Glucosylation prevents autotoxicity of stress inducible DOPA in maize seedlings

L-3,4-Dihydroxyphenylalanine (L-DOPA) is a compound with strong allelopathic effects on Brassicaceae, Asteraceae, Cucurbitaceae and Hydrophyllaceae species. Although Gramineae are less affected by L-DOPA with respect to their root growth, metabolic routes that protect them from L-DOPA toxicity are poorly understood. We identified a DOPA glucoside in maize ( Zea mays L., Gramineae) roots and leaves, but DOPA aglycon was not detectable in maize. Accordingly, when maize seedlings were exposed to L-DOPA solution, DOPA glucoside concentrations increased in maize leaves, suggesting that absorbed L-DOPA is rapidly converted to glucoside conjugate. When DOPA glucoside solution was applied to lettuce seeds ( Lactuca sativa ; Asteraceae), lettuce radicle growth was less inhibited compared to free L-DOPA. Considering that maize radicle growth is less affected by free L-DOPA, it is likely that maize seedlings protect themselves from toxicity by L-DOPA glucosylation. Interestingly, a developmental stage dependent variation in DOPA glucoside concentration was observed with highest level of metabolite detected in L2 stage maize leaves. As DOPA glucoside also increased in maize during herbivory by the bird cherry-oat aphid ( Rhopalosiphum padi L.) and the Graminae generalist armyworm ( Mythimna loreyi ), as well as in response to treatment with the plant hormones, we propose that DOPA glucoside might be involved in various stress responses and/or defense in maize seedlings.