Grass roots chemistry: meta-Tyrosine, an herbicidal nonprotein amino acid

Fine fescue grasses displace neighboring plants by depositing largequantities of an aqueous phytotoxic root exudate in the soilrhizosphere. Via activity-guided fractionation, we have isolatedand identified the nonprotein amino acid m-tyrosine as the majoractive component. m-Tyrosine is significantly more phytotoxicthan its structural isomers o- and p-tyrosine. We show that mtyrosineexposure results in growth inhibition for a wide range ofplant species and propose that the release of this nonproteinamino acid interferes with root development of competing plants.Acid hydrolysis of total root protein from Arabidopsis thalianashowed incorporation of m-tyrosine, suggesting this as a possiblemechanism of phytotoxicity. m-Tyrosine inhibition of A. thalianaroot growth is counteracted by exogenous addition of proteinamino acids, with phenylalanine having the most significant effect.The discovery of m-tyrosine, as well as a further understanding ofits mode(s) of action, could lead to the development of biorationalapproaches to weed control.