Stable-isotope-assisted metabolomics enables the study of (a)biotic stress-related metabolism of tyrosine compared to phenylalanine in wheat

To investigate the role of the previously little-studied tyrosine (Tyr), we have used stable isotope-assisted metabolomics to compare its metabolism to the well-studied phenylalanine (Phe) in wheat. Flowering wheat ears were treated either with 13C9-Phe or 13C9-Tyr as metabolic tracers under both control and stress conditions (addition of 0.2 mg per ear of the Fusarium mycotoxin and virulence factor deoxynivalenol (DON)). Overall, 115 Phe- and Tyr-derived wheat metabolites were detected. For 13C9-Tyr, the overall uptake and metabolization of the initially applied tracers (0.5 mg each) was about 40 % compared to 80 % of Phe, and 48 downstream metabolites were shared between both tracers. About one-third of the detected metabolites were found to be significantly induced by DON treatment (DON+). Those belonged to the classes of hydroxycinnamic acids (HCAs), HCA-amides, -glycosides, -quinates and lignans, suggesting their role as precursors for cell wall reinforcement and antifungal compounds. Evaluation of 13C isotopolog signals revealed that Tyr was incorporated into common downstream metabolites at a rate of approximately 1/6 (mock) and 1/4 (DON) of that compared to Phe. Tyr incorporation was induced upon DON stress compared to control conditions, whereas Phe showed indifferent median incorporation rates. This study highlights the added value and complementarity of using labeled Tyr and Phe as tracers in isotope-assisted metabolomics to improve annotation confidence and biological interpretation of plant metabolic responses to stress.