title: Identification and localization of bioactive naphthoquinones in the roots and rhizosphere of Paterson's curse (Echium plantagineum), a noxious invader.
abstract: Bioactive plant secondary products are frequently the drivers of complex rhizosphere interactions, including those with other plants, herbivores and microbiota. These chemically diverse molecules typically accumulate in a highly regulated manner in specialized plant tissues and organelles. We studied the production and localization of bioactive naphthoquinones (NQs) in the roots of Echium plantagineum, an invasive endemic weed in Australia. Roots of E. plantagineum produced red-coloured NQs in the periderm of primary and secondary roots, while seedling root hairs exuded NQs in copious quantities. Confocal imaging and microspectrofluorimetry confirmed that bioactive NQs were deposited in the outer layer of periderm cells in mature roots, resulting in red colouration. Intracellular examination revealed that periderm cells contained numerous small red vesicles for storage and intracellular transport of shikonins, followed by subsequent extracellular deposition. Periderm and root hair extracts of field- and phytotron-grown plants were analysed by UHPLC/Q-ToF MS (ultra high pressure liquid chromatography coupled to quadrupole time of flight mass spectrometry) and contained more than nine individual NQs, with dimethylacrylshikonin, and phytotoxic shikonin, deoxyshikonin and acetylshikonin predominating. In seedlings, shikonins were first found 48h following germination in the root-hypocotyl junction, as well as in root hair exudates. In contrast, the root cortices of both seedling and mature root tissues were devoid of NQs. SPRE (solid phase root zone extraction) microprobes strategically placed in soil surrounding living E. plantagineum plants successfully extracted significant levels of bioactive shikonins from living roots, rhizosphere and bulk soil surrounding roots. These findings suggest important roles for accumulation of shikonins in the root periderm and subsequent rhizodeposition in plant defence, interference, and invasion success.
authors.affiliation: Graham Centre for Agricultural Innovation (Charles Sturt University and NSW Department of Primary Industries), School of Agricultural and Wine Sciences, Wagga Wagga NSW 2678 Australia.
authors.forename: Xiaocheng
authors.initials: X
authors.lastname: Zhu
authors.affiliation: Graham Centre for Agricultural Innovation (Charles Sturt University and NSW Department of Primary Industries), School of Agricultural and Wine Sciences, Wagga Wagga NSW 2678 Australia.
authors.forename: Dominik
authors.initials: D
authors.lastname: Skoneczny
authors.affiliation: Department of Chemistry, Geology and Physics, Ashland University, Ashland, OH 44805 USA.
authors.forename: Jeffrey D
authors.initials: JD
authors.lastname: Weidenhamer
authors.affiliation: Graham Centre for Agricultural Innovation (Charles Sturt University and NSW Department of Primary Industries), School of Agricultural and Wine Sciences, Wagga Wagga NSW 2678 Australia.
authors.forename: James M
authors.initials: JM
authors.lastname: Mwendwa
authors.affiliation: Graham Centre for Agricultural Innovation (Charles Sturt University and NSW Department of Primary Industries), School of Agricultural and Wine Sciences, Wagga Wagga NSW 2678 Australia.
authors.forename: Paul A
authors.initials: PA
authors.lastname: Weston
authors.affiliation: Graham Centre for Agricultural Innovation (Charles Sturt University and NSW Department of Primary Industries), School of Agricultural and Wine Sciences, Wagga Wagga NSW 2678 Australia Institute of Applied Ecology, Fujian Agriculture & Forestry University, Fuzhou 350002, China.
authors.forename: Geoff M
authors.initials: GM
authors.lastname: Gurr
authors.affiliation: Division of Biological Science, University of Montana, Missoula, MT 59812, USA.
authors.forename: Ragan M
authors.initials: RM
authors.lastname: Callaway
authors.affiliation: Graham Centre for Agricultural Innovation (Charles Sturt University and NSW Department of Primary Industries), School of Agricultural and Wine Sciences, Wagga Wagga NSW 2678 Australia leweston@csu.edu.au.
authors.forename: Leslie A
authors.initials: LA
authors.lastname: Weston
mesh_terms: D002851:Chromatography, High Pressure Liquid; D036661:Echium / Q000378:metabolism*; D058865:Introduced Species; D013058:Mass Spectrometry; D009285:Naphthoquinones / Q000378:metabolism*; D009517:New South Wales; D018517:Plant Roots / Q000378:metabolism; D057685:Plant Weeds / Q000378:metabolism*; D058441:Rhizosphere; D013050:Spectrometry, Fluorescence
chemical_list: D009285:Naphthoquinones
keywords: Localization; SPRE; periderm; plant secondary products; rhizosphere; shikonins; soil microprobes; transport.