurposing invasive macroalgae into biocidal and defense-enhancing tools against tomato late blight (Phytophthora infestans).

Marine seaweeds are increasingly recognized as sources of bioactive compounds with potential applications in sustainable agriculture, including as biocontrol agents. The selection of invasive species such as Asparagopsis armata and Sargassum muticum offers a dual benefit: obtaining bioactive metabolites from a low-cost resource while simultaneously mitigating their ecological impacts using this harmful biomass. In the present study, crude extracts from A. armata and S. muticum were applied to tomato plants and were effective as biocontrol agents against Phytophthora infestans. Treatments modulated key physiological and metabolic processes, including photochemical energy distribution, membrane lipid composition, redox balance, and secondary metabolism, effectively eliciting plants for enhanced defense. Asparagopsis armata elicited particularly strong responses, whereas S. muticum induced moderate but consistent effects across multiple pathways. These changes were correlated with the pathogen inoculation outcome and improved plant resilience during infection. These findings provide physiological insights into how these macroalgae can suppress pathogen activity while enhancing host defenses, highlighting their potential as sustainable biocontrol agents. Exploiting A. armata and S. muticum in this context presents a resource-efficient biocontrol strategy that links plant adjuvant defense enhancer effect with the valorization of underexploited marine biomass, reinforcing their potential within sustainable agricultural systems.