Indole-3-carboxylic acid is a tryptophan metabolite produced by microbiota in the human gut. Indole-3-carboxylic acid isolated from Streptomyces sp. is active against Gram positive bacteria, yeast and fungi. Indole-3-carboxylic acid induces resistance in Arabidopsis adult plants against the necrotrophic fungus Plectosphaerella cucumerina through primed callose accumulation. Indole-3-carboxylic acid is a useful skeleton for the design and synthesis of bioactive molecules.