Production of bioactive napthoquinones by roots of Paterson's Curse (Echium plantagineum L.): implications for invasion success?

Paterson's curse roots were studied with respect to their ability to produce coloured secondary products, napthoquinones, in living roots. Young roots produced large quantities of red coloured anthro- or napthoquinones in outer layers of root periderm. In contrast, mature or aged roots exhibited blackened periderm containing accumulations of dark coloured secondary products, possibly due to oxidation or polymerization of these compounds over time. Ethanolic extracts of young root periderm tissues were bright red or pink in colour and contained several unusual napthoquinones, including acetylshikonin, and 1,3 dihydroxy -3- methylanthraquinone, as detected by LC/MS and GC/MS analyses. Mature or aged root extracts were colourless in appearance, and contained 1,3 dihydroxy-3-methylanthraquinone and other likely related constituents. Production of napthoquinones and colour of root extracts was clearly influenced by location of harvest of Paterson's curse, time of harvest and age of root tissue. Both young and aged root extracts exhibited strong inhibition of root growth of annual ryegrass, with young root extracts showing greatest phytotoxicity. Shikonin at 1 mg/ml was also phytotoxic to annual ryegrass growth, with increasing phytotoxicity noted with increasing shikonin concentration. The role of napthoquinones in plant invasion and their interactions in the plant rhizosphere require further elucidation, as novel napthoquinones exhibit potent antimicrobial, fungitoxic, and phytotoxic activity due to their impact on electron transport and cellular respiration processes.