Involvement of melanin synthesis and reactive oxygen species in phytotoxic action of l-DOPA in carrot cells

l-3,4-Dihydroxyphenylalanine (l-DOPA) is one of the most potent allelochemicals known. Our previous study indicated that this phenolic non-protein amino acid effectively suppressed root elongation in several plant species. In mammalian cells, l-DOPA has been reported to cause cell death by generating reactive oxygen species during its oxidative polymerization into melanin. Growth of carrot cells treated with l-DOPA for 8 days was decreased approximately 40% and 20% at 10−3 and 10−4 M, respectively. Chemiluminescence and lipid peroxidation increased significantly in l-DOPA-treated cells. These effects were reduced in the presence of ascorbic acid or α-tocopherol. The antioxidants partially prevented the growth reduction by l-DOPA. In l-DOPA-treated cells, the level of melanin increased continuously throughout the period of exposure (6 days); however, it did not increase in the presence of the antioxidants. These results suggest that the growth reduction of carrot cells by l-DOPA is at least partially due to oxidative damage accompanying the formation of melanin. This phytotoxicity of l-DOPA might be due to reactive oxygen species generated by the melanin synthesis pathway.