Velvetbean (Mucuna pruriens) plants impede the growth of neighboring plants. One compound, 3-(3',4'-dihydroxyphenyl)-L-alanine (L-DOPA), is responsible for the allelopathic capacity of velvetbean. This compound is an active allelochemical that decreases root growth of several plant species. In mammals, L-DOPA is a well-known therapeutic agent for the symptomatic relief of Parkinson's disease. However, its mode of action in plants is still not well understood. To address such issues, gene expression in Arabidopsis thaliana plants, which had been exposed to L-DOPA, was analyzed using DNA microarrays. After 6 h of L-DOPA exposure, the expression of 110 genes was significantly upregulated, and the expression of 69 genes was significantly downregulated. These induced genes can be divided into different functional categories, mainly on the basis of subcellular localization, metabolism, and proteins with a binding function or cofactor requirement. Based on these results, we suggest that L-DOPA acts by two mechanisms: it influences amino acid metabolism and deregulates metal homeostasis, especially that of iron, which is required for the fundamental biological processes of all organisms.