200 years ago, coumarin was purified from plant extracts for the first time. Later, it was shown that this newly identified molecule can influence various physiological processes in plants. However, its biosynthesis, perception, and signaling have never been studied at the molecular level. To unravel the physiological relevance of coumarin in plants, we recently revisited this field of research. This led to the identification of CIC1, an enzyme involved in the conversion of coumarin to dihydrocoumarin (DHC). Since cic1 mutants are insensitive to coumarin, while DHC, like coumarin, has an inhibitory effect on plant growth, we concluded that CIC1 is crucial for the observed coumarin activity. This project builds on these findings. CIC1 will be characterized in detail, and we will further unravel the molecular mechanism of coumarin by focusing on DHC as the active molecule.