Interaction of seaweed metabolites with plants to enhance protection against biotic and abiotic stresses

Biotic and abiotic stresses severely compromise economically important food crops' nutritional quality, growth, and yield. Conversely, the conventional reliance on chemical fertilisers and pesticides has generated substantial environmental and health risks, necessitating the development of sustainable alternatives. Seaweeds are rich sources of bioactive primary and secondary metabolites, and also promising natural biostimulants for enhancing plant resilience and productivity. Specific seaweed-derived metabolites function as molecular elicitors, mimicking pathogen-associated molecular patterns (PAMPs) and activating multi-layered plant defence mechanisms. This review aims to capture recent literature on the biological efficacy of seaweed extracts and their constituent metabolites, such as polysaccharides, phenolic compounds, and fatty acids, against diverse biotic stressors (e.g., bacteria, viruses, oomycetes, fungi (ascomycetes and basidiomycetes), nematodes, and herbivorous insect pests) and abiotic stressors (such as salinity, drought, extreme temperatures, and heavy metals). The biochemical, physiological, and molecular mechanisms by which seaweed-derived bioactive compounds modulate plant defence responses and stress tolerance pathways are also discussed in detail. In conclusion, seaweed extracts and derived metabolites show promising stress-type-specific effects against biotic and abiotic stresses through diverse mechanisms. However, field validation, dosage optimisation, and the discovery of novel bioactives are essential to harnessing their potential fully in sustainable agriculture