Abstract: The biologically active substance is obtained or capable of being obtained by alkaline extraction, using an aqueous solution of at least one base in the presence of at least one reducing agent, of at least one macroscopic edible mushroom reduced to powder, to obtain a mixture having a liquid portion containing the soluble extracted materials and a solid portion formed of insoluble solid particles; filtering the resulting mixture to remove the solid portion and optionally clarifying the liquid portion; neutralising the liquid portion using a cation exchange resin and optionally filtering/clarifying the liquid portion thus neutralised to obtain the biologically active substance in the aqueous phase; and treating the resulting aqueous phase, the treatment being selected from dilution in water, concentration, dehydration and lyophilisation to obtain a biologically active substance which is, respectively, more diluted in water, in an aqueous solution more concentrated, dehydrated in powder form or lyophilised.

Abstract: Use as a defense elicitor/stimulator in terrestrial plants of an extract or an extract fraction of at least one agarophyte red alga, said extract or said extract fraction containing at least one oligosaccharide having 2 to 50 ose units covalently bound to one another, and polysaccharides having more than 50 ose units covalently bound to one another which can be contained without exceeding 0.01% by weight relative to the total oligosaccharides/polysaccharides.