Tyramine pathways in citrus plant defense: Glycoconjugates of tyramine and its N-methylated derivatives

Glucosylated forms of tyramine and some of its N-methylated derivatives are here reported for the first time to occur in Citrus genus plants. The compounds tyramine-O-β-D-glucoside, N-methyltyramine-O-β-D-glucoside, and N,N-dimethyltyramine-O-β-D-glucoside were detected in juice and leaves of sweet orange, bitter orange, bergamot, citron, lemon, mandarin, and pomelo. The compounds were identified by mass spectrometric analysis, enzymatic synthesis, and comparison with extracts of Stapelia hirsuta L., a plant belonging to the Apocynaceae family in which N,N-dimethyltyramine-O-β-D-glucoside was identified by others. Interestingly, in Stapelia hirsuta we discovered also tyramine-O-β-D-glucoside, N-methyltyramine-O-β-D-glucoside, and the tyramine metabolite, N,N,N-trimethyltyramine-O-β-glucoside. However, the latter tyramine metabolite, never described before, was not detected in any of the Citrus plants included in this study. The presence of N-methylated tyramine derivatives and their glucosylated forms in Citrus plants, together with octopamine and synephrine, also deriving from tyramine, supports the hypothesis of specific biosynthetic pathways of adrenergic compounds aimed to defend against biotic stress.