Seaweed Polysaccharides and Derived Oligosaccharides Stimulate Defense Responses and Protection Against Pathogens in Plants

Plants interact with the environment by sensing “non-self” molecules calledelicitors derived from pathogens or other sources. These molecules bind to specificreceptors located in the plasma membraneand trigger defense responses leading toprotection against pathogens. In particular,it has been shown that cell wall and storagepolysaccharides from green, brown and red seaweeds (marine macroalgae) correspondingto ulvans, alginates, fucans, laminarin andcarrageenans can trigger defense responses inplants enhancing protection against pathogens. In addition, oligosaccharides obtained bydepolymerization of seaweed polysaccharidesalso induce protection against viral, fungaland bacterial infections in plants. In particular, most seaweed polysaccharides and derivedoligosaccharides trigger an initial oxidative burst at local level and the activation ofsalicylic (SA), jasmonic acid (JA) and/or ethylene signaling pathwaysat systemic level.The activation of these signaling pathways leads to an increased expression of genesencoding: (i) Pathogenesis-Related (PR) proteinswith antifungal and antibacterial activities;(ii) defense enzymes such as pheylalanine ammonia lyase (PAL) and lipoxygenase (LOX)which determine accumulation of phenylpropanoid compounds (PPCs)and oxylipins withantiviral, antifugal and antibacterial activities andiii) enzymes involved in synthesis ofterpenes, terpenoidsand/or alkaloids having antimicrobial activities. Thus, seaweedpolysaccharides and their derived oligosaccharides induced the accumulation of proteinsand compounds with antimicrobial activities thatdetermine, at least in part, the enhancedprotection against pathogens in plants.