Identification and localization of bioactive naphthoquinones in the roots and rhizosphere of Paterson's curse (Echium plantagineum), a noxious invader

Xiaocheng Zhu, Dominik Skoneczny, Jeffrey D. Weidenhamer, James M. Mwendwa, Paul A. Weston, Geoff M. Gurr, Ragan M. Callaway, Leslie A. Weston

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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 48 h 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.

Original languageEnglish
Pages (from-to)3777-3788
Number of pages12
JournalJournal of Experimental Botany
Volume67
Issue number12
DOIs
StatePublished - 2016

Funding

The authors would like to express their appreciation to the Australian Research Council Discovery Program which funded this study through DP130104346 grant awarded to LAW, GMG and RMC and to the Graham Centre for Agricultural Innovation which supported a research award to DS and a research initiative award to LAW. JDW acknowledges support for this project from a 2014 Endeavour Research Fellowship sponsored by the Australian Federal Government through the Department of Education. We also acknowledge the support of Dr. Robert Woolley at Coherent Scientific who assisted with laser confocal imaging.

FundersFunder number
RMC Research Corporation
Australian Research CouncilDP130104346

    Keywords

    • Localization
    • Periderm
    • Plant secondary products
    • Rhizosphere
    • SPRE
    • Shikonins
    • Soil microprobes
    • Transport

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