Seaweed extracts: Potential biodegradable, environmentally friendly resources for regulating plant defence

Marine macroalgae or seaweeds are the major components of the marine flora and are used as food, feed and fertilizer. Applications of seaweed extracts (SEs) from certain algae have the potential to improve plant growth and yield. The richness of polysaccharides, oligosaccharides, peptides, proteins and phytohormones in various SEs, favor the deployment of SEs as bio-elicitors for disease tolerance in plants. The SEs from some algae regulate the physiological, biochemical and molecular mechanisms of the plants to enhance defence against pathogens. The SEs also modulate the rhizosphere microbial composition, which contributes to regulation of plant defence responses. The regulation of salicylic acid (SA) and jasmonic acid (JA)-signaling pathways, reactive oxygen species (ROS) homeostasis and defence-related genes/enzymes by applications of SEs play a major role in the molecular regulation of defence response. This review focuses on the bioactive molecules of various SEs, functional mechanism of bio-elicitors, phytohormones, and molecular regulation towards disease tolerance in plants.