Alkaloids are part of the chemical arsenal designed to protect plants against an adverse environment. Therefore, their synthesis and accumulation are frequently induced in response to certain environmental conditions and are mediated by chemical signals, which are formed as the first responses to the external stimulus. A set of experiments using "Catharanthus roseus" seedlings is presented to illustrate this interaction. Using the well characterized induction of monoterpenoid indole alkaloid synthesis in response to methyl jasmonate, one chemical mediator; simple and reproducible experiments are proposed to demonstrate this type of plant-environment interaction to plant physiology or biochemistry B.Sc. students. After exposure to the inducer, seedlings are analyzed for alkaloid accumulation and for the increase of tryptophan decarboxylase (TDC) enzyme activity by means of thin layer chromatography, followed by UV spectrophotometry. Alkaloids should increase as a result of the exposure to the inducer, as well as TDC activity, which plays a critical role in channeling carbon skeletons from primary to secondary metabolism in this plant. (Contains 3 figures, 2 tables and 5 online resources.)