Thiophene derivatives are widely distributed in the Asteraceae (Compositae). They have been described as phytoalexins and thus play a role as natural protective agents. Several naturally occurring thiophenes show antibiotic, antifungal and nematicidal activities when they are applied to living systems in the presence of long-wave UV light (UV-A). The biological activity of thiophenes in the absence of light is still a matter of discussion. Some activity is still expected, since roots are the main site of thiophene accumulation. The thiophenes that accumulate in roots of Tagetes species are structurally related. Transformed roots cultured in vitro accumulate the same thiophenes. These in vitro cultured roots were used as a model to investigate plant-microbial interactions.